By : Arteche Power Quality Phone: 1-262-754-3883 (USA)

Objective: Redure third harmonics

A casino, located in Michigan, had 1000 slot machines and was planning an expansion that involved 500 additional slot machines. Their local electric utility measured voltage distortion of nearly 5% THD-v, plus the casino experienced interference on their CCTV security monitors. To improve the operation of their security systems and to meet the utility power quality requirements, they needed to solve the harmonics problems before adding 500 more harmonic
producing slot machines.

Problem : Harmonics caused by 1-phase power supply

Due to the accumulated effects of numerous switch-mode power supplies, (used in computers, video games and slot machines) the casino was experiencing several power quality issues:

• Harmonic voltage distortion was about 5% THD-v .
• Branch circuit current distortion was about 60% THD-i.
• 3rd harmonic current was as high as high as 50%.
• Neutral ground voltage measured several volts.
• UPS system fluctuations
• Surveillance camera distortion
• Corruption of access control data transmissions

The Solution: Adding a third harmonic filter (THF)

The Third Harmonic Filter is a series connected filter that is specially designed to minimize third harmonic currents in 1-phase circuits. In addition to reducing the 3rd harmonic by as much as 80-90%, this filter can also reduce the 5th, 7th and 9th harmonics. The THF filter is ideal for 3-phase, 4-wire power systems as found in casinos, office buildings and other commercial applications.

THF-60-0020-120-N1

The THF-60-0020-120-N1 is a branch circuit current rated filter and proved to be effective and convenient for this application which had as many as eight slot machines supplied from a single branch circuit. THF filters can be hard wired into the circuit and should be selected based upon the current rating of the branch circuit protective device.

Benefits:

THIRD HARMONICS REDUCED

• 3rd harmonic current was reduced to less than 10%.
• Reduced 3rd harmonic current on neutral.
• Reduced transformer load and temperature.

CONVENIENT RATINGS AND CONFIGURATIONS:

As shown below, the filter can be located in the pedestal beneath the slot machines and wired in series with the branch circuit conductors that supply several (typically 2-8) slot machines.

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Background
A large medical center in the southern United States experienced nuisance tripping of their equipment due to unexplained events on their electrical system. Realizing that nuisance tripping could be the result of any of a number of root causes, the facility contracted with S&C’s Power Systems Services Division to perform a complete power quality evaluation of their electrical system. The facility was also considering converting their existing “conventional” medium voltage source-transfer scheme to a high-speed S&C PureWave™ Source-Transfer System.

The facility is served through a common-bus preferredalternate scheme via two incoming 13.8-kV feeders. In the event of a voltage sag or outage on the preferred feeder the conventional source-transfer equipment is designed to transfer between preferred and alternate feeders in approximately 0.3 to 3 seconds. Unfortunately, contactors in voltage-sensitive equipment, as well as adjustable-speed drives, could still be affected under such a transfer scheme, resulting in nuisance tripping of this equipment.

Furthermore, in the event of short-duration sags due to faults in the utility system, the conventional source-transfer scheme will not even attempt to transfer. Thus, the equipment will be subjected to the voltage sags even though a healthy alternative feeder may be available. On the other hand, a STS will minimize the impact of any sags or outages on the facility’s electrical equipment. The STS accomplishes source transfer in response to voltage sags and outages typically 100 to 250 times faster than the conventional scheme, eliminating any impact of these kinds of disturbances on electrical equipment.
Results
A review of the serving utility’s and facility’s electrical systems were conducted during an on-site power quality evaluation. Records of disturbance events and their impact on electrical equipment in the facility were also reviewed.

The specific condition of the equipment in the facility’s electrical system and operation of electrical equipment was carefully examined during this on-site visit. Specific equipment problems were identified and the undervoltage ride-through characteristics were obtained from equipment manufacturers.

During the second phase of the evaluation, continuous monitoring of the phase voltages on the two incoming medium voltage feeders at the service entrance to the facility was conducted over a period of three months. In addition, continuous monitoring of phase voltages and currents was also conducted at the terminals of a 4160-volt chiller and a 480-volt fan drive over an approximate two-month period.

These units had been identified as the equipment most sensitive to voltage sags. Since monitoring was being done during the storm season, a large number of voltage sags related to faults on the utility’s transmission and distribution systems were recorded. A smaller number of these disturbances resulted in the actual tripping of the facility’s electrical equipment. The measured ride-through performance of the chiller and fan units was compared to the CBEMA curve. Results of the comparison indicated that the facility’s equipment generally had a slightly higher tolerance to disturbances than equipment covered by the CBEMA curve.

Comparison of the two feeder voltages during disturbances indicated that no significant disturbances occurred simultaneously on both feeders. Thus, at any given time there was at least one healthy feeder available to serve the facility.

Occasionally slight voltage sags (more than 80% of nominal voltage) affected both feeders. These sags were not severe enough to cause the equipment to trip. More severe voltage sags (less than 80% of nominal voltage) appeared mostly as a result of faults on the distribution system on separate occasions on each of the two feeders. In cases where the facility’s equipment tripped due to a voltage sag, the associated fault duration was on the order of 0.3 seconds. In such cases, the most sensitive drive was found to ride through at least 0.03 seconds (i.e., 3 to 10 times longer than the time it would take the STS to transfer). These statistics, combined with the results of a voltage regulation study, indicated that a STS can be effectively applied to significantly reduce the occurrences of nuisance tripping of the facility’s electrical equipment.