* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Survey

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Document related concepts

Three-phase electric power wikipedia, lookup

Resistive opto-isolator wikipedia, lookup

Immunity-aware programming wikipedia, lookup

Variable-frequency drive wikipedia, lookup

Current source wikipedia, lookup

Power electronics wikipedia, lookup

Switched-mode power supply wikipedia, lookup

Surge protector wikipedia, lookup

Voltage optimisation wikipedia, lookup

Stray voltage wikipedia, lookup

Opto-isolator wikipedia, lookup

Alternating current wikipedia, lookup

Buck converter wikipedia, lookup

Transcript

Combined Transient and Steady State Simulation November 2007 DIgSILENT GmbH Heinrich-Hertz-Strasse 9 D-72810 Gomaringen Tel.: +49 7072 9168 - 0 Fax: +49 7072 9168- 88 http://www.digsilent.de e-mail: [email protected] Published by DIgSILENT GmbH, Germany Copyright 2007. All rights reserved. Unauthorised copying or publishing of this or any part of this document is prohibited. Combined Transient and Steady State Simulation Table of Contents Table of Contents 1 Introduction ......................................................................................................................................... 4 2 Theoretical Background ....................................................................................................................... 4 3 PowerFactory Setup............................................................................................................................. 5 3.1 General description ................................................................................................................................ 5 3.2 Starting PowerFactory ............................................................................................................................ 7 3.3 Synchronizing the Slave with the Master................................................................................................... 7 3.4 Transmitter and Receiver Input Data ....................................................................................................... 9 3.5 Starting the Simulation ......................................................................................................................... 12 4 Example .............................................................................................................................................. 13 4.1 UKss Calculation, RMS Side ................................................................................................................... 16 4.2 Pext, Qext Calculation, RMS Side ........................................................................................................... 16 4.3 Convert_I, EMT Side ............................................................................................................................. 16 4.4 Convert_U, EMT Side ............................................................................................................................ 17 Combined Transient and Steady State Simulation 1 Introduction Combined transient and stability simulation means splitting the simulation of a power system into two parts, one performed in transient mode and the other in stability mode. The nonlinear nature of the power system implies that the responses at different frequencies are not completely decoupled. The steady-state behaviour of a component, which internally works with fast frequencies, may change as a result of interaction between fast and slow dynamics, hence changing the overall operating point of the system. The common practice is to use average steady-state models for such devices in system studies, but the question arises as to whether those models properly portray this interaction or not. The equivalent question for the new methodology is whether the partitioning of equations and solving them successively rather than simultaneously can reflect the phenomena of interest with enough accuracy. While the above issues deserve attention, they are not the main concern here. Rather our focus is to describe the implementation of RMS-EMT simulation in PowerFactory. In this report: The nature of data exchange between the simulators is explained in Theoretical Background. Then in PowerFactory Setup, it is shown how the communication between two PowerFactory simulators must be configured. A simple example is considered last. 2 Theoretical Background In stability simulation the electrical network is represented by phasors whereby only the synchronous frequency data is considered. This simplification yields conservative results, but is necessary in order to deal with the analysis of large-scale power systems. In transient simulation, on the other hand, the network is modelled by differential equations, and therefore, the electrical variables contain all the natural and forced frequencies. Stability simulation deals with the solution of differential-algebraic equations. The differential equations describe the mechanical as well as the rotor electrical dynamics, while the algebraic equations describe the network. It is helpful to review the nature of this combination as a similar situation arises when combining RMS and EMT simulations. The most important simplification in stability simulation is done with regard to synchronous machine modelling. The more accurate model of synchronous machine consists of a set of differential equations both in the rotor and in the stator. By neglecting the differential terms in the stator equations, the terminal voltage of the machine is represented by a phasor that can be used in the algebraic equations of the network. These equations yield the d and q components of the stator currents which are then used to calculate the internal voltage of the machine (voltage behind reactance). The internal voltage of the machine is treated as a state variable and used in differential equations together with the excitation voltage (see for example Kundur p.181). Combined Transient and Steady State Simulation As a result of this simplification, effect of fast transients of the network on the rotor variables is neglected. The slow frequencies, however, are represented well enough, and the slow variations of the rotor speed about the synchronous speed are calculated using a differential equation that includes the mechanical and electrical powers. For the range of frequencies of interest in stability studies (much below the synchronous frequency) the simplifications described above are reasonable. How is the combined RMS-EMT simulation justified? If the detailed model of a part of the network for example an electronic converter is available, and it is expensive or not straightforward to derive the average steady-state model, then the RMS-EMT simulation allows for the detailed model to be used for system studies. Instead of using an equivalent RMS model for the converter, it is represented in detail using an EMT model and interfaced with the rest of the network, which due to its size, is represented by an RMS model. Furthermore, if these periodically-switched devices go to their nonlinear region as the result of shift in the operating point (which is determined by the steady state of the whole system) or get disconnected from the system by protective devices which trigger upon fast transients, it might be important even from the stability point of view to take these fast phenomena into account for modelling. One obvious limitation of this simulation strategy is when at the point of split, the subsystems have loading effect on each other at frequencies other than the synchronous frequency (for example subsynchronous resonance). The combined simlulation method implemented in PowerFactory has a feedback nature. The master simulator (RMS) feeds voltages into the slave simulator (EMT), and receives currents from it. In the next section the setup details are explained. 3 PowerFactory Setup This section explains the settings that need to be done in order to establish communication between two PowerFactory simulators. 3.1 General description The simulation setup is depicted in Figure 1. Combined Transient and Steady State Simulation Bus RMS NETWORK EMT NETWORK PowerFactory 1 (Master) PowerFactory 2 (Slave) Transmits voltage Receives voltage Receives current Transmits current Figure 1- Splitting the task of simulation between two instances of PowerFactory The network to the left of Bus is simulated in the stability mode, and the network to the right of Bus in the transient mode in two PowerFactory simulators. The transfer of data between the simulators is realized through RPC (Remote Procedure Calls) which is based on a TCP/IP (Transmission Control Protocol/Internet Protocol). Two PowerFactory objects are used for this purpose; the transmitter (ElmTransm) and the receiver (ElmReceiv). The RMS network contains a load whose active and reactive currents are sent over by the EMT side. Similarly, the EMT network contains a sinusoidal voltage source whose magnitude and angle are sent over by the RMS side. Figure 2 shows this communication. Bus RMS NETWORK Load controlled from EMT side Z Bus EMT NETWORK Voltage controlled from RMS side Figure 2 – Communication between RMS and EMT networks Each network has a DSL model containing an ElmReceiv object and an ElmTransm object. In the RMS network the DSL model is connected to the load through the signals Pext and Qext. In the EMT network the DSL model is connected to the voltage source through the signals U_A, U_B, and U_C. Combined Transient and Steady State Simulation Since the RMS simulation only deals with one frequency, the currents calculated in the EMT section have to be filtered before being passed. Filtering the decaying DC component of the short circuit currents is of special importance, because the response of the RMS model to DC excitation is simply wrong. Z is the equivalent impedance of the RMS network at synchronous frequency. Inserting Z in the model means that the subtransient voltage (voltage behind reactance) has to be sent over from the RMS to the EMT side instead of the Bus voltage. This is advantageous because subtransient voltages are linked to the state variables and their values do not jump. 3.2 Starting PowerFactory When you start PowerFactory, make sure the “Master in Distributed Simulation” flag is checked for the master simulator, and unchecked for the slave simulator. This flag is found in the “Advanced” page of the logon window (Figure 3). Figure 3: Starting PowerFactory in master mode 3.3 Synchronizing the Slave with the Master The EMT simulator (slave) should be synchronized with the master. This is done in the “Real-Time” tab-page of the “Calculation of Initial Conditions” window. The settings are shown in Figure 4. Combined Transient and Steady State Simulation Figure 4: Settings of the Real-Time tab-page of Initial Conditions of the slave simulator Combined Transient and Steady State Simulation 3.4 Transmitter and Receiver Input Data Figure 5: Transmitter input data pages in RMS simulator Combined Transient and Steady State Simulation Figure 6: Receiver input data pages in RMS simulator Combined Transient and Steady State Simulation Figure 7: Receiver input data pages in EMT simulator Combined Transient and Steady State Simulation Figure 8: Transmitter input data pages in EMT simulator 3.5 Starting the Simulation After the simulators are initialized, master must be started first. After advancing one time step, it sends the results to the slave and stops to receive signals from the other side before continuing. Combined Transient and Steady State Simulation 4 Example The network in this example is trivial, and is meant only to show the modelling details with regard to RMS-EMT communication. Basically, the RMS and EMT networks are the same, as shown in Figure 9 and Figure 10. 20 kV Load ~ V Vrms Const. Current plini=10 MW qlini=0.1 MVAr Bus usetp= 1.0 p.u. X1=31.416 Ohm Source Controlled Load Figure 9: RMS network Load ~ Vemt V Reactor Const Current plini=10 MW qlini=0.1 MVAr xrea=31.416 Ohm Bus usetp=1.0 p.u. Controlled Source Load Figure 10: EMT network The load is a constant current load, so it represents a current source. Figure 11 and Figure 12 show the composite frames used to set up the communication. Combined Transient and Steady State Simulation Figure 11: Composite frame of the communication on the RMS side Combined Transient and Steady State Simulation Receiver * Reference Frq * RMS V_Meas * RMS I_Meas * 1 0 1 0 1 0 iq id ui ur ii ir 1 0 3 2 1 0 ComRMS: Communication on the RMS side ConvPQ * 1 0 Ukss * 1 0 Qext Pext 1 0 Load * fref u2 u1 2 1 0 Transmitter * DIgSILENT Figure 12: Composite frame of the communication on the EMT side Combined Transient and Steady State Simulation EMT i_Meas * EMT v_Meas * Receiver * 1 0 1 0 2 1 0 ii ir ui1 ur1 fref ui ur ComEMTframe: Communication in the EMT side 3 2 1 0 2 1 0 Convert_I * Convert_U * 1 0 2 1 0 u2 u1 U_C U_B U_A 1 0 2 1 0 Transmitter * Voltage * DIgSILENT The voltage and current are measured (using StaVmea and StaImea) at Bus on both simulators. The current measurement on both RMS and EMT networks is inserted in the cubicle facing the voltage source. 4.1 UKss Calculation, RMS Side ussr = ur - rs*ir + xss*ii ussi = ui - rs*ii - xss*ir inc(ussr) = ur - rs*ir + xss*ii inc(ussi) = ui - rs*ii - xss*ir vardef(rs)='Ohm';': Equivalent Resistance' vardef(xss)='Ohm';': Equivalent Reactance' 4.2 Pext, Qext Calculation, RMS Side Pext = -id*Unom*sqrt(3) Qext = iq*Unom*sqrt(3) inc(id) = -Pext/Unom/sqrt(3) inc(iq) = Qext/Unom/sqrt(3) ! The following two lines are needed to initialize the ! system when the load type is not specified. If ! the load type is set to Constant Current, the ! initialization is done without the following. !inc0(Pext) = 0 !inc0(Qext) = 0 vardef(Unom)='kV';': Load Nominal Voltage' 4.3 Convert_I, EMT Side umag = sqrt(ur*ur+ui*ui) cosphi = ur/umag sinphi = ui/umag id1 = ir*cosphi+ii*sinphi iq1 = -ir*sinphi+ii*cosphi idd = delay(id1,0.01) iqd = delay(iq1,0.01) Combined Transient and Steady State Simulation id = (id1+idd)/2 iq = (iq1+iqd)/2 ! Initialization inc(id) = ir*cosphi+ii*sinphi inc(iq) = -ir*sinphi+ii*cosphi inc(idd)=id inc(iqd)=iq inc(id1)=id inc(iq1)=iq inc(fref)=1 4.4 Convert_U, EMT Side phiref. = 2*pi()*fref*Fnom ualpha = ur*cos(phiref) - ui*sin(phiref) ubeta = ur*sin(phiref) + ui*cos(phiref) U_A=ualpha*Unom/sqrt3*sqrt2 U_B=(-0.5*ualpha+0.5*sqrt3*ubeta)*Unom/sqrt3*sqrt2 U_C=(-0.5*ualpha-0.5*sqrt3*ubeta)*Unom/sqrt3*sqrt2 sqrt3 = sqrt(3) sqrt2 = sqrt(2) ! Initialization !inc(phiref)=7.5*pi()/180 inc(phiref)=0.0 inc(ualpha)=ur*cos(phiref)-ui*sin(phiref) inc(ubeta)=ur*sin(phiref)+ui*cos(phiref) inc(U_A)=ualpha*Unom/sqrt3*sqrt2 inc(U_B)=(-0.5*ualpha+0.5*sqrt3*ubeta)*Unom/sqrt3*sqrt2 inc(U_C)=(-0.5*ualpha-0.5*sqrt3*ubeta)*Unom/sqrt3*sqrt2 inc(fref)=1 inc(ur)=19.55125 !1 inc(ui)=-2.477702 !0 vardef(Unom)='p.u.';': Nominal Line-Line Voltage' Combined Transient and Steady State Simulation ! ur and ui are in kV, therefore Unom = 1 ! If ur and ui are passed from the RMS simulator in p.u. ! then the value of Unom should be specified in kV vardef(Fnom)='Hz';': Nominal Frequency' Combined Transient and Steady State Simulation