含风电场的电力系统静态电压稳定性分析翻译.doc

燕 山 大 学本科毕业设计（论文）英文翻译课题名称：含风电场的电力系统静态电压稳定性研究 学院（系）：电气工程学院 年级专业：09级电力四班 学生姓名：张建春 指导教师： 王珺 完成日期： 2013.5.8 一 The interduction of steady-State Characteristics the speed-torque characteristic is quite linear around synchronous speed. If the rotor speed is below synchronous speed，the induction machine is operating as a motor and if the rotor speed is above synchronous speed，the induction machine is running as a generator.the mechanical power and the mechanical torque are given by the slip,rotor resistance and the rotor current.The speed-torque characteristic of the induction machine is quite linear around synchronous speed.the torque is proportional to the inverse of the rotor resistance. This implies that it is possible to have external rotor resistances connected in series with the existing rotor resistances of a wound-rotor induction machine. By changing the value of the external rotor resistance it is possible to change the slope of the speed-torque characteristic. One disadvantage with this method is that it is only possible to increase the slip using the external rotor resistances. This implies that if the induction machine is running as a motor, then an increased rotor resistance will decrease the rotor speed. On the other hand, if the induction machine is running as a generator, then if the rotor resistance increases, the rotor speed will also increase.Before semiconductors were available, one way of adjusting the slip was to introduce external rotor resistances. The external rotor resistance will cause additional losses in the rotor circuit. When semiconductors became available it was possible to recover the slip otherwise dissipated in the external rotor resistance.Thus, the slip power can be recovered into mechanical or electrical energy; therefore,this method is called slip power recovery. The rotor current must be rectified with a diode rectifier. For motor operation, the rotor circuit will see the diode rectifier as a resistance and therefore this method will work approximately in the same way as for the external rotor resistances. Note that the diode rectifier cannot be used in generator operation. The rectified current could be converted to mechanical power using a dc motor coupled to the shaft of the induction motor or fed back into the grid .Since Kramer drive require an extra dc motor it is of no interest, while the Scherbius drive is still in use.The main advantage of this configuration compared to the external rotor resistance is that the losses of the external rotor resistance can be recovered.If both stator voltage and frequency can be adjusted by an inverter, the torque-speed characteristic can be easily changed. When the speed is increased so that the stator voltage reaches maximum voltage, there is need for field weakening, the stator voltage is kept constant while the frequency is still increased. 二 The interduction of doubly-Fed Induction MachinesDoubly-fed machines can be used in variable-speed constant-frequency applications,such as wind turbines. The main advantage of a doubly-fed machine compared to a singly-fed for a variable-speed system is the reduced rating of the converters power rating. The reduction in power rating is dependent on the speed range of the drive.The standard doubly-fed induction machine is a wound rotor induction machine equipped with slip rings. The stator circuit is connected directly to the grid while the rotor circuit is controlled by an inverter via slip rings.The cascaded doubly-fed induction machine consists of two doubly-fed induction machines with wound rotors, that are connected mechanically through the rotor and electrically through the rotor circuits. The stator circuit of one of the machines is directly connected to the grid while the other machines stator is connected via an inverter to the grid. Since the rotor voltages of both machines are equal, it is possible to control the induction machine that is directly connected to the grid with the other induction machine. It is doubtful whether it is practical to combine two individual machines to form a cascaded doubly-fed induction machine, even though it is the basic configuration of doubly-fed induction machine arrangement. Due to a large amount of windings, the losses are expected to be higher than for a standard doubly-fed induction machine of a comparable rating.三 Stability of a Power SystemPower system stability is understood as the ability to regain an equilibrium state after being subjected to a physical disturbance. three quantities are important for power system operation: (i) angles of nodal voltages , also called power or load angles; (ii) frequency; and (iii)nodal voltage magnitudes . These quantities are especially important from the point of view of defining and classifying power system stability. Hence power system stability can be divided into: (i) rotor (or power) angle stability; (ii) frequency stability; and (iii) voltage stability.As power systems are nonlinear, their stability depends on both the initial conditions and the size of a disturbance. Consequently, angle and voltage stability can be divided into small-disturbance and large-disturbance stability. Power system stability is mainly connected with electromechanical phenomena. However, it is also affected by fast electromagnetic phenomena and slow thermodynamic phenomena. Hence, depending on the type of phenomena, one can refer to short-term stability and long-term stability.四 Connections of Wind FarmsAlthough the majority of wind turbines are situated on land, there is a growing demand for wind turbines to be placed offshore with some large wind farms now operational . This does not mean that offshore sites are always better than those onshore, as some onshore sites have better wind regimes than sites offshore.A common problem to all offshore energy conversion systems is the electrical cable connection to the onshore substation. and this then raises distance issues because all AC cables have high capacitance and the line charging current for long cable runs can be very high. while a number of independent cable runs may be necessary in order to transmit the required power from an offshore wind farm.Because of the large cable capacitance AC cables are currently limited to a distance under the sea of about 100-150 km with the maximum rating of three-core submarine cables currently being about 200 MW at 145 kV , although larger ratings are under development. Generally the outputs of a number of turbines are collected together at an offshore substation for onward transmission to shore. Once the output of a number of turbines has been collected, an alternative to AC transmission to shore is to use DC transmission. New DC transmission technology uses IGBT voltage source converters at the sending end (and possibly also at the receiving end) allowing total control at the sending end. For higher powers, conventional DC technology using GTOs can be used. Currently offshore wind farms are sufficiently close to shore that AC cables can be used, although a number of cables may be necessary to transmit the required power. One practical point to note is that the distance to shore also includes the shore-based cable run to the shore substation. In some situations this can be substantial. The problems associated with transferring electrical power to shore from offshore wind farms is also faced by tidal stream generators and wave generators. Tidal stream generators tend to be relatively close to shore, although laying cables in the strong currents where these turbines are situated is not straightforward. Wave energy is in its infancy with the large amounts of resource available. Harnessing this energy and transferring it to shore poses a significant challenge.五 Influence of Wind Generators on Power System StabilityThe synchronous generator is stiffly connected to the power system and exhibits an inherently oscillatory response to a disturbance because its power output is approximately proportional to the sine of the rotor angle. For small values of the rotor angle, power is proportional to the angle itself which produces spring-like oscillations. On the other hand, squirrel-cage (fixed-speed) induction generators are coupled to the grid less stiffly than synchronous generators. the torque of a fixed speed induction generator is proportional to the speed deviation (slip) hence providing inherent damping of oscillations. This positive influence is counteracted by the vulnerability of fixed-speed induction generators to system faults. Damping due to variable speed DFIGs depends very much on the particular control strategy employed. the DFIGs have good control capabilities due to the possibility of controlling both the magnitude and phase of the injected voltage. This makes it possible to design a power system stabilizer that improves the damping of power swings without degrading the quality of voltage control provided . Fully rated converter systems effectively decouple the generator from the grid, so they offer a very good possibility of improving the damping of power swings. Hence the general conclusion is that a partial replacement of traditional thermal plants employing synchronous generators, which exhibit a relative poor natural damping, by renewable generators, which exhibit a better damping, will improve the damping of electromechanical swings. This effect will be counterbalanced to some extent by the highly variable nature of renewable sources themselves, such as wind, marine or solar, but their variability may be effectively managed by either using energy storage or part loading one of the turbines in a farm and using its spare capacity to smooth power oscillations .The network effect of replacing large traditional generators by renewable ones will largely depend on the system in question. Recall that the stability of synchronous generators deteriorates if they are highly loaded, remote and operate with a low, or even leading, power factor. If renewable plants are connected closer to the loads, then the transmission networks will be less loaded, which will reduce reactive power consumption by the system and the voltages will rise. This effect can be compensated by reactive power devices, such as reactors or static VAR compensators, but this would require additional investment. If that is deemed uneconomical and the remaining synchronous generators are used for reactive power compensation, their operating points would move towards capacitive loading (leading power factor) so their dynamic properties might deteriorate. As the number of synchronous generators remaining in operation is reduced due to increased penetration of renewables, their overall compensation capabilities will also be reduced. Hence the overall effect might be a deterioration of the dynamic properties of the system .On the other hand, if the renewable sources are located further away from the main load centres, then power transfers over the transmission network will increase. Higher transfers will mean larger voltage angle differences between network nodes and deteriorated system dynamic properties (smaller stability margins).Increased penetration of renewables might also affect frequency stability. Due to its construction, a wind plant has smaller inertia and speed so that kinetic energy stored in it is reduced by a factor of approximately 1.5 when compared with a traditional plant of the same rating. The reduction in stored kinetic energy will have an effect on system operation and security because of the amplitude of frequency variations.六 Voltage Stability Voltage stability is the ability of a power system to maintain steady acceptable voltages at all buses in the system under normal operating conditions and after being subjected to a disturbance.Voltage stability can be attained by sufficient generation and transmission energy. Generation and transmission units have definite capacities that peculiar to them. These limits should not be exceeded in a healthy power system. Voltage stability problem arises when the system is heavily loaded that causes to go beyond limitations of power system.A power system enters a state of voltage instability when a disturbance, increase in load demand power or change in system condition causes a progressive and uncontrollable decline in voltage. The main factor causing instability is the inability of the power system to meet the demand for reactive power.The main reason for voltage instability is the lack of sufficient reactive power in a system. Generator reactive power limits and reactive power requirements in transmission lines are the main causes of insufficient reactive power.Synchronous generators are the main devices for voltage control and reactive power control in power systems. In voltage stability analysis active and reactive power capabilities of generators play an important role. The active power limits are due to the design of the turbine and the boiler.Therefore, active power limits are constant.Reactive power limits of generators are more complicate than active power limits. There are three different causes of reactive power limits that are;stator current, over-excitation current and under-excitation limits.The generator field current is limited by over-excitation limner in orde to avoid damage in field winding.In fact, reactive power limits are voltage dependent.However,in load flow programs they are taken to be constant in order to simplify analysis.七 Analysis of voltage stabilityThe most common methods used in voltage stability analysis are continuation power flow, point of collapse, minimum singular value and optimization methods. In this study, continuation power flow method is widely used in voltage stability analysis.So voltage stability can be analyzed by using continuation power flow.The Jacobian matrix of power flow equations becomes singular at the voltage stability limit. Continuation power flow overcomes this problem. Continuation power flow finds successive load flow solutions according to a load scenario.It consists of prediction and correction steps. From a known base solution, a tangent predictor is used so as to estimate next solution for a specified pattern of load increase.The corrector step then determines the exact solution using Newton-Raphson technique employed by a conventional power flow.After that a new prediction is made for a specifiedincrease in load based upon the new tangent vector. Then corrector step is applied. This process goes until critical point is reached. The critical point is the point where the tangent vector is zero. In continuation load flow, first power flow equations are reformulated by inserting a load parameter into these equations .Injected powers can be written for the i-bus of an n-bus system as follows （1） （2）where the subscripts G and D denote generation and load demand respectively on the related bus. In order to simulate a load change, a load parameter is inserted into demand powers and . ( 3 ) and are original load demands on i-bus whereas and are given quantities of powers chosen to scale appropriately. After substituting new demand powers in Equation (2) to Equation (3), new set of equations can be represented as: (4)where denotes the vector of bus voltage angles and V denotes the vector of bus voltage magnitudes. The base solution for =0 is found via a power flow.Then,the continuation and parameterization processes are applied.In prediction step, a linear approximation is used by taking an appropriately sized step in a direction tangent to the solution path. Therefore, the derivative of both sides of Equation（4） is taken. （5）In order to solve Equation 5, one more equation is needed since an unknown variable is added to load flow equations. This can be satisfied by setting one of the tangent vector components to +1 or -1 which is also called continuation parameter. Setting one of the tangent vector components +1 or -1 imposes a non-zero value on the tangent vector and makes Jacobian nonsingular at the critical point. As a result Equation 5 becomes: （6）where is the appropriate row vector with all elements equal to zero except the k element equals 1 .At first step is chosen as the continuation parameter.As the process continues, the state variable with the greatest rate of change is selected as continuation parameter due to nature of parameterization. By solving Equation 6, the tangent vector can be found. Then, the prediction can be made as follows: （7）where the subscript p+1”denotes the next predicted solution. The step size is chosen so that the predicted solution is within the radius of convergence of the corrector. If it is not satisfied, a smaller step size is chosen. In correction step, the predicted solution is corrected by using local parameterization. The original set of equation is increased by one equation that specifies the value of state variable chosen and it results in: （8）Where is the state variable chosen as continuation parameter and is the predicted value of this state variable. Equation （8） can be solved by using a slightly modified Newton-Raphson power flow method.八 Voltage Stability IndicesThe discussed coefficient may be treated as a measure of voltage stability margin from the point of view of demand increase. A voltage stability index based on the classical dQ/dV criterion can be constructed by observing that as the load demand gets closer to the critical value,