The harmonic currents generated by different types of loads, travel back to the source. While traveling back to the source, they generate harmonic voltages, following simple Ohm’s Law.
Harmonic voltages, which appear on the system bus, are harmful to other equipment connected on the same bus.
In general, sensitive electronic equipment connected on this bus, will be affected.
The Power Factor of almost all converter types of loads is low. It ranges from 0.6 to 0.9, and at best could be 0.945 in case of diode bridges.
Special care has to be taken while applying Power Factor Improvement Capacitors in such cases.
Harmonic Limits in Electric Power Systems * (IEEE 519 1992)
Harmonic limitations have been established by IEEE 519 1992 for the following reasons:
To limit the damage to power factor correction capacitors and harmonic filter systems caused by excessive harmonics.
To prevent series or parallel resonance in the electrical system.
To keep the level of harmonics at the PCC (Point of Common Coupling) from being excessive and distorting the system voltage and damaging other equipment on the system.
The PCC is defined as the electrical connecting point or interface between the utility distribution system and the customer's electrical distribution system.
The harmonic voltage limitations set forth by IEEE 519 1992 are:
Maximum Individual Frequency Voltage Harmonic: 3%
Total Harmonic Distortion of the Voltage: 5%
The harmonic current limitations are defined in the following table:
Maximum Harmonic Current Distortion in Percent of IL
120 Volt through 69 KV
Individual Harmonic Order (Odd Harmonics)
ISC/IL
h<11
11<h<17
17<h<23
23<h<35
35<h
TDD
<20*
4.0
2.0
1.5
0.6
0.3
5.0
20<50
7.0
3.5
2.5
1.0
0.5
8.0
50<100
10.0
4.5
4.0
1.5
0.7
12.0
100<1000
12.0
5.5
5.0
2.0
1.0
15.0
>1000
15.0
7.0
6.0
2.5
1.4
20.0
Even harmonics are limited to 25% of the odd harmonic limits
TDD refers to Total Demand Distortion based on the average demand current at the fundamental frequency and measured at the PCC (Point of Common Coupling).
*All power generation equipment is limited to these values of current distortion regardless of ISC/ IL value.
ISC = Maximum short-circuit current at PCC.
IL = Maximum demand load current (fundamental) at the PCC.
h = Harmonic number.
Different technologies are available mitigating the harmonics from the system.
These include:
Detuned or broadband harmonic filters: these filter banks are tuned to a frequency just below the predominant harmonic frequency. If the predominant harmonic frequency is say, 5th, it is normal practice to tune the filters to 189Hz, or 3.78 th harmonic, in 50Hz systems.
Active Harmonic Filters: these units are designed in such manner that, they will inject harmonic frequencies in the system, which will be in anti-phase of the load harmonic frequencies. This will effectively free the source being loaded due to harmonics.
Frequency response of tuned and detuned harmonic filters is illustrated in these figures.
Installing tuned filter traps.
Installing detuned filter traps.
Installing active harmonic filters.
Tuned filter traps: These are designed to absorb specific harmonic frequencies from the system. If the power system contains, 5th, 7th, 11th, 13th, etc. order of harmonics, separate tuned filters need to be installed for each frequency. This calls for detailed system study with respect to the impedance levels.
