Harmonics From Solar PV Inverters

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.

Typical Inverter Datasheet THDI 3 percent

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.

Harmonic Current Emissions Test - Part of ER G99 test requirements

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%).

RCCB Requirement in Domestic Electrical Installations

On 12 May 2023, the regulator Energy Market Authority (EMA) issued a circular that w.e.f 1/7/2023, all residential premises will be required to have a Residual Current Circuit Breaker (RCCB) installed. A two-year grace period is given. This circular was targeted to homes built in or before July 1985, as this RCCB requirement is already been in place since 1 July 1985.

Finer Points of this RCCB requirement

1. It refers to the 30mA sensitivity RCCB, meant for protection of socket outlets and lighting circuits.

2. The 30mA RCCB is a “Controlled Item”. In short, one should look out for the “Safety Mark”. Refer to Enterprise Singapore website for more details.

RCCB-ELCB Safety Mark
30mA RCCBs and older ELCB

3. 30mA RCCB used in Singapore has a faster tripping time, as compared to the IEC Standard. One can refer to Singapore Standard SS97 (Residual current operated circuit-breakers without integral overcurrent protection for household and similar uses (RCCBs) – General rules) for details.

Note: SS97 covers both Type A and Type AC RCCBs.

4. The RCCB has no overcurrent/overload protection. Hence it is important to ensure that there is a corresponding circuit breaker protecting it against overcurrent/overload. The 30mA RCCB comes in few typical ratings, 25A-40A-63A-80A-100A. One needs to ensure that the RCCB rating is higher or equal to its corresponding circuit breaker. In this example here, the 30mA RCCB is rated 40A, equal to the circuit breaker rating of 40A.

RCCB rating has to be greater or equal to MCB rating

Power Quality Tips for RCCB Selection

The 30mA RCCB is highly sensitive and can lead to cases of maltrip/nuisance trip. An RCCB is designed to trip anywhere above 50% of its sensitivity rating (For the 30mA RCCB, it may trip anywhere above 15mA). Below are couple of tips to follow, if you have been experiencing nuisance RCCB trip(s) with no clear fault/causes found.

1) One should not lump many circuits/appliances to one RCCB. Typically, for a HDB flat, 1 RCCB will suffice. But for larger homes, one may need multiple RCCBs.

The rule of thumb is to limit the standing/background leakage current to 20-25% of the RCCB sensitivity. If this is exceeded, additional RCCB(s) will need to be installed (eg. instead of 1 RCCB serving 10 circuits; re-wire 1 RCCB each to serve 5 circuits; total 2 RCCBs).

Note: Every equipment / appliance contributes a small amount of leakage current (from its mains filter etc).

Background Leakage Less Than 25%
Use a Leakage Clamp – Clamp on both Live and Neutral cables

2) The RCCB type should match the load(s) type. A mismatch has been known to cause both maltrip and non-trip cases.

RCCB Types
RCCB Types – Type AC most common in Singapore homes

Locally, it is being accepted that for typical residential power and lighting loads, the Type AC RCCB suffices. The price difference between a Type A and Type AC can be as much as 10x. Hence, it is rare to ever find a ‘Type A’ RCCB in homes. The type of RCCB can be differentiated from the ‘waveform logo’ on the RCCB itself, as circled in the photo above.

Loads today however are rarely sinusoidal in shape (as per the AC waveform seen). Here is a voltage/current waveform snapshot of my own home. The current waveform is on the right. It hardly looks sinusoidal.

Note: Some European countries have banned on the general use of Type AC RCCB.

Portable Residual Current Devices

In Singapore, all socket outlets intended for use by ordinary persons and are intended for general use needs to have RCD protection, with rated residual operating current of not more than 30mA.

In addition, the 30mA RCDs (type AC and A) used here have faster tripping time requirements, as shown below.

It is also a common practice in both industrial and commercial work settings, that the premises’ owner requires the vendor/contractor to use portable RCD (PRCD) whenever an electrical appliance / equipment is being connected to the socket outlet.

PRCD is designed to be plugged into any standard socket-outlet. An appliance / equipment can then be plugged into it. It provides additional protection against electrocution to the person in contact with the appliance/equipment (and its cable/wirings). Typical trip ratings are 10mA and 30mA.

Examples of PRCDs

It is sometimes taken for granted that this PRCD will trip first before the Distribution Board’s RCD when there is a fault at the particular appliance/equipment. While this is the ideal outcome, it is not necessarily so even if one were to use a 10mA PRCD.

Here, three numbers of 10mA PRCDs were tested under different scenarios.

RCD Trip Tests Using Fluke 1663 Multifunction Tester
Note: The Distribution Board has a background leakage current of about 6 to 7mA.
Test at Rated (0 degrees)
Test at Rated (180 degrees)

These simple tests have shown that it is possible for the Distribution Board’s RCD to trip due to a fault at the ‘PRCD-protected’ appliance/equipment.

It very much depends on

  • Background leakage current at the Distribution Board
  • Leakage / fault value at the appliance/equipment
  • Trip characteristics of the Distribution Board RCD and the PRCD

Context / Background of The Measurement Matters

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’.

  1. 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).
  2. Positive ‘Watts’ values
    (for a load centre).
  3. 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.

  1. The first two guiding principles were met.
  2. Voltage values seems to match the switchboard’s voltmeter.
  3. 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’

  1. It is at medium-voltage level (with an active voltage regulation).
  2. 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).
  3. 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.

  1. Wrong phase was clamped
    Clamp I1 was on Phase I3; Clamp I2 was on Phase I1; Clamp I3 was on Phase I2
  2. 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

Missing Earth Loop Impedance On One Phase

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.

– 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

– 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.

– 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).

Major Power Outage 18 September 2018

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.

Voltage & Frequency Trend

Power Quality Development in Singapore

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.


A paper on the same topic by Mr. Chang and the seniors of my old section can be found here.

PQSynergy International Conference and Exhibition 2017

PQSynergy 2017 Day 3 – Final day

The third and final day of PQSynergy 2017 ends with a sharing session by PQT’s Terry Chandler and Mirus’ Tony Hoevenaars on the topic of harmonics.

It has been another fruitful conference, with a good mix of local and international speakers. Thomas Pua’s (PSL) presentation on synchrophasors was particularly interesting and Bill Howe’s (EPRI) insights on the proactive use of PQ data is a welcome change for the industry.

I always look forward to these sharing sessions with fellow practitioners, something not common back home for me, especially in power quality.

This year, I shared some common and simple day-to-day PQ related cases encountered back in Singapore. The presented slides will soon be available for download at www.pqsynergy.com

PQSynergy International Conference and Exhibition 2016

My 2nd year presenting a topic in PQSynergy. It has been an enjoyable 2-day conference.
Made new friends and learnt new things from fellow practitioners. Glad to have met the guys from Sonel too.
Will definitely take a closer look at some of your instruments.

And congratulations to Terry Chandler and his Power Quality Thailand on another successful conference.
Happy 30th anniversary, PQT. Many more good years ahead.

PQSynergy 2016
PQSynergy 2016

Friends from PEA
Friends from PEA