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    Providing Transient Stability by Excitation System Response Improvement

    Methods through Long Term Contracts

    Ali Khandani1, Tel: (+98)-(918)-(3167748), Fax: (+98)-(23)-(33654089), E-mail: a.khandani@semnan.ac.ir

    Asghar Akbari Foroud2, Tel (+98)-(912)-(4618433), Fax: (+98)-(23)-(33654089), E-mail: aakbari@semnan.ac.ir

    1, 2 Faculty of Electrical and Computer Engineering, Semnan University, Semnan, Iran

    Abstract: Maintaining network stability and encouraging generation companies to involve in stability maintaining are 1

    major concerns of independent system operator (ISO) in deregulated power system. Excitation system of synchronous 2

    generators is effective and well-known equipment which has a significant effect on network stability. Therefore, 3

    improving performance of this system can enhance network stability. But utilizing of the methods, which improve 4

    performance of excitation system, imposes cost on generation companies. This paper proposes a motivation mechanism 5

    for enhancing transient stability which encourages generation companies to improve performance of excitation system. 6

    In this mechanism beside excitation system response improving methods, a transient stability constrained optimal power 7

    flow is solved in order to ensure maintaining transient stability in different contingencies. A 4-buses test system, the 8

    IEEE 14-buses test system, and the IEEE 118-buses test system are used to illustrate effectiveness of the proposed 9

    mechanism. Implementation results show that the proposed mechanism not only provides required transient stability 10

    margin with minimum operation cost, but also does not restrict generators production capability. 11

    Keywords- excitation systems; transient stability; Single Machine Equivalent (SIME); Transient Stability Constrained 12

    Optimal Power Flow (TSC-OPF). 13

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    1. Introduction 15

    Transient stability is the ability of power systems to maintain synchronism in the event of large disturbances. Such 16

    disturbances can increase generators rotor angle deviation; so if corrective actions fail, synchronization with the network will be 17

    lost [1]. Excitation system of synchronous generators is effective for primary control action which maintains transient stability 18

    of power systems. Excitation system supplies required field current to maintain generator synchronization with the network. 19

    Quick excitation system response can increase synchronization torque of generator in order to maintain synchronism with the 20

    network. As a result, response rate of excitation system has a significant effect on network stability. Various parameters affect 21

    performance of the excitation system. These parameters include ceiling voltage, ceiling current, nominal response of the system, 22

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    voltage excitation system response time and etc. Numerous methods were proposed to modify these parameters for improving 23

    performance of the excitation system which can be divided into three general categories: (1) expansion capability of excitation 24

    system, (2) improvement response of controller, and (3) modification input signal of controller. Over excitation limit exceeding, 25

    generator capability curve (GCC) expansion, control response improvement, and high side voltage control (HSVC) are some of 26

    these methods. These methods are called briefly improving methods in this paper. Simple implementation and operation, non-27

    restriction of generators production capability and good performance are some features of the improving methods. While, 28

    installation and operation cost of the improving methods discourage generation companies (GenCos) from utilizing these 29

    methods. If GenCos do not utilize the improving methods, other preventive control actions would be used to enhance network 30

    stability. 31

    Change in generators operation points is one of the preventive control actions for providing network stability. In this 32

    method, operation points of generators are modified to make system stable in probably disturbances. Transient stability 33

    constrained optimal power flow (TSC-OPF) is a preventive control action which is utilized to optimize operation point of 34

    generators in order to provide transient stability of the system. This topic is discussed in several articles [2-6]. In [2-4] rotor 35

    angle equations are considered as transient stability constraint in optimal power flow. As rotor angle equations are differential 36

    equations, these equations are converted into algebraic set of equations to solve TSC-OPF as a standard nonlinear optimization. 37

    Researchers in this field focus on converting differential equations and solving standard TSC-OPF. In [2] authors convert the 38

    differential equations into numerical equivalent algebraic equations and use standard nonlinear programming technique for 39

    solving TSC-OPF. In [3] authors proposed an enhanced discretization method to reduce converted system dimension and 40

    improve computational efficiency of the optimization algorithm. In [4] authors modelled transient stability as an objective 41

    function beside active power cost then solved this multi objective optimization by non-dominated sorting genetic algorithm II 42

    (NSGA-II). In [5-6] time domain simulation is performed to analyse transient stability of the system and simulation results are 43

    utilized in optimal power flow. In [5] simulation results determine critical and non-critical generators of the system. Critical 44

    generators are the generators which loss synchronism in the event of contingency. In that method, active powers of the critical 45

    generators are decreased to enhance the transient stability of the system. In [6] results of time domain simulation is converted to 46

    a single equation as transient stability constraint in OPF. Active power of generators, machine angle, magnitude and angle of 47

    buses voltage and other optimization variables are used to determine initial operation point to increase transient stability of the 48

    system. In that paper initial operation point is changed to satisfy transient stability constraint. Also some new methods based on 49

    modern heuristic methods were used to solve TSC-OPF [7]. In [7] authors proposed a method to estimate critical clearing time 50

    (CCT) by dual-kriging method. Result of this estimation is included in a single transient stability constraint of the TSC-OPF. 51

    Therefore differential algebraic equations are excluded from TSC-OPF and standard optimization method is used to solve 52

    problem. Authors in [8] reviewed different TSC-OPF methods and implemented some of these methods. Also merits and 53

    demerits of the studied methods in compare to each other are investigated in [8]. But it should be considered that all above 54

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    mentioned TSC-OPF methods cause to increase operation cost of the system, because active power outputs of generators are 55

    rescheduled in order to increase transient stability of the system. Moreover, restriction of active and reactive power generation 56

    due to changing of the generators operation points is the other disadvantage of those methods. 57

    As described utilizing excitation system response improving methods imposes costs on GenCos which discourage them 58

    from utilizing these methods. On the other side, other preventive methods for transient stability providing such as TSC-OPF 59

    cause to increase system operation cost and also restrict generators capacity. So, this paper proposes a motivation mechanism to 60

    encourage GenCos to utilize the improving methods. Transient stability enhancement by utilizing the improving methods does 61

    not change optimal operation point of the system in which system operation cost is minimal. If the desired transient stability 62

    margin is not provided only by utilizing the improving methods, then change in generators operation points is used to provide 63

    more needed stability margin. In this paper, TSC-OPF is used beside the improving methods to minimize the cost of active 64

    power rescheduling and provide the desired transient stability of the system. Innovations and contributions of this paper are 65

    summarized as follows: 66

    1) The proposed mechanism maximizes transient stability and minimizes operation cost of the system by considering 67

    effects of the improving methods on network stability. 68

    2) This mechanism provides required motivation for GenCos to utilize the improving methods. 69

    3) In this mechanism, GenCos capacity for power generating is not restricted. 70

    4) In this mechanism, operation point of the system for transient stability providing is the same operation point which 71

    minimizes network operation cost. 72

    5) In this mechanism, both of the improving methods and TSC-OPF are considered for providing transient stability. 73

    The rest of the paper is organized as follows. The second section describes the improving methods for excitation system 74

    response. The third section introduces the methodology used in transient stability analysis. The transient stability providing 75

    mechanism is presented in section 4. Section 5 is devoted to case study and section 6 concludes this paper. 76

    77

    2. Improving methods for excitation system response 78

    Performance of excitation system affects stability of the system. Results of previous researches show that the fast 79

    response AVR and high ceiling voltage exciter signif

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