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Three-Phase Network with Electrical Machines
During this session you simulate the three-machines system shown in this single line diagram.
Figure 1-19: Diesel Generator and Asynchronous Motor on Distribution Network
This system consists of a plant (bus B2), simulated by a resistive and motor load (ASM) fed at 2400 V from a distribution 25 kV network through a 6 MVA, 25/2.4 kV transformer, and from an emergency synchronous generator/diesel engine unit (SM).
A 500 kvar capacitor bank is used for power factor correction at the 2.4 kV bus. The 25 kV network is modeled by a simple R-L equivalent source (short-circuit level 1000 MVA, quality factor X/R = 10) and a 5 MW load. The asynchronous motor is rated 2250 HP, 2.4 kV, and the synchronous machine is rated 3.125 MVA, 2.4 kV.
Initially, the motor develops a mechanical power of 2000 HP and the diesel generator delivers 500 kW of active power. The synchronous machine controls the 2400V bus B2 voltage at 1.0 p.u. and generates 500 kW of active power. At t = 0.1 s, a three-phase to ground fault occurs on the 25 kV system, causing the opening of the 25 kV circuit breaker at t = 0.2 s, and a sudden increase of the generator loading. During the transient period following the fault and islanding of the Motor/Generator system, the synchronous machine excitation system and the diesel speed governor react to maintain the voltage and speed at a constant value.
This system has already been built in the Power System Blockset. Open the Demos library of powerlib and double-click the demo called Three-Phase Machines and Load Flow. A system named psbmachines opens.
Figure 1-20: Power System of Figure 1-19 Built with the Power System Blockset
The Synchronous Machine (SM) block is using standard parameters, whereas the Asynchronous machine (ASM) block is using S.I. parameters.
The other three-phase elements such as the inductive voltage source, the Y grounded/Delta transformer, and the loads are masked blocks built with standard single-phase Power System Blockset blocks. They are available in the Extra/Three-Phase library of powerlib. The 3-Phase Fault and the 3-Phase Breaker blocks are also available in the same library. If you open their dialog boxes, you see how the switching times are specified. Special measurement blocks provided in the Machines library are used to demultiplex the SM and ASM machine outputs.
The SM voltage and speed outputs are used as feedback inputs to a Simulink control system that contains the diesel engine and governor block as well as an excitation block. The excitation system is the standard block provided in the Machines library. The SM parameters as well as the diesel engine and governor models were taken from reference [1].
Figure 1-21: Diesel Engine and Governor System
If you simulate this system for the first time, you normally do not know what the initial conditions are for the SM and ASM to start in steady state.
Open the dialog box of the Synchronous Machine and Asynchronous Machine blocks. All initial conditions should be set at 0, except for the initial SM field voltage and ASM slip, which are set at 1 p.u. Open the three scopes monitoring the SM and ASM speeds as well as the ASM stator currents. Start the simulation and observe the first 100 ms before fault is applied.
As the simulation starts, you will notice that the three ASM currents start from 0 and contain a slowly decaying DC component. The machine speeds take a much longer time to stabilize because of the inertia of the motor/load and diesel/generator systems. In our example, the ASM even starts to rotate in the wrong direction because the motor starting torque is lower than the applied load torque. Stop the simulation.
| | Session 7: Three-Phase Systems and Machines | Load Flow and Machine Initialization | |