CHAPTER 5 POWER QUALITY IMPROVEMENT BY USING POWER ACTIVE FILTERS

Transcription

1 86 CHAPTER 5 POWER QUALITY IMPROVEMENT BY USING POWER ACTIVE FILTERS 5.1 POWER QUALITY IMPROVEMENT This chapter deals with the harmonic elimination in Power System by adopting various methods. Due to the development of Power Electronics technology, more Power Electronics appliances are used, which leads the serious harmonics pollutions. Using Shunt Active Filters we can eliminate these kinds of harmonics. The development of new Shunt Active Filter is presented. The concept of proposed Shunt Active Filter and its operating Principle, Control Theory is also discussed. The filtering scheme provides harmonics suppression at the source so that the source will supply high quality power to linear load. The Power Quality has become the buzzword in the last one decade due to increase in quality-sensitive load, like computers, non-linear switched devices, which are the sources of disturbance to create poor power quality & awareness of implications of power quality. 5.2 INTRODUCTION Quality means customer satisfaction, which cannot be defined absolutely. It is defined with reference to consumer expectations. The quality of a product is thus measured by using yardstick of consumer satisfaction. Electric power quality is satisfaction of its customer; a consumer is satisfied if he is able to use power through his equipment and devices to serve his purpose. This is possible only if his equipment and devices have Electromagnetic Compatibility (EMC) with the supply quality; Thus, EMC is the measure of power quality. The SIMULINK/MATLAB is a highly developed graphical user interface simulation tool. It has proved instrumental in

2 87 implementing the graphical based controller. The Simulation tool has been used to perform the modeling and simulation of the customer power controller for a wide range of operating conditions. The Simulation results of the proposed filter are discussed. The objective of this chapter develops the proposed new shunt active power filter for the current harmonics suppression using SIMULINK / MATALAB for power quality improvement.[42] 5.3 FILTERS Filters are used to restrict the flow of harmonics current in the Power Systems. It is a LC circuit, which passes all frequencies in its pass bands and stops all frequencies in its stop bands. There are two basic types of filters. The simplest method of harmonics filtering is with passive filters. It uses the reactive storage components, namely capacitors and inductors. It has two types. Shunt passive filter is the Combination of L and C elements, which are connected in parallel with the line. It will restrict the flow of harmonics through the line. Fig-5.1 shows the configuration of Shunt passive Filter. Non-linear load L supply Non-linear load L c supply c Fig 5.1 Shunt passive filters Fig 5.2 Series passive filters Series passive filter is the combination of L & C in parallel, which are connected in series with the supply as in Fig 5.2, which has the ability to eliminate harmonics amplification of shunt

3 88 passive filter. The series active filter needs a much smaller kva rating than a conventional shunt active filter, and as a result, the combined system has good filtering characteristics and high efficiency. This chapter presents a design of the shunt passive filter that makes possible a great reduction in the required kva rating of the series active filter. It can minimize the peak voltage across the series active filter and reduce the required kva rating of the filter to 60 percent. A computer simulation geared to practical applications of large three-phase. Thyristor rectifiers are used to compare the compensation characteristics of the optimized system with those of a combined system that uses a conventional shunt passive filter. Active Filters are newly emerging devices for harmonics filtering, which will use Controllable Sources to cancel the harmonics in the Power Systems. The basic principle of operation of an Active Filter is to inject a suitable non-sinusoidal voltage and currents in to the system in order to achieve a clean voltage and current waveforms at the point of filtering. Non-linear loads source Active filter source Nonlinear loads Active filter Fig 5.3 Shunt active filters Fig 5.4 Series active filters Shunt Active Filter is connected in parallel to the load. The system configuration is shown in Fig 5.3. It consists of a Voltage Source Inverter and a filter inductor connected in series. It performs a harmonics current suppression to the line. Where as series active filter shown in Fig

4 89 5.4, is connected in series with the load. The major advantages of the Series Active Filter are, it maintains the output voltage waveform as sinusoidal and balances the three-phase voltage.[43] 5.4 HARMONICS MEASUREMENT Importance of monitoring PQ In a case study where the end-user equipment knocked off-line 30 times in 9 months but there were only five operations on the utility substation breaker. There were so many events, which will result in end-user problems that never show up in the utility statistics. One example is capacitor switching, which is quite common and normal on the utility system, but can cause transient over-voltage that disrupt manufacturing of machinery. Another is a momentary fault any where in the system that causes voltage to sag briefly at the location of the customer, which might cause an adjustable-speed drive or a distributed generator to trip off, but the utility will have no indication that anything will miss on the feeder unless it has power quality monitor installed Harmonics study If a case study is conducted with two transformers, in which first transformer is supplying nonlinear load and there are 4 feeders on this transformer, whose rating is 2MVA which were supplying the non-linear loads. The harmonics distortion can be observed from Fig 5.5. Fig 5.5 Current Harmonics

5 90 Current Time Fig 5.6 Current waveform Table 5.1 Current Harmonic Amp A B C N Harmonics THD%f H3%f H5%f H7%f H9%f H11%f H13%f H15%f Voltage Time Fig 5.7 Voltage waveform

6 91 Voltage Harmonics Time Fig 5.8 Voltage Harmonics Table 5.2 Voltage Harmonic Volt A B C N Harmonics THD%f H3%f H5%f H7%f H9%f H11%f H13%f H15%f Table 5.3 Power & Energy P/V/I/pf A B C Total kw kva kvar pf (least) I rms V rms

7 92 It can be observed from the above waveforms and tables that the transformer contains current harmonics of around 16% and voltage harmonics of 4%. Major harmonics in Current waveform are 5 th and 7 th harmonics. Hence it is strongly recommended to mitigate the 5 th and 7 th harmonics in current waveform Power Quality Evaluation Transform rating: 2 MVA Electrical System = 415V,50 Hz, 3 Ph 3 Wires (With capacitor bank switched ON) Apparent Power, S = kva Real Power, P = 700 kw Reactive Power, Q = 500 kvar Power Factor, pf = 0.97(lagging) (5.1) Source Voltage, V (Phase-A), V A = 243 Vrms THDV fund, A = 4.1% (Phase-B), V B = 243 Vrms THDV fund, B = 4.4% (Phase-C), V C = 243 Vrms THDV fund, C = 4.2% (5.2) Load Current, I L (Phase-A), I L, A = 1500 A THDI fund, A = 15.2% (Phase-B), I L, B = 1260 A THDI fund, B = 16.0% (Phase-C), I L, C = 1560 A THDI fund, C = 13.2% (Neutral), I L, N = 120 A THDI fund, N = 20.6% (5.3) Power & Current are calculated by using CT Ratio 400:1 The total harmonic current for each phase is calculated as follows I H = THDI fund (i L (1+THDI 2 fund ))

8 93 I H, A = (1500 ( )) = A I H, B = (1260 ( )) = A I H, C = (1560 ( )) = A I H, N = (120 ( )) = A (5.4) Hence from the above calculation it is observed that the transformer contains a harmonics current of 200A per phase. Hence it is strongly recommended to reduce harmonics currents. THD results are obtained by FFT test. 5.5 HARMONIC ANALYSIS The case study conducted on 3 transformers in which one of them is of 1.5MVA, which drives a load of 50 kw with variable frequency. It is observed from Fig 5.1 that, the transformer is supplying a highly non-linear load current of 25% THD. In which major harmonics are 5 th and 7 th Power Quality Evaluation Transformer rating :(1.5MVA) Electrical System = 415 V, 50 Hz, 3 Phase 3 Wires Apparent Power, S = 834 kva Real Power, P = 815 kw Reactive Power, Q =180 kvar Power Factor, p f = 0.98(lagging) (5.5)

9 94 Source Voltage, V (Phase-A), V = Vrms THDV fund, A = 4..5% (Phase-B), V B = Vrms THDV fund, B = 4.2% (Phase-C), V C = Vrms THDV fund, C = 4.3% (5.6) Load Current, I L (Phase-A), I L, A = 1260 A THDI fund, A = 25.2% (Phase-B), I L, B = 1260 A THDI fund, B = 24.5% (Phase-C), I L, C = 1140 A THDI fund, C = 25.0% (5.7) Power & Current are calculated by using CT Ratio 600:1 The total harmonic current for each phase is calculated as follows I H = THDIfund (i L (1+THDI fund 2 ) I H, A = (1260 ( )) = A I H, B = (1260 ( )) = A I H, C = 0.25 (1140 ( )) = A (5.8) Hence from the above calculation it is observed that the transformer contains a harmonics current of 300A per phase. Hence it is strongly recommended to reduce harmonic currents [44].THD results are obtained by FFT test. 5.6 MECHANISM OF ACTIVE HARMONIC FILTER A 100 Amp Active Harmonics filter may be installed in PDB2 of Plant III supplied by Tx1. The Plant III has got 30 Ring frames, each ring frame having VFD (Variable Frequency Drive) of 50 kw rating. 30 Ring frames have been grouped into four groups. A Power Distribution

10 95 Board (PDB) supports each group. PDB2 supports 8 numbers of ring frames. Single line diagram of the same installation is given below in Fig 5.9. They were facing a Problem of high current harmonics more than 25% in PDB2. Tripping of the Circuit breaker and higher temperature of cable and transformer (Tx1). The 100 Amp APF Units may be installed across the load of PDB2. VFD 8 Incomer V p I p V s I s Transformer Tx 1 Source point Is 1 I H I L VFD 2 VFD 1 PDB 2 Other loads PFC Solid state Harmonics filter Fig 5.9 Single Line Diagram of Active Filter Transformer: 4 MVA, 33 kv /440 V, /, PFC: power factor correction capacitors (50 kvar cap Bank) Solid-state harmonics filter rating: 415 V, 50 Hz, 3-Ph-3 wire, 100 Amp AHF. VFD Rating: Each VFD is of 50 kw, 440 V, 50 Hz (5.9)

11 96 Slight Distortion in current wave is, because of higher harmonics current presents in the system than the rating of the AHF installed at the time of measurement. Due to variation in load sometimes-harmonics current exceeds rating of AHF & work in full correction mode voltage waveform. 5.7 RELATIVE STUDY Easy Installation. Installation without affecting the Production. Time required to install the AHF is less than 45 Min. User Friendly control Panel. Maintenance can be done easily without disturbing load efforts. Current transformers are to be connected at load side for Current Sensing. R, Y, & B terminals of AHF to be connected In shunt with the Load point. Current THD: Before AHF Installed Current THD (I THD ) was: 30-31% After AHF Installed Current THD (I THD ) brought down to: 5-6%. Voltage THD: Before AHF Installed Current THD (I THD ) was: 6-7% After AHF Installed Current THD (I THD ) brought down to: 4-5%. Improved power factor (up to Unity) without power factor correction capacitors.[45]

12 97 Parameter Table 5.4 Variation of Parameters Before AHF installation (Cap Bank ON) After AHF installation (Cap Bank OFF) Avg Amp Avg kva Max kva kw pf kvar I THD 24.5% 6.5% V THD 6.4% 5.3% Voltage Table 5.5 Transformer Primary Side Parameter Before AHF Installation After AHF Installation I THD % V THD 2.7% 2.4% Voltage 33KV 33KV PF at TX Max kva Table 5.6 Transformer Secondary Side Parameter Before AHF After AHF Installation Installation I THD 20.5% 16% V THD 4.7% 4.2%

13 ACTIVE FILTER CONTROL ALGORITHM Three phase voltage source i a i b i c Non-linear loads Active filter Fig 5.10 Block diagram of the system Source 3-ph i source i comp i load V source P controller V dc i comp ref Coupling coil Inverter module DC bus Load 3-ph i load Shunt Active Filter Fig 5.11 Shunt Active Filter Description of System configuration The system configuration of the proposed shunt active filter is shown in the Fig It consists of a three-phase source, which is connected to a three-phase non-linear diode bridge rectifier circuit. The Shunt Active Power Filter is connected in shunt with the load. It consists

14 99 of the voltage source inverter in series with the inductor and capacitor. The triggering for the inverter circuit is given through the control circuit. The inverter can be implemented by IGBTs operating in hard-switched Pulse-Width Modulation (PWM) mode to provide sufficient bandwidth for the filtering function Operating Principle Three-phase bridge rectifier with RL loads (non-linear loads) is connected to the three phase three wire distribution system as shown in Fig Due to the nature of the non-linear loads, harmonics are injected in to the system. Shunt Active Power Filter is connected in shunt with the load to suppress the harmonics. The Voltage Source Inverter (VSI) generates a compensating harmonics current in to the phase conductors through the inductor and capacitor sets connected in series with it. The generated harmonics currents cancel each other with out affecting the fundamental part of the source current Control Strategy There are three stages in the active filtering technology. In the first stage the essentials voltage and current signals are sensed using power transformers and current transformers to gather accurate system information. In the second stage, compensating commands in terms of current or voltage levels are derived based on control methods and AF configuration. In the third stage of control, the gating signals for the solid-state devices of the AF are generated using PWM techniques. In this we have used the instantaneous p-q theory for deriving the compensating signal. [46] Control Algorithm The generalized theory of the instantaneous reactive power in three phase circuits is also known as instantaneous power theory, or P-Q theory. It is based on instantaneous values in

15 100 three-phase power systems with or without neutral wire, and is valid for steady state or transitory operations, as well as for generator voltage and current waveforms. The p-q theory consists of an algebraic transformation of the three-phase voltages and currents in the a-b-c coordinates to the - -0 coordinates, followed by the calculation of the v 0 1/ 2 1/ 2 1/ 2 v a v = 2/3 1-1/2-1/2 v b v 0 3/2-3/2 v c (5.10) p-q theory instantaneous power components. As explained above the first step in p-q theory is to transfer the a-b-c frame of voltage and currents into - -0 coordinates. i 0 1/ 2 1/ 2 1/ 2 i a i = 2/3 1-1/2-1/2 i b i 0 3/2-3/2 i c (5.11) The instantaneous powers p and q are calculated using the equations given below p 0 = v 0 i 0 instantaneous zero sequence power. p = v i +v i instantaneous real power. q = v i -v i instantaneous imaginary power

16 101 p v v i = q -v v i (5.12) Where p = p + p ~ q = q + q ~ p 0 = p 0 + p ~ 0 p 0 = Mean value of the instantaneous zero sequence power corresponds to the energy per time unity which is transferred from the power supply to the load through the zero sequence components of voltage and current. p ~ 0 = Alternated value of the instantaneous zero sequence power. It means the energy per time unity is exchanged between the power supply and the load through the zero sequence components. The zero-sequence power only exists in three-phase system with neutral wire. Furthermore, the system must have unbalanced voltages and currents and/or 3 rd harmonics in both voltage and current of at least one phase. p = Mean value of the instantaneous real power, corresponds to the energy per time unity, which is transferred from power supply to the load, through a-b-c coordinates, in a balanced way (it is the desired power component). p ~ = Alternated value of the instantaneous real power, it is the energy per time unity, that is exchanged between the power supply and the load, through a-b-c coordinates. q = Instantaneous imaginary power, corresponds to the power that is exchanged between the phases of the load. This component does not imply any transference or exchange of energy between the power supply and the load, but is responsible for the existence of undesirable

17 102 currents, which circulates between the system phases. In the case of a balanced sinusoidal voltage supply and a balanced with or without harmonics, q(the mean value of the instantaneous imaginary power) is equal to the conventional reactive power (q=3.v.i 1.sin ) Vc 120 Vb 120 Va Power supply N Linear loads Balanced unbalanced Non-linear loads Balanced unbalance Shunt Active power filter C V dc Fig 5.12 Shunt Active Filters with Linear Loads As in Fig 5.12, is usually desirable p-q theory power component. The other quantities can be compensated using a shunt active filter. Can be compensated without the need of any power supply in the shunt active filter. This quantity is delivered from the power supply to the load through the active filter. This means that the energy previously transferred from the source to the load through the zero sequence components of voltage and current, is now delivered in a balanced way from the source phases. It is also concluded from Fig 5.12 that the active filter DC Bus is necessary to compensate input power P i and out put power p o, since these quantities must be stored in this component at one moment to be later delivered to the load. The instantaneous imaginary power (q), which includes the conventional reactive power, is compensated without the contribution of the DC Bus. This means that, the size of the DC

18 103 Bus does not dependent on the amount of reactive power to be compensated. The compensation reference currents in - -0 components can be calculated by using the equations (4.13) & (5.14) i c i c = 1 v - v p - p 0 v 2 2 +v v v q (5.13) Since zero sequence current must be compensated, the reference compensation current in the 0 coordinate is i 0 itself. i c0 = i 0, In order to get the compensating currents in a-b-c reference frame the inverse transformation of - -0 to a-b-c is applied. i ca 1/ i c0 i cb = 2/3 1/ 2-1/2 3/2 i c i cc 1/ 2-1/2-3/2 i c (5.14) Fig 5.13 Active & Reactive power control

19 Features of p-q theory The above Fig 5.13 synthesizes the reference current calculations of instantaneous p-q theory It is inherently a 3-phase system theory It is based on instantaneous values, allowing excellent dynamic response Its calculations are relatively simple (it only includes algebraic expressions that can be Implemented using standard processors). It can be applied to any three-phase system (balanced or unbalanced, with or without Harmonics in both voltages and currents) [47] 5.9 SIMULATION RESULTS The SIMUINK/MATLAB (version 7) is a highly developed graphical user interface simulation tool. It has proved instrumental in implementing the graphical based controller. The Simulation tool has been used to perform the modeling and simulation of the custom power controller for a wide range of operating conditions. The Simulation results of the proposed filter are discussed in this chapter. The Proposed scheme of Shunt Active Filter is shown in Fig For both with and without filter. The active filter is connected in parallel with the 3 phase supply. The load taken for the simulation is Diode and Thyristor bridge rectifier with RL load. The Tables 5.7 & 5.8 show the values of R and L for both diode and thyristor bridge rectifiers respectively.

20 105 Table 5.7 Parameters for Diode Bridge Load Resistance =2 ohms Inductance =1.5mH Load parameters Active filter parameters IGBT Inverter DC Bus = 900V Inverter Coupling Inductance/Phase =1.5mH Ripple Filter Parameters Rect./phase = 4 ohms Cap/phase =36 µf Cap/phase =72 µf Table 5.8 Parameters For Thyristor Bridge Rectifier Load Resistance =2 ohms Inductance =1.5mH Load parameters Active filter parameters IGBT Inverter DC Bus = 900V Inverter Coupling Inductance/Phase =1.5mH Ripple Filter Parameters Rect./phase = 4 ohms Cap/phase =36 µf Cap/phase =72 µf As explained in the control algorithm, there are basically three main steps in instantaneous p-q theory. First step is converting the voltage and currents in ABC reference (stationary) frame into alpha beta zero sequence (which is also stationary) frame as shown in Fig 5.15.

21 106 Second step is to calculate the instantaneous real and reactive powers and extracting reference compensating currents for compensating the harmonic currents and reactive power at the source side of the system as shown in Fig 5.16 Third step is to generate the reference compensating current by using an IGBT based voltage source inverter as shown in Fig 5.14 The results waveform will depict the effectiveness of the active filtering algorithm. It can be observed from Fig 5.17 that the source current has become a pure sinusoidal wave and it has been observed form POWER GUI tool that source current THD has been reduced from 24% to 2%. This can be observed from Fig 5.18 that, load power oscillations are reduced with active filtering.

22 107 Fig 5.14 Simulated Diagram of Active Harmonic filter

23 108 Fig 5.15 Simulated Diagram for ABC to Alpha Beta ZeroTransforamtion Fig 5.16 Simulated Diagram for Reference current calculation

24 109 Fig 5.17 Source and load current waveforms (Time on x-axis, V s & I L on y-axis) Fig 5.18 Source and load power waveforms (Time on x-axis, P s & P L on y-axis) The same active filter is connected to thyristor based bridge rectifier which is containing more harmonics in current compared to the diode bridge rectifier as mentioned in Fig The current THD is reduced from around 44% to 3% hence the designed active filter is effective with high percentage of THD. The designed active filter is capable of reducing around 50 amps of harmonics current in load. The simulation diagram for Active harmonics filter with thyristor based bridge rectifier load is shown in Fig 5.19

25 110 Fig 5.19 Simulated diagram of Active harmonic filter with thyristor bridge load Fig 5.20 Shows that the load current waveform has been improved and the source becomes a pure sinusoidal current waveform. It can also be observed from Fig 2.21 that the power variation at the load side is reduced compared to source side, in addition to this the active harmonics filter is capable of supplying the reactive power to the load and power factor at the source side is also increased.

26 111 Fig 5.20 Source and load current waveforms (Time on x-axis, I s & I L on y-axis) Fig 5.21 Source voltage and power waveforms (Time on x-axis, V s & P on y-axis)

27 CONCLUSION The purpose of this chapter is to review the results obtained during the present work before proceeding with the conclusions of the work done, the objective of the thesis stated in the first chapter are recalled. The primary objective of this chapter is to model and develop the proposed new Shunt Active Power Filter for the current harmonics suppression using SIMULINK/MATLAB (Version 7) simulation tool. The model of the proposed new Shunt Active Power Filter is realized. The developed new Shunt Active Power Filtering technology is implemented for a system feeding a non-linear load. It is simulated using the highly developed graphic tool SIMULINK available in MATLAB. The results reveal that the proposed new Shunt Active Filtering technology is simple and effective and is suitable for practical applications. Table 5.9 Harmonic Distortion of Load Voltage Dead V ab _50Hz THD HS H7 8kHz Time(µs) V(rms) (%) (%) (%) (%) From Table 5.10, it is observed that by the suppression of voltage and current harmonics using new active filtering technology, the power quality can be improved as discussed in the various chapters.