“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).
Harmonics gets people confused all the time. Some typical remarks will be like; “the neutral current harmonics are extremely high” or “burnt marks on the isolator were caused by harmonics”.
Here are 3 simple guidelines for harmonics in general LV applications.
Keep Voltage THD% below <5%. While there has been revisions in standards like IEEE-519 (revising upward from 5% to 8% for LV), trust me, keeping VTHD < 5% will make life easier for everyone (utility, facilities dept and end-users).
“Total harmonic content of the load current not exceeding 5% of rated current.” This is stated as one of the normal service conditions for a standard distribution transformer. Thus, it is a good practice to keep the total current harmonics < 5% of the transformer’s rated current (eg. 1MVA 22kV/0.433kV TF – rated current 1333A; total harmonic current shall be less than 67A).
Current THD% may gives you misleading results (see my other post – Mr. Harmonics I). One needs to see the actual current harmonics in absolute terms (amperes), total RMS current and make reference to the corresponding cable / circuit breaker sizes, before deciding on the next course of action.
Earlier this evening, there was a Transmission-level fault that occurred at approximately 7pm. The following is a waveform captured at 22kV incomer (monitored at L-N), located in the city area.
Here, it suggests that there was a Single-phase fault (L2) and that it originated upstream at higher voltage levels. There were reports of voltage dips from other PQ monitors in other transmission blocks as well.
Update: Suspected fault originated from Genco facilities in the west.
Earlier this morning, there was a Transmission-level fault that occurred at approximately 10:07AM. PQ monitors in the city-area captured the following waveforms at the 22kV incomer and at the 400V LT incomer side.
Here, the benefit of monitoring at L-N instead of L-L at 22kV is being showed clearly here. It is evident that there was a single-phase fault (L2) and that it originated upstream at higher voltage levels.
Note. A single-phase fault at 22kV will cause the other two phases to swell (not seen here).
There were reports of voltage dips in other transmission blocks from other PQ monitors. With this kind of magnitude observed here at 22kV, it is very likely that this fault originated in the same transmission block as well.
Update: 230kV cable damage along Keppel Road (South Region).
22kV Incomer – Monitored at L-N (South region)
LT Incomer – Monitored at L-N (South region)
When a 230kV fault occurs, a quarter of Singapore will feel the worst dip magnitude (the region where the fault occurs), with the other 3 regions observing shallow voltage dips. Below is an example (via a Dranetz HDPQ) in the North region of Singapore during this same incident.
Earlier in the week, areas in Tampines, Loyang, Bedok, Pasir Ris will have experienced voltage dips. It was made known later that it was due to a 22kV cable fault in Tampines area.
Few sets of our portable PQ analyzers placed in Loyang was able to capture this event. It registered voltage dips (at low voltage) of about 20%, lasting around 60-80ms. Recorded data showed that it was a three-phase fault.
One may be curious, how could a fault in Tampines result in a voltage dip in places like in Loyang or Bedok?
Our local distribution grid is densely interconnected, resulting in a high-reliability electricity network. Total blackouts are rare. But interconnectivity brings about a small disadvantage. Any fault will be seen/felt (in the form of a voltage dip) by everyone who is connected. The seriousness of this dip will be dependent on the types of faults and the electrical distance between you and the fault point.
Here in our case, the fault was in Tampines area. Based on experience, for a voltage dip ~ 20% to be seen at another 22kV network, it will need to be a significant fault causing a voltage dip in the range of 80-90% in the fault area (Tampines). Loyang (where our PQ analyzers were) is connected to Tampines at 66kV level.
Came across an interesting case recently, whereby the engineers taking care of a particular shopping mall had failed to obtain an earth loop impedance value on just one of the phases. Issue came to light as the electrical inspectors failed to obtain a complete earth loop results for the new tenants that are setting up shop in the mall.
Was called in to investigate if power quality is a factor here.
Background
– Similar “incomplete” earth loop values were obtained at the LV Main Switchboard. “OL” when measured between Phase L1 and Earth. Had used different sets of earth loop testers.
– Thorough checks were conducted on the distribution transformer, earth cables and the neutral-ground connection at the transformer.
– Loads on this particular switchboard include the building’s chiller systems and some tenants’ loads.
– Was reported normal earth loop values could be obtained when the chiller system is not in operation.
Recall:Earth Loop Impedance Test
– One of the tests conducted for an Electrical Installation “Pass” Certificate.
– To ensure when a fault occurs in an electrical installation, sufficient current will flow to operate the fuse or circuit breaker protecting the faulty circuit within a pre-determined time.
Recall: Earth Loop Tester
– Operates by inducing a current from the Supply system, by introducing a calibrated load between the phase conductor and the protective earth.
– And then monitors the voltage difference.
– Comes in various forms (Multifunction tester vs. Dedicated tester); (3-wire types L+N+E vs 2-wire types L+E)
Earth Loop in a TNS
Findings
– Conducted separate earth loop tests at LV Main Switchboard and monitored the quality of supply.
earth loop tester and pq monitor
– Apart from one failed attempt via the multifunction tester, subsequent tests produced repeatable earth loop impedance values across all the 3 phases.
– “OL” when measured between Phase L1 and Earth, used “K-brand” earth loop tester(s).
– VTHD on “high side” but expected due to the a few nos of variable speed drives being used at the Chiller system.
– Phase L1 – noticed multiple zero crossings.
Conclusion
– The quality of the supply waveform on Phase L1 affected / influenced the operability of the other earth loop testers, resulting in giving “OL” readings. An interesting case!
– Newer earth loop testers do have a ‘harmonic component’ – to cater for (IEC 61557-3:2007).
Much has been said about the blackout on Tuesday morning. Thus far, these are the official findings
Disruption lasted for about 38 minutes, between 1:18AM and 01:56AM on the 18 September 2018.
19 areas in Singapore were affected: Boon Lay, Choa Chu Kang, Clementi, Jurong, Pandan Loop, Aljunied, Geylang, Tanjong Rhu, Mountbatten, Kembangan, Bedok, East Coast, Ang Mo Kio, Bishan, Thomson, Mandai, Admiralty, Sembawang and Woodlands (~146,000 customers).
Caused by the tripping of power generation units belonging to Sembcorp Cogen and Senoko Energy.
Dubbed the worst major power outage since 2004.
Previous major blackout (albeit much smaller scale than this incident) was on 1 June this year confined to the CBD area.
We had one advanced energy logger on-site (an office in Cecil Street – CBD area), doing harmonics compliance measurement during the incident. Here, the trending graphs showed that frequency ‘dipped’ down to as low as 48.83 Hz. The journal logs showed that it returned to its normal operating range (49.5 Hz to 50.5Hz) in about 18 seconds (Nation-wide under frequency relays automatically kicked in for load shedding during these 18s).
Unfortunately, we did not use our higher-end equipment here (Dranetz HDPQ for instance). Hence the limited information (eg. Waveforms, detailed rms trends, etc could not be shown).
This particular office was not affected by the blackout.
I was invited to a talk by my former Deputy Managing Director, Mr. Chang Swee Tong at SP Group HQ.
It was a like a walk down memory lane, as he went thru various PQ-related initiatives that he led while at the helm.
Got to meet some old friends and mentors too.
Thanks again to SP Alumni Secretariat for the invitation.
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A paper on the same topic by Mr. Chang and the seniors of my old section can be found here.
