This example shows the use of the 3-Phase Dynamic Load and 3-Phase Programmable Voltage Source blocks
A dynamic load is connected on a 500 kV, 60 Hz power network . The network is simulated by its Thevenin equivalent (voltage source behind a R-L impedance corresponding to a 3-phase short circuit level of 2000 MVA). The source internal voltage is modulated in order to simulate voltage variation during a power swing. As the dynamic load is a nonlinear model simulated by current sources, it cannot be connected to an inductive network (R-L in series). Therefore, a small resistive load (1 MW) has been added in parallel with the dynamic load.
The dynamic load power is a function of its terminal positive-sequence voltage V. Open the Dynamic Load menu and notice that both exponents np and nq are set to 1 and that the specified minimum voltage Vmin is 0.7 pu . It means that the load active power P and reactive power Q are defined by the following equations:
If V > Vmin P = Po*(V/Vo); Q = Qo*(V/Vo) If V < Vmin P = Po*(V/Vo)^2; Q = Qo*(V/Vo)^2
In other words, as long as voltage is higher than 0.7 pu, the load current is constant. When voltage falls below 0.7 pu the load behaves as a constant impedance.
In order to simulate the variation of P and Q as function of voltage, the source internal voltage is controlled by the 3-Phase Programmable Voltage Source block. Open the source menu and notice that the specified type of amplitude variation is a sinusoidal modulation (Amplitude of the modulation = 0.5 pu, Frequency of the modulation = 1 Hz). Therefore, the source positive-sequence voltage varies between 0.5 pu and 1.5 pu. The initial source voltage is 1 pu. Modulation starts at t = 0.2 s and stops after 1 cycle at t = 1.2 s.
1. Initializing the dynamic load
In order to start the simulation in steady-state you have to specify the correct initial voltage Vo (magnitude and phase) corresponding to desired Po and Qo values.
The 'Machine Initialization' utility of Powergui block can be used to find this voltage and initialize the dynamic load. Open the Powergui and select 'Machine Initialization'. Specify the desired active power and reactive powers for the Dynamic Load (50 MW, 25 Mvar): Active Power = 50e6 Reactive Power = 25e6.
Then press the 'Compute and Apply' button. The phasors of AB and BC machine voltages as well as the currents flowing into of phases A and B are updated. The phase-neutral load voltage Uan is also displayed: ( 0.9844 pu - 1.41 degrees). If you now open the Dynamic Load dialog box you will notice that the values of Po, Qo and Vo have been updated.
2. Simulating the voltage swing
Start the simulation and observe load voltage, P&Q powers and current on Scope1. Observe that simulation starts in steady state. At t = 0.2 s, when voltage modulation is initiated, P and Q start to increase (trace 2), but, as np and nq are set to 1, the load current (trace 3) stays constant. When voltage falls below 0.7 pu the load behaves as a constant impedance. Therefore load current follows this voltage variation. Observe on Scope2 variations of instantaneous voltages and currents. Also, notice that computed Pand Q displayed on Scope3 are the same as P and Q internal signals returned by the Dynamic Load measurement output.