Electrical and Electronics Engineering: An International Journal (ELELIJ) Vol 2, No 2, May 2013 LOAD SHEDDING DESIGN FOR AN INDUSTRIAL COGENERATION SYSTEM Mukesh Kumar Kirar1, Renuka Kamdar2, Manoj Kumar3, Ganga Agihotri4 1 Department of Electrical Engineering, MANIT, Bhopal, India mukeshkirar@rediffmail.com, renukakamdar_123@yahoo.co.in, manoj11manit@gmail.com, ganga1949@gmail.com ABSTRACT This paper presents transient stability analysis and enhancement of Industrial Cogeneration Plant (ICP) using Artificial Neural Network (ANN) based adaptive load shedding. By selecting the total in-plant generation, spinning reserve, total plant load and rate of change of frequency as the input neurons of the ANN, the minimum amount of load shedding is determined to maintain in-plant load-generation equilibrium and to ensure continuity of power supply to critical loads of the plant. The comparison of under frequency relay based load shedding and ANN based adaptive load shedding is also preformed to evaluate effectiveness of the ANN based Load Shedding. The system frequency response for deferent generation-load scenarios is also determined. The industrial cogeneration is simulated on ETAP software and transient stability is analyzed by considering various contingencies and load-generation scenarios. ANN has been implemented on MATLAB. KEYWORDS Artificial Neural Network, Islanded System, Load Shedding, Electrical Transient Analyser Program (ETAP), Frequency Stability 1. INTRODUCTION Power supply to critical process loads is extremely important for an industrial cogeneration power plant not only for continuous production but also important for overall plant safety during severe disturbances. A sudden interruption in production may result in significant economic loss and raise safety concern. Most of the industrial customers with requirement of uninterrupted input of energy in the form of electric power and steam have installed cogeneration units. The cogeneration systems are broadly defined as the coincident or simultaneous generation of the combined heat and power (CHP) [1]. By installing cogeneration units industrial customer can achieve better efficiency of energy usage and enhance the reliability of electricity power supply [2]. The cogeneration has to be tied together with the Public Power Company (PPC) to cover the mismatch of load demand and power output by the cogeneration units in the plant and for the consideration of power quality. The several techno-economic studies [3-5] are required periodically throughout the operating life of the plant to ensure that a cogeneration plant will operate safely, reliably, and economically. To prevent total blackout and to stabilize the system under any abnormal condition, appropriate Islanding and Load Shedding (LS) strategies must be developed for industrial cogeneration system. The load shedding technique primarily can be classified as conventional load shedding technique and Adaptive or Intelligent load shedding technique. Conventional load shedding schemes, breaker interlock load shedding [11], under-frequency relay (81) load Shedding [6, 7, 10, 11], programmable logic controller-based load shedding are most common and easy way to isolate the excess amount of load during generation deficit in the islanded power system. The 35 Electrical and Electronics Engineering: An International Journal (ELELIJ) Vol 2, No 2, May 2013 conventional schemes are designed to work on worst system operating conditions. These schemes are not include, real-time system configuration, type and duration of the disturbances, as well as other important information and total loss of the system is an assumed possibility [14]. Conventional methods of system load shedding are too slow and do not effectively calculate the correct amount of load to be shed. Several schemes are reported in the literature [12-17] to overcome the shortcomings of the conventional load shedding schemes, by making it adaptive through complete understanding of power system dynamics. In this paper ANN based load shedding method in comparison to under frequency relay based load shedding is described. In order to illustrate the effectiveness of the proposed ANN based load shedding approach for the islanded ICP system, a large Oil Storage Terminal and refinery distribution system is investigated as a case study. Section 2 introduces system description and configuration of the ICP system. In Section 3, power studies are conducted, the frequency response of the system is analyzed. Section 4 illustrates ANN based load shedding, the 81-relay based and ANN based load shedding methods are compared in section 5 and concluded in section 6. 2. SYSTEM DESCRIPTION In order to determine transient performance of a power system, the sub-transient model of the generators, IEEE standard model of exciter and governor control systems of the cogeneration unit and static and dynamic model of loads have considered. The single line diagram (SLD) of ICP power distribution system is shown in figure 1. The industrial cogeneration is simulated on ETAP software. Figure.1 Single line diagram for the OTS 36 Electrical and Electronics Engineering: An International Journal (ELELIJ) Vol 2, No 2, May 2013 To provide continuous power supply for critical loads and enhance overall efficiency of the plant, ICP has installed two 10MW steam based cogeneration units STG-1 and STG-2. Both generating units in study are represented by detailed model with transient and subtransient circuits on both the direct and quadrate axes. The power is generated at 11kV and step-down to 6.6kV by generator transformers GT-301 and GT-302 and connected to Switchgear SG-301B. The block of the IEEE type 1 excitation model used for the both generators is as shown in figure 2. To achieve the quick response of the cogeneration unit output to the external disturbance, the single reheat steam governor-turbine system model as shown in figure 3 is used. Vref 1 1 + sTR Vt − + ∑ − SE = f (Efd) VRmax KA 1 + sTA ∑ − 1 K E + sTE + E fd VRmin sK F 1 + sTF Figure 2. Excitation system for SGT-1 and SGT-2 Wref P P W + ∑ − max K 1 + sTsr + ∑ 1 1 + sTc P = Pe Pref Pmin I soch Fhp 1 1 + sTch + 1 − Fhp 1 + sTrh ∑ + Pm Figure 3. Governor system for STG-1 AND STG-2 To improve system reliability and power quality, the ICP has Grid connectivity with public power company (PPC) at two points. PPC power is available at 66 kV voltage level through utility ties UTG-1 and UTG-2. Two on load tap changer grid transformers TR-301 and TR-302 step down the voltage from the 66 kV to the 6.6 kV level and connected to Switchgear SG-301A through cable. The Switchgear SG-401A, SG-401B, Motor Control Center MCC-401, Power and Motor Control Center PMCC-402 are supplied through transformers TR-401, TR-402, TR-403 and TR404 of 6.6/0.44 kV respectively. The motors above 160kW rating are connected to switchgear SG-401A and SG-401B at 6.6kV voltage level. During islanded condition, due to a utility service outage, in-plant generators will supply power to the plant load. The rated capacity of each generator is 10 MW and the total load connected is approximate 28.5 MW. 3. SYSTEM FREQUENCY RESPONSE ANALYSIS The dynamic performance of the system with respect to change in total generation and load can be represented by swing equation [17]. The relationship that define variation of frequency with total generation and load mismatch can be obtain from swing equation, = (1) 37 Electrical and Electronics Engineering: An International Journal (ELELIJ) Vol 2, No 2, May 2013 Where, G: nominal MVA of generator H: inertia constant δ: generator rotor angle f0: nominal frequency Pa : net accelerated or decelerated power (mismatch between generation and load) Consider the generator speed variation due to a disturbance which is given by, = + = 2 Where is the synchronous speed in rad/sec Differentiating above equation with respect to time, = = 2 (2) Substituting equation (2) in equation (1), we get = (3) 2 Equation (3) defines the rate of change of frequency in Hz with, total power mismatch Pa, system nominal frequency f0, and inertia constant H. The equation (3) can be used for an individual generator as well as for an equivalent generation in the system. For equivalent case, the inertia constant (H) can be derived from the following, = + + ⋯ + + + ⋯ + Where n is the number of generators in a power system The average rate of change of system frequency can be calculated by equation (4) df pL ( f 2 − f1 ) = dt H f 22 1 − 2 f1 (4) Where p = average power factor; H= inertia constant L= total lost generation/ total available generation f1 = nominal frequency; f2 = minimum allowable frequency 38 Electrical and Electronics Engineering: An International Journal (ELELIJ) Vol 2, No 2, May 2013 To find the proper relay setting for tie line tripping during fault in utility system and load shedding design to enhance system performance of an ICP power system during load generation mismatch, detail power system studies are required [20]. The computer simulations of load flow analysis, short circuit analysis and stability analysis for all possible load-generation scenarios and network configurations has been executed on ETAP software. The network configuration during normal operation is shown in figure 1. The load flow result for rated operating condition is shown in Table 1. Bus No. Bus in kV 6.6 6.6 0.415 0.415 0.415 0.415 SG-301A SG-301B SG-401A SG-401B MCC-401 PMCC401 Voltage Magnitude Voltage in %age Angle 98.53 -1.71 98.53 -1.71 101.1 -4.04 101.1 -4.04 98.4 -5.25 99.5 -5.32 Gene (kW) 9868 18000 0 0 0 0 Gene (kVAr) 3882 9432 0 0 0 0 Load (kW) 11066 10220 1243 1290 284 3521 Load (kVAr) 4661 4241 732 892 215 1778 Table 1. Load flow report The system frequency response without load shedding, for different generation-load scenarios and contingencies, as given in Table 2 is shown in figure 4. The contingencies considered for study includes, three phase fault in utility system, loss of utility supply and loss of generators STG-1. Bus frequency which starts falling after the tie CBs trip continuously along as the generator load deficit exists. Scenarios Scenario-1 Scenario-2 Scenario-3 Scenario-4 Total (MW) 19 17 15 12 Generation Total (MW) 20.786 20.486 24.206 24.626 Load Mismatch (MW) 1.786 3.486 9.206 12.626 Average -0.39 -0.76 -2.01 -2.75 Table 2. Generation and load mismatch at the time of tie-line trip 110 Frquency(in %age) 100 90 80 70 Scenario 1 Scenario 2 Scenario 3 Scenario 4 60 50 0 5 10 15 Time(sec) Figure 4. System frequency response for different load generation scenarios 39 Electrical and Electronics Engineering: An International Journal (ELELIJ) Vol 2, No 2, May 2013 4. ANN BASED ADAPTIVE LOAD SHEDDING An artificial neural network (ANN) is a flexible mathematical model consists of an interconnected group of artificial neurons which is used for modelling complex nonlinear relationships between input and output data sets [19]. Commonly neural networks are trained, so that a particular input leads to a specific target output. Once the ANN has been trained it can be used to classify unknown patterns. Figure 5 depicts three layer feedforward neural each layer has a weight matrix W, a bias vector b, and an output vector a. Where the input signal p with R variables is expressed as [p1, p2, p3, ……pR]T . Input-output data sets which are used to training, testing and validation of ANN are {(p1, q1), (p2, q2), (p3, q3), ……(pR, qR)}. Where q is desired output with R variables is calculated by system analysis. 2,1 1 n1 1,1 iw1,1 p1 1 2 a 1 f 1 2 n 1,1 S,R iw f 1 a 1 1 S1 n lw b S1 1 ( IW 1,1 p + b1 ) 3 a = f 3 ( LW 2,1 S 2 ,S1 a 3,2 f a 2 f f 2 a = 2 ( LW 2,1 1 2 ( LW 2.1 1 ( IW a +b 1,1 2 2 S2 f 3 a2 f 3 a S3 3 ) 3,2 S 3 ,S 2 a 1 ) p+b +b 3 n S3 lw 2 b S2 f a1 3 n2 2 2 b2 2 S2 2 f 3 3 b2 1 S1 3 f 3 2 2 2 1 f 3 n1 b1 n b2 = 2 a1 lw1,1 2 b1 n 1 f 2 1 a 3,2 2 n1 b1 p2 p3 p R1 1 a1 lw1,1 1 f 2 3 = 3 3 bS3 f 3 ( LW 3,2a 2 + b3 ) )+b ) 3 Figure 5. Multi-Layer Feed Forward Neural Network The Levenberg–Marquardt Back-Propagation (LMBP) algorithm is used for training of the ANN model because of the low error and least epochs. To prepare the training data sets for ANN, the transient stability analysis has been performed to solve the minimum load shedding for various operating scenarios with the help of ETAP software. The data is transmitted from the input layer, multiplied by their respective weights, to the hidden layers before reaching the final output layer. The error signals between the target and actual output at the output layer neurons are then propagated back to the hidden and input layers. The sum of square error is then minimized by adjusting the synaptic weights and bias in any layers during the training process of ANN model. For a multi-layer network, the net input nk+1(i) and output ak+1(i) of neuron i in the k+1 layer can be expressed as: sk n k +1 (i ) = ∑ wk +1 (i, j ) y k ( j ) + b k +1 (i ) (5) j =1 a k +1 (i) = f k +1 (nk +1 (i)) (6) By representing the sum of the output square error as the performance index for the ANN, the error function is given by E= 1 R 1 R ∑ (q r − a kr )T (q r − a kr ) = ∑ (er )T er 2 r =1 2 r =1 (7) 40 Electrical and Electronics Engineering: An International Journal (ELELIJ) Vol 2, No 2, May 2013 Where er = qr − a rk is the output error and a rk is the final output of the rth input. The Levenberg Levenberg– Marquardt algorithm is used to minimize the mean square error function in Eq. (7) The Simulation diagram of ANN in MATLAB is shown in figure fi 6.. The input is fed to the constant block x1 which is then processed and normalized by process input block. Inside this process input block the remove constantrows and mapminmax processes are being performed. The remove constantrows processes input and target data by removing rows with constant values. Constant values do not provide a network with any information and can cause numerical problems for some algorithms. Figure 6.. Simulink block diagram diag of ANN load shedding scheme Figure 7.. Regression plot for 20 & 10 neurons in 1st & 2nd hidden layer The mapminmax processes input and target data by mapping it from its original range to the range [-11 1]. The processed input is transmitted through a connection that multiplies its strength by the scalar weight w {1,1}, to form the product wp, again a scalar. The neuron neuron has a scalar bias b {1} that is simply being added to the product if input and weight (wp) at the summing junction. The bias is much like a weight, except that it has a constant input of 1. The sum is then fed to the TANSIG symmetric sigmoid transfer function. Transfer functions convert a neural network layer's net input into its net output. The output of this first hidden layer is then then treated as input to 41 Electrical and Electronics Engineering: An International Journal (ELELIJ) Vol 2, No 2, May 2013 the second hidden layer where the same repeats with the same TANSIG transfer function. The output of the second hidden layer acts as the input to the output layer and is multiplied by weight w {3,2} added by bias b{3} and fed to purelin transfer function to get the output. This output is then processed again to convert it back to the original form using mapminmax_reverse and remove constantrows_reverse process. The output can be viewed in the scope y1. PG (kW) 19000 18000 18000 18000 17000 17000 17000 16000 16000 16000 15000 15000 15000 14000 14000 14000 13000 13000 13000 12000 12000 12000 11000 11000 11000 10000 10000 10000 9000 9000 9000 PL (kW) 27763 26822 23378 21016 25455 23180 19468 26806 22364 19096 25410 23322 19878 21938 20960 17964 21420 18550 16584 20012 19482 16926 18312 16604 15772 14936 14440 13820 14768 14240 12472 PS (kW) 1000 2000 2000 2000 3000 3000 3000 4000 4000 4000 5000 5000 5000 6000 6000 6000 7000 7000 7000 8000 8000 8000 9000 9000 9000 10000 10000 10000 11000 11000 11000 df/dt -1.91 -2.14 -1.3 -0.73 -1.84 -1.35 -0.54 -2.94 -1.73 -0.84 -3.02 -2.42 -1.42 -2.47 -2.17 -1.23 -2.82 -1.86 -1.2 -2.91 -2.71 -1.79 -2.9 -2.22 -1.89 -2.15 -1.94 -1.67 -2.79 -2.54 -1.68 PDLS (kW) 8097 6550 2770 710 6261 3532 0 6660 2320 0 6620 4660 900 4150 3170 160 4620 1600 0 4220 3600 960 3510 1640 770 960 400 0 1915 1290 0 PANN (kW) 8050 6588 2862 722 6228 3622 134 6669 2292 8 6375 4620 906 4182 3206 204 4643 1588 25 4212 3577 865 3516 1628 803 925 433 -120 1928 1295 -20 % Error -0.16 0.13 0.32 0.04 -0.11 0.31 0.47 0.03 -0.09 0.03 -0.86 -0.14 0.02 0.11 0.12 0.15 0.082 -0.04 0.08 -0.02 -0.08 -0.33 0.02 -0.04 0.11 -0.12 0.11 -0.42 0.05 0.01 -0.07 Table 3. ANN-based load shedding results The number of neurons in input layer is equal to the number of inputs i.e. 4 while the output layer has one neuron. The selection of number of neurons for the two hidden layer is made on hit and trial method basis, comparing the regression plot of each and choosing the best among them. The 42 Electrical and Electronics Engineering: An International Journal (ELELIJ) Vol 2, No 2, May 2013 best performance is obtained with 20 neurons in 1st hidden layer and 10 neurons in 2nd hidden layer. The regression plot is shown in figure 7. By selecting the total in-plant generation, total load, spinning reserve of generators and rate of change of frequency as the input neurons of the ANN, the minimum amount of load shedding is determined. The input signal p with four variables is expressed as [ , " , # , ⁄ ]& and one output q as PDLS. The transient stability analysis for 121 load-generation scenarios have been carried out, with the values of inputs PG, PS, PL, and df/dt varied between 9000–18000KW, 2000-10000KW, 1946827868KW, and 0.54-3.02Hz/s respectively and ANN targets PDLS are determined. The 80% of the total cases is selected for the ANN training, 10% for testing and 10% for validation. The corresponding load shedding amount as calculated by ANN with LMBP algorithm (PANN) for 31 load-generation scenarios and fault in utility system with tie-line trip is shown in Table 3. 4. COMPARISON OF LOAD SHEDDING METHODS To demonstrate the effectiveness of the proposed methodology, system under study has been made to undergo a fault contingency and load shedding is performed by underfrequency relay and ANN based adaptive load shedding method. The underfrequency relays settings for the first-step load shedding will be activated simultaneously upon loss of the tie line. Underfrequency load shedding design, number of steps, step frequency, and percentage load shedding amount for 81L relay is shown in Table 4. Steps Step-1 Step-2 Step-3 Threshold frequency 49.5 48.5 48 %age LS 40 30 30 Time Delay (sec) 0.1 0.1 0.1 Table 4. Underfrequency relay setting for load shedding Figure 8 depicts frequency response of islanded system with 15000KW generation and 25410KW load, with underfrequency relay based load shedding and ANN based load shedding methods. In the case of underfrequency load shedding, at t=0.5 sec three phase fault is created in utility grid which is cleared by opening tie-line Circuit Breaker (CB-1 and CB-2) at 0.6 sec. As the system frequency reaches below 49.5 Hz at 1 sec, underfrequency relay is activated and first stage load shedding is implemented at 1.1 sec. 101.5 101 Frequency(in %age) 100.5 100 99.5 99 Relay Based UFLS ANN Based UFLS 98.5 98 97.5 0 5 10 15 20 25 30 Time(sec) Figure 8. System frequency with different load shedding methods 43 Electrical and Electronics Engineering: An International Journal (ELELIJ) Vol 2, No 2, May 2013 The system frequency still not recovered and it crosses 49 Hz (second step load shedding threshold) and second step load shedding is implemented at 1.46 sec. After 2-step load shedding system frequency is improved upto 49.5 Hz in 3.65 sec. It was found that the total amount of load shed using underfrequency relay method is 11500 KW. For the same contingency in case of ANN based adaptive load shedding, after the opening of tie-line Circuit Breaker (CB-1 and CB-2) at 0.6 sec the entire load shedding is performed at 0.74 sec with the calculation delay of 0.02 sec included. The amount of load shedding is 6620 KW of load which is 4880 KW less than the conventional scheme. The system electrical power and mechanical power variations for both the methods of load shedding is as shown in figure 9 and figure 10 respectively. 14 Electrical Power(MW) 12 10 8 6 4 ANN Based UFLS Relay Based UFLS 2 0 0 5 10 15 20 25 30 Time(sec) Figure 9. System electrical power with different load shedding methods 10 Mechanical Power (MW) 9.5 9 8.5 8 7.5 ANN Based UFLS Relay Based UFLS 7 0 5 10 15 Time(sec) Figure 10. System mechanical power with different load shedding methods 5. CONCLUSION In this paper an approach for improvement of frequency stability using ANN based adaptive minimum load-shedding scheme is developed for industrial cogeneration system. By executing the transient stability analysis for various operation scenarios of the ICP system, the training data set of ANN model, which includes, total system power generation, spinning reserve, total load, 44 Electrical and Electronics Engineering: An International Journal (ELELIJ) Vol 2, No 2, May 2013 and frequency decay rate as input, and the minimum amount of load shedding required as output, has been prepared. To verify the effectiveness of the proposed ANN based load shedding as compare to the present underfrequency relay based load-shedding, schemes are applied in the simulation on ETAP software to investigate the dynamic response of system frequency. It is concluded that the proposed ANN based methodology with two hidden layers and LMBP algorithm can achieve more effective load shedding to maintain system stability as compare to underfrequency based relay. REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] E. 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Distrib., 2009, Vol. 3, No. 1, pp. 99–114. 45 Electrical and Electronics Engineering: An International Journal (ELELIJ) Vol 2, No 2, May 2013 [19] Kuo-lin Hsu, Hoshin Vijai Gupta, Soroosh Sorooshian, “Artificial neural network modeling of the rainfall-runoff process” Water Resources Research - WATER RESOUR RES , vol. 31, no. 10, pp. 2517-2530, 1995. Authors Mukesh Kumar Kirar was born in Narsinghpur, India, in 06 Feb 1983. He received the B.E. (Electrical) degree from Government Engg. College, Ujjain, India in 2006 and M.Tech. (Power System) in 2008 and pursuing Ph.D from MANIT Bhopal, India. He is currently working as an assistant professor in the Department of Electrical Engineering, MANIT, Bhopal, India. His field of interests are power system stability and control, transformers and machines. Renuka Kamdar was born in Bhopal, India in 1987. She has received BE degree (2009) in Electrical and Electronics Engineering from Oriental Institute of Science and Technology Bhopal and pursuing her M. Tech degree in Power System from MANIT Bhopal. Ganga Agnihotri was born in Sagar, India, in 27 May 1949. She received the B.E. (Electrical) degree from MACT, Bhopal, India. She received the M.E. (Advance Electrical Machine) and PhD (Power System Planning Operation and Control) from University Of Roorkee, Roorkee in 1974 and 1989 respectively. She is currently working as a professor in the Department of Electrical Engineering, MANIT, Bhopal, India. She has 12 research papers in International journals, 20 research papers in National journals, 22 research papers in International Conferences and 70 research papers in National Conferences. Her fields of interest are Power System Planning, Power Transmission Pricing, Power System Analysis and Deregulation. Dr. Agnihotri has a membership of Fellow IE(I) and LISTE. 46