The PQUBE3 installed at my home’s incoming supply (230V Phase L3) detected an RVC (rapid voltage change) of 3.97% during the early morning of Thursday 22/02/2024.
And today, while retrieving a portable PQ monitor from one of our worksites in Banyan Drive, I noticed the following voltage trends (occurred at about the same time as my PQUBE3 event). This was recorded at 22kV.
For these 2 locations (one on the mainland in the western part of Singapore, and another on Jurong Island), to have similar voltage variations suggest that a transmission level fault had occurred during the said date/time.
We have two other sets of portable PQ monitors, one in Loyang (east of Singapore) and another in Marsiling (north of Singapore) which will be due for data retrieval next week. These PQ monitors would have similar stories to tell.
Updated – 28/2/2024
As expected, the other two set of portable PQ monitors captured the same event (at 22kV), with the PQ monitor at the northern part of Singapore saw the worst dip values, which was expected as the fault was made known to have originated from our northern neighbour (TNB – Malaysia).
Here, it can be deduced that it was a single-phase fault on the Blue phase (Phase L3).
I had a PQUBE3 installed at my home’s incoming supply for about six months now and this was the first voltage dip (or sag) that it captured.
Apparently there was a transmission level cable fault in the Utility’s network. This would mean everyone in Singapore would have ‘seen’ it one way or another (in varying degrees, depending on locations).
My home (western part of Singapore) is taking in 230V single-phase supply (on Phase L3).
The PQUBE3 recorded a voltage dip of 18.44%, for a duration of 60ms (Lowest voltage was 187.6V).
In general, current harmonics contribution from solar PV inverters do not pose much of a power quality problem. Its ITHD is usually small and negligible as compared to a harmonics-producing load such as a variable speed drive (ITHD for a typical 6-pulse drive ranges between 30% – 50%).
Typically, one will find a Current Total Harmonic Distortion of 3% stated in the datasheet for a quality-brand inverter, as seen here.
In Singapore, for a Grid-Tied Solar PV connection, the Licensed Electrical Worker (LEW) (i.e Qualified Person) will have to submit the inverter’s PQ-related type test report to the Grid operator (SP Group). Below is one such example – here it shows the portion whereby the inverter was tested as part of the UK Engineering Recommendation G99 test requirements. Values stated for quality-brand inverters will have its harmonic current emission values well within the limits.
You may wonder – One inverter is ok but how about a number of them accumulatively? I had the opportunity to measure numerous sites whereby the rated PV output was accumulatively more than 1MWac.
Here are two sites whereby the background harmonics can be considered to be on the low side and as such the effects of the on-site inverters were more representative (limited ‘contributions’ from the localized electrical network).
All measurements were done using an IEC 61000-4-30 Class A certified Power Quality instruments.
The Current Harmonic Distortion (ITHD) in the trends below have been scaled to the respective aggregated inverters’ rated current (in other words, shown here as Total Demand Distortion (TDD) values).
As observed here, the TDD values were less than 3% and the sinusoidal shape of the current waveforms were very much still visible.
Note: IEEE 519 recommends TDD values of 5% for power generation facilities.
Site #1:
Premises Type: Warehousing / logistics PV Size: 1352.8 kWp Aggregated Inverter(s) Rated Current = 1613A @ 400V. Measurement Point: 2500A PV-AC DB, directly connected to the Premises 5000A Main Switchboard (served by a 3MVA transformer) via 3000A flexible CTs (clamped on 3 sets of 500sqmm cables per phase). CT direction towards MSB as Load, PV as Source. VTHD: 0.89% – 3.96% (CP95: 3.6%).
Site #2:
Premises Type: Solar Farm (On-site loads: Auxiliary power and lighting loads only) PV Size: 2652kWp (for CS1) Aggregated Inverter(s) Rated Current = 62A @ 22kV (for CS1). Solar inverters connected at 400V, stepped-up to 22kV via a 2.5MVA transformer. Measurement Point: 22kV Incomer 1 from PowerGrid (CS1) via VT and CT. Note: Solar Farm has 2 x 22kV intakes from PowerGrid – only one intake shown here. CT direction towards PV as Load. Solar Farm was connected to a Lightly-loaded 22kV distribution network. VTHD: 0.59% – 1.22% (CP95: 1.09%).
What happens if a voltage dip occurred, and for some reason(s), you had not set the PQ analyzer to capture it or that the PQ analyzer had ran out of memory for capturing of events? (Note: some brands of PQ analyzer do have a limitation on how much event(s) it can record).
The short answer is that you will not be able to get detailed waveforms (and trend) of that particular event. Hence, information such as how long was the dip would not be known.
But not to worry. Not all is lost. You will still be able to get the lowest (minimum) RMS voltage(s) recorded. Depending on the brand/model/software used, one will need to plot out the ‘per-cycle’ RMS voltage minimum trends.
Dranetz, for instance has incorporated these per-cycle trends automatically in their voltage/current trends. So the minimum/maximum value(s) that one see on a Drantez voltage/current RMS trend is really the lowest and highest value(s) recorded.
For others, you need to find things like “Detailed RMS trends”. Typically they termed this as the Vrms 1/2 trends, as seen here.
Vrms 1/2 Trend – Showing The Voltage Dip That Occurred on 14/11/2021 1600hrs (V1: 146V, V2: 198V, V3: 140V)
One must be careful not to plot the ‘normal’ RMS trends, which calculate out the RMS values based on sets of 10-cycle waveforms, as shown below. Values here will not show the true worst-case min/max values. Short events like a voltage dip which lasts 3-4 cycles, will get ‘diluted out’ here in a 10-cycle RMS calculation.
Vrms Trend – (V1: 210V. V2: 225V, V3: 212V) (also usually seen on standard power loggers)
What is seen on the second trend graph here is also what is usually obtained when one uses a standard power logger. Hence the difference (in capability) between a PQ analyzer and a power logger, is clearly illustrated here.
Trend Analysis
Without an event capture, it will be impossible to ascertain the actual duration of this voltage dip. However, based on the differences in the two trends (Vrms 1/2 vs Vrms) minimum values, we can tell that this dip was definitely less than 10 cycles ( < 200ms).
This monitoring was done at Low Voltage (monitored at 3P4W) in a commercial complex along Orchard Road, with V1 and V3 dropped significantly (dip by about 40%) and V2 slightly lesser (dip by about 17%). Comparing with its loading/current trend suggests this was an event upstream of the monitoring point.
Looking upstream at 22kV, dip magnitudes at 22kV will be similar (i.e 22kV dip magnitude V12 will be similar to LV V1). At 22kV, for a voltage dip of such characteristics, it was likely an event originating from a higher voltage level (you may refer to other voltage dip posts on this).
Feedback from other PQ monitors island-wide supports this. This was a transmission-level voltage dip event. The trend results suggest that it was a single-phase (L1) transmission level fault.
A critical step in conducting an accurate power or power quality monitoring is the measurement/monitoring setup itself.
There are generally three guiding principles, to verify that the voltage and current probes have been connected correctly, to the ‘Circuit Under Test’.
Voltage and Current phasor pairings are ‘together’ (An inductive circuit will have the I lagging the V, and a capacitive circuit will have the I leading the V).
Positive ‘Watts’ values (for a load centre).
The context/background of the measurement
Voltage/Current phasor paired together and Positive kW – an example
Many consider that the first two principles are sufficient to ensure a correct connection of the instrument to the ‘Circuit Under Test’.
Unfortunately, this is not 100% true.
The context of the measurement will still need to be considered closely.
For instance, monitoring at load-end and seeing power factor values in the region of 0.5-0.6 may be ‘normal’.
But to see these same poor power factor values at the main incomer circuit of a large electrical installation? Well, one should re-check at how the instrument was connected.
The following example illustrates the importance of context/background of the measurement.
In one simple glance, values shown on the analyzer seem ok.
The first two guiding principles were met.
Voltage values seems to match the switchboard’s voltmeter.
Current values seems to match the switchboard’s ammeter.
However, the power factor seems to be very poor, for an incomer circuit of a large electrical installation complex.
For context, these were the characteristics of the ‘Circuit Under Test’
It is at medium-voltage level (with an active voltage regulation).
At this MV level, the electrical installation will be penalized when the kVarh of the month exceeds 62% of its kWh consumption (translates to a power factor of poorer than 0.85).
Loadings at this incomer level are expected to be fairly balanced.
Voltage/Current Phasor – as found
Thus, it is highly unlikely that the phasor diagram obtained is true of the ‘Circuit Under Test’.
Further checks revealed that two different set of errors have occurred upon connection of the instrument to the circuit.
Wrong phase was clamped Clamp I1 was on Phase I3; Clamp I2 was on Phase I1; Clamp I3 was on Phase I2
Wrong direction The clamps ‘arrow’s were pointed towards source (instead of load).
Before (left) and After (right) Corrections
It was also observed that the clamps used in this measurement have not been labelled (I1, I2, I3, etc respectively). This probably contributed significantly in the errors above.
A good practice is to have all the voltage test leads and clamps test leads labelled (on both ends) before ever using the instrument on-site.
Labelling test leads on both ends upon purchase – is a must
“Does frequency changes during a voltage dip?” – A favourite question among many. The answer is “Yes”.
This will be quickly followed by “But why our power quality monitor did not show significant changes in the frequency values even though a voltage dip event was captured?” The general answer is that the standard (IEC 61000-4-30) does not require it to measure such detailed changes in frequency values.
With the voltage waveforms captured, it is technically possible to calculate the frequency values during a particular event (say a voltage dip). Here’s an example, where the event waveforms were post-processed (via the Dranetz Dranview 7 software).
Such post-processing feature is very useful especially in generator load-test applications, where one will be interested to know both the voltage and frequency changes during every step-load change. An example is shown below. One just need to ensure that the pre/during/post event waveforms were set to be captured properly (i.e enough cycles). The software will do the rest.
Earlier this evening, there was a voltage dip that occurred just before 7pm. The following is a waveform captured at Low Voltage (monitored at L-N), located in the city area.
Reports of voltage dips from other PQ monitors in other transmission blocks suggest this was a Transmission-level fault.
Voltage Dip 13/09/2020 1857hrs (LV L-N)
Voltage waveform seen here was at LV (L-N). It mirrored what was seen at 22kV and above (at L-L). From here, it can be inferred that there was a transmission level single phase fault (Phase L1).
I had the privilege in toying around with the new HDPQ Xplorer earlier today, thanks to Terry Chandler of Power Quality Thailand. Definitely re-affirmed my belief that Dranetz PQ instruments and Dranview (especially) are at least one notch higher against its competitors.
