Advantage of using power quality analyzer with continuous waveform recording
Introduction
This page describes the advantages of continuous waveform recording over other traditional threshold based methods of events recording or other types of aggregated recording, which are not based on the waveform continuous concept.
The document includes real examples and case studies where:
- Continuous waveform recording is the only method to catch the event or to analyze a problem or phenomena.
- It is almost impossible or extremely inconvenient to catch the event or to analyze it.
The following page includes 6 chapters, each one comprised of real case studies and links to an article with additional cases of the same subject.
Compliance with Grid Code
Wind and solar farms are connected to the grid at the Point of Common Coupling (PCC). At that point, electrical parameters fluctuate continuously depending on many factors such as wind, sun, and more.
In addition to external factors, the power quality at the PCC is affected by the functionality of the equipment at the farm. At that point, there might be disagreements on the source or reason for failure or non-compliance with grid codes requirements including ride through, harmonics, sag/swells, etc. In many cases, when the grid operator encounters a problem, they try to locate the source by addressing the issue to all power providers connected to the network. As a result, the power providers find themselves constantly having to prove whether or not they are responsible for the problem. An on-and-off non-ended dialog started, causing severe disturbances to the farm operators, impacting the many farms under their responsibility.
Continuous waveform recording provide legal proof determining whether the source was caused by the farm or not. The reports are provided to the grid operator on demand and most of the times the case is closed, saving unnecessary discussions between parties.
Example: Operational Behavior of Wind Power Plants during Power Black-out in Turkey on 31st of March 2015
Solar Farm Operation
Figure 1 describes the daily changes of the power and the voltage at 1 cycle resolution. Figure 2 is a zoom in to a particular time when the voltage is ranging between 250.2 V to 250.6V, the converter’s power is going down from 41 KW to 28 KW and as a result the voltage is going down by 1.7V. After one minute, the power and the voltage are going up again. When the voltage is reaching up to a 250.2 V again, the power decreases immediately to 28 KW.
Figure 1: Daily changes of power
Figure 2: Zoom into 5 minutes
Conclusions:
Using PQA with continuous high resolution recording is the only way to identify anomalies such as the above, since the voltage values are within the standards and 10/15 minutes average cannot expose this phenomenon.
More Studies:
Read more on the advantage of continuous power quality analysis in photovoltaic systems.
Long Term Event Investigation
There are cases in which the consequences of the events should be traced or followed up on a long time before or after the events.
In this example, the voltage dropped at a copper mine in the Democratic Republic of Congo.
The average power of the plant was almost 16 MW. The event started as a short voltage sagof approximately 100 msec and then gradually, the voltage dropped within approximately 60 seconds to almost 0. The plant was able to recover after only an hour.
The continuous high resolution measurements allowed us to analyze in detail how the event developed during a relatively long time of 60 seconds until the power crashed completely.
Figure 3 below provides a general view of the first 60 seconds of the event and Figure 4 is a zoom-in of the voltage sag.
Figure 3: General view of the beginning of the event
Figure 4: Zoom in the 6 seconds voltage sag
More Studies:
Read more cases of long term event investigation
Unpredictable trigger
There are cases in which the traditional events such dips / sags, swells, RVC, etc. are not the cause of a failure / incidence. Therefore reviling the root cause of the failure become almost impossible without a high resolution continuous recording.
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.
Conclusions
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.
More Studies:
Read more cases with unpredictable triggers
Wide Area Investigation
Large areas can be defined as large plants with a few voltage levels, ranging from high voltage to low voltage, spread over a large area. In these sites, a large number of recording devices can be found relative to other areas.
A common event in part of the equipment can be reported due to threshold definition, or a different intensity of the event may be measured by each of the individual devices. In such an instance, part of the information is lost as a result of different threshold or intensity which affects the recording of the event.
Permanent recording saves all of the information without any loss of data, allowing for better analysis of the event consequences in the future.
The following example is an investigation of 1.5MW motor startup event in large area (pumping power station in New-Zealand) as reported by Siemens.
Figure 10 below describes two measurement points located a far distance from one another. The utility substation point was measured at 66KV and the pumping substation point was measured at 11KV.
Figure 10: Electrical diagram of a network in a large area
Figures 11 below shows that in both places described by figure 10 above, the voltage drop at 66KV network during 1.5 MW motor startup is 1.42% with compensation and 4.1% without compensation. The instance above mentions voltage levels that are not normally used as a threshold to record events.
The record however shows that the continuous measurements caught the event and the network behavior was then investigated with and without compensation during motor startup.
Figure 11: Recording of events in a large area.
R&D, Universities and Research Institutes
These applications are mainly used by utilities and universities. In these cases, initiating measurements based on triggers is not convenient since trigger setup may change frequently according to the test or application being performed. Continuous measurement improves operations due to the fact that all tests are recorded without requiring a change of threshold each time and subsequent value set verification.