World Academy of Science, Engineering and Technology 48 2008 Design and Economics of Reactive Power Control in Distribution Substation Khin Trar Trar Soe 4. Long and overloaded 11kV and subtransmission lines. 5. Poor voltage regulation on 11 kV and L.T lines, voltage drops being extended beyond permissible. 6. Under loading of distribution transformers. 7. Absence of shunt compensation in the subtransmission and distribution system; therefore, necessary to improve the working of the power distribution systems to reduce the unfavorable conditions and there by reduce losses, improve voltage regulation, etc. The system improvement has to be planned properly with the following objectives in mind. 1. To reduce losses in the distribution and subtransmission system. 2. To improve the voltage regulation so as to bring it within the prescribed limit. 3. To improve the power factor in the subtransmission and distribution system so as to get optimum utilization of /subtransmission/distribution capacities. Abstract—An electrical power system consists of three principle components that are generation station transmission line and distribution systems. A distribution system connects all the individual loads in a given to the transmission lines. All inductive loads require two kinds of power to operate with active power(kW) and reactive power (kVAR) in design and operation of alternating current electric power systems. A significant factor reactive power has been recognized. There is important interrelation between active and reactive power transmission. There are not purely sinusoidal wave forms, especially when it is compensated reactive power. The state controlled reactive power sources almost always produce harmonics. In a design of static compensators, harmonics should be considered individually. For a given distribution of power, the losses in the system can be reduced by minimizing the total flow of reactive power stability and voltage control in reactive control need about the use fixed shunt reactors, shunt capacitors, series capacitors, synchronous condenses and modern static compensator needed for reactive power control. Reactive power compensating mainly transmission system installed at substation is considered. The location of reactive power control in distribution substation can be seen that reactive power control, inrush current, shunt capacitors, series capacitors, shunt reactors, harmonics effect, ,economical considerations and selection of using apparatus. II. SHUNT CAPACITORS Shunt capacitors can be used on the distribution system to improve the voltage regulation of the system. The shunt capacitors, if connected to utilization equipment and switched on in accordance with the load, reduce the voltage drop in the distribution system and thus help in obtaining better voltage regulation. If the utilization equipment draws a current which is fairly constant, the voltage regulation by the shunt capacitor is more effective. Shunt capacitors installed on a distribution system reduce energy losses in every part of the system between capacitors and generators. The use of shunt capacitors improve the voltage regulation of the system, The size of the shunt capacitor banks varies from individual units of 5 to 25 kVA connected to the secondary or primary circuits of a distribution system to a bank of capacitors of large-size kVA connected to the bus of substation at the primary voltage side. Keywords— reactive power control, economical consideration, inrush current, harmonics effect. I. INTRODUCTION D UE to system expansion without proper and adequate planning and financial provision for the works in time, a large number of distribution systems have run into problems such as poor voltage regulation, poor power factor, high losses and poor efficiency, over loading and less reliability for continuity of supply. The causes for high losses and poor voltage regulation in the distribution and subtransmission system are: 1. Low power factor of the consumer installations. 2. Long and over loaded L.T lines. 3. Distribution transformers’ centers located away from the load centers. III. SHUNT COMPEMSATION AT THE HT SUBSTATION The benefits of shunt compensation at the HT substations are (i)the MVAR loading on the generating stations wherever it is overloaded is reduced ;(ii) release in transmission system capacity and reduction in transmission losses is released; and (iii) release in losses in the subtransmission lines. Benefits under (iii) can be worked out by considering the improvement Ms. Khin Trar Trar Soe is student of Mandalay Technological University (e-mail: khintrartrarsoe@ gmail.com). 416 World Academy of Science, Engineering and Technology 48 2008 in power factor and the consequent reduction of the line current. The revenue due to kWh saved on subtransmission lines may be calculated at average cost per kWh at 11 kV bus. The revenue due to release in transformer capacity may be worked out on the additional kWh. Imax = The HT capacitors, 11 to 132 kV may be of the switched and no switched type, depending on the minimum loading, maximum voltage conditions of feeders or substations. In case of no switched capacitors, the switchgear and damping reactors are not required. It has been found economical to install fixed capacitors and heavily-loaded 11 kV feeders for compensation up to 30% of kVAR of average feeder load. For switched capacitor banks, the switching and damping of inrush currents and the suppression of harmonics need special consideration. In the case of single capacitor banks, the damping reaction is not normally required from the consideration of inrush currents at the time of switching. The system reactance including that of the transformer at which the capacitor bank is installed is adequate enough to bring down the value of inrush currents within safe limits of the capacitor on switchgear. When a number of capacitor banks are used in parallel, it may become necessary to use series reactors for limiting the inrush currents. Reactors, like capacitors, are basic to and an integral part of both distribution and transmission power systems. Depending on their function, reactors are connected in shunt or in series with the network; singularly (current limiting reactors, shunt reactors) or in conjunction with other basic components such as power capacitors (shunt capacitor switching reactors, capacitor discharge reactors filter reactors). Reactors are utilized to provide inductive reactance in power circuits for a wide variety of purposes. These include fault current limiting, inrush current limiting for capacitors and motors, harmonic filtering, VAR compensation, reduction of ripple currents. Reactors may be installed at any industrial, distribution, or transmission voltage level. Shunt reactor compensation is typically required under conditions that are the opposite of this requiring shunt capacitor compensation. The maximum peak inrush current can be approximately given by the formula: (1) Imax = IC1 [ 1+XC1/XL1 ] Where, IC1 = Capacitor’s rated current (fundamental wave) rms XC1 = Capacitor reactance (fundamental wave) XL1 = Total inductive reactance of the system including capacitor bank (fundamental wave) The inrush current comprises a steady component of forced oscillation at supply frequency and a free oscillation of frequency. Inrush current frequency 1 L 1C 1 R2 4 L21 2 Neglecting terms, R 4 L 21 VII. LT CAPACITORS’ INSTALLATION LT capacitors are installed on the distribution system on individual lines or consumers motors to reduce system losses system losses and improve the system voltage and capacity. In addition, they provide other advantages for the consumer, such as reduction in kVA demand, losses and stable voltage . The optimum benefit desired from the capacitors largely depends on the correct positioning of the capacitor in the system. (2) VIII. SERIES AND SHUNT CAPACITORS because R is very small as Capacitors aid in minimizing operating expenses and allow the utilities to serve new loads and consumers with a minimum system investment. Series and shunt capacitors in a power system generate reactive power to improve power factor and voltage, thereby enhancing the system capacity and reducing the losses. In series capacitors the reactive power is proportional to the square of the load current, whereas in shunt capacitors it is proportional to the square of the voltage. There are certain unfavorable aspects of series capacitors. Generally the cost of installing series capacitors is higher than that of a corresponding installation of a shunt capacitor. compared to L1 f0 = 1 × 2π (4) VI. REACTORS V. INRUSH CURRENT 1 × 2π C1 L1 Where, C1 = equivalent capacitance of the circuit in µF L1 = equivalent inductance between the energized banks and bank to be energized in µF EN = line to neutral voltage Thus it may be desirable to install parallel capacitor bank with series reactors. The most important point to check is that such capacitors must have matched voltage rating with respect to reactors. Series reactors are normally installed to limit inrush currents and to prevent excessive harmonic voltages. Series reactors chosen with respect to harmonics are large enough that inrush currents cause no problems for capacitors and circuit breakers. IV. HT SHUNT CAPACITORS’ INSTALLATION REQUIREMENT f0 = 2 × EN × 1 L 1C 1 (3) In use of parallel banks, which already energized, the inrush current is mainly governed by the momentary discharge energized capacitor bank and since the impedance between the energized capacitor bank and the capacitor bank to be energized may be small, it may result in high peak inrush current. The maximum peak current is given by the expression: 417 World Academy of Science, Engineering and Technology 48 2008 3. Harmonic studies to determine the series and parallel resonance points in the system with connection of filter banks. Resonance occurs whenever an electrical circuit’s inductive and capacitive reactance connected either in parallel or series are equal at some frequency. The frequency at which a circuit is in resonance is called the natural frequency of the circuit. A shunt capacitor bank forms a resonant circuit with system inductive elements. This resonance condition can be excited by remote system disturbances such as remote bank switching or sources of harmonics current. Resonance can cause excessive over-voltages and currents possibly resulting in failure of equipment such as capacitors, surge arresters, instrument transformers, and fuses. This is because the protective equipment for a series capacitor is often more complicated. The factors which influence the choice between the shunt and series capacitors are summarized in Table 1. TABLE1. SERIES AND SHUNT CAPACITORS Preference Sr.No. Objective 1 2 Series capacitor Shunt capacitor Improve power factor Second First Improve voltage level in an overhead line system with a normal and low power factor First Second 3 Improve voltage level in an overhead line system with a high power factor Not used First 4 Improve voltage in an underground line system with a normal and low power factor First Not used 5 Improve voltage in an underground line system with a high power factor Not used Not used 6 Reduce line losses Second First 7 Reduce voltage fluctuations First Not used A. Harmonic Resonance Capacitor banks may resonate with harmonic currents produced else where on the system. Harmonic current flow into the capacitor bank may excite parallel resonance between the system inductance and bank capacitance. Parallel resonance causes high oscillating current between inductive and capacitive energy-storage elements. High oscillating currents cause excessive voltage distortion. Installing current-limiting reactors in series with the shunt capacitor bank can tune the bank to the offending harmonic’s frequency and eliminate parallel resonance. Parallel resonance is avoided since harmonic current cannot flow between the system inductance and the bank’s capacitance. X. THE DEGREE OF COMPENSATION Due to various limitations in the use of series capacitors, shunt capacitors are widely used in distribution systems. For the same voltage improvement, the rating of a shunt capacitor will be higher than that of a series capacitor. Thus a series capacitor stiffens the system, which is especially beneficial for starting large motors from an otherwise weak power system, for reducing light flicker caused by large fluctuating load, etc. The degree of compensation being decided by an economic point of view between the capitalized cost of compensator and the capitalized cost of reactive power from supply system over a period of time. In practice a compensator such as a bank of capacitors (or inductors) can be divided into parallel sections, each Switched separately, so that discrete changes in the compensating reactive power may be made, according to the requirements of the load. Reasons for the application of shunt capacitor units are because of 1. Increase voltage level at the load 2. Improves voltage regulation if the capacitor units ar properly switched. 3. Reduces I2R power loss in the system because of reduction in current. 4. Reduces I2X kVAR loss in the system because of reduction in current. 5. Increases power factor of the source generator. 6. Decrease kVA loading on the source generators and circuits to relieve an overloaded condition or release capacity for additional load growth. 7.By reducing kVA loading on the source generators additional kilowatt loading may be placed on the generation if turbine capacity is available. 8. To reduce demand power is purchased. Correction to 100 percent power factor may be economical in some cases. IX. RESONANCE AND HARMONIC For capacitor banks connected to high-voltage system series reactor must be used (a) for limiting the inrush current on energisation of bank and (b) to suppress the harmonics in order to prevent harmonic overloading of the bank as well as to avoid undesirable parallel resonance with the system reactance. It is therefore advisable for economic reason, to combine the power factor correction and harmonic filtering in the same bank. However depending on the most prominent harmonics in a particular installation, a number of banks may be necessary and needs to be determined by following system studies. 1. Short circuit study to evaluate the range of various of system impedance at the point of connection of compensation equipment. 2. Load flow study to evaluate the range of vibration of system voltage. 418 World Academy of Science, Engineering and Technology 48 2008 as small as possible to achieve maximum economy for a given amount of energy supplied. Assuming a fixed maximum power requirement, this can only achieved by power factor correction. It is possible to correct low power factor up to unity power factor, thus making the power factor in kW and kVA loading on a power system equal. But owing to the cost of power factor correction equipment this is never economically justified. Hence the load is partially compensated ( i.e. | Qγ| < |QL| ) the degree of compensation being decided by an economic between the capital cost of the compensator ( which depends on it’s rating) and the capitalized cost of obtaining the reactive power from the supply system over a period of time. 9. Reduces investment in system facilities per kilowatt of load supplied. kV A R Q2 P φ1 kW φ2 S2 Q2 Q1 S1 XII. ESTIMATION OF MOST ECONOMICAL POWER FACTOR Fig 1. Phasor Diagram of Improving Power Factor p.f1=φ1 p.f2 = cos φ2 -1 φ1 = cos (p.f1) Q1 = P tan φ1 φ2 = cos-1(p.f2) Q2 = P tan φ2 Size of capacitor to improve power factor from p.f1 to p.f2 Qc = Q1- Q2 = P tan φ1 - P tan φ2 = P(tan φ1− tan φ2) XI. ECONOMIC OF REACTIVE POWER CONTROL The electrical loading on electrical apparatus in power systems is a kVA loading. Such apparatus is designed to work at a definite voltage and not to exceed a definite maximum current. Both the operating voltage and the current, core losses and there together must not exceed the power which the apparatus can dissipate without exceeding its maximum working temperature. For a particular power system, voltage is constant and current is limited by the losses. Therefore, the volt-ampere (or kVA) has a maximum value and from P =VI cos φ the greater the value of cos φ the greater the power transmitted. It is thus an economical to work with low power factor since the power transmitted by the apparatus is reduced. It is also advantages, when a given amount of energy is to be transmitted, that this is done at lower power level over a long period of time, i.e. with a high load factor. Thus kVA loading is reduced by having both the high load factor and a high power factor. In order to induce consumers to work with minimum kVA and also to make those pay most who make the most demand on the power system, a two tariff may be used. A consumer’s annual cost is there of the form(AM + keU) kyats Where A = kyats per annum per kVA maximum demand M= maximum demand Ke = a charge ( kyats) per kWhr for each energy consumed U = energy consumed in a years ( average load) From the above expression, the consumer should make ‘M’ Fig 2. Power diagram for Estimation of Power Factor P = VI cos φ1 = VIa (5) Consider a load of power (kW) per phase at a lagging power factor of cos φ. It is required to correct the power factor at the consumer’s terminal by connecting power factor correction capacitance ‘C’ to give the most economical power factor. Since the load power is constant, only the reactance component of I2 (the current taken from the supply after power factor correction) is variable. Before power factor correction, Annual cost (1) = AM1 + keU (6) After power factor correction, Annual cost (2) = AM2 + B (kVAR1 –kVAR2) + keU (7) where B = the annual charge per kVAR of the power factor correction equipment kVAR1 = reactance power from the supply before power factor correction kVAR2 = reactance power from the supply after power factor correction The power triangle before and after power factor correction may be drawn as show below. 419 World Academy of Science, Engineering and Technology 48 2008 A. Configuration Fig 3.Triangle Diagram From the above two equations, Annual saving = cost (1) – cost (2) = A [M1-M2] – B [kVAR1- kVAR2] (8) = A [P sec φ1 - P sec φ2] – B [P tan φ1 - P tan φ2] Differentiating the saving with respect to the variable φ2 and equating to zero for maximum saving, A P [sec φ1 tan φ2] + P B [-sec2 φ2] = 0 A tan φ2 = B sec φ2 sin φ2 = B A XIV. COMPENSATED CONDITION TO INCREASE THE VOLTAGE OF SITTAUNG 33 KV TO 32.488 KV (9) is the condition for minimum cost. Hence φ2 will be the most economical power factor. Rs =8.15993Ω Xs = 19.14005Ω P =5 MW QL = 3.096 MVAR Reactive power supply from system is Qs = 1.49572MVAR Q γ = 1 MVAR XIII. REACTIVE POWER CONTROL IN DISTRIBUTION SUBSTATION FROM BAYARGYI TO SITTAUNG One parallel capcitor banks 1 MVAR is applied in this distribution substation. After compensation Supply voltage= E = V + ∆V = 33.7526+j 2.57 = 33.85 ∠ 4.3542 kV The total current in the supply line, Is = 0.1606 ∠ -16.6698 kA The compensator current, I γ =j 0.0307kA The current flow into the load = I L=0.199 ∠ -39.488 kA Power factor =cos16.6678 = 0.9579 (lag:) Voltage regulation = 1.575% Fig 4. One line diagram from Bayargyi to Sittaung TABLE 2.CALCULATION RESULT FOR REACTIVE POWER CONTROL(POWER FACTOR CORRECTION) AND VOLTAGE REGULATION WITH CAPACITOR BANKS From To kV Length (km) Conductor size (mcm) code Rdc( per mile) Rac XL Z (Ω) Bayargyi Sittaung 33 9.75 28.19 12.73 ACSR 397.5 ACSR 397.5 ACSR 397.5 Ibis 0.2306Ω Ibis 0.23033Ω Ibis .023.32Ω 1.57163 Ω 3.6024Ω 1.571+j3.60 4.5387Ω 10.8342Ω 4.538+j10.83 2.0496Ω 4.7.345Ω 2.049+j4.703 XV. CONCLUSION One of the reasons for improving the power factor is to decrease the reactive power. Another reason for improving for the power factor is to avoide poor voltage regulation. Power factor improvement may be achieved the use of synchronous motor. But this paper use the capacitor banks because it has no moving parts , initial cost is low, reaction in failures. This paper will help and give the knowledge of the power factor correction for distribution substation and calculation of size of capacitor banks to improve p.f and voltage regulation. We can calculate the economics about power factor correction 420 World Academy of Science, Engineering and Technology 48 2008 (reactive power control) .If we know about the consumer kVA, those pay demand on the power system, a two tariff ,we can calculate the maximum economy for amount of energy supplied. This paper can see and calculate for minimum cost and the most economical power will be φ2 ACKNOWLEDGMENT Firstly, the author would like to express her indebtedness and gratitude to her beloved parents, for their kindness, support, understanding during the whole course of this work and encouragement to attain ambition without any trouble. The author is indebted to all her teachers who give her knowledge from M.T.U and Y.T.U in Myanmar. REFERENCES [1] T.J.E.MILLER, 1982. “Reactive Power Control in Electric Systems” 1982 by Jihn Wiley & Sons Inc. [2] R.K.Mukhopashyay and T.Choudhury, S.P. Choudhury, Samiran Choudhuri, F.I.E, Power System for the year 2000 and beyond. “Reactive Power Compensation in Industrial Power Distribution System” [3] A.S pabla, “Electric Power Distribution” (Fourth edition) Tata McGraw-Hill Pubkishing Company Limited. [4] Williiam D. Stevenson, Jr, “Elements of Power System Analysis” (Third ediion) 1955,1962,1975 by Mc Graw-Hill, Inc. [5] ]Bernhardt G.A.SKROTZKI,. “Electric Transmission & Distribution.” 1954 Jersey Central Power and Light Cmpany. [6] Glen Ballou, "Electrical Engineering HandBook”. 1999. [7] Ed LL.Grisby Boca Ratton,.” Electrical Power Engineering.” 2001 [8] .R.S.ARORA. “Handbook of Electrical Engineering.” 2004. (Fourth edition), New Dehli. [9] A.Johnson, “Electrical Transmission and Distribution Reference Book”. Oxford & IBH publishing Company. Ms. Khin Trar Trar Soe received her M.E degree in Electrical Power Engineering from Yangon Technological University, and then following three months training in industry; joined the Department of Electrical Power Engineering at Technological University (Loikaw, Myanmar) where she taught courses in Transmission and Distribution for five months. Her interests include Transmission and Distribution in Station and substation..She is a student of Mandalay Technological University. 421