CPS TECHNICAL BULLETIN 
Apparent & Reactive Power Capabilities
November 2018

This CPS Technical Bulletin highlights the complexity of utility requirements for inverter functionaility and provides information on CPS Apparent & Reactive power capabilities.

Power Triangle

As power is transferred along a transmission line, it does not consist purely of real power that can do work once transferred to the load, but rather consists of a combination of Real and Reactive power, called Apparent power. Where Reactive loads are present, such as with capacitors or inductors, energy storage in the loads results in a phase difference between the current and voltage waveforms. 

Power factor describes the amount of Real power transmitted along a transmission line relative to the total Apparent power flowing in the line. The power factor of an AC electrical power system is defined as the ratio of the Real or Active power absorbed by the load to the Apparent power flowing in the circuit. A power factor of less than one indicates the voltage and current are not in phase, reducing the instantaneous product of the two.

Definitions

Real or Active Power; “P” Measured in kilowatts ( kW ). It does the “work” for the system. 

Reactive Power: “Q” Measured in kilovolt-amperes-reactive ( kVAR ), doesn't do useful “work”. It simply sustains the electromagnetic field. 


Apparent Power: “S” Measured in kilovolt-amperes ( kVA ). Apparent Power is the vector-sum of Active Power and Reactive Power. 


Power Factor: “PF” is the ratio of Real or Active Power to the Apparent Power. PF = cos(φ).


Power Factor

Tight synchronization with the grid by ensuring the inverter output current is in phase with the voltage is essential for seamless delivery of power by grid-tied PV inverters. At the heart of grid-synchronization methods, PLL (Phase Lock Loop) algorithms can ensure proper operation despite adverse operating conditions, capacitive/inductive loads, even grid faults. For CPS String inverters, an integrated DSP controller with on-chip ADCs and SVPWM signal along with a sophisticated PLL algorithm and control mechanism ensures effective grid synchronization of the inverter output current to the grid voltage.This tight synchronization to the grid voltage enables CPS inverters to operate at unity power factor of > 0.99, yet also offers them the ability to operate at leading or lagging power factor of up to 0.8 PF when required or mandated by the Area EPS operator.

"kVA Headroom"

CPS String inverters, specifically the 50kW, 60kW, 100kW, and 125kW models, have a Selectable Apparent power (kVA) rating. They are delivered from the factory configured with the default maximum Apparent power rating set equal to the Active power (kW) rating. A password protected parameter that is stored in nonvolatile memory enables the CPS String inverters to be configured for a maximum Apparent power rating of 55kVA, 66kVA, 111kVA, and 132kVA. This is colloquially referred to as " kVA Headroom ".
With “ kVA Headroom ” the 66kVA rated inverter has a maximum AC current limit of 79.4Arms and can deliver full rated “P” (60kW) up to 0.91PF. 
CPS understands that the Selectable Apparent power rating may not be needed or desired for all installations and is precisely the reason CPS made it configurable. When the maximum Apparent power rating is left to the default configuration of 60kVA, the maximum AC output current limit is set to 72.2Arms. When the maximum Apparent power is set to 66kVA, the maximum AC output current limit is set to 79.4Arms, which will affect conductor, OCPD, and XFMR sizing.

As shown in the figure below, when the kVA Headroom or maximum Apparent power rating is set/configured for 66kVA, the CPS 60kW String inverter is capable of providing Reactive power (kVArs) up to a leading or lagging PF of 0.91 before the current limit is reached and the Active power (kW) production begins to reduce. Conversely, without kVA Headroom, when the maximum Apparent power rating is set/configured for 60kVA, the CPS 60kW String inverter will immediately begin to reduce the Active power (kW) production when providing Reactive power (kVArs) or operating under a non-unity PF.

Grid Support

Many utilities have begun to require or mandate that DERs (Distributed Energy Resources), ie grid-tied PV inverters, operate with a lagging PF and deliver Reactive power (absorbing VArs) to offset AC BOS inductance on the feeder and minimize/mitigate voltage rise. With the introduction of Rule 21 in California requiring the Grid Support autonomous Volt-VAr function, and in general higher PV penetration in the USA, delivery of Reactive power by CPS String inverters from unity PF to 0.91 PF (leading or lagging) while still enabling full Active power improves utility acceptance and project ROI.
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If you have any questions about CPS products or applications related to the topics in this bulletin, please email: Carlos Abad

CA Rule 21 Volt-Var Curve