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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 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.

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

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