CHAPTER 4 HARMONICS AND POWER FACTOR

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1 4.1 Harmonics CHAPTER 4 HARMONICS AND POWER FACTOR In this research a comparative study of practical aspects of mixed use of diode and Thyristor converter technologies in Aluminium Smelters has been carried out. The nonlinear nature of these converters creates significant line current harmonics. The current drawn by the converters from line is distorted and resulting in a high Total Harmonic Distortion (THD). The harmonics has a considerable effect on the system Power Factor (PF) [38]. These two factors provide: a quantitative measure of power quality in an electrical system is Power Factor (PF) and Total Harmonic Distortion (THD). Therefore it is essential to address about harmonics and power factor in this chapter [39],[40]. Harmonics were always present in the power systems. Earlier harmonics were minor and had no detrimental effects and today the advent of power electronics in every field and proliferation of non-linear loads in industrial power applications has given rise to the problem of harmonics. The application of AC-DC converters in the Aluminium industry is also a big source of harmonics in power system. Harmonics as a function of power frequency are produced on both AC and DC side of the converter. The magnitude of each harmonic is inversely proportional to its order. Thyristors have brought vast improvement in control design and are being used increasingly in AC-DC converters, but have one disadvantage i.e. they produce harmonics and harmonics has considerable impact on other equipment and on the power system. The major impact is on system power factor. Lower order harmonics are more troublesome because of their large amplitude and difficulty in designing of filter circuit to eliminate the same. We cannot wish away harmonics from the present day power systems so we need to tackle them. In Aluminium Smelters, AC-DC converters generate harmonic currents and inject into the power supply network and their effects on power quality is a serious topic of concern [41]. Harmonics is a term, used to describe the shape or characteristic of a voltage or current waveform, with respect to the fundamental frequency in an electrical

2 distribution system. Harmonic distortion is a kind of pollution in the electrical supply. Harmonics of complex current waveform is a way to define the current or voltage waveform. If no harmonics exist, then the waveform is described as the fundamental sine wave (50Hz).The harmonic frequencies are, exact multiples of the fundamental supply frequency (which is typically 50 Hz), thus a voltage with a frequency of 100Hz, would be the 2 nd harmonic in a 50 Hz electrical system. Typical harmonics for a 50 Hz system are the 5 th (250 Hz), the 7 th (350 Hz) and the 11 th (550Hz). A normal supply system operates at 50 Hz power frequency, However, due to any reason if voltage/current signal of higher frequencies (higher than 50 Hz), are introduced in the supply system, they are termed as harmonics, e.g. if a waveform of 250 Hz is introduced in a 50 Hz system, one can say that the system has 5 th harmonic present in the system [42]. Truly speaking harmonics are a mathematical model of the real world. Any nonsinusoidal wave which is periodic in nature can be mathematically decomposed into sine waves of fundamental and higher frequencies, which are multiple of the fundamental frequency. Power system harmonics is an area that is always receiving a great deal of attention in Aluminium Smelters due to non-linear load of AC-DC converters as the load of AC-DC converters is very large portion of connected load [12,23] Linear Load: Linear load is having constant impedance and the resulting current follows the supply voltage changes. In this type of loads, when the supply voltage is sinusoidal, current will also be sinusoidal. Examples of such loads are: i) In candescent lamps ii) Resistance Heating loads Non-Linear Load: A load draws a non-sinusoidal current wave when supplied by a sinusoidal voltage source. The impedance of this type of load varies within each cycle and draws current disproportional to the supply voltage. At each and every point of the waveform, the load impedance varies resulting into higher or lower current than it should be. This 78

3 makes the load current to be non-sinusoidal, resulting into harmonic creation or distortion. Some of the typical non-linear loads are given below: i) AC-DC Converters ii) AC Drives (VFDs) iii) Solid state UPS iv) Induction furnace/arc furnace v) Switch mode power supply vi) UPS vii) Computers Effects of Harmonics on the Power System: Equipment Problems Generators Overheating in coils & cores Transformer Generation of beat noise due to core's magnetostriction phenomena caused by the harmonic current. Reduction of capacity due to increase of core and copper loss caused by harmonic voltage and current Capacitors & Overheating, burnout, resonance Transductors MCCBs Faulty operations due to excessive harmonic current Power fuses Blowing out due to excessive harmonic current Neutral cables Overheating of neutral lines Rectifier controller Faulty control due to phase shifting of control signals Relays Faulty operations due to phase variation caused by harmonic current or voltage Computer Adverse influence on performance Watt-hour meter Measuring error due to non-linear characteristics of effective voltage & current flux Burn out of current coils due to inflow of excessive harmonic current 79

4 4.2 Harmonics due to AC-DC Converters The Aluminium Smelters use only two types of AC-DC converters i.e. diode converters & thyristor converters. A diode converter is a device for converting an AC to DC into a unidirectional or approximate direct current. It is known fact that diode has a low impedance in forward direction to current flow and in reverse direction a nearly infinity impedance to oppose the flow of current. So when an AC voltage is applied to a diode current will flow through its positive half cycles only and current will be zero during negative half cycles. A typical voltage and current wave of half wave rectification is shown in Fig 4.1 is produced by using single diode. Fig. 4.1: Diode converter output voltage/current waveform In thyristor converter the amount of power delivered to a load can be controlled. A basic circuit is used to produce necessary gate current to turn the thyristor on. If the thyristor is fired at angle α then the current flowing in the load has a wave form and the sharp rise shown as in Figure 4.2, however gives rise to harmonics. Fig. 4.2: Thyristor converter output voltage/current waveform It is clear from Fig. 4.1, that the angle between voltage source and fundamental component of current is zero. Then the displacement power factor (DPF) for a diode converter is unity. But for thyristor Idc = Im/π (1+cosα) Irms = π/2 Idc (1+cosα) 80 80

5 So, in case of fully controlled AC to DC thyristor converter, the DPF will be same as cosine of the delay angle α resulting further deterioration power factor [35]. The input current of these AC-DC converters comprises of large discontinuous peak current pluses that results in high input current harmonic distortions. This high distortions input currents owes due to the fact that these AC-DC converters conduct for a short period. Therefore it is clear that AC-DC converters introduces large amount of harmonics in to the source current. If the source is having inductance then the voltage distorted at point of common coupling shall be substantial [43-44]. The generation of harmonics for the considered converters of HINDALCO can be explained as fallows with the help of Figures 4.3 (a) and 4.3 (b) The AC power supply flows from Renusagar Power Company to HINDALCO Rectifier station and from Rectifier station, AC power is fed to the AC-DC converters through their Rectifier transformers. From this it is clear that Fundamental current flows from the source (generator) towards the Smelter. However, the harmonic current generated by these AC-DC converters, flow from the load end towards the source. In this process, the injected harmonic current passes through the end users transformers, its main incomer power transformer, cables, and overhead wires up to the generating station. Fig. 4.3 (a) and 4.3 (b) As there are other loads connected to the main bus, which is normally applicable to most of the power distribution systems, the harmonic tend to flow into them in case they offer a lesser impedance compared to the network, for example, capacitors offer a very low impedance to higher frequency current and hence tend to absorb a lot of harmonics from a polluted system

6 4.3 Permissible Levels of Harmonics: It is very important to know about the harmonic distortion limits for this research work. Recommended practices of IEEE Standards helps to limit the percentage of harmonic distortions generated by AC-DC converters. These limits will help in designing the filter banks for reactive compensation for systems with harmonic distortion [41]. Harmonic Distortion (%) limits as per IEEE Standards : Table 4.1: Harmonic distortion (%) limits as per IEEE Std Current Distortion Limits for General Distribution Systems (120 V through V): Maximum Harmonic Current Distortion in % of I L Individual Harmonic Order (Odd Harmonics) I sc /I L < >34 %TDD <20* < < < > Even harmonics are limited to 25% of the odd harmonic limits above. Current distortions that result in a dc offset, e.g. half wave converters, are not allowed. *All power generation equipment is limited to these values of current distortion, regardless of actual I sc / I L. Where, I sc = Maximum short circuit current at point of common coupling (PCC) I L = Maximum demand load current (fundamental frequency component) at PCC PCC = Point of Common Coupling Voltage Distortion Limits Table 4.2: Voltage Distortion (%) Limits as per IEEE Std Bus Voltage at PCC Individual Voltage Distortion (%) Total Voltage Distortion THD (%) 69 KV and below KV through 161 KV KV and above

7 Two important points must be made in reference to the above: i) The customer is responsible for maintaining current distortion to within acceptable levels, while the utility is responsible for limiting voltage distortion. ii) The limits are only applicable at the point of common coupling (PCC) between the utility and the customer. The PCC, while not explicitly defined, is usually regarded as the point at which the utility equipment ownership meets the customer's or the metering point. Therefore, the above limits cannot be meaningfully applied to, say, distribution panels or individual equipment within a plant. The entire plant must be considered while complying with these limits. Here it is necessary to mention about the concerns of voltage and current distortions and understand why it is more important to consider current harmonics only in this research? Power systems have low source impedance and well regulated voltage. For given amount of harmonic current flow, the resulting voltage distortion will be relatively small except harmonic resonance conditions whenever harmonic levels are in excess of 5%, and then it is always about current harmonics [17]. In this research work also the focus is on current harmonics than voltage harmonics. 4.4 Concerns for Current Distortion: The existence of a distorted current wave shape does not always mean that there will be problems. The power system, because of its stiffness, will be able to absorb substantial amount of harmonic currents without problems. For a problem to exist, the distorted current must flow in a path with high impedance, resonance or significant exposure to communication circuits. If the current flows through a path of high impedance or into a resonant circuit, the voltage will become distorted. This results in the current distortion. Due to the current distortion there is the additional heating and losses which can occur for elements in the current path e.g. transformers. When transformers are designed for known harmonic producing loads, manufactures make special design modifications to accommodate the harmonic currents by considering K-Factor. However, there is a 83

8 need to calculate the THD and design filter bans to keep the THD as per the limits recommended in IEEE Standards THD Calculations: There are several measures used for evaluating the influence of harmonic current on the power system. One of the most common is the "Total Harmonic Distortion (THD)", which gives a measure of the effective (RMS) value of the harmonics relative to the fundamental component. The RMS value of a wave comprised of several harmonics is the square root of the sum of the squares of the individual harmonics. Therefore the THD is computed as follows: Where, I 2, I 3, , I n = Individual RMS harmonic current components I 1 = Fundamental frequency RMS current. When THD is large then there is considerable distortion takes place, which lowers the power factor [29]. 2 1/2 THD = [ (I2 2 + I In ) ] / I Methods to Control Harmonics: There are many methods available to control the harmonic levels of the loads. It is always easier to solve the problems if the basics of problem are known. The load, which is not linear or draws less or more current at any part of the waveform, creates non sinusoidal current. The following methods are common for reducing harmonics in the power circuits: 1) Use of multiphase systems 2) Using shunt filters 3) Injecting harmonic current of suitable frequency Based on the above, the following methods are widely used to control harmonics: 1) Usage of passive filters such as: a) Series Filter b) Shunt Filter 2) Phase Multiplication 84

9 Series Filter is connected in the incoming power line before applying it to the load. Since the series filter is tuned to a particular frequency, it offers very high impedance for that frequency. Rest of the harmonic frequencies are not attenuated to that extent. Hence this type of filter can be used to control specific harmonic frequencies e.g. 3 rd, 5 th or 7 th harmonic. Shunt filter offers a low impedance path for specific frequency tuned. This is useful to trap particular harmonics e.g. 5 th, 7 th, 11 th, 13 th etc. Various shunt filter configurations are being used. But in HINDALCO Smelter, the shunt filter configuration is used as shown in Fig. 4.4 The capacitance used in the filter circuit is also utilized to supply reactive power to improve power factor. Fig. 4.4: Configuration of Shunt Filter Phase Multiplication: In AC-DC conversion the basic configuration for connecting semiconductor devices is six pulse bridge connections. This 6-pulse converter will have 5,7,11, 13 etc. harmonics. These lower order Harmonics are more troublesome because of their large amplitude and difficulty in designing of filter circuit to eliminate them. The number of pulse can be increased through parallel connection of two or more six pulse converters. A 12-pulse converter will have 11, 13, 23, 25 etc. orders of harmonics. Due to increase of pulse numbers, harmonics present in the system decease. In 12 pulse rectifier, the lowest order input current harmonics are 11 th and 13 th, with a balanced input voltage source. Where, the input current harmonics 5 th and 7 th are 85 85

10 theoretically eliminated but as the high pulse converters are formed from six pulse connections, 5 th and 7 th order harmonics will always be present though their amplitude is much smaller than that obtained with six pulse converter[32,44]. Harmonic Current Injection: By injecting harmonic current of suitable harmonic number at dc side of the converter transformer the wave can be considerably improved in ac side though this method may be effective but not in practice in Aluminium Smelters so far. Conclusions: a) Harmonic pollution to be tackled with due seriousness. b) Harmonics do not damage only our own equipment but also that of our other plants equipment. c) Wherever the harmonics are found exceeding the limits, suitable filters may be one of the solutions. d) Due care should be taken during selection of all new equipment & systems. e) Special efforts are required to create the awareness to mitigate harmonics pollution. Otherwise this becomes a major problem to lower the power factor. f) The use of non-linear loads is inadvertent as per a survey, it is assumed that use of non-linear devices will grow further and it may become 50%-70% of the total load in near future. g) Harmonics can be termed as "Evil" in the modern electrical system. New advancement in various fields of electrical engineering like Control system, Power electronics, Computers etc., is resulting in continuous increase in the usage of non-linear loads. Non-linear loads are always with generation of harmonics and indirectly lower the power factor of the power system. A greater awareness about harmonics, its cause, effects and solution will help us in improving the quality of Power System. 4.7 Power Factor: Power factor of an AC system is defined as the ratio of real power to the apparent power in the circuit and it is an indicator of the power quality [16]. Power Factor= Active power /Apparent power 86

11 Apparent Power = (Activepower) 2 (Re activepower) 2 Power factor is also the cosine angle between voltage and current in ac circuit. Generally there is a phase angle φ difference between voltage and current in ac circuit, cos φ is called as the power factor of the ac circuit [39]. The power factor varies between 0 and 1 and it can be either inductive (legging) or capacitive (leading). Real power always does the useful work. Reactive power is the power required to produce the magnetic fields (lost power) to enable the real work to be done where total power is considered. Power factor represents a figure of the merit of the character of power consumption. A low power factor indicates poor utilization of the source power capacity needed by the load. Correcting the power factor frees up MW capacity for future plant growth and lowers current power costs [12]. The waveform of the current in the input AC line which feeds the phase controlled converter is not a pure sinusoidal. It consists of a fundamental component and a series of super imposed harmonic components. The displacement angle denoted by Ø is defined as the angular displacement between the fundamental component of the AC line current and the associated line voltage. In all the phase controlled converter circuits, the fundamental components of current lags behind the associated voltage or in the limiting case it is in phase with it. The displacement factor is defined as the cosine of the displacement angle. Therefore power factor is also defined as the ratio of the total mean input power to the total RMS input in VA. Current Harmonic Distortion (µ) The distortion factor of the current in a given input line is defined as the ratio of the rms. amplitude of the fundamental component to the total rms. amplitude. I 1 n 2 2 (I 1 ) (I n ) n 2 I 1 Irms If the Voltage waveform is sinusoidal then the input power factor can be defined as p.f. = V₁I₁ Cos (Ø)/V₁I rms. = I₁/I rms. x Cos (Ø₁) = µ Cos (Ø) Where Cos (Ø) is the displacement power factor (DPF) between the fundamental components of voltage and current, and µ is the current distortion factor. This 87

12 commutation angle is due the process of current transfer from one device to other device takes place in finite time. This is because of the fact that current cannot change suddenly through a device due to its di/dt limitation and secondly the presence of commutating reactance present in the circuit does not allow current to change suddenly. During this process dead circuit exists. Due to this commutation angle, the available DC output voltage is always less in comparison to no commutation delay[45].thus unity power factor can only be achieved when µ =1, since Cos (Ø) cannot be greater than one Reasons for Poor Power Factor. AC-DC Converters produces a non-sinusoidal line current due to non- linear input characteristics of the converters. Due to non- linear behavior of converters, the line current is distorted and results in high Total Harmonic Distortion (THD) and low Power Factor (PF) [39]. For a sinusoidal input voltage, the PF, before the input filter is expressed by NO PEF Q RJ ;STU< V ^ V WXY Z V \ ] \ _ Therefore reduced THD helps to improve the power factor [44]. Poor power factor caused due to the source inductances and commutation delay in converters which results an overlap period µ. Therefore current lags voltage and degrades power factor. The power factor of diode converters before power factor correction is in the range of 0.89 to 0.93 near rated output. The power factor of a thyristor converter however depends on the firing angle of the system. At HINDALCO Smelter the Thyristor convertor system of pot line 7 is operating at firing angle α = 13. The power factor before power factor correction was measured between 0.82 to 0.84 but with larger firing angles, the power factor reduces even further. 88

13 Need of Power Factor Correction. There is utmost need of power factor correction due to following reasons: 1) Power factor correction reduces cost to the consumers for electricity charges and helps to maintain good power quality. The power quality has serious economic implications for customers. 2) Power factor correction leads to a big reduction of apparent power drawn from the AC source which in turn saves energy and minimizes the transmission losses. 3) For better utilization of electrical machines. If power factor correction is not done then the size of generators, conductors, transformers, and switch gears would be increased in in size and cost to carry the extra current [24] 4) To improve rectifier efficiency by reducing large r.m.s values of the input currents [11] Methods to Improve Power Factor 1) Phase shifting transformers are used to have multi-pulse operation o f converters to eliminate lower order harmonics. This multi-pulse system reduces current THD value and helps to improve power factor. 2) Power factor improvement can be done either by increasing active power or by reducing reactive power but increasing active power is not the solution but to reduce the reactive component. Power factor correction is not simple with distorted waveform. Capacitors are added to improve the displacement factor if the voltage is sinusoidal. To improve the distortion factor, filters are added or higher pulse numbers are used. The power factor of diode converters after power factor correction is in the range of 0.95 to 0.99 near rated output. The power factor of a thyristor converter however depends on the firing angle of the system. The value of firing angle at pot line 7 is being kept α = 13 when the thyristor converter system is operating for required output voltage with 12-pulse. The power factor after power factor correction is in the range of 0.90 to 0.93 with 12-pulse, however with 24-pulse converter system the power factor will be better. But with larger firing angles, the power factor will reduce even further.