Monitoring location : A factory in Southeast Asia, 1-phase 2-wire, 100V circuit
The power supply is damaged
1. Voltage fluctuation
The following power characteristics were concluded from analyzing this 2-week voltage fluctuation graph.
Unfortunately, sufficient event data was not recorded in No. 1 and No. 2, so that detailed analysis was not possible. The important point to note is that a large voltage fluctuation occurred between 9 p.m. and 9 a.m. everyday (No. 3), and the fluctuation was large at about 50V (between 75Vrms and 125Vrms).
2. Event Data
The voltage dip (instantaneous voltage drop) occurred frequently at night. Only 5 voltage dips were detectable in a 1s period. All voltage dip situations demonstrated the same tendency. This is the analysis of one voltage dip event.
- When the depth of the voltage dip reaches 90Vrms, the power supply is switched from the commercial supply to the UPS.
- When the power supply is switched to the UPS, the voltage RMS value increases to 116Vrms (125Vrms maximum).
- The voltage waveform changes from the sine wave to the square wave in the UPS supply.
- The square wave continues for about 1.25s.
- The power supply is changed from the UPS to the commercial supply later. Upon this switching, the voltage drops to 78Vrms (75Vrms minimum() for a short period.
3. Summary of analysis
- Worse power supply quality occurs frequently at night (9 p.m. to 9 a.m.)
- Worse power quality phenomena starts when the voltage dips.
- oltage swell occurs when switching from the commercial power to the UPS due to the voltage dip.
- Voltage dip occurs when the commercial power recovers and the power supply is switched from the UPS to the commercial supply.
1) Primary solution
Frequent switching to the UPS because of a voltage dip is not favorable. It appears that overload current flows to the equipment due to voltage dips and swells. To solve this problem without fail, the power supply should be stabilized to prevent the occurrence of a voltage dip.
2) Alternative solution
The UPS is used as a “stand-by power system (SPS)”. By changing it to an although more costly “on-line UPS system”, the dips and swells can be reduced during UPS switching.
The following is the type of UPS specified in the JIS C4411-3, 2004 standard.
Source : Guidebook for Power Quality Measurement, HIOKI E.E. Corporation