Optimal Allocation of Distributed Generation (DGs) and Static VAR Compensator (SVC) in a power system using Revamp Voltage Stability Indicator Abhilipsa Rath Sriparna Roy Ghatak Parag Goyal Department of Electrical Engineering Department of Electrical Engineering Department of Electrical Engineering KIIT University KIIT University KIIT University Bhubaneswar, India Bhubaneswar, India Bhubaneswar, India Abstract The power system round the globe functions at highly loaded conditions resulting in poor voltage profile, voltage instability and high system losses. Electrical power increment and difficulties in providing required capacity provide a spur to appoint a Distributed Generation (DGs) and Static VAR Compensator (SVC) option. DGs and SVC integration ameliorate voltage profiles, system efficiency, Voltage stability and power quality and to lower system losses, while neglecting consequential venture in transmission and distribution systems. The device does not work satisfactorily if it is placed in any random location in the system therefore It is necessary to determine the optimal location of the devices (DG and SVC), as the computational time requirement can be reduced by performance analysis of the device at weaker bus location. In the present work on an IEEE 14 bus system, the weak bus identification is determined by pioneering and ingenious approach called Revamp Voltage Stability Indicator (RVSI). The optimal allocation of the device has been done based on its performance assessment at different identified weak bus locations using 3 different indices Voltage Profile Improvement Index (VPII), Line Loss Reduction Index (LLRI) and Revamp Voltage Stability Indicator (RVSI). The Electrical Transient Analyzer Program (ETAP) software is being used for the load flow analysis of the IEEE 14 bus system. Keywords Revamp Voltage Stability Indicator; Distributed Generation; Static VAR Compensator; VPII; LLRI I. INTRODUCTION The power system across the world operates at highly loaded conditions resulting in voltage instability, high system losses and decreasing voltage profile. The different economic, technical environmental and legal constraints have already provided limitations towards the extension of our existing power system as a result of which with the day to day increase in load demand, the Power systems across the world are being forced to operate their existing equipments closer to their stability limits (maximum capacities). The power system planners and researchers in the recent years have become more concerned about the problem of voltage stability and have considered it as a serious cause of power system insecurity, instability and voltage collapse. Voltage stability can be described as the ability to retain their bus voltages within desirable limits even after following sudden big disturbance in a power system. An increase in loading of the system causes an increased shortage of real and reactive power of the system, which in turn causes a fall in the bus voltages of the system [1, 2]. The multi-objective PSO Algorithm has been solved to identify the ideal size and location of the device and also voltage stability index has been solved to identify the weak bus [3].Distributed Generator (DGs) can play an important role in distribution system planning. The DGs are directly attached to the load. It capacity varies from few KW to few MW [4]. Researcher have used Tabu algorithm to find less system losses optimal location of distributed generator [5]. Authors have disputed about the benefit from DG investment for the utility [6].Various soft computing methods have been presented for study the profits of DG located in an electrical Network [7]. More than one DG can be placed in an electrical network which can minimize the losses [8, 9]. SVC retains the network voltage uniformly at a set value.it is a shunt compensating FACTS device which is analogous to a parallel combination of a variable shunt inductor and a variable shunt capacitor.each of them can be varied to get desirable voltage control and reactive power injection at the terminally connected control bus. SVCs are apt of providing faster and smoother voltage control as compared to the conventional shunt compensating devices with the help of power electronic devices. A practical SVC is composed of thyristor controlled shunt reactor and a bank of switchable fixed capacitors. Switching of capacitors is usually conducted with thyristors which can be directed with the help of thyristors which can be controlled with the help of voltage sensing control systems. SVCs along with providing voltage control also reduces the 978-1-4799-5141-3/14/$31.00 2016 IEEE
active and reactive power losses in the network thus contributing to some economic welfare[10,12].one of the effectual technique to rescue the electrical network from voltage collapse is to lower the VAR load or adjoin VAR sources by establishing devices like SVC under stressed condition. The SVC plays a important and affirmative influence on enhancing the system voltage profile, voltage stability, ameliorate the power factor and decrement in power loss [13]. The weak bus locations of a system can be recognized with the help of the various available voltage stability indices such as,, [14]. The weak zone of the system is being obtained with the help of an innovative voltage stability indicator RVSI. It is not practically possible to conduct the performance assessment of SVC and DG at all the bus locations of a network. Therefore, in order to reduce the computation time required for determining optimal allocation, it is more useful to conduct the performance assessment of the devices only at the weak bus locations (buses which are more prone to voltage instability) of the system. The performance of the SVC and DGs are analyzed only at the 4 weak bus locations of an IEEE 14 bus system at 180% loading. The analysis exhibits that all the parameters (LLRI, VPII and RVSI) are observed to improve with the optimal placement of the devices and the most appropriate result is being obtained when the devices are placed at bus 14.The better result is being shown by DG than the SVC when connected at the optimal position. II. Distributed Generation (DGs) The Distributed Generation refers to small generating sources or units using real and reactive power to uplift voltage profile established near the local loads and load centers. This restricts the requisite of network augmentation. The DG can be expounded in several ways. The Electric power research institute recognizes a DG ranging from a few KW to 50 MW[4].The International Energy Agency(IEA) describes a DG as a generating plant serving consumer onsite or yielding aid to a electrical system linked to grid at various voltage levels. The DGs are of different types ranging from conventional fossil fuel based combustion to the renewable energy including micro turbines, wind, photovoltaic, CHP, small hydro turbines or hybrid. The word Distributed Generation is impressionistic and varies with locality III. STATIC VAR COMPENSATOR(SVC) The primary requirement to avoid voltage instability is that the power system should be able of moving reactive power from source to customer throughout the steady operating situation. That s why the targeting for the SVC is the main aim for power utilities. SVC is a first generation FACTS device and it reacts as a reactive power compensator. SVC control reactive power for bus voltage regulation. SVC continuously compensates the reactive power, so that Power factor and power quality both increase. It is a shunt FACTS device and it can be work as both inductive and capacitive compensation. SVC is unable to exchange the active power from the system. A couple of thyristor which are attached in a back to back arrangement is used to control the current through the SVC. IV. VOLTAGE STABILITY Voltage Stability is defined as the capability of an electrical network to retain the potential difference between the buses which is acceptable. For voltage stability analysis, various voltage stability indices such as [14] is being used. In this paper we are using an innovative concept namely Revamp Voltage Stability Indicator (RVSI). It is used to test the system s voltage stability. The aim of voltage stability analysis is to locate the weak zone or the critical lines which are on the margin of instability. A. Revamp Voltage Stability Indicator(RVSI) A Revamp voltage stability indicator is commenced by the quadratic equation of voltage at the sending end for 2-bus Network. Here in the network where the line for which RVSI is near to 1.00 reached its instability limit. It could be the reason of unexpected voltage fall of the corresponding bus caused by the variation of reactive load. The derivation RVSI [1] is discussed in details in the Appendix. RVSI = ( ) [1] Here Z= line impedance, X = line reactance, = receiving end VAR, = voltage at the sending end, R = line resistance, = power angle. V. APPROACH TO QUANTIFY THE BENEFITS OF DG AND SVC A. Voltage Profile Improvement Index(VPII) The addition of DG and SVC changes the voltage profile at different bus to a desired limit. The Voltage Profile Improvement Index (VPII) gives us information about the increment in voltage. It can be observed from the following index [3]. For DG, For SVC, VPII = VPII = / / [2] The established expression for Voltage Profile is given as [3] VP = V L K [4]
Here is the voltage magnitude, is the load represented as complex power, is the weighting factor and N is the of buses. If VPII<1 then Voltage in system has reduced. If VPII=1 then no change in voltage If VPII>1, then Voltage in system has increased. B. Line Loss Reduction Index(LLRI) Real power losses indices are presented as by following equation [3]. For Devices, LLRI = / [5] / Here / is real losses with devices and / is real losses without devices. LLRI<1 Losses is reduced LLRI=1 No change in loss LLRI>1 Losses is increased method using ETAP software separately with DG and SVC. Step 7: Calculate the performance indices LLRI, RVSI and VPII. Step 8: The VPII, LLRI and RVSI were compared with the base case values and the new solution was stored. Step 9: End VII. RESULTS AND DICUSSION The optimal allocation of SVC and DG is implemented on an IEEE 14 bus system at 180% loading. The various details of an IEEE 14 bus system are mentioned in the Appendix. Fig.1. shows the RVSI values for the different lines of the system calculated at 180% loading. The lines 12-13, 13-14 and 9-14 show higher RVSI values as compared to the other lines. Therefore, these lines are chosen as the weak lines of the system and the corresponding sending and receiving end buses (bus Number (9, 12, 13 and 14) are chosen as the weak bus locations of the system. The System Losses decreases with the deceasing value of LLRI. VI. IMPLEMENTATION In interconnected system with 180% loading, the voltage profile is proficient enough, so the voltage compensation is not required. However the power system round the globe is being forced to operate under heavily loaded condition because of which power system suffers from considerable drop in voltage profile, stability and high system losses. Therefore, in this piece of work, the analysis of optimal placement of SVC and DG has been carried out on a 180% loaded system with the use of appropriate technical parameters. The implementation is done using ETAP software described below using following steps. Step 1: The required power system data is assembled, studied and the system is constructed. Step 2: The loads of the 14 bus system is increased to 180% of their base values (180% loading). Step 3: Load flow study was done in Newton Raphson method using ETAP software. Step 4: For the various line of the system RVSI calculation is done for identification of weak bus. Step 5: To improve the voltage profile, stability and to reduce the system losses, DG and SVC were penetrated at the recognized weak bus. Step 6: Load flow study was done again in Newton Raphson Fig. 1. RVSI of Lines at 180% loading without devices Table I shows specification of DG which we have used in this system. DG is equipped with the system of 5% of the total load TABLE I. Optimal DG Sizes DG IN MW Power Factor 23 0.85 Table II shows specification of SVC which we have used in this system. SVC is equipped with a nominal inductive MVAR rating of 3.162 MVAR and a slope of 1.6 % along with a proper capacitive MVAR rating required for bringing the voltage at the connected bus as close as possible to 1p.u. TABLE II. Optimal SVC Sizes Capacitive MVAR Inductive MVAR Slope in % 23.62 3.162 1.67
Table III shows the comparison VPII of distinct values obtained with the use of SVC and DG at the different weak bus locations. The VPII is highest when SVC is placed at bus 14. It shows that voltage profile for devices at bus 14 has increased to maximum among all buses. It shows that with the DG placement voltage profile is improved more in comparison to SVC. TABLE III. VPII with DG and SVC SVC at bus VPII DG at bus VPII 14 1.096 14 1.137 13 1.058 13 1.087 12 1.036 12 1.054 9 1.012 9 1.023 Table IV shows the comparison of LLRI different values obtained with the use of SVC and DG at the different weak bus locations of the IEEE 14 bus system. The LLRI is the lowest when SVC and DG is placed at bus 14. It shows that Loss for devices at bus 14 has reduced maximum among all buses. It shows that with the DG placement Losses is reduced much in comparison to SVC. Fig. 2. Voltage Profile of DG in 14 Bus System TABLE IV. LLRI with DG and SVC SVC at bus LLRI DG at bus LLRI 14 0.889 14 0.830 13 0.901 13 0.841 12 0.923 12 0.856 9 0.924 9 0.867 In Fig.2 and 3 shows the comparison of the per unit bus voltages of the different buses of the system at 180% loading before and after placing SVC and DG. Use of DG and SVC causes significant increase in the bus voltages of the system. Fig. 3. Voltage Profile of SVC in 14 Bus System Fig.4. and 5 shows the values of RVSI for the different lines of the system calculated at 180% loading. The value of RVSI is maximum in case of 180% loading and continuously decreasing as we are moving from bus 9 to bus 14 when DG and SVC are placed at these buses. The most optimal place for DG and SVC location is bus 14 as it is giving the minimal value (near to 1) for RVSI
Fig. 4. RVSI of DG at 14 Bus System We are using a Revamp Voltage Stability Indicator (RVSI) to calculate the voltage stability of the system where the value of load angle is taken into consideration. Using ETAP software the receiving end and sending end angle for buses were calculated along with the load angle. Let Sending end and Receiving end voltage between a Transmission line is and. is the current flow through Sending End and Receiving End. Here = [1] And, again = [2] Compare [1] and [2] and solving We get an equation for =0 Where = For the solution of the real part of the equation, Equation Discriminant should be greater than 0. We get 4 0 Thus 4Z Q 1 V ( ) Which gives Revamp Voltage Stability Indicator Fig. 5. RVSI of SVC at 14 Bus System VIII. CONCLUSION In this work the weak zone of the system is being obtained with the help of new and innovative voltage stability indicator RVSI. In order to reduce the computational time required to determine the optimal location and the performance of the device is analyzed only at the 4 weak bus locations of an IEEE 14 bus system at 180% loading. The present analysis exhibits that all the parameters (LLRI, VPII and RVSI) are observed to improve with the placement of the device and the best results are obtained when the devices are placed at bus 14. Therefore it is selected as the optimal location for the placement of the device. From the results it can be concluded that DG has more role in loss reduction, voltage profile improvement and voltage stability as compared to SVC. APPENDIX A. Revamp Voltage Stability Index RVSI = A. IEEE 14 Bus System 4Z Q V ( ) Fig. B1. Single line Diagram of IEEE 14 Bus system
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