Three-Phase Programmable Source, PLL, Voltage and Power Measurement

Description

A 25kV, 100 MVA short-circuit level, equivalent network feeds a 5 MW, 5 Mvar capacitive load. The internal voltage of the source is controlled by the Discrete 3-Phase Programmable Voltage Source block.

Open the programmable source dialog box and look at the parameters controlling the voltage and frequency. A 60 Hz, positive-sequence of 1.0 pu, 45 degrees is specified.

At t = 0.5 s, a sinusoidal modulation of the frequency (amplitude 3 Hz, frequency 0.4 Hz) is started. The modulation stops at t =3 s so that a full cycle of modulation can be observed.

The Three-Phase V-I Measurement block is used to monitor the three load voltages and currents. Open its dialog box and see how this block allows to output the three voltages and currents in p.u.

The Discrete 3-Phase PLL block measures the frequency and generates a signal (wt output) locked on the variable frequency system voltage. The PLL drives two measurement blocks taking into account the variable frequency: one block computing the fundamental value of the positive-sequence load voltage and another one computing the load active and reactive powers. These two blocks and the PLL are initialized in order to start in steady state.

The whole system, (power network, PLL and measurement blocks) is discretized at a 50 us sample time.

Simulation

Start the simulation.

Observe on Scope1 block how the PLL tracks the changing system frequency (trace 1). As expected, the system frequency varies between 57 Hz and 63 Hz with a maximum rate of change of 7.5 Hz/s. Traces 2 and 3 show the variations of magnitude and phase of positive-sequence voltage as the frequency is changing. Traces 4 and 5 show the variations of active and reactive powers.

Observe on Scope2 that the (wt) ramp output of the PLL stays locked on zero crossings of phase A voltage.