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Improving Electric Power System Reliability

Autor: Radomir GoNO”;~~ and Stanislav RusEK*  ~Department of Electrical Power Engineering, Faculty of Electrical Engineering and Computer Science, VSB-Technical University of Ostrava, 17, Iistopadu 15, 708 OO Ostrava, Czech Republic  E-mail .’

The paper deals with analysis of data about distribution network device failures and electric power supply outages from distribution companies. There are also suggestions how to improve reliability using new materials. Since each company has different structure of data monitoring the uniform methodology was created to enable comparison of processed results. Data structure, the way of distribution network device reliability evaluation and some of the preliminary results of the analyses are presented in this paper. There are also described the basic principles of the Reliability Centered Maintenance-RCM and its application to electric distribution network devices. Its aim is more effective maintenance program of equipment. The inputs of such analyses are databases of outages, maintenance, equipment condition and financial flow. The last part of the paper consists of introduction of fuel cell systems reliability in context of the grid-1inked operation.


Institutional changes associated with market liberalization taking place all over the world change drastically the approach to power supply quality. It develops towards a purely commercial matter between suppliers and their customers. The supply that does not comply with agreed qualitative parameters will lead to trade disputes and financial settlements. So-called undelivered energy, including its valuation, arrives on the scene.

Electricity market liberalization has started in both countries. It is a little bit faster in the Czech Republic than in Japan. The market will be opened completely for all consumers from I st January 2006 in the Czech Republic.

Current trends toward deregulation and heightened competition make distribution companies provide a stable and reliable supply of electricity. That is why they have to seek suppliers offering the highest quality, competitively priced products and services available in the global marketplace today.

The two following aspects of supply quality may be considered:

  • Supply reliability-relating the availability of electricity in the given location, by another name lower number of interruptions.
  • Voltage quality-relating the purity of characteristics of the voltage waveform, including the absolute level of voltage and frequency.

This paper deals in more detail with the former point.

Lowest price in fact means reasonable, balanced price of electricity as a compromise between costs and certain reliability level.

Since this is journal of materials engineering we would like to point out new materials that are used for improving reliability of electric power network.


The principal goal of monitoring events occurring in the distribution networks is to secure the reliable supply of electrical energy to consumers in accordance with the Distribution Network Grid Code Ref. [1] . The rate of reliability may be determined from databases of events by means of global indices of supply reliability or reliability indices of particular elements.

The world centers of reliability analyses provide electronic databases of information about availability of electronic and no electronic components and distribution function of kind of failure. They contain not only resultant failure rate but we can even get to the producer, operation conditions, etc. It is possible to use these databases for prediction of availability of complicated systems. Unfortunately, databases do not contain data about electric power equipments operated in our conditions.

In the former Czechoslovakia exclusive database of failures, outages and damaged equipments in whole electric power system began to rise in 1975. Unfortunately, the database filling has been stopped since 1990 because of political and social changes. Distribution companies obtained independence and started introducing their own systems.

Later, according to the new world trends, distribution companies decided to uniformly monitor global indices of reliability and also reliability of chosen pieces of equipment. That necessary data for analyzes are stored centrally at our research workplace where item reliability is also processed Ref. [2] . Data are hand on and process since the year 2000.

In Japan There is The Federation of Electric Power Companies that stores this data.

2.1 Databases of pieces of equipment

The majority of utilities create statistics for the reliability of network components, including lines, transformers, etc. They are especially collected to identify an unreliable piece of equipment and to be used as the input into probability calculations of system behavior.

Basic data on the reliability of equipment and elements of systems is as follows:

  • Failure rates of particular pieces of equipment and elements.
  • Outages of the piece of equipment due to maintenance and inspections.
  • Outages of the piece of equipment due to operating works on the piece itself and labor safety securing in the vicinity of live parts of the distribution system.

For consequential evaluation of reliability it is necessary to have data on the number and range of the examined piece of equipment.

Thus it is not only the case of data on e.g. the number of failures and the mean duration of a failure of the line of the certain type and voltage level, but also the case of data on the total range of the observed piece of equipment, i.e. here on the total length of the line of the certain type and voltage level.

The result of analyses is the determination of the failure rate and mean failure duration for particular items of equipment or for group of equipments. With more detailed databases, other pieces of information may be found that are important for operators, such as the most frequent cause of failures, networks with the greatest amounts of undelivered energy, etc.

This data also serves to evaluate properties of pieces of equipment already operated (or a piece of equipment of the certain type of the selected supplier) , to select new pieces of equipment, to assess the time suitable for restoring pieces of equipment at the end of their lives, to choose the operating mode of the HV network node, and others. Unfortunately, some distribution companies monitor only failures of major units because they do not interested in determination of item reliability at all.

The concrete reliability indices of the item of equipment are calculated according to following methodology Ref. [3] .

For the failure rate the following relation may be written:

N  the number of failures
Z  the number of elements of the certain type in the network
X  the considered period (year)

For the mean duration of the failure the following is valid:

Nr  the number of failures of the element of the certain type
Ti   the duration ofthe failure of the element ofthe certain type (h)

2.2 Data Structure and Range

Eight distribution companies from the Czech Republic and one from the Slovak Republic furnish Technical University of Ostrava with the databases of outages and failures. Individual companies release the data different formatted in various data files; thus it is necessary to choose different approaches to data conversion to unified database. Thirty one attributes were chosen in amount attributes for the data analysis. These attributes are depicted in Table 1.

Today database contains more than 400 thousand records on voltage levels 110 kV, MV and partially LV.

We use an n-ary relation, where n=31, to model the data. If queries for values of all attributes will be defined we may apply a multidimensional data structure for indexing the relation. We can apply R-tree or Ub-tree. The data structures provide better efficiency to querying many-attribute relation than classical B-tree applied in mainline DBMSS (database management system) . In the case the indexed space is space of dimension 3 1 .

2.3 Some results

The determination of item reliability from total database has been so far provided for the first level structuring (according to type of equipment) : distribution transformer station MV/LV, cable line, transformer station MV/MV and switching station MV and overhead line.

The graphic tendency of the failure rate Eq. (1) and the mean failure duration Eq. (2) interpreted from data mentioned above is displayed for 22 kV cable in the Figure. I .

Great differences between particular periods with one company are evident as well as those between particular companies. These results are to be accepted with reserve, because the statistical reliability of data from the five-year period is insufficient in the area of electrical power engineering and the method of event observation is still different with particular companies. From this the necessity of the long-term observation of failures and outages and the need to unify observation methodologies follow. With more detailed databases, other pieces of information may be obtained that are important for operators. In Figure. 2 there is an analysis of causes of failures.

2.4 How we improve parameters

There are some possibilities as for how to improve reliability:

  • Maintenance free equipment-eliminate planned outages. The certain element is not capable to perform its function because it is unavailable during maintenance.
  • Indirect live-wire operations shorten working interruptions-new insulating materials for working tools.
  • Remote controlled disconnecting switches that can cut off nominal currents-eliminate manipulating time and money. They are also maintenance free.
  • Distribution companies in the Czech Republic started to install insulated overhead lines in forest areas.
  • Sulfur Hexafluoride or vacuum is used for circuit breakers instead of oil.
  • Paper-insulated cables were replaced by polyethylene-insulated cables. There is a comparison of databases 1975-1990 and 2000-2004 in Table 2.

Figure 1 Tendency of reliability indices

Figure 2 Failures according to their causes


This part of the paper is related to lifetime estimation of used materials and effective spending of resources.

Maintenance is, from the point of view of reliability, such state of the piece of equipment when the certain element (or a group of elements) is not capable to perform its function because it is unavailable due to maintenance. Then each maintenance outage means, in principle, a decrease in the certain system reliability.

If an element is under maintenance, it is neither in operation, nor is available. In the case of the series reliability system any maintenance outage causes the outage of the whole system. In the case of the parallel reliability system, any maintenance outage causes a decrease in the total reliability of the system; thence it affects the intensity and the mean duration of outages.

RCM is a decision-making tool that makes it possible to control or improve the maintenance schedule. RCM provides underlying data for adequate and logical decisions and is applied outside existing control maintenance systems. Views obtained by means of the RCM method are then utilized in modifying or redefining the existing maintenance schedules. If this method is employed properly, RCM may make the existing maintenance schedules more efficient and optimized.

The goal of reliability-centered maintenance is to formulate such a maintenance strategy so that the total operating costs may be minimized at keeping the necessary degree of the reliability, safety and environmental soundness of equipment operated.

It is necessary to take many steps first that can be briefly summarized in the following points Ref. [4] :

  • The determinations of all items of equipment that are subject to maintenance and thus take part in the RCM process itself.
  • The determination of functions of these items of equipment.
  • The determination of a resultant model of equipment aging.
  • The determination of equipment importance.
  • The identification of equipment failures and their consequences.
  • The setting up of the equation of total operating costs of  equipment and the finding of the most suitable forms of maintenance.

First step at the RCM implementation is decision for all maintained equipment what type of maintenance will be applied to them. Generally it is possible to choose from following:

  • To keep existing system of maintenance (periodical according to Preventive Maintenance Order) .
  • Operation to failure (Corrective Maintenance) -regular inspections and necessary measurements required will be performed for recognition if the equipment is safe operating.
  • Periodical RCM.
  • On condition RCM.

From the point of view of the technical condition determination of equipment and also its reliability, we can differentiate two kinds of RCM-periodical (optimization of the maintenance cycle) and on condition (determination of the order of components for maintenance) Ref. [5].

In the area of transmission and distribution networks such items of equipmefit as transformers, outdoor and cable lines, switching elements, protection devices, etc. will be included into the RCM system.

In the case of these elements, the foundation for RCM applications is to determine the aging model and the so-called importance of the element. This is mostly expressed by the costs of element maintenance, the costs of element repair and costs of element outage.

At the Department of Electrical Power Engineering at Technical University of Ostrava we have already been concerned with the development of a methodology for RCM for some years.  Our main objective is its practical utilization and inclusion into the system of maintenance of the electrical power company. Because the RCM system utilizes many information sources and will optimize the maintenance of several thousand components, it was necessary to design a software tool for useful processing just the same amount of data.

After studying the theory of the RCM system we chose two basic approaches to RCM implementation in the framework of distribution power networks. One approach leads to the optimization of the maintenance cycle for all components of the certain type or groups of components of the same type. The other approach leads to the optimization of condition-based maintenance (on-condition maintenance) , i.e, to the determination of optimum order of maintenance of particular components of the same type (Ref. [6] ) .

The approaches will be applied according to a specific component of the distribution network. The comparison of the approaches is as follows:

  • Optimization of the maintenance cycle-the number of components ofthe certain type is high; generally, each component of the certain type has low importance, costs of the specific component of the certain type cannot be obtained, at the analysis of the event (failure, outage) the specific component cannot be found.
  • Determination of the order of components for maintenance-the boundary must be defined from when performing maintenance is reasonable not only from the economical point of view, equipment monitoring is possible (e.g. on-1ine monitoring) , we must be able to determine the condition and importance of equipment.

The block diagram of the program is given in Figure. 3 . The basic inputs are databases of the distribution company (technical information system-TIS and financial information system-FIS), from which required data will be read. The inputs, which will be entered by the program operator, are mainly criteria for the determination of the condition of the component (weights of particular influences) and criteria that will serve the determination of importance of particular components. Furthermore, control on the part of authorities must be taken into account, such as penalties imposed for not obeying the standards prescribed for electricity supplies.

For the first group of types of components the output of the program is the optimum maintenance cycle, for the second group of types of components then the optimum
maintenance (coordinates of components condition and importance of them) .

3.1 Optimization of the maintenance cycle

The determination of the optimum maintenance interval is based on the cost function. For each item of equipment, an equation of the total operating costs as function of maintenance rate is necessary to be set up and its local minimum is to be found. The simplified cost equation will consist of three basic parts and will express the total operating costs per year of operation of the piece of equipment:

  • Maintenance costs Nu
  • Repair costs No
  • Outage costs Nv

Additional costs are omitted. The cost function has the following form:

Nc=Nu+Na+Nv (CZK. year~*) (3)

The main simplifying assumption for the quantification of particular cost items is the fact that these particular cost items will not change with time, or that their percentage increases will be roughly equal.

For the sake of simplification it is possible to say that the maintenance costs and repair costs depend on the maintenance and repair rates, and thus on the kind and the condition of the certain element (on its aging model). The outage costs depend on the kind and condition of the element and, in addition, on the location of the element in the electrical power system (depending upon system configuration) ; i.e. on the importance of the element.

Detailed information about particular parts of cost function is introduced in Ref. [7] .

With reference to the fact that any “importance” cannot be assigned to any specific component (neither FIS, nor TIS divides data up to a specific piece of equipment) , it is necessary to proceed to data division into groups. Then, maintenance intervals of the groups are different. Input data for the division of components into groups by importance are as follows:

  • For all components of the certain type-coefficient for consumer evaluation, the number of groups for division and their limits and the type of component.
  • Separately for each component-identification number, the number of connected consumers by type, possible another division of the component.

The result of the division of components into groups by importance is the determination of the amounts of components in particular groups and the assignation of a group number to each component.

Figure 3 Block diagram of the program

Input data for the RCM analysis itself are maintenance costs, repair costs, failure rate, total time of failures, time of scheduled outage, number of all consumers, including their types, number of outages at not obeying the standards, penalties, price of undelivered electrical energy for specific types of consumers, relationship between costs of undelivered energy by particular types of consumers, relationship between costs of outage by specific groups, maintenance rate and the average power passing through the certain component. The given data are related to the period under consideration of one year.

Sources of these input data are exports from technical records, failure databases and financial databases, or the data are entered directly by the keyboard and are stored in a special file.

There is created maintenance schedule for all equipments of particular group on the basis of the calculated optimal maintenance rate. For example if the optimal maintenance rate is at the 0.2 (Figure. 4), it means once every five years, maintenance will be performed every year approximately for one fifth from all equipments.

3.2 Condition-based maintenance

By means of monitoring systems and various diagnostic methods the condition of a piece of equipment is determined. On the basis of the condition one may assess how long a piece of equipment will be probably run until a functional failure occurs.  With reference to the fact that this is a rather expensive matter, this type of maintenance will be applied especially to expensive and operationally important pieces of equipment, such as EHV, HV transformers, etc.

The structure of input data depends on the specific component. Generally, they may be divided into three groups. For instance, for  110 kV power circuit breakers the structure of input data is as follows:

a) Identification of the specific component-identification of the  specific circuit breaker, electric station, field/outlet, year of putting  into service, type of circuit breaker, extinction medium, serial number of circuit breaker, year of circuit breaker manufacture,  kind of drive, type of drive, serial number of drive, year of drive manufacture .

Figure 4 Graphical Representation of the Cost Function

b) Data determining the condition of this component-condition of circuit breaker, date of the last action, tightness of extinguishing chamber, date of the last overhaul of contacts, number of engine hours of compressor after overhauling the contacts, number of close/open (CO) cycles after the overhaul of contacts, date of the last overhaul of compressor (drive) , number of engine hours of  compressor after overhauling the compressor, number of CO cycles after overhauling the compressor (drive) , date of diagnostic  tests, evaluation of diagnostic tests of circuit breaker, date of technical condition evaluation, climatic conditions, CO number, number of compressor engine hours, condition of metal parts, earth wire condition (protection against dangerous contact with non-live parts) , condition of insulators.

c) Data determining the importance of this component-importance of circuit breaker, circuit breaker location, type of line, possibility  of backup, importance of consumption, energy transmitted per  year.

The result of maintenance by condition and impedance is a graph with the layout of particular pieces of equipment (Figure. 5) . On the basis of this graph, the optimum order of components  for maintenance is then determined.


In the so called “White Report” the EU member states have committed themselves to roughly double the share of renewable energy sources in covering their gross domestic energy consume by 20 lO. This strategic target is based on the premise that each nation shall contribute by its own input in accordance with the national conditions. According to EU agreement the Czech Republic will produce 896 of electricity from renewable sources in  2010. But there is a great problem with continuity and reliability of supply from most of renewable energy sources. Our university research focuses on applications of fuel cell. We would like them to support increasing share of renewable energy sources as a back up and some kind of energy store.

The reliability of the current generation of fuel cell power plants is one of the most critical issues. Analyses of the reasons for the reduced reliability of many fuel cell power plants in US and Japan have shown that by far the vast of forced shutdowns have been caused by the failure of the balance of plant components.

 Figure 5 Maintenance by condition and importance

Ultimately, fuel cell plants can have on-stream availabilities of 98-99%, values that are currently achieved in conventional hydrogen plants (Ref. [8] ).

The high reliability of a fuel cell system will largely result from the modularity of the stacks (their ease of maintenance-only partial shutdown is necessary) , but should also be attributable to  their lack of highly stressed moving parts operating under extreme conditions and to their ease of maintenance. The use of modular units permits a site layout that that can be designed to permit the replacement of complete modules, which not only allows for a more economical use of spare parts but also minimizes lost output. Also, by replacing spare modules, a plant could be operated at full power during periods of routine maintenance, should be this necessary. Even without spare parts, plants could be designed so that only partial shutdown will be necessary in the event of failure.

The following conditions are required for grid-connected plants:

  • To maintain high quality of electricity (higher harmonics, voltage regulation, frequency change, and so on) .
  •  To provide suitable protection in the event of system faults.

In the case of “no supply of electricity to the grid” the plant should be equipped with a “reverse power protection system,” which may be a dummy load or other appropriate equipment.

Fuel cell generators are being considered for grid-connected  (grid-parallel) , grid-connected is landing (provide power to local  loads at the time of grid outage), grid-independent (stand-alone) as well as grid linked (allowing power import but not export to grid) operations (Ref. [9] ) .

Although the utility grid is generally a reliable power source, its design allows substantial voltage surges or sags, and allows momentary interruptions to clear faults. Recently, the grid outages are occurring more frequently. When a combination of a fuel cell power plant and grid as a further back up are used, unprecedented availability of the power supply can be achieved.

Fuel cell systems have high-energy efficiency, zero or near-zero emission of harmful exhaust gas (using hydrogen) . However,  there are several problems that keep FC from commercial use.

4.1 Appropriate type of FC

For transportation and residential power-output up to 100 kW-are used low temperature FC and for power plants-output more than I MW-are used high temperature FC.

There are mentioned only main material requirements for FC in this paper. For all FC types sulfhr substances have poisonous  impact.

  • PEMFC (Proton Exchange Membrane Fuel Cells-solid  perfluorinated-sulfonic acid) -require polymer electrolyie with a  good conductance of protons, very pure hydrogen as the fuel, input gases must be humidified, expensive platinum catalyst, they have high performance degradation and there is a poisonous impact of  CO.
  • FC (Alkaline Fuel Cells-liquid potassium hydroxide) -require pure hydrogen and pure oxygen as the reactants and there is a  poisonous impact of CO and C02.
  • AFC (Phosphoric Acid Fuel Cells-1iquid phosphoric acid dispersed on Teflon-bonded silicon carbide) -use expensive noble metals catalysts, Iiquid electrolyte that can migrate, and there is a poisonous impact of CO.
  • CFC (Molten Carbonate Fuel Cells-molten alkali (potassium or sodium) carbonate retained in a lithium aluminium oxide  (LiAI02) matrix) -have high temperature liquid electrolyie that  can migrate, properly engineered stainless steel.
  • OFC (Solid Oxide Fuel Cells-solid yttria-stabilized zirconia) -require high temperature components with the same thermal expansion coefficient and a good seal management. Unfortunately, all ceramic construction cause higher internal resistance.

4.2 Fuel

The fuel cell reaction requires hydrogen, but there are several methods ofproducing hydrogen from fossil resources. If electricity is generated by natural energy (solar power, wind power, etc) ,  water could be electrolyzed to obtain hydrogen gas.

  • Hydrogen-only water emissions (depends of production) .
  •  Methanol-1iquid fuel (concentrated energy) but requires reforming heating to about 250~C.
  •  Gasoline-widespread available but requires reforming heating to about 800 “C . If gasoline includes sulfur, it cannot be reformed  easily.
  • Biodiesel is similar like gasoline but does not include sulfur, is not toxic and is biodegradable.
  • Natural gas-internal reforming in high temperature FC, the sulfur compounds must be removed first.

In area of Ostrava with heavy industry we would like to utilize bio-gas from waste-water treatment or farming and methane from coalmines.

 4.3 Storage of hydrogen

There are several methods of hydrogen storage but also with their problems:

  • High-pressure hydrogen tank-high pressure (e.g., 35MPa) requires robust tank that are heavy.
  • Adsorption Storage-metal hydride and carbon nanotubes alloy-has a very high storage capacity but those storages are quite heavy and very expensive.
  • Liquid-hydrogen requires heat-insulating tank-extremely low temperature (-253’O cause material shortness.

4.4 Price

The price is one of the most critical issues facing the commercialization of this technology today. In case of today price of fossil fuels, FC do not have a chance to compete with other  sources.


Electric power distributors have to be prepared that they will be penalized for unsupplied electricity. Monitoring of failures and outages in the transmission and disinbution of electrical energy is necessary for determination of the reliability of network elements and the supply of electrical energy to consumers. Incorrect input data leads, of course, to false results even if the correct computing method is used. For consequential evaluation of reliability it is necessary to have data on the number and range of the examined piece of equipment. We have been canying out the observing and analyzing of failures at the research workplace of the Department of Electrical Power Engineering since the year 2000. With reference to a rather small number of failures in the area of electrical power engineering, the results will be reliable only after several years. The larger data range, the more accurate will be statistical results. That is why we would be glad to have also a possibility to enhance the databases by data from other countries. Knowledge of the item reliability allows finding out defective production runs of the equipment of distribution network, oftenest cause of failures, how long outage causes particular equipment in certain year, areas of the greatest amounts of unsupplied energy, cost of outage etc. Knowledge of the item reliability is necessary for reliability calculations for connection of wholesale consumer that are more and more requested today.

Distributors also have to spend their resources effectively to be able to compete in the electricity market. This is impossible without application of new materials that increase equipment reliability, extend time between inspections and reduce eventually completely cancel time required for maintenance. Without the item reliability is impossible to introduce Reliability centered maintenance system. In this contribution, the RCM theory is summarized in a nutshell. The main problem is always to find reliable updated input data. Thus the primary task is a change in the existing structures of particular databases in the certain regional power distribution companies. The program was developed with the aim to be universal, so that it may solve both the approaches the optimization of the maintenance cycle and determination of the order of components for maintenance. All variables of the program may be entered from input databases and edited by means of the keyboard. The novelty of our approach is its applicability to real data from distribution company. This is verified because one distribution company included our results to maintenance schedule for several types of equipment.

The first experience with fuel cells shows that islanding option allows power delivery to the local critical load during grid outage.

This capability also allows a stand-alone ftiel cell system to operate at the base load and draw peak power from the grid. Most fuel cell systems are being designed to provide an availability of  >959i~. Several of these plants can be connected in parallel to  achieve an even higher level of availability. It is necessary to develop new cheaper materials that ensure reliable operation of  FC.


This work is supported by The Ministry of Education, Youth and Sports of the Czech Republic. Project MSM6198910007.


[1] Distribution companies of the Czech Republic, “Distribution  Network Grid Code”, Appendix No. 2-Methodology of reliability determination of electric power supply and distribution network equipments, Prague, (2001) .

[2]  Rusek, S., Proch~zka, K., “Metodika ur~0v~ni spolehlivosti dodavky elektrick6 energie a prvku disinbu~n~rch soustav  (Methodology for the determination of reliability of elecincal energy supply and elements of distribution systems) ,”Conference ~K CIRED, pp. 4/16-4120, Tabor, (1999) .

[3]  Rusek, S. “Spolehlivost elektrick~ych siti (Reliability of  electric networks) ,” V~B-TU Ostrava, ISBN 80-7078-847-X,  (200 1 ) .

[4]  Moubray J “Reliability centred Maintenance”, Butterworth-Heinemann, (1997).

[5]   Skog, J., “Maintenance Task Interval Determination,”  Maintenance and Test Engineering Co. USA ( 1999) .

[6]   Rusek, S Raska T , “Zuverlassigkeitsorientierte  Wartungsplanung im Verbundnetz,” Jubilaumsforum  Netzbau-und Betrieb, Potsdam, (2002).

[7] Rusek. S.. Gono. R., “Reliability Centered Maintenance and its Application to Distribution Networks,” International Scientific Conference EPQU 2003, Cracow, pp. 529-536, ISBN 83-914296-7-9, (2003).

[8]   Blomen, L.J.M.J., Mugelwa, M.N., “Fuel cell systems,” Plenum Press, 614 p., ISBN 0-306-44158-6, (1993)

9] Farooque, M., Maru, H.C., “Fuel Cells-The Clean and Efficient Power Generators,” Proc. IEEE, vol. 89, No. 1 2. p 1819-1829, (2001)

Effect of utility-induced surges in a steel mill with variable speed drives (Practical Grounding, Bonding, Shielding and Surge Protection)

Posted Feb 12 2014 by Edvard in Energy and Power, Variable Speed Drives on Electrical Engineering Portal

Case study - Effect of utility-induced surges (A steel mill with variable speed drives); photo credit:

Case study – Effect of utility-induced surges (A steel mill with variable speed drives); photo credit:


A steel mill with variable speed drives (VSDs) had problems of frequent tripping of the VSDs with the indication ‘overvoltage in AC line’. Each tripping caused severe production disruption and resulted in considerable monetary loss due to lost production.

Steady-state measurements by true RMS voltmeter showed that voltage was normal and within the specified operating range of the VDS. A power line monitor was then used in the distribution board feeding the VSDs and the incoming power feeder to the mill. At both locations, the monitors showed transient overvoltages of damped oscillatory type waveform with an initial amplitude of over 2.0 pu and a ringing frequency of about 700 Hz.

The timing of disturbances coincided with the closing of capacitor banks in the utility substation feeding the steel mill (refer Figure 1a below).

Figure 1a - Distribution arrangement

Figure 1a – Distribution arrangement


It was confirmed by the VSD manufacturer that the VSDs were provided with overvoltage protection set to operate at 1.6 pu voltage for disturbances exceeding 40 µs.

Since the switching transients were above this protection threshold, the VSDs tripped.

It may be noted that switching on a bank of capacitors results in high charging current inrush. When this current passes through the line’s inductance L, a momentary voltage surge occurs. Further interaction of the capacitor C with inductance L results in an oscillatory flow of current, which is damped by the resistance R in the system.

The oscillatory disturbance superimposed over the normal power-frequency voltage wave caused the overvoltage protection to operate.


The solution lies in reducing the transient peak to a value that is below the overvoltage protection threshold.

This was achieved in this case by installing a surge protection device (SPD) in each VSD. The SPD clamped the transient to a peak value of 1.5 pu thus avoiding the operation of overvoltage protection.

Figure 1b - After additions

Figure 1b – After additions
Another possible solution would have been to install an inductor L1 in the switching circuit of the capacitor for a few seconds and then shunt it by switch S.

Since the voltage seen by the incoming feeder to the mill would be the combination across C and L1, the transient will have a smaller amplitude. This solution will however call for cooperation from the utility as it involves additional equipment to be installed by them (refer Figure 1b above).

Reference: Practical Grounding, Bonding, Shielding and Surge Protection – G. Vijayaraghavan, Mark Brown and Malcolm Barnes (Get your harcopy from Amazon)

Guidelines for producing Power Quality case studies for the web

Here are some guidelines to produce an informative Power Quality case studies that will help to sell your skills or mitigating solutions. These sections should be covered:

  • Introduction – The problem statement and the consequences
  • Analysis – what are the steps taken to analyze the problem
  • Solution – what solution has been chosen to mitigate the problem
  • Conclusion – show how effective is the solution Continue reading

Identification of Unhealthy Power Systems with Non-Charateristic Harmonics

International Journal of Energy Engineering 2011; 1(1): 12-18 DOI: 10.5923/j.ijee.20110101.03

Identification of Unhealthy Power Systems with Non-Charateristic Harmonics

Xiaodong Liang1,*, Y. Luy2

1Edmonton Product Center, Schlumberger, 10431 35A Ave., Edmonton, Alberta, T6J 2H1, Canada 2Research and EMS, Schlumberger, 42 Rue Saint Dominique, Paris, 75007, France

1. Introduction

Due to the wide application of non-linear loads, harmonics pollution is one of the main concerns for the power systems. Although great progress has been made on harmonic mitigation by manufacturers of nonlinear devices, industry facilities and utility companies, serious harmonic issues can still be present in electrical power systems especially when ill conditions happen either caused by utility companies or by end users in industrial facilities.

Variable frequency drives (VFDs) are most widely used in industrial facilities. Characteristic harmonics defined by IEEE std. 519-1992 are based on various system configurations of VFDs[1]. Except some special loads such as arc furnaces[1] and railway traction systems which produce even harmonics and other non-characteristic harmonics into the system, most industrial facilities with VFDs only have characteristic harmonics and some small amount of non-characteristic harmonics which are usually under the even and triplen harmonics limits proposed by IEEE std. 519-1992. Therefore, when large amount of non-characteristic harmonics are present in a power system, it usually indicates that the system is unhealthy and that troubleshooting is required to determine the root causes of the problem.

Non-characteristic harmonics are investigated in[2-9]. It is explained in[2] that non-characteristic harmonics are caused by unbalanced voltage magnitude or phase non-symmetry. The amplitude of non-characteristic harmonics increases with increasing voltage non-symmetry.

Two cases are investigated in this paper for two industrial facilities showing large content of non-characteristic harmonics. The first case is for a mining power system with two rectifiers rated at 12MW each and with a set of harmonic filters installed. Extensive investigation is conducted for this system based on both measurements and computer simulation results. The second case is for an oil field distribution system with multiple VFDs. The issue for this system is that current waveforms at the input of VFDs are seriously distorted.

2. Characteristics and Noncharacteristic Harmonics of Rectifiers

IEEE std. 519-1992[1] proposes harmonic currents generated at a bridge rectifier for an ideal condition. “Ideal” is based on an assumption that dc current has no ripple and the dc current is transferred from one phase to another the instant the voltage on the incoming phase exceeds the voltage on the outgoing phase[1]. The harmonic current components for ideal condition are derived by the following equations[1]:

equa 1 equa 2+3


h : harmonic order

k and m : any positive integer

q : pulse number of the rectifier circuit

Ih: the amplitude of the harmonic current of order h

I1 : the amplitude of the fundamental current

For a 6-pulse rectifier or a variable frequency drive (VFD), the characteristic harmonic currents are 5, 7, 11, etc. For VFDs using phase multiplication technique such as 12-pulse and 18-pulse mitigating input harmonics, some harmonic currents can be cancelled compared to 6-pulse drives. As states in IEEE Std 519-1992, if m six-pulse rectifier sections [1]:

  • Have the same transformer ratio
  • Have transformers with identical impedances
  • Are phase shifted exactly 60/m degrees from each other
  • Are controlled at exactly the same delay angle, and
  • Share the dc load current equally

Table 1. Voltage harmonic distortion limits based on IEEE std. 519-1992

Voltage Harmonic Distortion Limits
Bus Voltage at PCC Individual Voltage Distortion, % Total Voltage Distortion, %
69 kV and below 3.0 5.0
69.001 kV through 161 kV 1.5 2.5
161.001 kV and above 1.0 1.5
NOTE: High-voltage systems can have up to 2.0% THD where the cause is an HVDC terminal that will attenuate by the time it is tapped for a user

Table 2. Current harmonic distortion limits based on IEEE std. 519-1992

Current Harmonic Distortion Limits for General Distribution Systems (120 V Through 69 000 V)
Maximum Harmonic Current Distortion in Percent of IL
Individual Harmonic Order (Odd Harmonics)
Isc/IL <11 11<h<17 17<h<23 23<h<35 35<h TDD
<20* 4.0 2.0 1.5 0.6 0.3 5.0
20<50 7.0 3.5 2.5 1.0 0.5 8.0
50<100 10.0 4.5 4.0 1.5 0.7 12.0
100< 1000 12.0 5.5 5.0 2.0 1.0 15.0
>1000 15.0 7.0 6.0 2.5 1.4 20.0
Even harmonics are limited to 25% of the odd harmonic limits above.
Current distortions that result in a dc offset, e.g., half-wave converters, are not allowed
*All power generation equipment is limited to these values of current distortion, regardless of actual Isc/IL
Where Isc= maximum short-circuit current at PCC. IL= maximum demand load current (fundamental frequency component) at PCC

Then the only harmonic present at the input of the drive will be of the order of kq ± 1 as shown in Equation (1). For example, characteristic harmonics for 12-pulse VFD systems with two rectifiers phase shifted by 30° are 11,13, 23, 25th,… For 18-pulse VFD systems with three rectifiers phase shifted by 20° the lowest characteristic harmonic is 17th. For 24-pulse VFD systems with four rectifiers phase shifted by tioned in[1] that no two rectifier sections are identical in all these respects. Therefore, non-characteristic harmonics will always be present to the degree that the above requirements are not met in practice.

IEEE std. 519-1992 proposes the recommended harmonic distortion limits (Tables 1 and 2), which are widely accepted by various industries. It is specified in Table 2 that the even harmonics are limited to 25% of the odd harmonic limits in the table[1].

In a 6-pulse rectifier or a variable frequency drive (VFD), the characteristic harmonic currents are 5, 7, 11, etc. For VFDs using phase multiplication technique such as 12-pulse

3. Case 1: Even Harmonics

Even harmonics are usually present in very small amounts and are not a concern for power systems under normal operating conditions. However, large amount of even harmonics could be generated in some ill conditions such as equipment malfunction. The situation could be amplified if the system contains harmonic filters that might excite an even harmonic resonance.

Case study 1 addresses a serious even harmonic issue happened in a large mining facility consisting of two large 6-pulse rectifiers rated at 12MW each. The system configuration is shown in Figure. 1.

Fig 1Figure 1. System configuration for Case 1.

The rectifiers are connected to a 10 kV common bus with Figures. 2 and 3 show seriously distorted current wavetwo groups of harmonic filters installed. The first group has a forms. The harmonic content shown in Figures 4 and 5 5and a 7single tuned harmonic filters. The second group contains large amount of even and 3harmonic currents. In has a 11 and a 13 single-tuned harmonic filters and a 17 order to trace the source of these harmonics, measurements high pass harmonic filter. The two rectifiers are connected to at the inputs of the two rectifiers were taken. The current the common bus through two 7MVA transformers with a 30° waveforms for the two rectifiers are shown in Figures. 6 and phase shift angle. Such configuration constructs a quasi 7. The corresponding current harmonic spectrums are shown 12-pulse rectifier system. For the case that two rectifiers in Figures 8 and 9. have exactly same loading during the operation, harmonic 18% cancellation of the 5and 7 harmonic currents will be thebest. When the loading of the two rectifiers are not equal, for example, 65% load factor for one rectifier and 80% load Harmonic current in % of fundamental factor for another rectifier, most 5,7, 17, and 19har onic currents are still cancelled and only small amount of these harmonics are left in the system.

Large even harmonic currents were detected at key loca tions of the system in March 2004. Large 3rd harmonic cur rents were also found. Such high even harmonics cause serious concerns and an investigation was performed to find out the root cause of the problem.

Even harmonics were first detected at the circuit breaker CB3 on the secondary of the 25MVA main service transformer. The measured current waveform at CB3 is shown in Figure. 2. For further verification, another measurement was taken at the circuit breaker CB6, feeding the 10KV common bus, “Rectifier Main Bus”. The measured current waveform t CB6 is shown in Figure. 3. The corresponding harmonic current spectrums at CB3 and CB6 are shown in Figures. 4 and 5, respectively.

Identification of Unhealthy Power Systems with Non-Charateristic HarmonicsFigure 2. Current waveform at CB3 on the secondary of the 25MVA main transformer 1 (CT ratio is 3000:1).

Fig 3

Figure 3. Current waveform at CB6, main feeder for two rectifiers (CT ratio is 2000:5).

Figures. 2 and 3 show seriously distorted current wave-forms. The harmonic content shown in Figures 4 and 5 contains large amount of even and 3rd harmonic currents. In order to trace the source of these harmonics, measurements at the inputs of the two rectifiers were taken. The current waveforms for the two rectifiers are shown in Figures. 6 and 7. The corresponding current harmonic spectrums are shown in Figures 8 and 9.

Fig 4

Figure 4. Harmonic current spectrum measured at CB3.

Fig 9

Figure 5. Harmonic current spectrum measured at CB6.

Fig 6Figure 6. Current waveform at CB7 at the input of Rectifier 1 (CT ratio is 600:5).

Fig 7Figure 7. Current waveform at CB8 at the input of Rectifier 2 (CT ratio is 600:5).  Fig 5   Figure 8. Harmonic current spectrum measured at CB7 at Rectifier 1.Fig 8Figure 9. Harmonic current spectrum measured at CB8 at Rectifier 2.

Due to large amount of the 2, 3and 4harmonic currents as shown in Figures. 8 and 9, the current waveforms at the two rectifiers in Figures. 6 and 7 do not show the typical current waveform shape of 6-pulse rectifiers. For the comparison purposes dominant harmonic currents at key measurement points are summarized in Table 3. Dominant harmonic currents measured at the two rectifiers in April 2002 are also included in the same table.

Table 3. Measured harmonic current spectrums in March 2004 and April 2002 at Key location of the facility.

Harmonic order Harmonic current in percent of the fundamental, %
March 2004 April 2002
CB 7 Rectifier 1 CB8 – Rectifier 2 CB3 – Secondary main TX CB6 – Feeder to Rectifiers Rectifier 1 or 2
2 16.8 34.3 7.0 12.2 6
3 4.8 17.2 6.9 11.7 1.8
4 8.1 6.9 15.4 26.9 2.3
5 24.8 17.1 2.1 1.8 29.1
6 4.1 6.0 5.3 10.9 1.0
7 9.1 8.7 1.0 2.1 2.3
8 1.8 3.7 1.5 2.5 1.4
9 1.8 0.9 0.5 1.2 0.5
10 1.4 3.4 0.3 0.4 1.1
11 6.5 2.8 0.1 0.5 4.9
12 0.8 1.1 0.1 0.2 0.1

Table 3 indicates that 16.8% and 34.3% 2nd harmonic currents in percent of the fundamental were generated by Rectifiers 1 and 2 in March 2004, respectively. The 4th harmonic currents are the second largest even harmonics present in the amount of 8.1% and 6.9% of the fundamental for Rectifiers 1 and 2. The two rectifiers also generated large 3rd harmonic currents in the amount of 4.8% and 17.2% of the fundamental. The 3rd harmonic currents will be discussed separately in the next section for Case 1.

Characteristic and non-characteristic harmonic currents were flowing upstream of the rectifiers. They first went through CB6, the main feeder of the two rectifiers, thendistributed to other parts of the distribution system. Some characteristic harmonics such as the 5th harmonic currents met at the common bus, “Rectifier main bus”, and most of them were cancelled due to the phase shifting of the two transformers. As shown in Table 3, the 5th harmonic current is 24.8% at Rectifier 1 and 17.1% at Rectifier 2, most of the 5th harmonic currents are cancelled at the Rectifier main bus, and the 5th harmonic current remains only 1.8% at CB6.

As per the non-characteristic harmonics, the 2nd harmonic current is reduced to 12.2% of the fundamental at CB6 from originally 16.8% and 34.3%, which is reduced. However, the 4th harmonic current increases to 26.9% of the fundamental from originally 8.1% and 6.9%. Similarly, the amplified 4th harmonic current, 15.4% of the fundamental, is also found at CB3. The cause of the amplification of the 4th harmonic current at the upstream electrical circuit is investigated by performing the harmonic and resonance study.

A resonance analysis indicates that due to the connection of the two groups of harmonic filters, the peak impedance points created are located at the 4th harmonic frequency (240Hz for the 60Hz system). Since the rectifiers generated a significant amount of even harmonic currents, serious resonance and amplification took place and resulted in large 4th harmonic currents flowing in the system. The system frequency response characteristics at the 10 kV “Rectifier Main Bus” are shown in Figure. 10. The frequency response characteristic curve at 10 kV “Main Bus” connected to the secondary of 25MVA transformer is very similar to Figure 10.

Figure 10 indicates due to the connection of the 5th, 7th, 11th, and 13th single-tuned harmonic filters a few that the peak impedance points located at 240Hz, 360Hz, 480Hz and 720Hz are created. For a 60Hz system, these frequencies correspond to the 4th, 6th, 8th and 12th harmonics. Peak impedance points are also known as resonance points. The 4th and 6th harmonic currents in Table 3 are significantly increased at CB6, this is the result of the harmonic resonance.The 8th and 12th harmonic currents in Table 3 do not show obvious amplification at CB6 because the inductance of the two 7MVA transformers at the rectifiers branches block part of these higher order even harmonic currents going through to the upstream system.

The harmonic current spectrum taken at one of the recti-fiers in April 2002 shows the harmonic content of the recti-fiers under normal operating conditions (Table 3). It is found that non-characteristic harmonics including the 2nd, 3rd, 4th, 6th etc were very small at that time.

The analysis indicates that the two rectifiers generate large amount of non -characteristics harmonics. Thenon-characteristic harmonic currents in the  4th, 6th harmonic frequencies are amplified due to parallel resonance in the system. That is the reason that a 26.9% 4th harmonic current was detected a t the 10 kV “Rectifier Main Bus”, CB6. It is very likely that a malfunction on the rectifiers caused the non-characteristic harmonics problem.

Subsequent troubleshooting of the rectifiers verifies that a malfunction on the rectifiers caused this problem. The malfunction was corrected, the large amount of non-characteristic harmonic currents disappeared from the system.

Fig 10Figure 10. Frequency response characteristic at 10KV “Rectifier Main Bus”.

4. Case 1: Third Harmonics

Large amounts of 3rd harmonics were also found in Case 1 during the malfunction of the rectifiers in March 2004 (Table 3). The worst case was at Rectifier 2 with 17.2% of 3rd harmonic current. The 3rd harmonic current also appeared in the upstream circuit at CB 6 and CB 3.

References[8,9] provide the explanation that under the conditions of utility voltage unbalance, triplen harmonics such as the 3rd and 9th harmonic current can appear at the converters or rectifiers. Two examples are given in[9] with different line voltage unbalance conditions using a 460V 30kVA VFD. The 3rd harmonic currents in percent of the fundamental are 19.2% and 83.7% corresponding to 0.3% and 3.75% line voltage unbalance, respectively. For the case that there is no obvious line voltage unbalance, the 3rd harmonic current is 2.1% for the same drive[9].

Based on the same principle, the 3rd harmonic currents shown at the rectifiers in the mining facility in March 2004 were also caused by line voltage unbalance. The three phase-to-phase voltages at CB 8 for Rectifier 2 were measured for 6 hours with one measurement point every minute on March 17 2004. The line voltage unbalance is calculated for each measurement point. The voltage unbalance calculation method is based on an equation provided in[4]. According to[4] the voltage unbalance in percent is defined by the National Electrical Manufacturers Association (NEMA)

in Standards Publication no. MG 1-1993 as follows:

equa 4Note that the line voltages are used in this NEMA standard as opposed to the phase voltages. When phase voltages are used, the phase angle unbalance is not reflected in the % Unbalance and therefore phase voltages are seldom used to calculate voltage unbalance[4].

The calculated line voltage unbalance during the 6 hour trending measurement is shown in Figure. 11. Figure. 11 indicates that the voltage unbalance at Rectifier 2 for the measurement period ranges between 0.4% and 0.7% with most of the values falling between 0.5% and 0.6%. The calculated line voltage unbalance values explain why the 3rd harmonic current was as high as 17.2% at Rectifier 2.

Fig 11Figure 11. Calculated line voltage unbalance at CB8, Rectifier 2 based on measured three-phase line-to-line voltages for Case 1.

5. Case 2: Triplen Harmonics

Case 2 addresses a triplen harmonic issue in an oil field distribution system with multiple variable frequency drives (VFD) in operation. The current waveforms at the inputs of the VFDs are seriously distorted. A root cause analysis was required to find a solution to this problem.

As the first step for troubleshooting, measurements were taken at the input of each VFD. The measured current waveform for one of the VFDs is shown in Figure. 12. Other VFDs have similar current waveforms at their inputs. It is found that the two humps are not in the same magnitude for each half cycle in the current waveform.

Fig 12Figure 12. Measured current waveform at the input of VFD 1.

The corresponding harmonic current spectrum for the measured current waveform is shown in Figure. 13. This current harmonic spectrum contains 23% 3rd harmonic current and 13% 9th harmonic current in percent of the fundamental. On the other hand, even harmonics are small and all within 1.5%. Therefore, current waveforms distortions are caused by triplen harmonics only.

Fig 13

Figure 13. Harmonic current spectrum at the input of VFD 1.

Similar to Section 4, the line voltage unbalance for the 480V low voltage system is calculated by using measured three phase-to-phase voltages for VFD1. The measurement trending period is more than 5 days. The calculated line voltage unbalance during the measurement period is shown in Figure. 14. This figure shows that the line voltage unbalance values range between 0.2% and 0.9% with most of the values falling between 0.3% and 0.6%. With such line voltage unbalance from the utility power supply triplen harmonics are generated at the inputs of VFDs and thus further lead to serious current waveform distortions at the drive inputs.

Therefore, it can be concluded that the root cause of Case 2 is the voltage unbalance from the utility power supply.

Fig 14

Figure 14. Calculated line voltage unbalance at the input of VFD1 based on measured three-phase line-to-line voltages for Case 2.

6. Conclusions

Non-characteristic harmonics including even and triplen harmonics are investigated in this paper. Two case studies are conducted.

Case 1 deals with even harmonics generation caused by equipment malfunction of an industrial facility. Even harmonics, particularly the 4th and 6th harmonic currents are significantly amplified by resonance due to the presence of the 5th and 7th single-tuned passive harmonic filters in the system. Case 1 also shows that the root cause for a high 3rd harmonic current (up to 17% at one rectifier), is caused by a supply line voltage unbalance.

Harmonic current in % of fundamental

Case 2 presents a serious current waveform distortion issue in an oil field distribution system with multiple VFDs. Triplen harmonic currents are found at the input of the VFDs, but even harmonic currents appear to be normal. All drives in the system show similar situations. The calculated line voltage unbalance at the input of one VFD ranges between 0.2% and 0.9% based on measured three phase-to-phase voltages. It is concluded that the root cause of Case 2 is line voltage unbalance from the utility power supply.

Based on investigations in this study, when significant amount of non-characteristic harmonics appear in industrial facilities, it indicates that something can be seriously wrong in the system. A detailed root cause analysis and troubleshooting should be conducted to identify the root cause of the problem and to resolve it before equipment gets damaged or personnel gets injured.


[1] IEEE Std 519-1992, “IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems”, The Institute of Electrical and Electronics Engineers, Inc., ISBN 1-55937-239-7, USA

[2] Vaclav Kus, Zdenek Peroutka, Pavel Drabek, “Non-Characteristic Harmonics and Interharmonics of Power Electronics Converter”, 18th International Conference and Exhibition on Electricity Distribution (CIRED), 2005, PP. 1-5

[3] M. H. J. Bollen, S. Cundeva, S. K. Ro nnberg, M. Wahlberg,

0.6 Kai Yang, Liangzhong Yao, “A Wind Park Emitting Characteristic and Non-Characteristic Harmonics”, 14th International Power Electronics and Motion Control Conference (EPE/PEMC), 2010, PP. S14-22 -S14-26

[4] X. N. Yang, M. X. Han, H. Ding, “Non-Characteristic Harmonics Analysis of Double 12-Pulse Series Converters Based on Modulation Theory”, International Conference on Electric Utility Deregulation and Restructuring and Power Technologies, 2008, PP. 2091-2095

[5] A. D. Graham, “Non-Characteristic Line Harmonics of PWM AC-DC Converters”, Proceedings of 9th International Conference on Harmonics and Quality of Power, 2000, Vol. 3, PP. 955-960

[6] A. I. Maswood, Shen Wei, “A Twelve-Pulse Converter under Unbalanced Input Voltage”, the 7th International Power Engineering Conference (IPEC), 2005, Vol. 2, PP. 809-814

[7] Paul C. Buddinggh, “Even Harmonic Resonance – An Unusual Problem”, IEEE Transactions on Industry Applications, Vol. 39, No. 4, July-August 2003, PP. 1181-1186

[8] Sebastião E. M. de Oliveira and José Octávio R. P. Guimarães, “Effects of Voltage Supply Unbalance on AC Harmonic Current Components Produced by AC/DC Converters”, IEEE Transactions on Power Delivery, Vol. 22, No. 4, October 2007, pp. 2498-2507

[9] Annette von Jouanne and Basudeb (Ben) Banerjee, “Assessment of Voltage Unbalance”, IEEE Transactions on Power Delivery, Vol. 16, No. 4, October 2001, PP. 782-790

[10] Seung-Gi Jeong and Ju-Yeop Choi, “Line Current Characteristics of Three-Phase Uncontrolled Rectifiers under Line Voltage Unbalance Condition”, IEEE Transactions on Power Electronics, Vol. 17, No. 6, November 2002, pp. 935-945

[11] Arshad Mansoor, Jim McGee and Fang Zheng Peng, “Even-Harmonics Concerns at an Industrial Facility Using a Large Number of Half-Controlled Rectifiers”, Proceedings of IEEE 13th Annual Conference Applied Power Electronics Conference and Exposition, 1998, APEC’98, Vol.2, 15-19 Feb 1998, pp. 994-1000

[12] Ray P. Stratford, “Rectifier Harmonics in Power Systems”, IEEE Transactions on Industry applications, Vol. 1A-16, No.2, March/April 1980, pp. 271-276

[13] David E. Rice, “Adjustable Speed Drive and Power Rectifier Harmonics -Their Effect on Power Systems”, IEEE Transactions on Industry applications, Vol. 1A-22, No.1, January/February 1986, pp. 161-177

Harmonics Case Study

food processing harmonics

Harmonics Case Study:

Secure Food Production by Mitigating Harmonics

Highly automized processes often contain VFDs, which cause harmonics in the power supply. This can affect machinery and can cause downtime in production lines. This was the case at a major food processing plant in Dortmund, Germany.


Harmonics profile before AHF

High harmonic current distortion
Large disturbance of production

The food processing plant is part of a federation of over 300 independent retail dealers and suppliers to approximately 540 grocery stores. The center is a 100.000 m² warehouse and distribution center with affiliated butcher. After the butcher was destroyed in a fire in 2009, it had been rebuilt and expanded in 2011 into an 18.000 m² large butcher shop. Since completion at the end of 2011, the new processing plant produces 250 tons of meat for the grocery stores and up to 25 tons of sausages every day. The new butcher shop contains state of the art logistics and meat processing systems.


The VFDs connected to the new meat cutter created typical current harmonics of a six-pulse-converter (5th, 7th, 11th, 13th, etc). These harmonics resulted in commutation notches in the voltage, which influenced the whole production process. The harmonics caused blackouts and disturbances to the lighting. It also influenced the sausage stuffing and caused long production stops disturbing the process flow. Occasionally up to half of the employees had to be sent home for the day due to disturbances in the process.

Harmonics profile after AHF

Low harmonic current distortion
No disturbance of production

Solution – Active Harmonic Filter with Harmonics Compensation

The food processing plant decided to use an Active Harmonic Filter to eliminate the problems in the power supply. The installed system consists of one filter with a capacity of 300 A harmonics compensation to provide the needed harmonic power for the meat cutter.


After the installation and a short commissioning, all disturbing harmonics were cancelled and the commutation notches disappeared. All meat cutters could now, for the first time, be started at the same time, without any effect on lighting or the sausage stuffing process.

Source: Active Harmonic Filters

Harmonics Mitigation on VFD

Harmonics mitigation of variable frequency drives

Harmonics Mitigation Increases Output with 30%

The water treatment process involves a large number of variable-speed pumps to process large amounts of fluids. By using Active Harmonic Filtering technology to optimize the electrical behaviour of their variable frequency drives, a major water processing plant in Sweden managed to increase the maximum output capacity of their systems by 30%.

Thanks to Active Harmonic Filtering we can now process 30% more fluids during periods of peak demand. We also save energy.

– Project Manager


The treatment plant is part of the sewage works in Gothenburg. It is one of the largest in all of Scandinavia and central to saving the environment from pollution. With heavy demands, the two basic requirements on this critical regional infrastructure are constant operation and sufficient treatment capacity. UPS power backup systems are crucial to ensure a secure and stable power supply.

Harmonics mitigation

To increase plant capacity, 17 new VFD-controlled pumps were installed at the processing plant. This increased harmonic distortion on the electrical system significantly and the resulting overcurrents caused the UPS fuses to melt. To avoid this, the pumps were run at reduced speed. This was however a temporary solution as it reduced treatment capacity below demand. Frequency converters are a well-known source of potentially damaging harmonics.

Harmonics Elimination – the Challenge

After consulting a premier Swedish consulting firm, the processing plant announced a public procurement process seeking the implementation of active harmonic elimination technology. The target was to retrieve full treatment capacity by eliminating the harmonics.

Solution – Active Filter Units

The result was the installation of two 600 kVA Active Harmonic Filter units to manage two transformers supplying 2160 kVA. There were a number of characteristics of the specific harmonic filter solution that led to the plant’s choice:

  • Flexible connection and system dimensioning
  • Reduced maintenance costs to other connected equipment
  • Disturbance free electrical environment
  • Reduced energy consumption through decreased transformer losses

Harmonics Eliminated – the Result

Eliminating the harmonics in the system resulted in an increased max output capacity of 30%.

In addition, the processing plant now manages to operate below the threshold value for harmonic distortion (SS 421 1811). The plant now enjoys further benefits with a reduced energy consumption and improved environmental performance.

Source: Active Harmonic Filters

Flicker Compensation Case Study

AHF Flicker compensation welding

AHF Reduces Flicker from Radiator Production

Today’s industry constantly faces new challenges. As the local community grows, large businesses with energy intensive production processes are faced with the challenge of reducing their effects on the grid. High flicker emission levels can potentially disturb other industries on the public grid.


Flicker levels before AHF compensation

Flicker Level Before Compensation

The plant is a 55000 square meter radiator factory in Belgium, It consists of six production lines that in total can produce about 5000 radiators a day. The production process consists of presses, seam welding and spot welding. The process is inherently very energy demanding and so puts great demands on the power grid. These processes combined to create large voltage drops in the feeding substation with the result of too high Pst values. The problems at the plant caused flashing lights when the local utility company would connect other consumers to the same transformer.

Flicker level after AHF compensation

Flicker Level After Compensation


The local utility company demanded that the Pst 95% value could not exceed 0,7. Measured values during 2009 showed tops in the Pst equivalent to 1,6. Achieving this goal was no small feat due to the rapidly fluctuating load, and the many different load patterns that could occur with such a high number of welding machines.

Active Harmonic Filters – the Solution

The market leading response time for the active harmonic filter was a necessity for the customer to reach the values that the utility company demanded. The system consists of six units, making it a total of 2,1 MVAr continous power to compensate for the voltage drops.

Installation of Active Harmonic Filters Creates Results

After installing the active harmonic filters, the plant has managed to keep their Pst value below 0,63, regardless of how many welding lines are run simultaneously. The reference values have been measured by external consultants and approved by the local utility. As a side effect of the lowered flicker value, the plant now also enjoys stabilized production environment.

Source: Active Harmonic Filters

STATCOM Case Study

STATCOM AHF Reduces Flicker

Due to an increased usage of the local grid, a stricter limit of flicker emission became a necessity. One of the largest steel wire producers in Europe managed the problem by installing a STATCOM solution and lowering their flicker emissions.


Flicker before statcom compensation

Flicker Before STATCOM Compensation

The processing plant is the largest steel wire producer in Europe, part of a German industry group founded in 1856. The company produces steel reinforcement mesh grids. The production line is made up of various welding equipment, including spot welders from Schlatter AG. As is the case with all powerful spot welders, the abrupt current consumption causes voltage variations, which in turn produces flicker.


Due to expansion of the area and increased amount of renewable energy sources, it became necessary to lower flicker contribution. Early reference measurements showed flicker levels up to Pst 2 and Plt 1.4. A futher

Flicker after STATCOM compensation

Flicker after STATCOM compensation

complication the the case was caused by a lot of switching activity in the surrounding electrical grid. This contributed to higher background flicker and making measurements more difficult. Finally, depending on the production type currently running, the flicker will vary.

STATCOM by AHF – the Solution

The solution was offered in cooperation with Schlatter AG, who also delivered the welding lines.

A STATCOM solution consisting of 7 300/690W water cooled active harmonic filters was installed. The nominal installed power is 2.5 MVAr. The fully water cooled STATCOM follows the load dynamically. Neither changes in the production nor newly installed equipment leads to any need for adjustment of the compensation system. The AHF units were installed via a dedicated transformer and use their own medium voltage measurement point.

Delivered Results

The STATCOM solution reduced the flicker level to Plt 0.6. Reduced reactive power lowered current consumption 25-40%. Voltage dips on the 20 kV rail were lowered from around 500V to around 100V.

Source: Active Harmonic Filters

Harmonics and Notches in Dynamic Test Load

Active Harmonic Filters Improve Dynamic Test-Bed

A major pioneer in the manufacturing industry caused problems on the power supply network with their dynamic test bed. Here, an installation of the right combination of active harmonic filters now compensates harmonics up to the 100th order with great results. Both harmonics and voltage notches are reduced to enable top performance of the equipment.

Case Background

Dynamic test load before harmonics compensation

The test benches, owned by the development department of a major European production plant, are used to test components in the development phase. Varying test conditions can be programmed, which gives the test bench very dynamic properties.

Harmonics Compensation Challenge

The same transformer is connected to two parts of the test bench. With a very dynamic load whose load current amplitude can change from zero to maximum in approximately 100 ms, it was impossible to run both parts of the test bench simultaneously. The voltage notches of up to 25% in combination with very high harmonic disturbances prevented this. This caused serious delay in the testing facility as well as exceeding the limits in EN61000-2-4.

Active Harmonic Filters – the Solution

To solve the power quality issues, several active harmonic filters were installed to compensate the disturbances. Two 200/480V filters were installed together with one 100/480V filter that in combination compensate all frequencies up to the 100th harmonic order. The first two filters can be used to compensate lower harmonics while the third compensates for higher order harmonics and interharmonics. The three units were configured to share the

load with the 100/480V filter working on higher orders only. This resulted in extremely short response times and considerably lowered load disturbances.

Dynamic load after harmonics compensation

Harmonics Compensation – the Result

Thanks to the active harmonic filter installation, voltage notches could be reduced to 10%. In addition, harmonics were lowered to the required level stipulated in EN61000-2-4. Now, both test benches can be run simultaneously without any of the problems caused by poor power quality.

Source : Active Harmonic Filters