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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 GSFs from other grid codes that uses 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 GSF 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 GSF 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

  

NameDescriptionUnit

Nominal Frequency

Nominal grid frequency

Hz

Minimum Active Power

Minimum active power output capacity

pu

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 – 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 -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

-

Active Power Priority

When selected, reactive power output (Qr) is limited to keep apparent power less than or equal to 1 pu

-

Sampling Time

Specify the timestep for solving equations inside the secondary block

s

Initialization Freeze Time

The Pr and Qr outputs are 1 pu and 0 pu, respectively, during this interval of time directly after starting the simulation.

s

Voltage Support with Reactive Power Control Tab

 


 


NameDescriptionUnit
Constant Power Factor

Constant Power Factor Excitation

Specify the constant power factor setpoint to be set as either under-excited (reactive power absorption) or over-excited (reactive power injection)

-

PF Limits Injecting [Min Max]

Specify the minimum and maximum limits during over-excited mode

-

Pf Injecting

Specify the power factor injection reference

-

Pf Limits Absorbing [Min Max]

Specify the minimum and maximum limits during under-excited mode

-

Pf Absorbing

Specify the power factor absorption reference

-

Open Loop Response Time

Desired open-loop settling time of this function

s

Voltage-Reactive Power (Volt-VAR)

Reference Voltage

Specify the voltage reference setpoint (center of the Volt-VAR curve)

pu

Voltage Points

Specify the voltage points of the Volt-VAR curve

pu

Reactive Power Points

Specify the reactive power points of the Volt-VAR curve

pu

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

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

s

Open Loop Response Time

Desired open-loop settling time for the transient response of this function

s

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

Active Power Points

Specify the active power points of the Watt-VAR curve

pu

Reactive Power Points

Specify the reactive power points of the Watt-VAR curve

pu

Open Loop Response Time

Desired open-loop settling time of this function

s

Constant Reactive Power

Reactive Power Excitation

Specify the reactive power excitation setpoint to be set as either under or overexcited

-

Reactive Power Limits Injecting [Min Max]

Specify the minimum and maximum limit for the reactive power injection

pu

Reactive Power Injecting

Specify the reactive power injection reference value

pu

Reactive Power Limits Absorbing  [Min Max]

Specify the minimum and maximum limit for the reactive power absorption

pu

Reactive Power Absorbing

Specify the reactive power absorption in per unit

pu

Open Loop Response Time

Desired open loop settling time for the transient response  of this function

s

Voltage support with Active Power Control Tab

NameDescriptionUnit

Voltage Points [V1 V2]

Specify the voltage points for the Volt-Watt curve

pu

Active Power Points [P1 P2]

Specify the active power points for the Volt-Watt curve

pu

Open Loop Response Time

Desired open-loop settling time for the transient response of this function

s

Frequency Support with Active Power Control

NameDescriptionUnit

Under-frequency Deadband

Specify the deadband value for under frequency

Hz

Under-frequency Droop Slope

Specify the droop slope for under frequency

% (f_pu/P_pu)

Over-frequency Deadband

Specify the deadband value for over frequency

Hz

Over-frequency Droop Slope

Specify the droop slope for over frequency

% (f_pu/P_pu)

Open Loop Response Time

Desired open-loop settling time for the transient response of this function

s

Inputs, Outputs and Signals Available for Monitoring

Inputs

NameDescriptionUnit

P

Measured inverter output active power

pu

Q

Measured inverter output reactive power

pu

Vrms_avg

Average of the RMS value of three-phase grid voltages in per unit

pu

Freq

Measured frequency of inverter output voltage

Hz

Plim

Active power limit setpoint

pu

Disable

Signal used to disable the inverter. When Disable = 1, the power references are forced to 0.

-

Outputs

NameDescriptionUnit

Pr

Active power reference (Commonly used as a limiting value)

pu

Qr

Reactive power reference

pu

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

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