Introduction

This example implements the control for a three-phase central PV inverter. The system uses a boost in order to interface the PV panel with the intermediate DC bus. As such, the system is similar to that of AN003, except that the inverter is a three-phase variant.


Downloads

Two sets of files are proposed, suitable for implementing the control and simulating its behavior in MATLAB Simulink or Plexim PLECS environment.

Simulink model

PLECS model

Minimum requirements:

  • Imperix ACG SDK 3.5.1.1 or newer.
  • For control code development in Simulink:
    • MATLAB Simulink R2016a or newer.
    • Plexim PLECS VIEWER 4.4.2 or newer (free)
  • For control code development in PLECS:
    • Plexim PLECS 4.4.2 or newer. The HIL version also required the PLECS_RT_Box target support package.

Control implementation

The overall control implementation corresponds to the following choices:

Grid-side control

  • Synchronization with the grid is made using a conventional quadrature PLL.

  • The grid current control is achieved using a conventional vector current control in stationary reference frame (dq). The corresponding technique is detailed in TN106.

  • The DC bus voltage is controlled from the grid side, using a cascade on top of the current control. A conventional PI controller is used. More details on the approach are available in TN108.

PV-side control

  • On the PV side, a basic current control is implemented (see details in TN105). The current reference is set from a Maximum Point Point Tracking algorithm, as detailed in TN117.

Precharge and operation state machines

  • The DC bus precharge and the operation of the converter stages is managed by a Finite State Machine.
  • The user action variable is "activate". When set to 1, the precharge of the DC bus from the grid starts. The Precharge state machine closes the precharge relay and waits until the DC bus reaches its minimum value. Then, it closes the bypass relay and outputs "Ready to operate".
  • Once the Precharge state machine indicates "Ready to operate", the Operation state machine activates the converters and closes the PV-side relay. This starts the MPPT algorithm (boost current control) and and the cascaded DC bus and grid current regulations.

Screenshots of the control implementation in both Simulink and PLECS are shown below:

Simulation results

The example models available for download at the top of this page have the following simulation scenario, which is illustrated by the simulation results below:

  • At t=0, the DC bus is already precharged at 500V and the PWMs are not yet activated.

  • At t=0.1s, the "activate" variable is set to 1. The PWM are immediately activated (FSM is bypassed in simulation), the DC bus voltage is regulated at 750V, and the MPPT algorithm converges to gradually to the MPP (corresponding to a PV current reference around 18A).
    After the MPPT algorithm has converged (after t=0.22s), the Perturb and Observe algorithm gives a PV current reference that oscillates around the MPP current.

  • At t=0.5s, the DC voltage reference is reduced to 700V.

  • At t=0.8s, the solar irradiance is reduced (simulation of passing clouds). The MPPT algorithm gradually decreases the PV current reference and reaches the new MPP at around t=1s (corresponding to a PV current reference around 15A).

HIL simulation results

HIL simulation allows further tests on the Precharge and Operation state machines. The following results are obtained when executing the control code on a B-Box RCP, and the plant model on a RT-Box.

The figure below shows the evolution of the DC bus voltage during the precharge, when starting from an empty bus. The point (1) corresponds to the user changing the variable "activate" from 0 to 1. This has the effect of closing the precharge relay and the DC bus is charged from the grid, through the precharge resistors and the converter diodes. The point (2) is where the voltage reaches the minimum bus voltage, which closes the bypass relay. 200 ms later, at (3), the boost and inverter stages are activated and the DC bus is regulated at 700V.

During that procedure, the grid currents are shown below. At steps (1) and (2), they are rectified currents flowing through the converter diodes, and after (3), during normal operation, they are sinusoidal currents with d- and q-axis components controlled by the algorithm.

During normal operation, the grid current amplitude will depend on the power produced by the PV panel. The figure below shows the effect of a step in the solar irradiance on the grid current amplitude:

Quick-start guide

An obvious prerequisite of grid-tied operation is that all necessary precautions are used in order to make the flow of uncontrolled currents through the inverter diodes impossible. This notably requires that the DC bus is pre-charged by suitable means before the connection to the grid. Useful information regarding how to pre-charge the DC bus in grid-tied applications is given in TN131.

Don’t forget to properly configure protection thresholds on the B-Box RCP before starting experimental activities. The related documentation can be found in PN105.

Required hardware

Experimentation on this Application Note can be made using standard imperix equipment:

  • 1x “Type C” rack with 4x PEB8024 modules

  • 1x passive filters rack or:

    • 1x 2.5mH inductor (Lg)

    • 1x 2.5 mH inductor (Lb)

  • 4x DIN800V voltage sensors

In addition, the following equipment is needed:

  • 1x PV panel or PV panel emulator

  • 1x AC disconnector, or better controllable relay as well as a circuit breaker.

  • 1x isolation transformer (if panel is grounded)

  • (1x pre-charge circuit - optional, not shown)

I/O signals

Controller inputs (sensors)

Analog input channelMeasured quantitySensor
AI0PV current (Ipv)embedded on PEB8024
AI1PV voltage (Vpv)DIN800V
AI2DC bus voltage (Vdc)DIN800V
AI3-5Grid voltages (Vg_abc)embedded on PEB8024
AI6-8Grid currents (Ig_abc)embedded on PEB8024

Controller outputs (PWM signals)

PWM channel (lanes)PWM signals
CH0 (LN0-1)Boost converter (high-side signal for reverse conduction in the MOSFET) 
CH1 (LN2-3)Grid-tied inverter, phase a
CH2 (LN4-5)Grid-tied inverter, phase b
CH3 (LN6-7)Grid-tied inverter, phase c

Start-up procedure

The following steps can be used as a brief reminder of a reasonable start-up procedure:

1) Before addressing this application, run a simpler control code in order to validate that:

All measurements are correctly acquired, with the appropriate gain and sign (notably currents).

Protections are correctly configured and do trigger at the appropriate value.

The switched voltage (EMF) at the converter-side of the grid inductor has no obvious phase mismatch with the grid voltage (e.g. operating in open-loop while disconnecting the inductor). There are indeed numerous risks of polarity errors…

2) Pre-charge the DC bus to a sufficient voltage to avoid any risk of uncontrolled diode conduction. In single-phase application, the minimum DC bus voltage is defined by .

Double-check this condition.

3) Physically connect the converter to the grid, either using a controllable relay or using manual wiring.

4) In BB Control, enable PWM operation. Related getting-started instructions are available in PN137.

Experimental results

tbc

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