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 Pmax and Pmin curves. These curves can contain between 2 to 10 points, and up to three independent Pmax and Pmin 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 Pmax (blue) and Pmin (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 (Wmax). 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 Pmax curve, the FSTART frequency will always be the first point in the array, while the FSTOP frequency will be the last. For the Pmin curve, the FSTART frequency will always be the last point in the array, while the FSTOP frequency will be the first.
In general, the Pmax curve is intended to reduce the power being delivered to the grid during over-frequency conditions, while the Pmin 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 Pmax 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 Pmin 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 Pmax curve and above the values specified by the Pmin curve. For power commands less than ±100% Wmax, the PCS will linearly adjust the commanded power from the respective min/max FSTART frequency to the corresponding min/max FSTOP frequency. For example, if the user has commanded 50% power and the frequency exceeds FSTART_MAX, the commanded power will be limited linearly from 50% at FSTART_MAX to -100% at FSTOP_MAX.