Power Quality Analysis of Unpredictable Events
Transient and short duration variation cannot be measured in a traditional continuous manner as they occur only momentary during long period of time. The recording of such events is usually based on threshold setup required by the regulator/utility known as the Grid Code. however, some phenomenons are not covered by the Grid Code, or the Grid Code thresholds requirements are irrelevant to a specific installation. for example, a large motor causes a substantially voltage drop to all nearby plants during startup. The complaint is addressed to the power utility company, that measure the voltage at the substation located few tens of k”m away. Which, in turn claims that such an event was not seen or recorded on their devices.
Other examples and real cases are covered in the following article.
Error in Transformer Tap Changer
A continuous power quality audit was conducted by the Israeli Electrical Company (IEC) to the request of one of their HV customers. The G4500 power quality analyzer was installed on the primary side of the distribution transformer. At the end of the audit, no traditional event was detected by the power quality analyzer. Figure 1 below shows the 1/2 cycle RMS values for 15 hours. The chart shows that the voltage level is 3.1% above the nominal value of 22kV and that voltage drop of 4% of nominal are present. The measured values are within the EN 510160 standard.
Figure 2 below is a zoom in to one of the voltage drops, the voltage dropped in steps of 200V every 40second and rise up at the same steps but is shorter duration. The total duration of the event is 8 minutes. This indicates on a failure of the tap changer controller. This information triggered a maintenance activity to be done, and the tap changer was repaired as a result.
Figure 1: 15 hours of 1/2 cycle RMS values
Figure 2: Zoom in to a voltage drop
Automotive Industry, Sport Welding Application at Car Industry
Figure 3 below provides a general overview of the voltage fluctuations at the automotive plant. During this 20 minutes the voltage fluctuarated rapidly to -15% from the voltage nominal. Setting thresholds of 10% as required by the standard will result in event saturation that would fill up the memory.
Continuous measurements at 1/2 cycle resolution over a long period of time enables investigation of the intensity of the drops caused as result of spot welding operations occurring at the same time. A drop below a certain level may cause a severe damage in production quality, which necessitates taking out the defective chassis for repair and returning it to the production line following maintenance.
Figure 4 shows a zoom in during 8 seconds on the voltage fluctuation.
Figure 3: General view of the level of voltage drop
Figure 4: Eight second zoom in of the voltage fluctuation
When using a real-time compensation system (EQ), a substantial improvement in flicker levels and voltage drop intensity was able to be achieved.
Troubleshooting a Customer With Flicker Issues
In the following example the customer complained that their lights are flickering. It was decided to install a power quality analyzer for 1 week for investigation.
Figure 5 below provides a general view of the voltage RMS at 1 cycle resolution and the PST values for more than 8 days. Unexpectedly the PST values are at the allowed range (below 1).
Figure 5: General view Voltage and PST for 8 days
Figure 6 below shows the THDV and the 7th harmonics values at 1 cycle resolution. The graph clearly shows that the pattern of both THDV and the 7th harmonic is the same. Figures 7 and 8 zoomed in to a particular time when the 7th harmonic is active. It shows that averaging the harmonics values to 10min interval, as required by the standard, will “filter” this phenomena. Figure 9 clearly shows the waveform envelopes caused by the 7th harmonics.
The reason for the high 7th harmonic was partly a resonance in the network close to the 7th harmonic that occurred when some capacitor banks were connected. The reason it was not purely a 7th harmonic, was that there was a small scale hydro power plant which generated the harmonics. The generator in this power plant was an asynchronous generator, and as the rotor then lags the 50 Hz voltage with a slip, a 7th harmonic from the rotor becomes an inter-harmonic near the 7th harmonic.
Tap Changer Pre-fault Analysis
The Israeli Electric Company reported transformer taps changing on DATE. When this occurred, the measurements showed that during the operation of a tap change in HV, the voltage changed by approximately 0.5%. This change level cannot be recorded since it was far away from any threshold level used for recordings. What this means is that the event was not recorded. It also should be noted that in term of power quality, this event was not deemed significant.
In figure 10 below, there are three steps presenting three voltage levels. In the first step, there is a high THDV above 3.2% (THDV is measured at 1 cycle resolution).
Zoom-in at the first step of figure 11 below shows that the voltage was also distorted. This indicates a potential problem in the tap changer. This information triggered maintenance activity to be done, and the tap changer was repaired as a result.
Figure 10: Records of tap changer operation in three steps
Figure 11: Zoom in on the first change (between 97.2-93.3)
When using a real-time compensation system (EQ), a substantial improvement in flicker levels and voltage drop intensity was able to be achieved.