Technical Articles
In continuation to the earlier article on solutions for accurate PF Correction in establishments with Solar Installations using 4 quadrant APFC relays, we shall now explain about the impact on Harmonic Distortion in such plants due to solar installations. Here also, we are taking the cases where industries / buildings that have solar power generating stations, in addition to regular electricity supply from Utilities.
Non-linear loads (UPS, VFD, LEDs etc.) present in any industry would generate current harmonics and the same result in Voltage distortions, proportional to system impedance. However, Solar inverter which converts generated DC power to AC power, employs IGBTs & internal harmonic filters. Hence harmonic distortions in AC output of the solar inverter would be under limit set by respective Renewable energy regulatory bodies. In other words, harmonics measured in any industry with solar is always due to non-linear loads and not because of solar inverter. However, it is often observed that the Total harmonic distortion (%THD) increases, the moment Solar panels starts generating power. Hence many misunderstands that solar inverters are the reason for sudden increase in harmonics. This is depicted in diagrams 1 & 2.
I-THD = (Harmonics content) / (Fundamental current)
It can be noted in the above drawings that the THD values are increasing from 20% to 50% as soon as solar inverters are switched ON. This is because for the same harmonics current (200 A in both), the fundamental current is reducing from 1000 A to 400 A.
As per latest CEA-2019 regulations, customer should maintain the harmonic levels in their industry below the limits as specified in IEEE 519: 2014. The rationale behind the harmonics limits is the current harmonics injected by individual consumers to the grid should be such that the grid voltage is not distorted to unacceptable values. IEEE 519 specifies the harmonic limits as Total Demand Distortion (TDD) and not as Total Harmonic Distortion (THD). %TDD represents the percentage of harmonics with respect to the maximum load current drawn for 12 months period. Whereas %THD represents the percentage of harmonics content with respect to the load current, at the instant of measurement. It is important to note that a small load current may have a high %THD value but may not be significant enough to distort the grid voltage. At TDD the possibility of voltage distortion is high.
When solar is generating (refer Diagram 4) power, the net current at main incomer reduces from 1000 A to 400 A. Hence the %THD increases (from 5% to 12.5%) for the same harmonics content. The higher value of 12.5% is %THD and not %TDD as the load current is only 400 A, against the full load current of 1000 A. IEEE 519 guidelines specifies TDD as limits at PCC level and not THD. The point to be noted is that there is no need to panic about high percentage values of THD when solar is generating power. For the given load, whether solar is ON or OFF, the harmonics current values would be same and only the fundamental current (50 Hz) varies depending upon the generation. Hence the %THD increases when solar is generating for the same load and in most cases no need to worry.
We shall now understand the above discussed concept with help of actual site observations at a Hospital in Aurangabad city.
Average maximum load current over last 6 months: 800 A
Installed Solar panel capacity: 400 kW
APFC panel with detuned reactors: 250 kVAr with 4 quadrant sensing APFC controller AHF: 150 Ampere 3 Phase 4 wire
IEEE 519 limit for I-TDD: 8%
| Parameters | Solar OFF | Solar OFF | Solar OFF | Solar OFF |
| AHF OFF | AHF ON | AHF OFF | AHF ON | |
| Time | 15:04 to 15:15 | 15:25 to 15:29 | 15:49 to 15:55 | 15:59 to 16:09 |
| Load Current (Amps) | 757 | 737 | 541 | 532 |
| Displacement PF | 0.997 | 1 | 1 | 1 |
As shown in above table, Power factor is maintained close to unity with help of proper 4 quadrant APFC controller. When solar is OFF, the current harmonics is reduced to 5.3% and AHF is functioning normally. When Solar is ON, harmonics is relatively higher in both AHF ON and OFF conditions. The percentage values have increased, but looking into the actual harmonics current (Load current x I-THD% x 100), the values remain almost same (38 to 39 A). This means, the AHF is functioning normally and reducing almost same amount of harmonics current at the main incomer. But only the %THD value changes, due to change in fundamental current.
Moreover, as per IEEE 519, it is clearly specified that the harmonics (%TDD) should be measured only while drawing current equal to 12 months average of maximum value currents at PCC. Refer below the excerpt from IEEE 519 – 2014 (Page 7).
This means, while solar is ON, the current measured at PCC will not be maximum and hence the corresponding harmonics values are not TDD. In other words, IEEE 519 standards are applicable only without solar or while drawing maximum current fully from the grid. Hence in order to check the compliance of IEEE 519, measure harmonics at full load & without solar.
Consumers with solar installations in their industrial premises are suggested not to panic about %THD levels at PCC level. One should monitor the %TDD value (when solar is OFF) & ensure it is being maintained below IEEE 519 limits by installing suitable Harmonic filters as required at site. It is advised to consult with our PQ expert team when someone wants solution in such scenarios.
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