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Volt/VAR Smart Inverter Functionality

To support grid voltage stabilization during over and under-voltage conditions, UL1741 certified smart inverters, such as the OZpcs-RS40, can be configured to automatically absorb or inject reactive power based on grid voltage measurements. This behavior is commonly referred to as Volt/VAR control and is implemented using a configurable array of points, that when combined, define a linear, piece-wise curve that results in the desired Volt-VAR behavior.

The following figure illustrates a basic Volt/VAR curve that consists of four points. At the endpoints, to the left of the lowest point and to the right of the highest point, the VAR level remains constant (i.e. flat lines). The voltage points are defined as a percentage of the nominal grid voltage. The corresponding reactive power points are defined as a percentage of the inverter’s maximum reactive power set point.

Referring to the figure, when the measured grid voltage lies between V2 and V3, the inverter’s reactive power command is zero. If the grid voltage begins to exceed the upper limit defined by V3, the inverter will begin to absorb reactive power, linearly increasing the reactive power command between V3 and V4.  Once the voltage exceeds V4, the reactive power absorption is clamped at Q4.

On the contrary, when the voltage begins to fall below the lower limit defined by Q2, reactive power is injected into the distribution system to help maintain the nominal voltage.When the voltage falls below V1, the reactive power injected is clamped at Q1.

The OZpcs-RS40 provides the capability of defining three curves, each containing between 1 to 12 points. The ability to configure three curves allows the system designer the flexibility of defining different Volt/VAR behaviors and dynamically switching between them as operating conditions dictate.  The figure below provides an example of aggressive, moderate, and mild Volt/VAR configurations.

The OZpcs-RS40 is a MESA/SunSpec compliant, rack mounted PCS, specifically designed for grid-tied and micro-grid ESS applications.  Download the product brochure (DR-0110) for additional information.