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.

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 Ppre is the stored pre-disturbance active power, Prated is the rated active power, Pmin is the minimum active power constraint, f is the frequency, kuf and kof are the droop constant for under and over frequency, respectively, and dbuf and dbof 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 Pmin value of 0.2):

Example of the Frequency-Watt curve with kuf = kof =0.05 and dbuf = dbof = 0.036 Hz . The curves are traced for different values of pre-disturbance active power.

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

Name	Unit	Description

Name	Unit	Description
Nominal Frequency	Hz	Nominal grid frequency.
Minimum Active Power	pu	Minimum active power output capacity.
Voltage Support with Reactive Power Control		Constant Power Factor (CPF) - Specify this option to maintain a constant power factor at the point of coupling.
		Voltage-Reactive Power (Volt-VAR) - Specify this function to output reactive power reference as a function of the measured point of coupling voltage.
		Active power- Reactive power (Watt-VAR or PQ) - Specify this function to output reactive power reference as a function of the measured active power output.
		Constant reactive power - Specify this function to output the desired reactive power reference.
Voltage Support with Active Power Control		None - No function is activated.
Voltage Support with Active Power Control		Voltage – Active Power (Volt-Watt) - Specify this function to output active power reference as a function of the measured point of coupling voltage.
Frequency Support with Active Power Control		None - No function is activated.
Frequency Support with Active Power Control		Frequency -Watt (Droop) - Specify this function to output active power reference as a function of the measured point of coupling frequency.
Select Power Priority		Reactive Power Priority - When selected, active power output (Pr) is limited to keep apparent power less than or equal to 1 pu. Accordingly, active power will be curtailed for the required reactive power.
Select Power Priority		Active Power Priority - When selected, reactive power output (Qr) is limited to keep apparent power less than or equal to 1 pu.
Initialization Freeze Time	s	The active power output (Pr) and the reactive power output (Qr) are 1 pu and 0 pu, respectively, during this interval of time directly after starting the simulation.

Voltage Support with Reactive Power Control Tab

Constant Power Factor mode

Name	Unit	Description

Name	Unit	Description
Constant Power Factor Excitation		Specify the constant power factor setpoint to be set as either Under-Excited (VAR Absorption) or Over-Excited (VAR Injection).
PF Limits Injecting [Min Max]		Specify the minimum and maximum power factor limits during over-excited mode (This parameter is activated when the Over-Excited (VAR Absorption) mode is chosen for Constant Power Factor and deactivated when the Over-Excited (VAR Absorption) mode is selected for Constant Power Factor).
Pf Injecting		Specify the power factor injection reference (This parameter is activated when the Over-Excited (VAR Absorption) mode is chosen for Constant Power Factor and deactivated when the Over-Excited (VAR Absorption) mode is selected for Constant Power Factor).
Pf Limits Absorbing [Min Max]		Specify the minimum and maximum power factor limits during under-excited mode (This parameter is deactivated when the Over-Excited (VAR Absorption) mode is chosen for Constant Power Factor and activated when the Over-Excited (VAR Absorption) mode is selected for Constant Power Factor).
Pf Absorbing		Specify the power factor absorption reference (This parameter is deactivated when the Over-Excited (VAR Absorption) mode is chosen for Constant Power Factor and activated when the Over-Excited (VAR Absorption) mode is selected for Constant Power Factor).
Open Loop Response Time	s	The desired open-loop settling time of this function.

Volt - VAR mode

Name	Unit	Description

Name	Unit	Description
Reference Voltage	pu	Specify the voltage reference setpoint (center of the Volt-VAR curve).
Voltage Points	pu	Specify the voltage points of the Volt-VAR curve.
Reactive Power Points	pu	Specify the reactive power points of the Volt-VAR curve.
Enable Autonomous Vref Adjustment		When enabled, the voltage reference (center of the Volt-VAR curve) dynamically changes to the average value of the grid voltage.
Vref Adjustment Time Constant	s	Specify the voltage reference filter time constant. Vref is defined as the moving average of the measured positive sequence voltage at the point of coupling over a user-defined time window. (This parameter is activated when the Enable Autonomous Vref Adjustment is enabled).
Open Loop Response Time	s	The desired open-loop settling time of this function.

Watt - VAR mode

Name	Unit	Description

Name	Unit	Description
Active Power Points	pu	Specify the active power points of the Watt-VAR curve.
Reactive Power Points	pu	Specify the reactive power points of the Watt-VAR curve.
Open Loop Response Time	s	The desired open-loop settling time of this function.

Constant Reactive Power mode

Name	Unit	Description

Name	Unit	Description
Reactive Power Excitation	pu	Specify the reactive power excitation setpoint to be set as either under or overexcited.
Reactive Power Limits Injecting [Min Max]	pu	Specify the minimum and maximum limit for the reactive power injection (This parameter is activated when the Over-Excited (VAR Absorption) mode is chosen for Reactive Power Excitation and deactivated when the Over-Excited (VAR Absorption) mode is selected for Reactive Power Excitation).
Reactive Power Injecting	pu	Specify the reactive power injection reference value (This parameter is activated when the Over-Excited (VAR Absorption) mode is chosen for Reactive Power Excitation and deactivated when the Over-Excited (VAR Absorption) mode is selected for Reactive Power Excitation).
Reactive Power Limits Absorbing [Min Max]	pu	Specify the minimum and maximum limit for the reactive power absorption (This parameter is deactivated when the Over-Excited (VAR Absorption) mode is chosen for Reactive Power Excitation and activated when the Over-Excited (VAR Absorption) mode is selected for Reactive Power Excitation).
Reactive Power Absorbing	pu	Specify the reactive power absorption (This parameter is deactivated when the Over-Excited (VAR Absorption) mode is chosen for Reactive Power Excitation and activated when the Over-Excited (VAR Absorption) mode is selected for Reactive Power Excitation).
Open Loop Response Time	s	The desired open-loop settling time of this function.

Voltage support with Active Power Control Tab

Name	Unit	Description

Name	Unit	Description
Voltage Points [V1 V2]	pu	Specify the voltage points for the Volt-Watt curve.
Active Power Points [P1 P2]	pu	Specify the active power points for the Volt-Watt curve.
Open Loop Response Time	s	The desired open-loop settling time of this function.

Frequency Support with Active Power Control

Name	Unit	Description

Name	Unit	Description
Under-frequency Deadband	Hz	Specify the deadband value for under-frequency.
Under-frequency Droop Slope	% (f_pu/P_pu)	Specify the droop slope for under-frequency.
Over-frequency Deadband	Hz	Specify the deadband value for over-frequency.
Over-frequency Droop Slope	% (f_pu/P_pu)	Specify the droop slope for over-frequency.
Open Loop Response Time	s	The desired open-loop settling time of this function.

Inputs, Outputs, and Signals Available for Monitoring

Inputs

Name	Description	Unit

Name	Description	Unit
Frequency	Measured frequency of inverter output voltage	Hz
Vrms_avg	Average of the RMS value of three-phase grid voltages	pu
P	Measured inverter output active power	pu
Q	Measured inverter output reactive power	pu
Plim	Active power limit setpoint	pu
Disable	Signal used to disable the inverter. When Disable = 1, the power references are forced to 0.	-

Outputs

Name	Description	Unit

Name	Description	Unit
Pr	Active power reference (Commonly used as a limiting value).	pu
Qr	Reactive power reference.	pu

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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HYPERSIM Documentation

GFL Secondary Control Three-Phase