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The secondary control block contains the logic to implement grid support functions (GSF) considering the specifications from IEEE Std 1547-2018 [1]. Therefore, it can be applicable to implement grid support functions from other grid codes that use similar specifications. It generates the active and reactive power references used by the primary control loops. Typically, the active power reference is used as a limiting value for active power injection. The secondary control block allows the user to select which grid support functions are activated and define their respective parameters. The GSF are separated into three categories: Voltage Support with Reactive Power Control, Voltage Support with Active Power Control, and Frequency Support with Active Power Control. This structure allows to have multiple grid support functions operating simultaneously while also ensuring their mutual exclusivity. 

Mask and Parameters

The Secondary Control block contains the following configuration tabs:

• General

• Voltage Support with Reactive Power Control

• Voltage support with Active Power Control

• Frequency Support with Active Power Control

General Tab

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Voltage Support with Reactive Power Control Tab

The subtab and its corresponding parameters displayed on this tab vary depending on the choice made for 'Voltage Support with Reactive Power Control'. Four different subtabs and their parameters appear based on the choice of 'Voltage Support with Reactive Power Control'.

When 'Voltage Support with Reactive Power Control' is configured as 'Constant Power Factor (CPF),' the subtab labeled 'Constant Power Factor' and its associated parameters are shown.

When 'Voltage Support with Reactive Power Control' is configured as 'Voltage-Reactive Power (Volt-Var),' the subtab labeled 'Volt - VAR' and its associated parameters are shown.

When 'Voltage Support with Reactive Power Control' is configured as 'Active Power - Reactive Power (Watt-Var or PQ),' the subtab labeled 'Watt - VAR' and its associated parameters are shown.

When 'Voltage Support with Reactive Power Control' is configured as 'Constant Reactive Power,' the subtab labeled 'Constant Reactive Power' and its associated parameters are shown.

Voltage support with Active Power Control Tab

The parameters on this tab become active when the parameter 'Voltage Support with Active Power Control' is set to 'Voltage - Active Power (Volt-Watt),' otherwise, they are deactivated.

Frequency Support with Active Power Control

The parameters on this tab become active when the parameter 'Frequency Support with Active Power Control' is set to 'Frequency- Watt (Droop),' otherwise, they are deactivated.

Inputs, Outputs, and Signals Available for Monitoring

Inputs

Outputs

Description

The following sections describe every grid support function (GSF) implemented in the secondary control component.

Constant Power Factor (CPF)

The CPF function calculates the reactive power reference to maintain a constant power factor at the point of coupling (PoC). It is calculated based on the power factor setpoint and measured active power. The user must define the constant power factor excitation setpoint, its excitation (VAR absorption or injection), its limits, and the function open-loop response time. The function response time is defined as the duration from a step change in control signal input until the output reaches 90% of its final value. It is achieved using a first-order low-pass filter with its time constant adjusted to meet this requirement and is limited to a maximum value of 10 seconds.

Voltage- Reactive Power (Volt-VAR)

When activated, the Volt-VAR function outputs the reactive power reference as a function of the measured PoC voltage, based on a user-defined piecewise linear curve as shown in the following figure:

 The default curve uses values as specified in [1]. The function also supports autonomous voltage reference (Vref) adjustment, where the curve characteristic is automatically adjusted around Vref. Vref is defined as the moving average of the measured positive sequence voltage at the PoC over a user-defined time window. The function response time is configurable and implemented as explained previously.

Active Power- Reactive Power (Watt-VAR or PQ)

When activated, the Watt-VAR function outputs the reactive power reference as a function of the measured active power output, based on a user-defined piecewise linear curve as shown in the following figure:

The first active power point of the curve is limited to the greater of 0.2*Prated or Pmin, per [1]. The function response time is configurable and implemented as explained previously and is limited to a maximum value of 10 seconds.

Constant Reactive Power (Cons. Q)

When activated, the constant reactive power function outputs the desired reactive power reference. The user must define the reactive power setpoint, its excitation (VAR absorption or injection), its limits and the function response time. The function response time is implemented as explained previously and is limited to a maximum value of 10 seconds.

Voltage -Active Power (Volt-Watt)

When activated, the Volt-Watt function outputs the active power reference as a function of the measured PoC voltage, based on a user-defined piecewise linear curve as shown in the following figure:

The first active power point of the curve is set to Prated (1 p.u.) and the second point is limited to the lesser of 0.2*Prated or Pmin, per [1]. The function response time is configurable and implemented as explained previously and is limited to a maximum value of 60 seconds. It is important to note that the power reference Pr is defined as a limiting reference value. This implies that the active power output still depends on the available power in the DC link.

Frequency -Watt (Droop)

When activated, the Frequency-Watt function outputs the active power reference as a function of the measured PoC voltage frequency, based on available active power, pre-disturbance measured active power, and a deadband around the nominal frequency as defined in [1]. When the frequency is inside the defined deadband, the Pr output of the secondary block is the active power limit, which is defined by the Plim input and by the power priority configuration. When the frequency moves outside the deadband, the Pr output signal is a limiting value defined by the droop curve. The Pr signal is defined as follows [1]:

where  is the stored pre-disturbance active power,   is the rated active power,  is the minimum active power constraint,  is the frequency,  and are the droop constant for under and over frequency, respectively, and and   are the lower and upper limits of the deadband, respectively. An example of the Frequency-Watt curve for different pre-disturbance values is shown as follows (with value of 0.2):

Example of the Frequency-Watt curve with and Hz . The curves are traced for different values of pre-disturbance active power.

Limitations

Although the Secondary Control block widely covers the GSF specified in [1], there are some limitations that need to be considered by the user for this release of the library:

• For constant functions such as CPF and Cons. Q, the reference can only be set on the configuration mask. However, the user can modify the subsystem block by disabling the library link and adding an external input block as shown:

References

[1] IEEE, "IEEE Standard for Interconnection and Interoperability of Distributed Energy Resources with Associated Electric Power Systems Interfaces," IEEE Std 1547-2018 (Revision of IEEE Std 1547-2003), no. doi: 10.1109/IEEESTD.2018.8332112, pp. 1-138, 2018.

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