Back to Basics – What is Voltage Dip?

Definition: Temporary reduction of the r.m.s voltage at a point in the electrical system below 90% threshold of the declared nominal voltage, between 10ms and up to a minute.

Voltage Dip (RMS Trend)
Voltage Dip (RMS Trend)

Simply speaking, a sudden voltage drop of more than 10% of the declared nominal voltage. The utility in Singapore has in place a Power Quality Monitoring System (PQMS) from 22kV voltage level upwards (all the way to 400kV). These are three-wire three phase systems; and hence line to line voltages are used in defining a dip.

There is a significance of such definition being used in the 66kV and 22kV networks, whereby it is a resistively earthed grounded system (thru the neutral ground resistor). Here, a single phase fault will not be a registered as a voltage dip as the other two non-faulted phases will swell; ‘compensating’ the faulted phase. This will result in a drop of voltage (line to line) of usually less than 10% (hence not a dip). The utility here described such events as ‘Voltage variation’.

Two things matter when it comes to describing a voltage dip.
1) Magnitude of the dip
This typically reflects the fault severity and also the proximity of the monitoring point to the fault location.
2) Duration
The timer starts when the voltage falls below the 90% threshold and ends when all voltages are equal to or above the 90% threshold. This is very much dependent on the time taken to isolate the fault and the nature of loads connected.

Normally a voltage dip here in Singapore will lasts less than 200ms (10 cycles). This is about the average time the primary protection takes to isolate the fault from the network.
Usually a longer duration will suggest a somewhat sluggish protection relay operation.

Typical causes of a voltage dip in Singapore:
1) Equipment / cable faults in the utility network.
2) Equipment / transmission line faults in Tenaga Nasional Berhad (TNB) network (Singapore is connected to Malaysia at 230kV).
3) Customer Installation faults.
4) Cable damage by earthworks.
5) And to a very small extent, load switching like motor starting.

Singapore’s electricity regulator, the Energy Market Authority (EMA) publishes cases of voltage dips on its website.

Dip due to a customer installation fault
Dip due to a customer installation fault

Back to Basics – True RMS

There used to be a time whereby the price between a non-True RMS (aka Average Responding) and a True RMS meter/clamp is pretty significant; so much so that if one is just to measure voltage or current for checking if the circuit is ‘Live’ or not, one will go for the cheaper Non-True RMS device.

Recently, price difference have narrowed down quite a bit and in my opinion, one should just get a True RMS meter/clamp straight away.

Loads today are pretty much almost non-linear these days anyway.

For a pure sinusoidal wave;

true rms vs average responding
true rms vs average responding

Hence for an average responding meter, it will scale the rectified average of the ac waveform by 1.11.
This holds true only for pure sinusoidal waveform; which do not exist in the practical world.

The difference in reading can vary from between 5 and 40%, depending on what type of waveform that is being measured.
An example below shows a difference of about 10%. The load being measured was a combination of a couple of CFL and LED bulbs.

It is funny to me to see some contractors out there who uses a non True RMS meter/clamp to verify the readings obtained from their expensive PQ meters.


True RMS vs Average Rectified Clamp
True RMS vs Average Responding Clamp

*Besides True-RMS capability, one should also check for its Safety CAT category. This is an important safety consideration that should not be disregard.


Causes of Power Quality problems

The causes of PQ problems can be divided into two main causes

1) Deficiencies and disturbances in the supply

and / or

2) Caused by the nature and behaviour of the consumer’s load and installation

Here it shows the results of a survey done by Georgia Power Co, from the perspectives of the Customer and Utility

causes of pq problems

However from my experiences, the percentage of blame towards the service provider takes almost 99% of the time here in Singapore.For instance, in a large manufacturing plant setting, the blame on any failure on the “Tools” side will almost exclusively go to the Facilities department (sometimes even without thorough checks on the equipment).
For smaller Customers, tripping issues on their equipment will usually leads to a call / complaint to the Utility almost immediately.

Back in my time working for the utility company, I have experienced countless times whereby my power quality measurements / monitoring showed that the cause of such trips, were actually caused by the Customer themselves (usually a case of high starting in-rush of Customer’s own equipment).

I guess this is the ‘Blame Culture’ here in Singapore. Nothing related to engineering here. Will love to hear experiences from overseas.

As with most things, solving a power quality problem ranges from the very obvious / simple (for instance the above example) to the very complex (especially when it comes to “intermittent” issues).

Personally, I have developed my own checklists (improved over time from own experiences and others) and takes a structured approach at every power quality problem. You should too, if you are new in this area.

Typically, it will begin with a site visit to gather facts/information from relevant parties and conduct baseline site measurements (eg snapshot PQ measurements, IR thermal checks, checking the earthing system etc). It is also important to ask many questions to different people involved. Sometimes you may even get contradicting answers from them! But nevertheless, their answers will give you some clues on what the problem is.

It is from the review of these data (facts and measurement results) that will enable the PQ investigator to form hypothesis what are the likely causes and determine the next course of action necessary to best serve the Customer’s needs.

If the problem happens to be very direct and obvious to identify, the investigation will end here and a report will be formulated on possible ways to solve the problem.

However most PQ investigations will require a further in-depth survey into the electrical system concerned to identify the underlying problem. This will usually require strategic placement of  a number of power quality meters simultaneously over a period of time (typically one business cycle) at various levels of the electrical network for data comparison and correlation. Things like sources of harmonics can usually be determined thru the use of power harmonic flow. Sources of flicker on the other hand can be determined thru comparison of its loadings and the flicker trends recorded.

I was a trained combat medic back in National Service, so I will like to use an analogy from the medical field here.

It is akin to seeing a doctor when one is ill. The doctor will firstly ask questions and conduct simple tests on the patient. If the illness is obvious, the patient will be prescribed specific medication and sent home. Otherwise, the patient will be recommended to undergo further tests at the hospital to narrow down / determine the causes of his illness.

What is a PQ problem?

What is a power quality problem, one may wonder?

There are many definitions to this out there but my personal favourite will be
“Any power problem manifested in voltage, current, or frequency deviations that result in failure or misoperation of customer equipment”.

The keywords here are “Customer Equipment”. If it does not affect the operation of the equipment, it is not a problem (yet).
For instance, almost every load that we have today is a non-linear load, contributing harmonic current and hence distorting the supply voltages.

At the beginning, it will probably not be a an issue, but over time if not keep in check and with more non-linear loads added, the supply voltages will be distorted further and eventually equipment may start to fail/misoperate.
The problem is made worse if design considerations were not made to ‘cater’ for this harmonic loads (eg undersized neutral cables) and no control was made to what type of equipment (whether it meets relevant product emission standards or not, etc) can be connected to the network. In this situation, a combination of equipment control and benchmarking of power quality indices is needed (more on this in my future posts).

Other than harmonic distortion, other power quality problems include voltage transients, frequency variations, flicker, leakage currents, in-rush currents and voltage dip/swell (just to name a few).

Here in Singapore. the utility company, SP PowerGrid, states in their FAQ that the most important power quality concern in Singapore is voltage dips. This is because a dip will have significant economic impacts on the sizeable semiconductor industries primarily located in Marsiling / Woodlands and Pasir Ris / Tampines area. These semiconductor plants have equipment which are sensitive to these milliseconds variation of the supply voltage. A dip may result in their equipment to trip, halting productions which eventually lead to a host of other costly problems.

My experiences showed that while many of such equipment in the “Facilities” side (Eg variable speed drives to the pumps) are quite well-protected (SEMI F47-rated at least), many equipment in the “Tools” side are not. And one has to take into consideration too that the SEMI F47 has its own limitations as well (more on this in my future posts).

This is not to say that other power quality problems do not exist in Singapore. Things like excessive harmonic distortion for instance are “localized” problems, usually more apparent in the Customer’s end (low voltage side). It is only seen in the Grid’s high voltage side (6.6kV and above) if the problem has escalated considerably.

Having worked for the utility, I do know that there are pockets of the transmission / distribution network that have breached the limits set aside for things like harmonic distortion and flicker. No equipment were known to be affected (yet) and at times, it can also be a complex problem for the utility company to pinpoint the chief cause of the problem, as the grid network is very vast. I believed this is a challenge for many other utility companies worldwide.

Linking back to my favourite definition of a power quality problem, it is a misconception that a PQ problem can only be solved by putting expensive PQ meters / monitoring instrument. A PQ meter is just one of the many tools to aid the PQ investigator. Other tools include earth loop measurement device, earth tester, Infrared Thermal Scan, etc. It is also another misconception that once a PQ monitor is installed, it definitely can pinpoint/solve the problem.

Remember the inputs to the PQ meter is just voltage & current. It is not some “magical”  troubleshooting instrumentation. The selection of what PQ meter to use, its settings, the placement of such meters and more importantly the experience of the PQ investigator matters.

p/s: I will share my experiences on the use of various PQ meters in future posts.