Higher Order Voltage Harmonics

[Thabiso]

Part A - Higher order harmonics (greater than 13^th)

1. What are current practices for measuring voltage harmonics on a utility level?

• Eskom/Locally
• Internationally (i.e what are the standards)

1. Does anybody require measurements beyond the 13^th harmonic

1. What are the effects of higher order harmonics on:

1. Utility equipment
2. Customer equipment

1. What equipment generates higher order harmonics?

1. What are the risks associated with higher order harmonics at present specifically for South African Utilities given the long transmission lines and the installation of capacitor banks on EHV? What impact do capacitors on the transmission network have on harmonic levels?

Part B ? Measuring harmonics with Capacitive Voltage Transformers (CVT?s)

On networks with voltages of 220 kV and higher, voltages are measured with CVT?s. CVT?s do not have a linear, flat frequency response and therefore measurements are only accurate close to nominal system frequency (i.e 50 Hz/60Hz)

1. How can accurate harmonic measurements be taken via CVT?s ?
2. What is the theoretical concept underlying measurement of harmonics via CVT?s
3. Are any commercial devices available that enables these measurements?
4. What are the costs?
5. What are the operational hurdles associated with such devices?

:roll:Your input is highly appreciated

 

[gar]

130109-0834 EST

A comment on a capacitive voltage divider. I have no experience with the device you are using, but from a theoretical and some practical experience one can can obtain wide bandwidth relative to 60 Hz.

Consider an oscilloscope probe. These are quite good from DC to 100s of MHz. Typical probes are 10 megohms into a 1 megohm scope input. The intent is a 10 to 1 voltage division. The probe has a 9 megohm resistor paralleled by a small adjustable capacitor.

The probe cable has a substantial shunt capacitance from the signal line to ground, and the scope input also has shunt capacitance across its 1 megohm input resistance.

If there is no capacitor across the resistor in the probe, then there is effectively a low pass filter between the signal being measured and the scope amplifier input. Thus, poor bandwidth. In the frequency range to about 100 MHz simple design can provide good square wave response by adding a small adjustable capacitor across the resistance in the probe. With little difficulty in the mid 50s on an automotive ignition research project I built a compensated probe for 50,000 V that was good from DC to 10 MHz for a scope input. This used a long spiral carbon film resistor in the probe, and on a plastic tube around the resistor i painted a distributed silver conductive band for the compensating capacitor.

I suspect in your power system application you hang a plate around the high voltage wire. This high voltage portion of the voltage divider probably has a very high resistance in shunt with that high voltage capacitor. This capacitor, then connects to the instrument input that is a capacitor shunted by a resistor. Now without compensation of the resistive component at the instrument input in the voltage divider there is effectively a high pass filter. Then your question would be how to compensate this circuit without putting a resistor in parallel with the high voltage capacitor portion of the probe.


Probably the first thing to do is at a low voltage, maybe 100 V, apply a square wave input to your instrumentation (the square wave to the high voltage wire) and see what happens. Instrumentation means the high voltage probe and its associated measuring instrument. You would simulate the physical structure of of your high voltage wire and apply the square wave to that wire.

.

 

[ghostbuster]

Be careful when using CVT's for measuring harmonics.Usually the CVT's are already connected to station monitoring equipment which can create significant harmonic load distortion.Would recommend removing station loads from CVT's prior to performing these harmonic measurements.

 

[Besoeker]

Hi Thabiso
And welcome.
Cape Town is a beautiful location.
I'll answer your questions the best I can but it is based on my own methods and experience. I can't tell you what is common practice.

Part A - Higher order harmonics (greater than 13^th)

1. What are current practices for measuring voltage harmonics on a utility level?

• Eskom/Locally
• Internationally (i.e what are the standards)

For harmonics at utility level I generally look at the secondary side of the metering VTs with a digital storage scope with a serial port and download the data to a spread sheet so I can do the number crunching.
I think others use something like a Dranetz power quality analyser.

1. Does anybody require measurements beyond the 13^th harmonic

In UK, analysis up to the 50^th is a usual requirement.

1. What are the effects of higher order harmonics on:

1. Utility equipment
2. Customer equipment

Higher losses is the general answer. How much and other effects is equipment specific.

1. What equipment generates higher order harmonics?

Anything that takes non-linear current. Generally harmonic current is a series with higher orders being of lower amplitude. A (very) rough guide is that the amplitude is the reciprocal of the harmonic number.
Variable speed drives and high current rectifiers are culprits but are usually considered as such and often mitigating measures taken for such installations. Less obvious perhaps, are things like computers, chargers, televisions.....in fact almost everything with electronics. Their individual contribution may be small - but the vast numbers of them can be a major contribution.

1. What are the risks associated with higher order harmonics at present specifically for South African Utilities given the long transmission lines and the installation of capacitor banks on EHV? What impact do capacitors on the transmission network have on harmonic levels?

Two points come to mind.
Harmonic voltages will result in higher capacitor currents which distress them.
There is a risk of parallel resonance between the capacitors and the line impedance that may be coincident with one of the harmonic frequencies.

Part B ? Measuring harmonics with Capacitive Voltage Transformers (CVT?s)

I have no experience with CVTs. Probably others on this forum do.
Good luck.

 

[robbietan]

Part A - Higher order harmonics (greater than 13^th)

1. What are current practices for measuring voltage harmonics on a utility level?

• Eskom/Locally
• Internationally (i.e what are the standards)

1. Does anybody require measurements beyond the 13^th harmonic

:roll:Your input is highly appreciated

in our neck of the woods (Philippines), there is a standard for Voltage THD and Current TDD. there is a 5% VTHD and 5% TDD on distribution voltage (34.5 kV, 13.8 kV, 480V, 400V and 230V). however, still no standard of how to measure, when to measure and what device to measure. no need to measure higher order harmonics, the total VTHD is the only standard listed

 

[Electric-Light]

I don't think its necessary to measure higher order harmonics beyond utilization point. The very higher orders can't get past iron core transformers anyways.

The measurement instrument bandwidth is likely no issue, because even the lowest end equipment will far surpass the ability of potential transformer used to allow measurement of high voltage line to instrument which is the bottle neck in higher frequency pass-thru.

 

[wirenut1980]

I work for a utility and have done a fair amount of monitoring at transmission voltages 69 KV, 138 KV, 230 KV, and 345 KV. I have never seen harmonics above about 13-15 in any significant amount. In fact, I ould say anyting above the 7th has been insignificant at the transmission level. We monitor harmonics on our transmission capacitor banks to protect against resonance. I'm not sure at what level of voltage THD they alarm at. Other than that, we don't really look at harmonics at the transmission level, because overall, the THD has been so low when we have looked on occasion.

I have heard that CVT's (I call them CCVT's here in the US) do have some error when looking at higher frequencies than the fundamental. But I have nothing concrete to back that up.

As far as standards, we use IEEE 519 for harmonics. That standard outlines total harmonic distortion limits, as well as individual harmonic limits.