IEEE Power & Energy Magazine - March/April 2015 - 32

II
Q
Overexcited
Active Power (W) Consumption

I
Overexcited
Active (W) Power Feed-In
Power Factor (cosϕ)
Limitation (Overexcited)

Reactive Power (var)
Capability at Low or
Zero Power

Apparent Power (VA)
Limitation

Working Area for Fixed Reactive
Power (var) (Overexcited)

Working Area for Power
Factor (Overexcited)

P
Working Area for Power
Factor (Underexcited)

Working Area for Fixed Reactive
Power (var) (Underexcited)

Active Power (W)
Limitation
Power Factor (cosϕ)
Limitation
(Underexcited)
III
Underexcited
Active (W) Power Consumption

IV
Underexcited
Active (W) Power Feed-In

figure 1. A P-Q diagram of an inverter, showing the device's working areas in terms of active and reactive power and different control modes, in the generator reference frame (source: AIT).

Due to their nature as power electronic devices, DER
converters can modify their electrical characteristics through
software controls, advanced function parameter settings, and
local measurements to adjust their active and reactive power
output by autonomously responding to local grid conditions,
external signals, or schedules. Technically speaking, their

Normalized
Reactive Power Q/Qmax

Positive Q
Overexcited

Negative Q
Underexcited

Exemplary Volt/Var Curve
V 1/Q 1

Vmin

Vnom
V 2/Q 2

V 3/Q 3

Vmax
Grid Voltage
V/Vn

V 4/Q 4
Normal Operating Voltage
Range of DERs, According
to Interconnection
Specifications

figure 2. An exemplary curve used for volt/var control,
based on voltage and var pairs (source: AIT).
32

ieee power & energy magazine

controllability is limited only by the basic capabilities of the
device and the availability of the primary power source.
The fundamental capabilities of a DER inverter to provide
active and reactive power are commonly described by its socalled P-Q diagram. Figure 1 shows the possible working
areas of the device in a four-quadrant view and the limits in
terms of maximum and minimum active and reactive power
that can be delivered. The DER inverter is limited in software
to operate within a power circle to avoid exceeding the current rating of the power electronics.
Inverters designed for use in generation systems are normally capable of operating only in quadrants II and III (the
consumer reference frame), while DER inverters for use with
storage systems are designed to work in all four quadrants
because they can consume active power from the grid for
charging electrical storage. Theoretically, a power electronic
inverter would be capable of operating anywhere within the
semicircle of these quadrants. Device capabilities are constrained in software to the pie-slice-shaped working area
region in power factor (PF) mode and a rectangular area for
var operation mode. Additional constraints typically apply
for the operation in fixed-var mode at very low or even zero
active power feed-in, indicated by the shaded gray area in
Figure 1. Within its working areas, the inverter is free to
march/april 2015
Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - March/April 2015
