Harmonic measurement in electrical networks
Contents
Procedures for harmonic measurement
Harmonic measurements are carried out on industrial or commercial sites:
- Preventively, to obtain an overall idea on distribution-network status (network mapping),
- In view of corrective action, to determine the origin of a disturbance and determine the solutions required to eliminate it,
- To check the validity of a solution (following modifications in the distribution network to check the reduction of harmonic disturbances)
The harmonic indicators can be measured:
- By an expert present on the site for a limited period of time (one day), giving precise, but limited perception,
- By instrumentation devices installed and operating for a significant period of time (at least one week) giving a reliable overview of the situation,
- Or by devices permanently installed in the distribution network, allowing a follow-up of Power Quality.
One-shot or corrective actions
This kind of action is carried-out in case of observed disturbances, for which harmonics are suspected. In order to determine the origin of the disturbances, measurements of current and voltage are performed:
- At the supply source level,
- On the busbars of the main distribution switchboard (or on the MV busbars),
- On each outgoing circuit in the main distribution switchboard (or on the MV busbars).
For accurate results, it is necessary to know the precise operating conditions of the installation and particularly the status of the capacitor banks (operating or not, number of connected steps).
The results of measurement will help the analysis in order to:
- Determine any necessary derating of equipment in the installation, or
- Quantify any necessary harmonic protection and filtering systems to be installed in the distribution network, or
- Check the compliance of the electrical installation with the applicable standards or Utility regulations (maximum permissible harmonic emission).
Long-term or preventive actions
For a number of reasons, the installation of permanent measurement devices in the distribution network is very valuable.
- The presence of an expert on site is limited in time and it is not always possible to observe all the possible situations. Only a number of measurements at different points in the installation and over a sufficiently long period (one week to a month) provide an overall view of operation and take into account all the situations that can occur following:
- Fluctuations in the supply source,
- Variations in the operation of the installation,
- The addition of new equipment in the installation.
- Measurement devices installed in the distribution network prepare and facilitate the diagnosis of the experts, thus reducing the number and duration of their visits.
- Permanent measurement devices detect any new disturbances arising following the installation of new equipment, the implementation of new operating modes or fluctuations in the supply network.
- For an overall evaluation of network status (preventive analysis), this avoids:
- Renting of measurement equipment,
- Calling in experts,
- Having to connect and disconnect the measurement equipment. For the overall evaluation of network status, the analysis on the main low-voltage distribution switchboards (MLVS) can often be carried out by the incoming device and/or the measurement devices equipping each outgoing circuit,
- For corrective actions, it is possible to:
- Determine the operating conditions at the time of the incident,
- Draw-up a map of the distribution network and evaluate the implemented solution.
The diagnosis may be improved by the use of additional dedicated equipment in case of specific problem.
Harmonic measurement devices
Measurement devices provide instantaneous and average information concerning harmonics. Instantaneous values are used for analysis of disturbances linked to harmonics. Average values are used for Power Quality assessment.
The most recent measurement devices are designed referring to IEC standard 61000-4-7: "Electromagnetic compatibility (EMC) – Part 4-7: Testing and measurement techniques – General guide on harmonics and interharmonics measurements and instrumentation, for power supply systems and equipment connected thereto".
The supplied values include:
- The harmonic spectrum of currents and voltages (amplitudes and percentage of the fundamental),
- The THD for current and voltage,
- For specific analysis: the phase angle between harmonic voltage and current of the same order and the phase of the harmonics with respect to a common reference (e.g. the fundamental voltage).
Average values are indicators of the long-term Power Quality. Typical and relevant statistical data are for example measures averaged by periods of 10 minutes, during observation periods of 1 week.
In order to meet the Power Quality objectives, 95% of the measured values should be less than specified values.
Fig. M10 gives the maximum harmonic voltage in order to meet the requirements of standard EN50160: "Voltage characteristics of electricity supplied by public distribution networks", for Low and Medium Voltage.
Odd harmonic
non-multiples of 3 |
Odd harmonic
multiples of 3 |
Even harmonic | |||
---|---|---|---|---|---|
Order h | Relative amplitude U_{h}: % |
Order h | Relative amplitude U_{h}: % |
Order h | Relative amplitude U_{h}: % |
5 | 6 | 3 | 5 | 2 | 2 |
7 | 5 | 9 | 1.5 | 4 | 1 |
11 | 3.5 | 15 | 0.5 | 6...24 | 0.5 |
13 | 3 | 21 | 0.5 | ||
17 | 2 | ||||
19 | 1.5 | ||||
23 | 1.5 | ||||
25 | 1.5 |
Portable instruments
The traditional observation and measurement methods include:
Oscilloscope
An initial indication on the distortion affecting a signal can be obtained by viewing the current or the voltage on an oscilloscope.
The waveform, when it diverges from a sinusoidal, clearly indicates the presence of harmonics. Current and voltage peaks can be observed.
Note, however, that this method does not offer precise quantification of the harmonic components.
Digital analyser
Only recent digital analysers can determine the values of all the mentioned indicators with sufficient accuracy.
They are using digital technology, specifically a high performance algorithm called Fast Fourier Transform (FFT). Current or voltage signals are digitized and the algorithm is applied on data relative to time windows of 10 (50Hz systems) or 12 periods (for 60Hz systems) of the power frequency.
The amplitude and phase of harmonics up to the 40th or 50th order are calculated, depending on the class of measurement.
Processing of the successive values calculated using the FFT (smoothing, classification, statistics) can be carried out by the measurement device or by external software.
Functions of digital analysers
- Calculate the values of the harmonic indicators (power factor, crest factor, individual harmonic amplitude, THD)
- In multi-channel analysers, supply virtually in real time the simultaneous spectral decomposition of the currents and voltages
- Carry out various complementary functions (corrections, statistical detection, measurement management, display, communication, etc.)
- Storage of data
Fixed instruments
Panel instrumentation provides continuous information to the Manager of the electrical installation. Data can be accessible through dedicated power monitoring devices or through the digital trip units of circuit breakers.
Which harmonic orders must be monitored and mitigated?
The most significant harmonic orders in three-phase distribution networks are the odd orders (3, 5, 7, 9, 11, 13 ….)
Triplen harmonics (order multiple of 3) are present only in three-phase, four-wire systems, when single phase loads are connected between phase and neutral.
Utilities are mainly focusing on low harmonic orders (5, 7, 11, and 13).
Generally speaking, harmonic conditioning of the lowest orders (up to 13) is sufficient. More comprehensive conditioning takes into account harmonic orders up to 25.
Harmonic amplitudes normally decrease as the frequency increases. Sufficiently accurate measurements are obtained by measuring harmonics up to order 30.