Traditionally, ‘power quality’ is considered to be a compatibility problem: is the equipment compatible with the power delivered by the grid including its events, and vice versa? Hence, ac power quality involves voltage, its frequency and waveform. If the supply voltage is steady, its frequency close to the rated and the voltage waveform a smooth sinusoid, the power quality is said to be good. In power quality, thus defined, the quality of electric current is simply ignored! Hence, wouldn’t it be a more appropriate definition for ‘voltage quality’? The above misinterpretation of ‘power quality’ has had major consequences. While utilities have been making sincere attempts to control and improve the voltage quality, it has simply ignored the current! Unlike voltage, current demanded by the load is in the hands of the consumers, who, under the active energy tariff regime, are largely uncontrollable. Consumers have been observed, lately, to be connecting non-linear loads that are polluting the electrical system by injecting harmonics.
What are Harmonics?
Just as still water when disturbed form ripples, a standard sinusoidal supply when disturbed by connecting non-linear loads cause harmonic currents to be injected into the system affecting also the neighbouring consumers connected to the same bus.
The proliferation of solid state devices in lighting ballasts, computers, electronic and communication equipment, variable frequency drives for motors, DC-powered loads, to name a few are non-linear loads that are all pervasive. These loads distort the current waves from following the sinusoidal voltage waveform. This distorted periodic current waveform can be represented using a Fourier transform to discern the contribution of each frequency component which is but a multiple of the fundamental frequency (50 Hz). These higher frequency components, such as 3rd Harmonic (150 Hz), 5th Harmonic (250 Hz), etc., are in general referred to as harmonic currents.
Why Stress on Harmonic Currents and not Harmonic Voltages?
The voltage waveform of the power generated is fairly sinusoidal. Each utility company ensures operation free from faults and vibrations, and with high efficiency. All this helped keep the voltage a smooth sinusoid (Figure 2).
However, the same cannot be said about the current waveform. Current is in the hands of the consumers. The number of consumers is high – in lakhs, if not crores, per utility. Choice of purchase of non-linear loads, their installation and operation are all in the hands of the consumers who distort the current waveform. It is extremely difficult to control them. In ideal condition assuming harmonic voltage to be zero with only 3rd harmonic current flowing, the average active power consumed by the load due to the 3rd harmonic current is zero (see Figure 4). Hence, the average slope of the steadily increasing active energy characteristic curve remains constant regardless of the presence of the 3rd harmonic current (see Figure 5).
LED Lamps – A Case Study on Harmonics
In figures 6 and 7, two types of LED lamps are displayed from the market. The current waveforms of the 60W and 200W lamps have been captured in figure 8 and figure 9 respectively. None of them resemble the ideal sinusoidal current waveform (shown in Figure 1) due to the high levels of harmonic current injected by the respective lamps.
The LED lamps have been dismantled. It was found that the manufacturers of these low-cost lamps had eliminated the driver circuit. Their design team was least bothered about harmonics. Their focus was simply to light up the LEDs at a minimum cost. Giving due consideration to the unidirectional, non-linear characteristics of LEDs, the minimal circuit comprises of a bridge rectifier and a current limiter. While the bridge ensures unidirectional flow of current through the LED strings, the current limiter ensures fail safe operation beyond the knee point of the LED characteristic curve.
Both lamps have approximately 70 numbers of 1W LEDs per string. Considering an LED drop of 3 volts (knee point in LED characteristics), the current will be essentially zero during the time period when the instantaneous voltage is below the knee point i.e (3 x 70) = 210 Volts. From figures 8 and 9, one can identify the sections where the current is essentially zero. What differentiates the two lamps are the current limiting circuits, resulting in unique distortions(harmonics).
Harmonic Menace Solutions
Nowadays, a popular solution to suppress harmonic currents is to insert filters in the system. There are two types of filters, namely active and passive. An active filter can be designed using a three-level IGBT bridge topology that offers a good approximation to a sine wave with low switching losses. A passive filter has a combination of capacitors and inductors that are tuned to resonate at a single frequency, or through a band of frequencies.
It is possible to have series and shunt configurations of the filters. A ﬁlter connected in series is designed to present a high impedance to the harmonic frequency that needs to be blocked.
A current (harmonic) source should be ideally short circuited. Hence, a shunt configuration is more common where filters are connected in parallel to divert harmonic currents to ground, and simultaneously provide reactive power to correct the power factor (as they are designed to be capacitive at the fundamental frequency). In the example below (Figure 11), a commonly used passive, single-tuned (single frequency) or notch or series filter in a shunt configuration has been chosen.
One such example on PSCAD X4 to illustrate the use of the series filter is simulated. The circuit in figure 12 represents a system that has X/R = 10 (Q) and a load that injects typical harmonic currents generated by a twelve-pulse converter. This type of converter injects high levels of 11th and 13th harmonic currents into the AC side.
The results obtained thus show a reduction of 96 per cent in the magnitude of the 11th and 13th harmonic currents (see Figure 14).
Though the solution is effective, it involves cost. Unless there is a lucrative incentive to install active or passive filters, consumers would resist. In an illiterate society, the onus is rightfully pushed over to the electrical appliance manufacturers to ensure that they produce goods that exhibit an ideal resistive load, void of harmonics.
Apparent Energy Metering and Tariff
Effective Value of Current
The RMS value of a complex current wave is equal to the square root of the sum of the squares of the RMS values of its individual components [1, p. 315]. Using standard notations, the RMS value of the complex current, I, can be given as follows.
Similarly, the RMS value of a complex voltage wave, E, can be given as follows.
The apparent power S in Volt-Ampere (VA), is measured as the product of the RMS voltage (E) and RMS current (I): S = E I or
The unit of apparent power is volt-amperes (VA) or kilovolt-amperes (kVA). This power is then integrated to get apparent energy (kVAh). For a constant line power loss, and a constant load RMS voltage E, the apparent power is the maximum active power that can be transmitted through the line. In other words, maximising the amount of power transmitted while keeping the thermal stress of the line constant.
Apparent power is a measure of the maximum heat generation potential in the load, and its value does not drop when the load becomes inefficient. This enables to effectively use apparent power as a basis and reference for obtaining a measure of efficiency of the load either in generating useful heat, useful torque, or useful light in the load for a certain effective or RMS value of current.
Outdated Definition of Power Factor and Apparent Power
Multiple definitions of apparent power have been causing confusion. By comparing the definitions (as per IEEE Std 1459-2010) with those used by the meter manufacturers, one can conclude that mostly all meters manufactured so far measure fundamental reactive power Q1 and use it to compute apparent power S1. Hence, the quantities those meters recording are actually of fundamental frequency, and not inclusive of harmonics. Hence, the harmonic currents go undetected in those meters!
Correct Definition of Power Factor and Apparent Power
Here, the IEEE 1459-2010 standards is quoted to justify the choice of parameters – namely true power factor (PF) and apparent power (S) respectively for the design and development of the apparent energy meters. That is inclusive of all harmonics.
Total Harmonic Distortion – Current: THDI
The harmonic currents can be quantified by the parameter – Total Harmonic Distortion – Current (THDI) which is given as the summation of all the harmonic components of the current waveform compared against the fundamental component of the waveform.
However, the above equation requires separation of the fundamental component from the higher harmonics. An easier technique to measure the dimensionless parameter, THDI, is by using the approximation formula below, which is accurate within 1 per cent for voltages with THDV < 0.05 and for currents with THDI > 0.4. For values outside this range, it is an indicative approximation.
The non-fundamental apparent power SN in Volt-Ampere (VA) is measured according to . For the same condition, namely THDV < 5 per cent and THDI > 40 per cent the following expression holds.
What is noteworthy is the sequence in which the parameters are computed. The most fundamental parameters, such as apparent power, S, are computed in the beginning, offering higher accuracies, while the most complex derived parameters, say of THDI, are computed at the end. The apparent energy meter offers accuracies of 1 per cent for S and 5 per cent for THDI respectively. This means that computation of S is simple, straightforward and hence, viable for large scale production and proliferation of apparent energy meters.
Smart Apparent Energy Meters
In 2011, the apparent energy meters (originally developed in 1997) have been we redesigned using the energy metering chip, EM773 from NXP semiconductors, that has an inbuilt metrology engine to compute S, as per the new IEEE 1459 (see Figure 15). Further, to offer a smart solution, the EM773 chip was connected to a two-way RF transceiver – OL2381 (also from NXP), that gave it a wireless M-Bus communication capability.
Figure 16 shows the user interface offered by such a smart apparent energy meter. The differences between S and S1 or PF and PF1 for a typical household appliance that injects harmonics (THDI = 133.6 per cent) can be observed from the readings displayed. In Table 1 similar differences observed when different appliances were chosen at random are shown. This strengthens the belief that nowadays, mostly all appliances are injecting high levels of harmonics (THDI > 120 per cent).
Apparent Energy Tariff
Earlier, apparent energy (S) based tariff (unit kVAh) is introduced that is to be the only single parameter-based tariff that is fair, incorporates an embedded power factor-based discount mechanism and meets the above objectives. It can play a vital role particularly in the developing countries that are facing a crisis of high losses and low degree of utilisation (high degree of blocking) of transmission and distribution equipment due to highly inductive loads, loads injecting harmonics, and switching loads. It is a win-win solution where consumers can avail tariff discounts by becoming more efficient while electric utilities can increase its revenue both through line loss reduction and through the collection of penal charges from defaulting consumers.
Street Lighting Pilot
Empowered with recommen-dations from the Ministry of Power, and two separate directives from the Gujarat Electricity Regulatory Commission (GERC), in the year 2011, Centre for Apparent Energy Research’s R&D centre successfully demonstrated technical loss reduction to the tune of an amazing 92.7 per cent in a pilot project involving three street lighting feeders of Anand Municipality, Gujarat. Smart dimmable fluorescent lamps were used. If we consider a typical street light feeder with 80 lamps, this represents a savings of Rs 880 per month in terms of line losses.
More importantly, this pilot has given a basis for our estimate of national level savings potential of `15 million per hour. Through the implementation of an apparent energy tariff with 25% discount, we were able to demonstrate its effectiveness in reducing, both the harmonic current and reactive current levels. Table 3 shows the street lighting (SL) tariff rates as offered by GERC. It is observed that the apparent energy tariff carries a 25 per cent discount over the active energy tariff rates. See how the incentive mechanism works.
If a consumer decides to operate non-linear loads that inject significant amount of harmonics and makes no effort to curb the same, then due to low power factor load (PF < 0.75), he would get billed more under the apparent energy (S, kVAh) tariff in comparison to the active energy (P, kWh) tariff. The lower the PF, the greater the difference or higher the bill under the kVAh tariff!
If the consumer puts in limited effort to only obtain a partial improvement in his PF such that it rises and stays at the threshold point of PF = 0.75, then in this situation, the bills would be identical under both the tariffs.
On the other hand, if the consumer puts in the complete effort, as intended under the new apparent energy tariff mechanism to shift to linear loads (PF @1), then he will avail the complete discount of 25 per cent and his bills will be lower by 25 per cent under the kVAh tariff.
Unlike the kWh tariff, the new kVAh tariff has an inbuilt discount or penalty mechanism to lure the consumers, to exercise their choice, to opt for efficient (linear) loads and save. If a consumer cannot discard his non-linear load, he has the option of inserting filters in parallel to sink the harmonics instead of injecting them into the distribution system, and still avail the discount in the bills. In this manner, his load would appear as a linear load to the utility feeding him, thereby minimising the utility’s line and transformer losses.
Shift from Pilots to Large Scale Implementation
A project is considered to be truly successful when it is bankable and repeatable on a larger scale. Nandesari Industrial Estate, in Vadodara District of Gujarat state is selected for this purpose in 2015. A total of over 800 number of lamps were systematically replaced by smart and efficient dimmable LED street lamps. Metering and tariffs were based on apparent energy. As a result, Nandesari Industries Association has been reaping a 25 per cent discount in their electricity bills.
Challenges posed by Harmonics
Is it possible to identify the location of the harmonic current source by simply measuring the electrical parameters at the PCC (point of common coupling) or metering point? The answer is no. There could be multiple 3rd harmonic current sources located at different consumer installations in the vicinity (a realistic scenario today), and no clear technique to identify each of their locations from the PCC (though there are a number of published papers claiming to do so!).
A simpler and more elegant mechanism would be to shift to apparent energy metering and tariffs. As it is observed earlier, apparent energy simply deals with scalar quantities, and thus, effectively breaks down the need to have a vector relationship. This does not only eliminate the overhead of locating the source of harmonic currents, but automatically incorporates a community or co-operative feature that would help curb harmonics. This is how the mechanism works:
Here a current harmonic source is treated like just like any other polluting source in another medium, say air or water. Currents are shared in a distribution system like air or water with others. Just as an entity polluting the air or water would irk others in the neighbourhood, so also would injection of harmonic currents into the distribution system.
The usage of apparent energy tariff system compounds the problem. The neighbour does not only get irked by the damage these harmonic currents do to his appliances, but also due to the higher bills that he would be paying if he draws those harmonic currents. Such a condition will invigorate ‘Community behaviour’ which means that everyone would aspire to belong to a good neighbourhood, where one gets the benefits of a better quality of power supply, and lower bills!
Today, when energy resources are becoming scarce, it is a crime to spoil or waste energy and to charge only for the active energy consumption is no longer considered acceptable. Even the component of energy not contributing towards any useful work should be accounted for. In other words, wasted energy should be treated as energy consumed. Apparent energy is a measure of the ‘Energy Delivering Potential’, and, therefore, is considered most appropriate parameter for measurement of energy and tariffs. Armed with the correct definition of apparent energy and low cost apparent energy meters based on simple digital signal processing technology, It is shown how an electrical revolution can be effectively triggered – where market sees rapid sales of efficient electrical appliances. Once the revolution sets it, It is expected a disruptive but pleasant change where even appliance labels would display ratings in terms of VA instead of W.
Could you kindly tell me what is the impact of harmonics on kVARH readings?