In a previous post, I discussed how the Volt/VAR function can be used to provide grid voltage stabilization during over and under-voltage conditions. In addition to stabilizing out of tolerance voltage conditions, over and under-frequency grid conditions can also be mitigated using Frequency/Watt functionality. In a manner similar to Volt/VAR, the Frequency/Watt function will automatically generate real power commands based on grid frequency measurements.

The OZpcs-RS40 implements the Frequency/Watt function using a configurable array of points that are used to specify frequency-based P_{max} and P_{min} curves. These curves can contain between 2 to 10 points, and up to three independent P_{max} and P_{min} curves can be configured. The ability to define three separate curves allows system integrators to pre-define aggressive, moderate, and mild behaviors and switch between them “on-the-fly”, depending on operating conditions.

The following figure illustrates basic Frequency-Watt P_{max} (blue) and P_{min} (red) curves, consisting of two points. Each point is defined by a frequency in Hertz, and a corresponding power limit defined as a percentage of the maximum output power set point (W_{max}). At the endpoints, to the left of the lowest point and to the right of the highest point, the Watt level remains constant (i.e. flat lines). For the P_{max} curve, the F_{START} frequency will always be the first point in the array, while the F_{STOP} frequency will be the last. For the P_{min} curve, the F_{START} frequency will always be the last point in the array, while the F_{STOP} frequency will be the first.

In general, the P_{max} curve is intended to reduce the power being delivered to the grid during over-frequency conditions, while the P_{min} curve will increase the power during under-frequency conditions. Note that battery based energy storage applications have the ability to absorb power. To support this ability the OZpcs-RS40 allows for “negative” power values, as illustrated in the example curves.

Additionally, the power limit is not required to stop at 0%, it may continue beyond the X-axis, as shown in the figure. For example, a P_{max} point (P2) that is configured with a negative value, indicates that for extreme over-frequency conditions, the PCS will be forced to absorb power from the grid. Similarly, a P_{min} point (P1) configured with a positive value indicates that the PCS will be forced to deliver power to the grid for gross under-frequency conditions.

When the Frequency-Watt curves are enabled, the user’s real power command will be clamped below the values specified by the P_{max} curve and above the values specified by the P_{min} curve. For power commands less than ±100% Wmax, the PCS will linearly adjust the commanded power from the respective min/max F_{START} frequency to the corresponding min/max F_{STOP} frequency. For example, if the user has commanded 50% power and the frequency exceeds F_{START_MAX}, the commanded power will be limited linearly from 50% at F_{START_MAX} to -100% at F_{STOP_MAX}.

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.