Technical Articles
This article is about the impact of harmonics on Power Factor value and measurement methodology followed by utilities while calculating the same.
In India, energy charges are assessed in either of below two ways:
kWh billing:
Active energy (kWh) is used to calculate the energy consumption. For charging reactive energy, Power Factor is measured separately and there is penalty / incentive for maintaining at certain levels.
Some states that follow kWh billing are Gujarat, Kerala, Karnataka, and few other states (marked in White colour).
Tamil Nadu follows kWh billing, but it also imposes penalty on harmonics separately to consumers above 33 kV HT supply.
kVAh billing:
Apparent energy (kVAh) billing is relatively new method to assess the energy consumption. Apparent power is the vector summation of active and reactive power. Hence kVAh will account for PF as well. Thus making the tariff simpler.
Some of the states that follow kVAh billing are Maharashtra, Telangana, Delhi, Bihar and 9 other states (marked in Green colour).
Impact of Harmonics on Power Factor:
In addition to the above information, it must be noted that harmonics are also reactive in nature. This means, presence of harmonics will increase the overall reactive power and hence reduce the power factor.
Hence the Power Factor (PF) with and without the presence of harmonics need to be represented differently, as given below:
True PF: It is the product of displacement PF and distortion PF.
Advanced portable power analysers (Fluke, Krykard, etc.) represent both True Power Factor (PF) and Displacement Power Factor (cos ϕ) separately, as shown:
In this, the True PF is less than the Displacement PF (cos ϕ). It is due to presence of harmonics in that system. Harmonics will reduce distortion PF and hence True PF also reduces.
Let us also see how the same is measured in Utilities’ tariff meters.
In all the state utility meters, harmonics (iTHD & vTHD) are not measured by default, unless utilities specifically ask for. Hence distortion PF is not getting calculated in utility meters with the formula stated above.
But at the same time, the measured Voltage and Current that are used to calculate kVAh, are basically Vrms and Irms. These RMS values are combination of fundamental and high frequency harmonic components.
Where, Vf is the fundamental voltage and Vh is the harmonic voltage (2nd, 3rd, 4th, etc).
An APFC controller has separate filter circuit to remove harmonic components, as capacitors can only improve displacement PF. But utility tariff meters do not filter-out the harmonics in the measured voltage & current values. Due to the presence of harmonics, the PF value in tariff meters will be close to True PF. Hence this True PF will be lesser than the Displacement PF which is maintained by APFC. Let us see an example in the below table:
In the above table, all the voltage, current and THD% values are taken from a power analyser. Using the values, Power Factor is calculated using different formulas as shown above. By comparing the “Theoretical True PF” and “Measured PF” (with RMS values), it is evident that the Measured PF with RMS values are very close to Theoretical True PF. This PF value is lesser than the displacement PF also. All the state utilities’ 3 phase tariff meters calculate Apparent Power (kVA) as vector summation of Active and Reactive Power. The Power Factor calculated in the tariff meters as per below formula, will also be close to True PF:
It is now clear that the Power Factor measured in 3 phase tariff meters supplied by utilities in India, actually measures Power Factor close to True Power Factor and not only the displacement Power Factor. In the above calculated example, the APFC controller will show PF as 0.994 (R ph), whereas the tariff meter will show the PF as 0.949 (R ph). If the consumer is in kWh billing state, incentive would have been lost and if in kVAh billing state, the number of consumed units would have been increased by 5.4%.
It is necessary to create awareness among the consumers about the methodology of PF calculation. With both current and voltage harmonics present in the system, it is wise to check on True Power Factor and not only the Displacement Power Factor. Consumers should also be aware that not the capacitors alone will improve True PF but harmonics also need to be mitigated. Only by doing both, True PF can be brought close to unity and consumers can reap maximum benefits.
Power capacitors that are used to improve Displacement PF can result in harmonic amplification & resonance. The amplified harmonics will further reduce the True PF. So consumers shall go with combination of passive detuned reactors along with capacitors to avoid resonance, amplification and Active Harmonic Filter to mitigate harmonics.
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