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The wave reference block generates the three-phase reference signal that drives the semiconductor bridge by regulating the real and imaginary components of the instantaneous currents (d and q) according to the input reference current Id and Iq. The wave reference block produces the three-phase reference signal used to drive the semiconductor bridge. This control is achieved by adjusting the real and imaginary components of the instantaneous currents (in the d and q coordinate frames) based on the input reference currents, Id and Iq. The output of the block can be configured to be a duty ratio or instantaneous voltage value. The block contains the current controller and also implements active islanding detection methods (IDM) conjointly with the protection system by modifying the reference current waveform. 

Mask and Parameters

The Wave Reference block contains the following tabs in the configuration mask:

• Configuration

• Current Controller

• Output Filter

Configuration Tab

Current Controller Tab

Open

Output Filter Tab

Islanding Detection Method Tab

This tab is activated when the islanding detection is enabled in the "Configuration" tab.

Inputs, Outputs, and Signals Available for Monitoring

Inputs

Outputs

Description

The Wave Reference block includes a current controller in the DQ frame and an additional loop for islanding detection.

Control System

The following figure shows a simplified control diagram for a grid-connected inverter system:

The amplitude () and phase () of the per-phase current reference (x={a,b,c}) is first calculated from the input peak value of the per-phase real () and reactive () component of the current as follows:

(1)

(2)

A Park transformation is used to rotate the reference frame of the per-phase current reference and to measure the inverter output current using the phase angle estimated by a PLL. The direct and quadrature components of the currents are compared, and the error signals are passed to the current controller (one controller per component). The current controller generates the reference voltage. The direct and quadrature terms are decoupled based on output filter topology. Dynamic saturation using the measured DC-link voltage limits the reference voltage. The reference voltage is then passed to the PWM modulator that drives the semiconductor bridge. If the disable signal is triggered, the output reference is forced to 0, hence blocking the firing of the semiconductor bridge.

The current controller type is selectable. The first option is a typical proportional-integral (PI) type controller. The discrete-time transfer function for the PI current controller implemented in the developed component is given by:

where  is the proportional gain and  is the integral gain of the PI controller. As shown by the transfer function, the Backward Euler integration method is used. Anti-windup using unity back-calculation is also implemented to avoid overshooting when the current reference is changed.

The user may also implement any other current controller using the custom transfer function option. For this purpose, the user may define the controller using a multiple-order transfer function as given below,

Either of the current controllers generates the reference voltage. When the output signal of the wave reference is selected as the inverter voltage, it outputs the balanced three-phase instantaneous waveforms that can be used to control an average converter model. When the output signal is selected as the duty ratio, it outputs the normalized reference signals (balanced) to be used by a modulator to generate the pulses for a switching converter model.

Active Islanding Detection Methods

The following active islanding detection methods are implemented.

• GE voltage scheme

• GE frequency scheme

• GE voltage + frequency scheme

These methods use positive feedback of the measured inverter output voltage magnitude and/or frequency in the direct and quadrature axis of the current control loops to drive away the voltage and/or frequency of the inverter when it is islanded until the protection elements operate.

The GE voltage scheme, as illustrated in the following figure,

The scheme uses the measured inverter output voltage magnitude to implement active islanding detection [1]. The direct axis component of the measured voltage is passed through a second-order band-pass filter, a gain, and a limiter and is added to the direct axis current reference. When the inverter is islanded, a slight deviation of the voltage will result in an increasing output active power, which in turn will increase the voltage further using this positive feedback loop.

The GE frequency scheme uses the same concept with the measured voltage frequency which is passed through a second-order band-pass filter, a gain, and a limiter and is added to the quadrature axis current reference. When the inverter is islanded, a slight deviation of the frequency will result in an increasing output reactive power, which in turn will increase the frequency further using this positive feedback loop.

Limitations

• It is not recommended to feed the wave reference three-phase block with highly unbalanced current as the implemented current controller, which is in the DQ frame, is currently implemented considering the typical balanced system operation.

• For LC and LCL filters, the current controller is implemented for grid-side current control with the assumption that the grid side and converter side current are the same.

References

[1] U.S. National Renewable Energy Laboratory, "Study and Development of Anti-Islanding Control for Grid-Connected Inverters," Golden, CO, 2004.

Intellectual Property Disclaimer

Natural Resources Canada owns all intellectual property rights in the Smart Inverter Modelling Toolbox software and related products.