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The Primary Control Loop block provides setpoints for the Inner Control Loop and aims to satisfy certain system-level specifications. These can range from supplying pre-specified active and reactive power to providing services such as inertia and primary frequency response. The General Structure of the Primary Control Loop contains a power synchronization loop and a voltage profile management. The X/R ratio of the power system the grid-forming inverter is connected with, determines the relationship between active power, reactive power, frequency, and voltage. Typically, at a higher X/R ratio, active power is linked with the frequency, whereas reactive power is correlated with the voltage. With the reduction of the X/R ratio, active power, reactive power, frequency, and voltage become coupled together, and finally the relationship completely reversed in a lower X/R ratio. In most of the power systems (transmission level), the X/R ratio of the lines is normally high. Therefore, in the Primary Control Loop, the grid forming control strategies are based on the direct linkage between active power and frequency/angle and between reactive power and voltage magnitude. Hence, the reference and measurement for the active power are given as inputs to the power synchronization loop, while the reference and measurements for the reactive power are given as inputs to the voltage profile management unit. Three Primary control loop structures, i.e., Droop Control, Virtual Synchronous Generator (VSG) control, and Synchronverter control have been integrated with the Primary Control Loop block. Droop and Virtual Synchronous Generator require an inner control loop, whereas Synchronverter doesn’t require any Inner Control Loop. A short description of the adopted Primary control strategies is given in the following sections.

Mask and Parameters

Mask Parameters vary with the selection of Primary Control Loop Type. If Droop Control is selected as the Primary control, the following information is required by the configuration mask of the Primary Control Loop block.

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If Virtual Synchronous Generator (VSG) Control is selected as the Primary control, following information are required by the configuration mask of the Primary Control Loop block.

If Synchronverter is selected as the Primary control, following information is required by the configuration mask of the Primary Control Loop block.

Inputs, Outputs, and Signals Available for Monitoring

Inputs

Outputs

Description

This block contains three conventionally adopted Primary Control Loop approaches, i.e., Droop Control, Virtual Synchronous Generator (VSG) Control, and Synchronverter Control. Brief descriptions of the adopted Primary Control Loop approaches are as follows.

Droop Control

Droop-controlled grid-forming inverters stem from synchronous generators' self-regulation capability, i.e., decrease the dispatch of active power when the grid frequency increases, and vice versa. Depending on the power references and operating conditions, Droop control sets the references of the angular frequency, phase angle, and magnitude of the virtual voltage source for the inner control loop. The control architecture of the Droop control is shown below.

In this figure,  m_P and m_Q are the droop coefficients for the active and reactive powers, respectively. Furthermore, P_meas and Q_meas are the measured output active and reactive power.  w_ref is the reference angular frequency.

Virtual Synchronous Generator (VSG) Control

grid former inverters controlled by Virtual Synchronous Generator mimic the synchronous generator's dynamic and inertial characteristics. Depending on the power references and operating conditions, the virtual synchronous generator (VSG) control sets the references of the angular frequency, phase angle, and magnitude of the virtual voltage source, for the inner control loop. The control architecture for the virtual synchronous generator is shown below.

The objective of a virtual synchronous generator VSG strategy is to generate the frequency and the angle of the inner source based on the swing equation of a synchronous generator. In this figure, J_s is the moment of inertia and D_P is the damping constant. Note that a virtual synchronous generator VSG is a strategy for controlling the active power (or frequency). For reactive power, droop strategy has been implemented, as shown above.

Synchronverter Control

Similar to virtual synchronous generator VSG, Synchronverter also mimics the behavior of a synchronous generator. However, unlike VSG, Synchronverter does not require any synchronization unit both for pre-synchronization purposes and during normal operation [1]. Therefore, during operation, depending on the power references and operating conditions, synchronverter directly generates the reference control signal in d-q coordinates and overcomes the requirement of the Inner Control Loop. The control architecture for the Synchronverter is given below.

The objective of a Synchronverter strategy is to generate the frequency and the angle of the inner source based on the swing equation and the magnitude of the inner voltage source based on the field electromagnetic equations. In the active power control part (i.e., Figure (a))  J_s is the moment of inertia of all the parts rotating with the rotor, T_e is the electromagnetic torques and D_P  is the damping factor. In Figure (b), M_f is the peak value of the mutual inductance between the rotor and stator windings, and i_f is the field current [2].  M_fi_f is calculated from the reactive power control loop (i.e., Figure (b)). T_e is calculated according to Equation (1). Reactive power does not need to be measured and can be calculated from Equation (2). The emulated three-phase voltages, E, are calculated using (3). The ABC parameter is then transformed into d-q coordinate and acts as a reference control signal.

References

[1] X. W. M. L. X. L. a. S. E. R. Rosso, "Grid-Forming Converters: Control Approaches, Grid-Synchronization, and Future Trends—A Review," IEEE Open Journal of Industry Applications, vol. 2, pp. 93-109, 2021.

[2] Q.-C. Z. a. G. Weiss, "Synchronverters: Inverters that mimic synchronous generators," IEEE Trans. Ind. Electron, vol. 58, no. 4, pp. 1259-1267, Apr. 2011.

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