含风电场的电力系统静态电压稳定性分析翻译

1、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：至8完成日期：2013. 5.8The interduction of steady-State Characteristicsthe speed-torque characteristic is quite linear around synchronous speed If the rotor speed is below synchronous speed, the induction machine is opera ting a

2、s 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 sy

3、nchronous speedthe 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 i

4、s 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 辻 the induction machine is running as a motor, then an increased rotor resistance will decrease

5、 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 resis

6、tance 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 po

7、wer 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 b

8、e used in generator operation.The rectified current could be converted to mechanical power using a de motor coupled to the shaft of the induction motor or fed back into the grid Since Kramer drive require an extra de motor it is of no interest, while the Scherbius drive is still in use .The main adv

9、antage 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 t

10、he stator voltage reaches maximum voltage, t here is need for field weakening, the stator volt age is kept constant while the frequency is still increasedThe interduction of doubly-Fed Induction MachinesDoubly-fed machines can be used in variable-speed constant-frequency applications, such as wind t

11、urbines The main adva nt age of a doubly-fed machine compared to a singly-fed for a variable-speed system is the reduced rating of the converter s power rating. The reduction in power rating is dependent on the speed range of the drive The standard doubly-fed induetion machine is a wound rotor indue

12、tion 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 ringsThe cascaded doubly-fed induetion machine consists of two doubly-fed induction machines with wound :rotors, that are connected mechanically th

13、rough 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 inducti

14、on machine that is directlyconnected to the grid with the other induetion machineIt is doubtful whether it is practical to combine two individual machines to form a cascaded doubly-fed induetion machine, even though it is the basic configuration of doubly-fed induetion machine arrangement Due to a l

15、arge amount of windings, the losses are expected to be higher than for a standard doubly-fed induction machine of a comparable rating.Stability of a Power SystemPower system stability is understood as the ability to regain an equilibrium state after being subjected to a phys ical disturbance. three

16、quantities are important for power system operation: (i) angles of nodal voltages , also call ed power or load angles; (ii) frequency; and (iii)nodal vol tage magnitudes These quantities are especially important from the point of view of defining and classifying power s ystem stability Hence power s

17、ystem stability can be divide d into： (i) rotor (or power) angle stability; (ii) frequen cy stability; and (iii) voltage stabilityAs 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 divi

18、ded into small-disturbanee andlarge-disturbance stability.Power system stability is mainly connected with elect romechanical phenomena However, it is also affected by fas t electromagnetic phenomena and slow thermodynamic phenomen a. Hence, depending on the type of phenomena, one can refer to short-

19、term stability and long-term stabilityConnections 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 bet ter t han t

20、hose onshore, as some onshore sites have bet ter wind regimes than sites offshoreA 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 char

21、ging 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 100T50 km with th

22、e 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 turb

23、ines 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 te

24、chnology using GTOs can be usedCurrently 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 th

25、e shore subs tat io n. 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 c

26、ables 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.Influence of Wind Generators on Power System StabilityThe

27、 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 Fot small values of the :rotor angle, power is proportional to the angle itself which

28、 produces spring-like oscillations On the other hand, squirrel-cage (fixed-speed) induetion generatots are coupled to the grid less stiffly than synchronous genera to rs. the torque of a fixed speed induction generator is proportional to the speed deviation (slip) hence providing inherent damping of

29、 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 cont

30、rolling 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 genera tor from the grid

31、, so t hey 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 rela tive poor natural damping, by renewable genera to rs, which exhibit a bett

32、er 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 lo

33、ading 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 high

34、ly loaded, remote and operate with a low, or even leading, power factor. If renewable plants are connected closer to the loads, then the transmission net works will be less loaded, which will reduce reac tive power consumption by the system and the voltages will rise This effect can be compensated b

35、y 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

36、 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 dyn

37、amic proper ties of the sys tem 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

38、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 traditiona

39、l plant of the same rating. The reduction in stored kinetic energy will have an effect on system operation and security becauseof the amplitude of frequency variationsVoltage StabilityVoltage stability is the ability of a power system to maintain steady acceptable voltages at all buses in the system

40、 under normal opera ting 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

41、system Voltage stability problem arises when the system is heavily loaded that causes to go beyond limitations of power systemA 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 d

42、ecline 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 transmissio

43、n 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 st ability analysis active and reactive power capabilities of generators play an important role. The active power limits

44、are due to the design of the turbine and the boiler .Therefore, active power limits are constantReactive 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-exci tation current and under-excit

45、ation limits. The generator fi eld current is limited by overexcitat ion limner in orde to avoid damage in field windingIn fact, reactive power limi ts are voltage dependent However, in load flow programs they are taken to be constant in order to simplify analysis 七 Analysis of voltage stabilityThe

46、most common methods used in voltage stability analys is are continuation power flow, point of collapse, minimum singular value and optimization methods In this study, con tinuation power flow method is widely used in voltage stabi lity analysisSo voltage stability can be analyzed by using continuati

47、on power flow. The Jacobian matrix of power flow equations becomes singular at the voltage stability limit Continuation power flow overcomes this problem Continuatio n power flow finds successive load flow solutions according to a load scenario. It consists of prediction and correctio n steps From a

48、 known base solution, a tangent predictor is used so as to estimate next solution for a specified patte rn 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 mad e for a specifieP

49、t = Pgi Pm, Qi = Qgl Qi)i(2)Pt = Pgi Pm, Qi = Qgl Qi)i(2)dincrease in load based upon the new tangent vector The n corrector step is applied This process goes until critic al point is reached. The critical point is the point where the tangent vector is zero.In continuation load flow, first power flo

50、w equations are reformulated by inserting a load parameter into these equations .Injected powers can be writ ten for the ibus of an n-bus system as followsnP =为 |W|M|(G& cos 久 + Bik sin 6k)Q = sin 6k - 乩 cos久)z(1)(7)where the subscripts G and D denote generation and load demand respectively on the r

51、elated bus.In order to simulate a load change, a load parameter 2 s inserted into demand powers and 如.Pm = Pl)io + 兄(PAdw)Ql)i =(3 )Pl)io and Qd10 are original load demands on i-bus whereas 皿宓 andare given quantities of powers chosen to scale 几appropriately. After substituting new demand powers in E

52、quation (2) to Equation (3), new set of equations can be represented as:F(/9,V,2)= 0(4)where & denotes the vector of bus voltage angles and V den otes the vector of bus voltage magnitudes. The base solutio n for 兄二0 is found via a power flow Then, the continuation and parameterization processes are

53、appliedIn prediction step, a linear approximation is used by ta king an appropriately sized step in a direction tangent to the solution path Therefore, the derivative of both sides of Equation (4) is taken.F別 O+FwIV + F加入=0F6, Fv, F. dV = 0OdZIn order to solve Equation 5, one more equation is neede

54、d since an unknown variable 几 is added to load flow equati ons This can be satisfied by setting one of the tangent ve ctor components to +1 or T which is also called continuati on parameSetting one of the tangent vector components +1 or -1 imposes a non-zero value on the tangent vector and makes Jac

55、obian nonsingular at the critical point. As a result Equation 5 becomes:(6)where ek is the appropriate row vector with all elements equal to zero excep t the k element equals 1 .At first step 久 is chosen as the continuation parameter .As the process continues, the state variable with the greatest ra

56、te 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:op+i歹pV=V+5dVA2dAwhere the subscript p+l” denotes the next predicted solution.The step sized(Qs-QL)/dV = O(V = Vx)kQ

57、 = Qg / dQi（d(Qs-QL)/dV = O(V = Vx)kQ = Qg / dQi（ 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 co rrected by using local parameterization. The origina

58、l set of equation is increased by one equation that specifies th e value of state variable chosen and it results in:Xk-fj(8)Where Xk is the state variable chosen as continuation param eter and is the predicted value of this state variable Equation (8) can be solved by using a slightly modified Newto

59、n-Raphson power flow method八 Voltage Stability IndicesThe discussed coefficient = Pax：Po 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 cons true ted by observing t hat as the l

60、oad demand gets closer to the critical value, both the equilibrium points move towards each other until they become one unstable point there is always a point between the equilibrium points of voltage Vx, such thatAs the power demand of the composite load increases, the voltage Vx tends towards the