1 Definition of Active Power, Reactive Power and Apparent Power
SMA Solar Technology AG
2
IPC_STP-TI-en-14
Technical Information
This document provides basic information on reactive power and how to set the inverter in order to feed reactive power
into the utility grid in compliance with standards and demand.
1 Definition of Active Power, Reactive Power and Apparent Power
Electrical power is the product of current and voltage. While current and voltage have stable values with direct current,
the strength and the direction of both current flow and voltage change regularly in alternating current. In the utility grid,
current and voltage have a sinusoidal shape, so that their product, electrical power, is also sinusoidal. In DC systems, the
sign of the power indicates the direction in which the electrical energy, in the form of active power, is transported.
In general, this also applies in an AC circuit. However, the power may not always be positive or negative, but rather its
sign can fluctuate periodically, causing the power to oscillate back and forth. This oscillating power does not do any work
and is therefore referred to as reactive power. The time delay between the current and voltage curve -- its so-called phase
shift -- is a value which is easy to measure and characteristic of the relation between active power and reactive power at
the point under consideration in the electric circuit.
1.1 Active Power P
With no phase shift between progression of current i(t) and voltage v(t) over time, both always have the same sign and
simultaneously reach their maximum and minimum values. The power oscillates between zero and the positive maximum
value. Averaged over time, this results in a positive power value P (unit: W; Watt) and only active power is generated P.
This behavior occurs only when ohmic loads are the only loads in the electric circuit. In a real utility grid, however, cable
inductance and capacitance ensure that the active power is always accompanied by a small amount of reactive power.
Figure 1: Pure active power: current and voltage are in phase
STP 12000TL-US-10 /
STP 15000TL-US-10 /
STP 20000TL-US-10 /
STP 24000TL-US-10 /
STP 30000TL-US-10
from firmware version
2.80.00
from firmware version
2.80.00
from firmware version
2.80.00
Device type
Reactive power provision
0
overexcited
to 0
underexcited
Q on Demand 24/7
Integrated Plant Control
