Basics of Voltage Harmonics
Before going to the case study, it is better to understand about voltage harmonics and its impact. Harmonics always originate as current harmonics and voltage harmonics are the results of current harmonics & system impedance. Current harmonics originate because of the presence of non-linear loads like variable speed drives, UPS, SMPS, LEDs, CFL lamps, welding sets, arc furnaces in the system. They act as harmonic current sources. The resultant current waveform can be quite complex depending on the type of load and its interaction with other components of the system.
The current and voltage harmonics are often expressed as Total Harmonic Distortion (THD) and Total Demand Distortion (TDD). THD is the ratio of all harmonic components to the fundamental component while TDD is ratio of all harmonic components upon the maximum load (not maximum demand of system) over a considerable period.
Origin of V-THD
The current harmonics flow through system impedance (source impedance and line impedances) and cause harmonic voltage drop across the impedances. This in turn will distort the supply voltage waveform and thus voltage harmonics are generated. Long cable runs, high impedance (%Z) transformers, DG sets etc. contribute to higher source impedance and hence, higher voltage harmonics would be generated.
Any VTHD above 5% in LT side of transformer is considered high and anything above 8% is very high. Textile, steel industries and commercial buildings, where the share of non-linear load is close to 100%, current harmonics will be very high. Many industries in these segments are very cost conscious. Hence they typically go for transformers with low fault level i.e., high %impedance (%Z), and they save in both transformer cost and switchgear cost. Here, the deadly combination of high current harmonics and high source impedance results in very high voltage harmonics.
Impact on adjacent industries / grid
An industry, say industry A, that has large non-linear loads will generate huge current harmonics in its system. A nearby industry, say industry B, connected to the same grid may not have non-linear loads, yet, it may be subjected to high voltage harmonics. These voltage harmonics are the result of high current harmonics and high transformer impedance of industry A. Thus, industry B would witness high voltage harmonics, even with low current harmonics.
Ill-effects of high V-THD
Capacitors offer low impedance to harmonic currents which can be explained by the formula Xc = 1/(2πFC). As the frequency increases, capacitive reactance decreases thus will act as sink for harmonics. In the above example, the moment industry B goes for power factor correction with capacitor banks, they act as sink for both voltage harmonics coming from the grid and harmonics generated from non-linear loads in industry B. This will result in high current harmonics in the APFC panels of industry B, leading to overloading of capacitors (2 to 3 times rated current).
In APFC panels, whenever the VTHD is goes above 15%, current THD can go more than 100%. The RMS current drawn by capacitors can go more than 2 times its rated current. In such scenarios, overheating of Short Circuit Protection Devices (SCPD) can happen due increased skin effect. Due to high VTHD, blast or nuisance operation of fuses can also happen.
Circuit breakers interrupt current flow at a current zero. A badly distorted current wave may contain current zeroes at multiple locations other than the normal zero of the fundamental sine wave, as shown in the figure. These spurious current zeroes could cause premature interruption and restrike during a circuit breaker opening operation. In the above waveform, it can also be observed that the peak current is very high and the crest factor is reaching above 3.0 (against the normal value of 1.414). High harmonics may also leads to issues in contactor coil like overheating, humming & burning and failure of coil’s RC surge suppressor
Voltage harmonics affect the entire system irrespective of the type of load. They affect sensitive equipment, electronic cards throughout the facility especially like those that work on zero-voltage crossing. High VTHD also affects induction motor’s performance, as it induces pulsating torque, there by resulting in failure of motor shaft & bearings. High harmonics also effect contactor’s coil like overheating, humming, burning & failure of RC surge suppressor.
Issues with regular 7% detuned reactors at high VTHD conditions
Major part of harmonics can be controlled if one can control the harmonic amplification and resonance caused by capacitors. Majority of the customers install 7% series detuned reactors along with 480 V / 525 V capacitor banks in APFC panel (tuning frequency of 189 Hz). Impedance of these reactors avoids resonance and offers higher impedance to 5th and above order harmonic currents.
But when VTHD is more than 5%, the impedance offered by 7% reactor will not be sufficient and so the harmonic amplification will not get controlled effectively [Refer the table in the next page]. At the same time, the 7% reactor starts saturating, resulting in humming sound, overheating (more than 100⁰C) and eventually burning.
In industries where there are very high 5Th harmonics (due to 6 pulse AC & DC drives), the voltage crest factor tends to reduce to less than 1.414 resulting in a flat peak, like a momentary DC (refer the waveform). This causes the regular 7% reactors to saturate momentarily and results in humming noise & overheating.
Most reactor manufacturers suggest to bypassing the series 7% detuned reactors in case of high V-THD conditions to avoid burning and connect only capacitor banks. But this will even worsen the harmonic levels (refer below table).
Solution - High V-THD detuned reactor
To overcome the above two problems, specially designed high V-THD reactors can reduce the harmonic amplification effectively, without overheating. These reactors can operate with very low heat generation and without humming noise, as compared to normal 7% reactors. The impact of high V-THD reactors is explained with the help of below calculation of RMS current and resultant current harmonics, for the given voltage spectrum:
As per above table, when the VTHD is at 13.6%, a 25 kVAr capacitor (without reactor) draws 44.8 A against rated current of 32.8 A. When 7% detuned reactor is added in such condition, the same capacitor still draws more than rated value of 39.7 A.
It is evident from the calculation that 7% reactors are not sufficient to control the current harmonics and it tends to overheat. Whereas high V-THD reactors gives much better reduction in harmonic amplification and also the current drawn by the capacitors comes close to rated values.
This ultimately results in cost effective hybrid solution, as it reduces the rating of Active Harmonic filter to mitigate harmonic levels below IEEE 519 recommended limits.
Let us verify the above with practical case study where the hybrid solution offered by LK at one of the sites with very high V-THD%.
Power quality study
A detailed power quality study has been conducted in the industry which revealed the following data:
Capacitor feeders were carrying almost double the rated current, due to low impedance & resonance, in the high VTHD system. Frequent failure of electronic cards also reported at this site. High Voltage and Current harmonic levels are also reason for very low True PF. True PF is the product of Displacement PF and Distortion PF. Where,
APFC panels can cater reactive power as per requirement to maintain displacement PF. In order to improve True PF, distortion PF also need to be improved. In order to do so, current harmonics need to be eliminated to the minimum possible level.
As per study done by a third party team, report suggested to install 800 ampere Active Harmonic Filter to mitigate harmonic levels – control distortion PF and achieve close to unity PF. But these high harmonic levels can be brought down if harmonic amplification by capacitors is controlled effectively. Consequently, AHF rating can also be reduced.
Level 1 solution – High V-THD reactors
Based on the readings obtained from the Power Quality study, customer installed capacitor duty contactor switched 350 kVAr APFC Panel with LK’s High V-THD reactors and 525 V LTXL Ultra Heavy Duty capacitors. The step sizes are 1x100 kVAr, 4x50 kVAr, 1x25 kVAr, 1x10 kVAr and 1x5 kVAr. The performance of high V-THD reactors can be understood by below mentioned observations:
After installing High V-THD reactors, they achieved a good PF with low heat generation and avoided PF penalty. Current and voltage harmonic levels came down effectively by restricting the harmonic amplification. The reactors are also working satisfactorily at normal temperature.
Level 2 solution - Active harmonic filter
To mitigate the existing harmonics below IEEE 519 limits and to fine tune PF close to unity, Active Harmonic Filter has to be installed. Now, 300 amperes is sufficient, against 800 amperes which was earlier suggested before installing high-VTHD reactors. The performance with the combination of AHF and high-VTHD reactors are given below:
After installing High V-THD reactors, they achieved a good PF with low heat generation and avoided PF penalty. Current and voltage harmonic levels came down effectively by restricting the harmonic amplification. The reactors are also working satisfactorily at normal temperature.
Conclusion
In the above case, high VTHD reactors + capacitor combination could achieve close to Unity PF in high Voltage Harmonics system. This also resulted in reducing the size and cost of Active Harmonic Filter. Without these reactors, customer would have installed AHF with rating 2.67 times the existing rating (300 A). Since installation of this solution there have been no card failures or cases of nuisance tripping.
It is always challenging to select electrical components in industries rich in Voltage Harmonics. High VTHD is relatively more dangerous than high ITHD. High current harmonics can be resolved by installing harmonic filters, but not always possible with high VTHD. However, with good application knowledge and right products, power quality solution is possible, even in such extreme cases. Addressing high VTHD with specially modified reactors is one such step to gain significant benefits in a cost effective way.
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