Grid-Link 3-Phase Inverter with PQ Control

This example simulation shows PSIM being used to control a grid link 3-phase inverter with real and reactive power control. Control in the dq reference frame is being implemented. The control scheme allows for real and reactive power to flow from the DC bus to the grid or from the grid to the DC bus. This simulation features the use of the abc-dqo and dqo-abc transformations and the conversion of P & Q references into the dq reference frame.

The Power Stage

The power stage, defined by red wires, is:

1. The DC source, modeled by an ideal battery
2. Inverter, modeled with ideal IGBTs
3. Inductors, modeled with a small series impedance
4. Grid, modeled by an ideal 3-phase source

 The power stage is providing 6 feedback signals:

• 3 current sensors provide the phase currents
• 3 voltage sensors provide the phase to neutral voltages

The power stage in this simulation is relatively simple, ground has been defined as the neutral of the grid–3-phase source at right.

The Control Circuit

The control reference inputs for the control circuit are:

• P_ref the reference power in watts, a positive magnitude has power flowing from the DC bus to the grid
• P_ref the reference reactive power in Var, a positive magnitude has reactive power being supplied to the grid

The power flow is bi-directional in any combination, negative magnitudes push active and reactive power from the grid to the DC bus.

In this implementation, the bi-directional flow is highlighted by a step voltage source that toggles a change in real power flow at 165ms from 5kW flowing from DC bus to grid to 10kW flowing from grid to DC bus. There is a change in reactive power flow from 0 Var to 15kVar at 300ms.

Field orientated control, FOC, is being implemented as we can see Id_ref & Iq_ref are being used to generate gating signals via a dqo-abc conversion.

The generation of Id_ref and Iq_ref is the aspect of this simulation that should be of most interest. The feedback phase currents and voltages are passed through an abc-dqo transformation to resolve the d-axis and q-axis components of the signals. You will see the voltage components are used to generate a signal labeled ‘abs’ which is used along with vd and vq to convert P_ref & Q_ref into d-axis and q-axis values for the current, Id_ref & Iq_ref.

The PI controllers implemented for Id_ref and Iq_ref should be designed before the PQ control is implemented.

Implementation considerations

This simulation showcases the implementation of PQ control, without considering the need to synchronize to the grid to generate theta for the dq transformations. The synchronization aspect has been simplified as theta, labeled in the simulation as ‘wt’, is being generated from a time source and scaled by 2*pi*50. Theta is not being generated by sensing the grid, and the simulation will not work if the grid frequency source is changed.