A Review of Adaptive Under Frequency Load Shedding Scheme on Islanded Distribution System Using MATLAB/SIMULINK Dhananjay 1, Vijay Garg 2 1 M.Tech Scholar, Electrical Engineering, U.I.E.T, Kurukshetra, Haryana, India 2 Assistant Professor, Electrical Engineering, U.I.E.T, Kurukshetra, Haryana, India 1 dhananjayyadav31@yahoo.com, 2 vkgarg_kuk@yahoo.com Abstract: Adaptive Under-Frequency Load Shedding (UFLS) scheme is a reliable and accurate method to maintain the frequency of the system in accessible limit by shedding the right amount of load from system during abnormality. The purpose of this paper is to review work carried on Adaptive UFLS. It consists of combination of under frequency relay and rate of change of frequency (RCOF) relay. During the abnormal condition, the frequency of system deviates below from its normal value. To bring back the frequency to their normal load shedding is only reliable method. Adaptive UFLS scheme works only if the frequency is dropping from their normal value. It plays a vital role in reducing the power deficiency stress from generator in the case of islanding operation. Under balance condition the frequency of system remain stable (50 Hz or 60 Hz). Behavior of frequency with different condition is shown in table. Keywords: Adaptive UFLS, Load Shedding, Under-frequency load shedding, RCOF, Power System Stability I. INTRODUCTION Large demand and deregulation of electricity are major factors, which force the interconnected power system to operate closer to its limit. Due to these factors, cases of cascade power outages rises which force the system to shut down. To face the situation of complete blackout, a fast restoration method intentional islanding is developed which split the system in number of island during initial stage of disturbance. In case of islanding, a portion of system is separated from grid and the supply of that portion is maintained by Distributed Generator (DG). DG has many benefits and it play a vital role in maintaining the continuous operation of electrical island. For Secure and reliable operation of electrical island load shedding scheme is integral part of strategy to maintain the power balance in islanded part. During the formation of electrical island the load demand is more than the generation capacity in islanded part. Due to load imbalance, the frequency of that part starts decreasing. To bring back the frequency to their normal value Adaptive UFLS scheme is implemented. Benefit of Adaptive UFLS is that it avoids unnecessary load shedding as in case of conventional load shedding. Adaptive UFLS sheds the load in number of stages for smooth transition. II. 37 LOAD SHEDDING BASIC Load shedding is process in which an excess load is shed from system to maintain the stability of system. Load shedding is helpful to enhance the transient stability of the power system. Total Generation (TG) = Total Load (TL) + Losses Fig 1 Test system load shedding for an islanded network Table I: Behavior of frequency with different condition System Frequency Increase No change Decrease System Condition TG > TL + Loss TG = TL + Loss TG < TL + Loss Load shedding is used in under frequency condition, when frequency is deviated below from normal value. There are many type of load shedding scheme but, three are main. Firstly, Traditional LS is simple but unreliable to shed accurate amount of load and also do not use sophisticated relays like RCOF. Secondly, Half Adaptive LS is similar to traditional relay at some extent but RCOF relays are used for shedding of right amount of load. Thirdly, Adaptive LS
use both under frequency relay and rate of change of relay to shed the right amount of load. Criteria for Load Shedding Load shedding relay should be able to shed load equal to maximum value of overload. Load should be shed in number of stages, rather than in single stage for smooth transition and accurate load shed Pe =Total Load Power. H = Inertia Constant H = Stored kinectic enaergy at syncronous s speed MJ Generator MVA rating For Multi-machine, H = 1 JWs 2 2 MVA Frequency setting should be done for maximum and minimum setting. 2H 1 df 1 fo dt = Pm 1 Pe 1 macine 1 III. Each stage should shed enough loads to handle the next, more serious contingencies. Time delay should be short for frequency relay. Load shedding relay should be spread throughout the interconnected system to avoid unnecessary power loss and undesirable islanding. Load priorities should be created for load shedding. 38 ADAPTIVE UNDER-FREQUENCY LOAD SHEDDING. This technique is works only if the system frequency falls below a certain threshold value.adaptive UFLS scheme is an advanced technique of load shedding. It consists of UFLS relay and sophisticated relays RCOF to shed the accurate amount of load shed with enhanced reliability. During the initial stage of electrical island, generation capacity is less than the total demand. Due to this power imbalance, frequency is start decaying to bring back the frequency to normal value Adaptive UFLS scheme is best suited scheme of load shedding. In this scheme RCOF calculate the total amount of power imbalance and shed the right amount of load by both relays in number of stages for smooth transition. The objective of adaptive under frequency control mechanism of the power system to maintain the power balance between the generation and consumption by shed a right amount of load with safe and smooth and thus, guarantee the reliability and the security of the system. The first step of scheme is to estimate the total power mismatch between load and generator. Total power mismatch can be determined as follows. Swing equation for single machine is given by Where, fo = rate frequency, Pm = Total Generated Power 2H df = Pm Pe fo dt Heq = Pm eq = Pe eq = 2H 2 df 2 fo dt = Pm 2 Pe 2 macine 2 2H n df n fo dt = Pm n Pe n macine n individual inertias of eac macine for all te macines in system. individual mecanical power of eac macine for all te macine in te system. individual electrical power of eac macine for all te macine in te system. The relation between power imbalance and rate of change of angular speed are given by: P = 2H d( w) + D w dt Where P is power imbalance, w is angular speed, D is damping factor, H is inertia constant. A. Different Component used in Adaptive UFLS Scheme. 1) Frequency and RCOF Detector Frequency measuring device measure the frequency of the system it is connected to the low voltage side of the potential transformer. The output of the frequency measurement device is feed to the load shedding controller. The RCOF is to calculate the total load mismatch between generated power and load power. RCOF relay is fast acting relay. Expression of initial rate of change of frequency df dt = P 2H 2) Load Shedding Controller The output of the frequency measuring device and rate of change of frequency is feed to the load shedding controller. Load shedding controller is responsible for control load shedding with the use of frequency and RCOF. It also checks whether the system is in island mode or not. Output of load shedding controller is feed to the circuit breakers which connect loads. If frequency decays, reaches the first limit then certain amount of load will be shed and if the
frequency still decays then LSC wait for second limit and so on until the frequency restore to normal value by cutting suitable load at each stsge. A certain delay time like communication time, calculation and circuit breaker opening time is included. Fig 2 Load shedding at different frequency limit IV. MATLAB SIMULATION In this test model 502 Mw load is feed by 2 X 200 MVA generator with 30 MVA spinning reserve capacity and rest power is supplied by grid. Power balance and frequency is shown in fig 4 and fig7. Grid is disconnected from the network by a circuit breaker. Circuit breaker connecting the grid with electrical island shows the status of islanding. After disconnection of grid the supply of 503 MW is maintained by 2X 200 MVA generators, but the generation capacity is less than the total load. This create a power imbalance and frequency of the system starts decay as shown in fig 5 and fig 8.To bring back the frequency to normal value Adaptive UFLS scheme initiated after the sensor sense the condition of power imbalance because of islanding. In this scheme, load shedding activated as the frequency decay to stage 1 but due to of large imbalance it still decay to stage 2 and so on up to stage 5. In each stage certain amount of load is shed and after stage 5 frequency again restore to 50 Hz as show in fig 9 and also the power is balanced after load shed as shown in fig 6. There is certain delay in each step of load shedding to avoid unnecessary load shedding. Fig 3 Isolated Electric Island 39
V. SIMULATION RESULT Fig 4 Power balance before islanding. Fig 6 Power imbalance correction using Adaptive UFLS Fig 5 Power imblance after grid disconnected. Fig 7 Frequency before grid disconnected. 40
International islanding with Adaptive UFLS provides more security and reliability to the electric island and power system. In this paper Adaptive UFLS scheme in electric island is sucessfully demonstarted with the help of MATLAB Simulink. With the use of Adaptive UFLS codition of overloading in electric island can be smoothly eliminated with enhanced stability. In this scheme RCOF and frequency relays sheds right amount of load and avoid the unnecessery load shedding. VII. REFERENCES [1] Alireza Saffarian and Majid Sanaye-Pasand, "Enhancement of power system stability using adaptive combinational load shedding", IEEE Transaction, 2012,Vol 26, No 3,pp. 1010-1020, 2012. [2] Diana Zalostiba, "Power System blackout prevention by dangerous overload elimination and fast-restoration", IEEE, Vol. 4, pp. 001-005, 2013. Fig 8 Frequency variation after grid disconnected. [3] Haibo You and Zhong Yang Self Healing in power systems: An Approch using islanding and rate of frequency decline-based load shedding, IEEE Transactions,, Vol 18, No 1.pp:174-180, 2003. [4] Mokhlis and Abu Bakar, A new under frequency load shedding scheme for islanded distribution network. IEEE PES, 2013. [5] A. A. M. Zin, H. M. Hafiz, and W. K.Wong, Static and dynamic under frequency load shedding: A comparison, IEEE/POWERCON, pp. 941-945, November 2004. [6] P.Kundur, Power System Stability and Control, Newyork: Mchraw-hill Inc, 1994. [7] M.Karimi, and H.Mohamad, "Under-Frequency Load Shedding for islanded distribution network connected with mini grid", IJEPES, 2012, pp. 127-138. Fig 9 Frequency recovery after grid disconnected using Adaptive UFLS. VI. CONCLUSION 41