Published by Terry Chandler, A Power Quality Practitioner ™ for 30 years,
Director of Engineering Power Quality Thailand LTD/Power Quality Inc. USA,
Consultant for Dranetz Corporation USA (Asia business unit General manager)
Harmonics defined by multiple standards around the world.
 IEC 6100047 Class I defines harmonics and how to measure them.
 IEEE 5192014 defines harmonic limits.
 This paper is focused on the measurement technique, the accuracy of the measurement and identifying the source of the harmonics in a repeatable, industry recognized method.
Why do we need to measure harmonics?

Some local regulations require documented harmonic levels and maximum allowable harmonics
 Many countries/states/city/electric utilities have these regulations
 Many countries/states/city/electric utilities have these regulations

Electrical system issues related to harmonics:
 Overheating transformers
 Overheating neutrals
 Power Factor Capacitor failures
 Notching in the sinewave causes high frequency interference
 Mystery breaker tripping
 Transformer or panel audible noise
 Unexplained electrical system interference and operations.
 Low Power Factor
 Overheating transformers
Measurement techniques are documented in detail by IEC 61000430 class A Edition 3
 IEC 6100047 Class I standard (harmonics standard referenced)
 All modern PQ meters that are rated Class A measure harmonics to this standard.
 The instruments digitize the waveform and execute a FFT to calculate the harmonic values of THD and individual harmonics values The Harmonics FFT algorithm is specified exactly in the standard
 Harmonics are calculated on 10/12 cycle samples and must be GAPLESS. (Some instruments state 99.9% gapless??)
Harmonic Current Flow
Figure 1: Distorted – current induced voltage distortion.
Figure 2
Harmonic Load Current
Impedance:
The AC circuit equivalent of resistance in DC circuits. Impedance is the opposition to AC current flow made up of the available circuit elements of resistance, capacitive reactance, and inductive reactance. Each AC current frequency (harmonic) may be subject to a different impedance in the circuit. – Mike Lowenstein is president of Harmonics Ltd., Monroe, Conn.
Figure 3
Voltage Distortion (result of current)
Figure 4
Note : the distorted current is constant the voltage distortion changes based on the Measurement point.
Harmonic Flow on Feeders
Normal path:
Figure 5: Harmonic currents tend to flow from the harmonic source (nonlinear load) into the utility source because it is the lowest impedance.
Flow Path Altered by Capacitor – II
 Adding a capacitor causes current to flow in abnormal paths
Figure 6
Monitoring for Harmonic Studies
 How long to monitor
 What to monitor “voltage, current and power harmonics”
 Where to monitor
 Monitoring during a test
Monitoring Duration

The only constant in harmonics is the varying rate of change
 Monthly
 Weekly
 Daily
 Hourly
 By the minute
 By the second
 By the cycle
 Monthly
 Data is needed for the amount of time it takes to clearly define the harmonic situation under ALL conditions.
How many simultaneous monitoring points? It depends on the situation and loads.
Figure 7
Is voltage distortion from the voltage or the current?
Figure 8: Harmonic voltage and current trend – data with weak correlation
Figure 9: Harmonic and current trend – data with strong correlation.
The variations are useful time is in seconds!!
Harmonic Power Flow
 Which direction is the harmonic power flowing? from the source to the load, or, from the load to the source?
 A controversial topic, the most commonly accepted practice is to measure the harmonic watts. The phase angle, or the relationship between the voltage and current for a particular harmonic. (note: NOT THD!)
 The same rules that would be applied to a pure sine wave of voltage and current (which has only a fundamental frequency component) would be applied here.
Harmonic current flow?
 Why is harmonic current flow is always indicated as from the nonlinear load/s end towards the source end (“backwards” from normal load current flow) ?

Jbartos (Electrical) 20 Apr 01
The nonlinear load is a source of harmonics. If electrical equivalent circuits are drawn, one for each harmonic frequency, the different frequency sources (harmonic frequency sources) will be located at the nonlinear load location and the harmonic currents will be flowing from it. This assumes a perfect source with no harmonic distortion!! In the real world of electrical networks, the source is not perfect. – Terry C.
 This assumes a perfect source with no harmonic distortion!! In the real world of electrical networks, the source is not perfect. – Terry C.
Measuring Harmonic Power Flow Low Voltage (Direct Connection)
 Sampling frequency
 (# of samples per cycle divided 2)
 Voltage resolution (What is the minimum voltage a meter can make with defined accuracy?)
 For example, if the 9^{th} harmonic voltage is .1% of the 230v fundamental then actual voltage will be 0.23 Vrms. Most PQ meters minimum accuracy specification is 1vrms or 10 Vrms or a % of full scale at low levels.
 Current resolution of the meter. Specification is typically +/ 0.1% reading +/ 0.05% of full scale. So, with a full scale of 100.00 amps of 50 Hz current and a 9^{th} harmonic of .1 %, the current value of the 9^{th} harmonic would be 0.1 amp. But the measurement accuracy would be +/ (.05X10=.5amps!!)

Low voltage 10 amp
– Frequency response
– Low current accuracy (typical 10% to 90% rated)
CT’s – TR2510A
Measuring Harmonic Power Flow (in a transmission or distribution system)
 PQ meter capabilities (can it determine the phase angle of harmonic voltage and current?)
 Sample frequency, Voltage resolution, Current resolution
 PT
 Frequency response
 Voltage output resolution relative to instrument
 HV/Medium CT
 Frequency response
 Output current magnitude
 Low voltage CT
 Frequency response
 Output current magnitude
Typical PQ meter Voltage Accuracy
 Permanently installed (DBMI 61K)
 0 – 600V, 1000Vpk, AC/DC coupled
 RMS: 512 Samples/cycle, +/ 0.1% Reading, +/0.05%FS, over 7KHz bandwidth
 IEC61000430 Class A compliant
 Harmonics: IEC6100047
 Flicker: IEC61000415
 PQ: IEEE1159, IEEE519
PQ meter Current Accuracy (voltage input without I to V transducer)
 Range: 0 – 1.5V, DC coupled (AC/DC)
 RMS: 512 Samples/Cycle, +/ 0.1% Reading, +/0.05%FS, 3KHz bandwidth
 IEC61000430 Class A compliant
 Harmonics: IEC6100047
 Flicker: IEC61000415
 PQ: IEEE1159, IEEE519
Medium Voltage or HV Voltage and Current Transducers
 Voltage transducers
 PT Potential Transformers (inductive)
 CVT Capacitive voltage transducer
 Resistive divider
 Current transducers
 Inductive
 Fiber optic
Voltage Transducers (PT or VT)
 Required to reduce the voltage level to safe to connect meter.
 CVT (Capacitive coupled)
 Inductive Transformer
 Resistor divider
Frequency Response Transformer Type PT (Typical?)
Figure 10
Transducer Output Value at Harmonic Frequencies
Example:
 VTHD Voltage Distortion of 2% at 110kV
Assume 50% of the THD is the 5^{th} harmonic (1% 5^{th} harmonic) 1 % of 110kV = 1.1kV of the 5^{th} harmonic (250 hertz) / PT ratio 110kV to 100V divide by 1000.
Actual 5^{th} harmonic voltage presented to instrument is 1.1 volt.
If 7^{th} Voltage harmonic is 0.1% then 110/1000 = 0.1VAC is presented to voltage channel.
If 9^{th} voltage harmonic is 0.01% .010 VAC is presented to the voltage channel.
Voltage channels are typically specified from 1 or 10vrms to 600v or 1000vrms.
VT Errors at Range of Frequencies
Figure 11: Transmission errors with various different inductive instrument transformer types.
Errors in Inductive Voltage Transformers PTs
Figure 12: Amplitude and phase errors of an inductive voltage transformer at various different frequencies.
HV/MV Inductive Current Transducers
 Accuracy and frequency response
Figure 13: Amplitude errors in an inductive current transformer at various different frequencies.
Secondary Current Transducer
 Accuracy varies with frequency.
 Frequency response is not linear.
 Precision measurement of very low levels of harmonic power is limited by phase angle error of CT.
 Example, if primary current is 50% of full load. CT secondary current would be 50% of 1 or 5 amps. (0.5 or 2.5 amps). If 5^{th} harmonic current is 1%, = 0.005 amps or 0.025 amps. Clamp on CT minimum rated current is 0.1 amp.
Is it possible to identify the harmonic source on distribution substation buss? YES! But it’s not easy and it is expensive.
 What is needed:
 PQ instruments on the buss CT and PT that can record the V harmonics, I harmonics with phase angles and periodically record the 10 cycle sample waveforms of at least 128 samples per cycle.
 PQ instruments on each feeder that is a significant load. (IE greater than 5%)
 Software that can post process the data to align the time stamps, calculate all harmonic parameters including power from the waveform samples.
 PQ engineering training to analyze the data.
 Patience!
Conclusions
 Monitoring and evaluating harmonics in electrical system is a complex task due to complexity of the measurements and interaction of the various loads.
 Determining the source of the harmonics on the transmission or medium voltage grid has additional complexity of the voltage and current transducers.
 With detailed simultaneous measurements of all feeders, PQ engineering and software it is possible to deduce the source of harmonics on a substation bus bar.
 The only constant in harmonics is the varying rate of change!
Contact the author TerryC@powerquality.co.th
If you can’t explain it to a 6^{th} grader, you don’t understand it yourself ~ Albert Einstein
This is a very informative site. Keep up the great work.
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