Electric power quality: which parameters must be measured if harmonic distortion occur in the network?

In modern homes, the use of electrical appliances requiring switch-mode power supply (PC’s, UPS’) has largely increased. In addition, many powerful electrical appliances, like electronic speed controllers and rectifier chargers, consuming much energy, are largely in use.

They produce non-linear loads to the network and cause distortions to the sine waves presenting voltage and current which results in harmonic pollution of the electrical network.

This is the reason why electrical network on most industrial enterprises and office buildings are polluted.

Harmonic pollution of the electrical network may lead to various types of damage of electrical equipment obstructing the whole technological process:

1. Breakdown of PF correction capacitors and light-emitting capacitors.
2. Overheating in neutral wires due to adding up of the third-harmonic currents and the current of a neutral wire exceeding over two times the rated value of the phase current.
3. Overheating in transformers with high power consumption.
4. Overheating of electric motors.
5. Degradation of the electromechanical properties of electric motors.
6. Nuisance tripping of overcurrent protection devices, overheating of fuses, nuisance tripping of protection due to the leakage currents.
7. Damage to winding insulation in transformers and electric motors.
8. Overheating of feed wires and electric cables.

9. Inaccurate readings of measuring instruments and individual feedback sensors in process automation systems.

Even short-term harmonic pollution may result in breakdown of capacitors and interference in the operation of electronic equipment.

Under certain circumstances, when using high-power electronic devices, the harmonics arising in the network at one production enterprise can be transmitted to the networks of other enterprises through the power grid and cause damage to them.

In practice, a significant part of electricians is not aware of the range of values characterising harmonic distortions, the existing standards for these values, and the measuring equipment required for the timely detection of these distortions.

This article determines the basic characteristics of harmonics and lists equipment for measuring these characteristics and alarm warning. These pieces of equipment are also capable to launch protection shutdown of capacitors and enable various stages of filters partially discharging the loads.

The basic characteristics of harmonic distortions:

1. Harmonic spectrum – contains the effective values (RMS AC) of voltage and current of harmonics up to the 40th or 63rd. Measurement results can be presented both in the form of tables and corresponding graphs.

   The necessary measurements, alarm warning and protection switching should be carried out using the following SATEC instruments: PM 296, C 191 HM.

2. THD (V)% (Total Harmonic Voltage Distortion).

   The necessary measurements, alarm warning and protection switching should be carried out using the following SATEC instruments: PM 296, C 191 HM, PM 172E, PM 172EH, RPT 091.

3. THD (I)% (Total Harmonic Current Distortion).

   The necessary measurements, alarm warning and protection switching should be carried out using the following SATEC instruments: PM 296, C 191 HM, PM 172E, PM 172EH, RPT 091.

4. TDD % (Total Demand Distortion), evaluating harmonic distortion against the full load demand of the electrical system within a certain period of time (T).

   In most cases, we can assume that the full load demand (I) for a certain period (T) is equal to the rated value with the specified period (T) varying from 1 minute to 60 minutes.

   The necessary measurements, alarm warning and protection switching should be carried out using the following SATEC instruments: PM 296, C 191 HM, PM 172E, PM 172EH.

5. K-factor, evaluating additional heating caused by harmonic distortion.

   The necessary measurements, alarm warning and protection switching or partial load shedding of transformers should be carried out using the following SATEC instruments: PM 296, C 191 HM, PM 172E, PM 172EH, RPT 091, PM 130EH, RPM 072E.

Measurement of current, voltage, power and other characteristics

To evaluate the true values (True RMS AC) of the characteristics of the polluted network, specific equipment capable of making a large number of instantaneous measurements of the current values of the curve, generally not less than 32 measurements per period. These measurements should be carried out using the following instruments: PM 172E, PM 172EH, PM 130 EH.

If it is needed to evaluate True RMS AC taking into account harmonics up to the 63rd, not less than 128 measurements per period should be taken. These measurements should be carried out using the following instruments: PM 296, C 191 HM, PM 172E, PM 172EH, RPT 091.

SATEC manufactures a wide range of high-tech measuring instruments for electrical networks that are heavily polluted with harmonics. These instruments are capable of measuring all characteristics required in order to evaluate the level of harmonics and verify the compliance with standards.

1. PM 296 with internal storage is capable of performing harmonic analysis including voltage, current and power spectra, taking into account harmonics up to the 63rd. It also analyses network interference, short-term harmonics, and various events caused by deviations from the standard values ​​(voltage, current, frequency, THD, etc.).

   This device is recommended for use at critical facilities with sensitive equipment, such as hospitals, airports, manufacturing enterprises with a continuous technological cycle, as well as at crowded venues (large shopping centres, office buildings, etc.).

2. PM 172E, PM 172EH, RPT 091, C 191 HM, PM 130 EH: are capable of evaluating TDD (I)%, THD (I)%, THD (U)% in addition to regular characteristics. These devices are recommended for installation at venues where significant harmonic distortion is predicted, for example, in manufacturing enterprises with a large number of frequency controllers, in large office buildings, etc.

Summary

1. The vast integration of electronic devices causes significant pollution of electrical networks with harmonics which affect these very devices mostly

2. Under these circumstances, only digital measuring devices based on microprocessors are capable of taking accurate measurements (True RMS AC) of current, voltage and power.

3. To ensure proper operation of the equipment and prevent damage, it is necessary to regularly evaluate the level of harmonics in the network by measuring the following parameters: THD, TDD, k-factor and the harmonic spectrum for both permanent harmonics that are always present, and for short-term harmonics.

4. It is necessary to regularly take measurement of the current in the neutral wire of the supply network and to receive warning signals when it exceeds the standard value.

5. SATEC has developed the entire range of instruments for measuring harmonics, alarm warnings, enabling filters, shutting down capacitors and shedding loads partially.