Three-Phase Network with Electrical Machines

The two-machine system shown in this single-line diagram represents a diesel generator and asynchronous motor in a distribution network:

This system consists of a plant (bus B2) simulated by a 1 MW resistive load and a motor load (ASM) fed at 2400 V from a distribution 25 kV system through a 6 MVA, 25/2.4 kV transformer, and from an emergency synchronous generator/diesel engine unit (SM).

The 25 kV system 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.

This system is modeled in the Emergency Diesel-Generator and Asynchronous Motor example.

The SM parameters and the diesel engine and governor models are based on reference [1].

Initially, the motor develops a mechanical power of 2000 HP and the diesel generator is in standby, delivering no active power. The synchronous machine therefore operates as a synchronous condenser generating only the reactive power required to regulate the 2400 V bus B2 voltage at 1.0 pu. 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.

When 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.

These initial conditions are:

Open the Synchronous Machine and Asynchronous Machine blocks. All initial conditions are set at 0, except for the initial SM field voltage and ASM slip, which are set at 1 pu. Open the three scopes monitoring the SM and ASM signals and the bus B2 voltage. Start the simulation and observe the first 100 ms before fault is applied.

As the simulation starts, note that the three ASM currents start from zero 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 starts to rotate in the wrong direction because the motor starting torque is lower than the applied load torque. Stop the simulation.

To start the simulation in steady state with sinusoidal currents and constant speeds, all the machine states must be initialized properly. This is a difficult task to perform manually, even for a simple system. In the Tools tab of the powergui block dialog, click the Load Flow Analyzer button. Use the Load Flow Analyzer app to initialize the machines.

Using the Phasor Solution Method for Stability Studies

When you increase the complexity of your network by adding extra lines, loads, transformers, and machines, the required simulation time becomes longer. Moreover, if you are interested in slow electromechanical oscillation modes (typically between 0.02 Hz and 2 Hz on large systems) you might have to simulate for several tens of seconds, which can result in long simulation times. The conventional continuous or discrete solution method is therefore not practical for stability studies involving low-frequency oscillation modes. For these studies, use the phasor technique (see Introducing the Phasor Simulation Method).

For a stability study, you ignore the fast oscillation modes that result from the interaction of linear R, L, C elements and distributed parameter lines. These oscillation modes, which are usually located above the fundamental frequency of 50 Hz or 60 Hz, do not interfere with the slow machine modes and regulator time constants. In the phasor solution method, these fast modes are ignored by replacing the network differential equations with a set of algebraic equations. The state-space model of the network is therefore replaced by a transfer function evaluated at the fundamental frequency and relating inputs (current injected by machines into the network) and outputs (voltages at machine terminals). The phasor solution method uses a reduced state-space model consisting of slow states of machines, turbines, and regulators, thus dramatically reducing the required simulation time. Two solver types are available for phasor models: continuous and discrete. The type of solver is specified in the powergui block by setting Simulation type to either Phasor (continuous) or Discrete phasor. The continuous phasor solution uses a Simulink^® variable-step solver. Continuous variable-step solvers are efficient in solving this type of problem. An example of a continuous variable-step solver you can use in this situation is the ode23tb with a maximum time step of one cycle of the fundamental frequency (1/60 s or 1/50 s). Discrete phasor uses a local solver to discretize and solve the phasor model at a specified sample time. The Discrete phasor simulation method allows you to use Simulink Coder™ to generate code and simulate your model in real time.

Apply the phasor solution method to the two-machine system you simulated in the Emergency Diesel-Generator and Asynchronous Motor example with the conventional method. Open the Emergency Diesel-Generator and Asynchronous Motor example.

In the powergui block, set Simulation type to Phasor. Specify the fundamental frequency used to solve the algebraic network equations. Enter 60 in the Frequency field. Note that the words Phasor 60 Hz now appear on the powergui icon, indicating that this new method is used to simulate your circuit. To start the simulation in steady state, you must first repeat the machine initialization procedure.

Observe that simulation is now much faster. The results compare well with those obtained with the continuous mode simulation.

You may also try the discrete phasor simulation. In the powergui block, set Simulation type to Discrete phasor and specify a sample time of 4e-3 sec.

The synchronous machine waveforms are compared on the following figure for three simulation types:

Both phasor models (continuous and discrete) compare well with the continuous model.

Contrary to the continuous phasor solver, which uses a full set of machine differential equations for modeling stator and rotor transients, the discrete phasor solver uses simplified machine models where differential equations on the stator side are replaced with algebraic equations. These lower-order machine models eliminate two states (phid and phiq stator fluxes) and produce simulation results similar to commercial stability software. Compared with the continuous phasor solver, the discrete phasor solver produces cleaner waveforms. For this example, you can observe that in the discrete phasor model, the speed (w) and terminal voltage (Vt) high-frequency oscillations are eliminated and the Vt voltage glitch observed at the breaker opening is also eliminated.

The discrete phasor solver has also two additional advantages:

The phasor solution method is illustrated on more complex networks in the following examples:

The first example illustrates the impact of PSS and use of a SVC to stabilize a two-machine system. The second example compares the performance of three different types of power system stabilizers on a four-machine, two-area system.

The phasor solution method is also used for FACTS models. See Improve Transient Stability Using SVC and PSS and Control Power Flow Using UPFC and PST.