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 progression, meaning 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 constantly positive or constantly 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 – the so-called phase shift – is a value which is easy to measure and characteristic of the relationship between active power and reactive power at the point under consideration in the electric circuit.
Active Power P
With no phase shift between the 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 P is generated. This behavior only occurs when ohmic loads are the only loads in the electric circuit. In a real utility grid, however, the line inductances and capacitances alone result in the active power always being accompanied by a small amount of reactive power.
Reactive Power Q
With a phase shift phi of 90°, the maximum current occurs precisely when the voltage crosses zero, resulting in the power oscillating between positive and negative values. The average over time is therefore zero. This is known as pure reactive power Q (unit: Var, from the French volt-ampère-réactif), which moves “back and forth” in the lines.
Apparent Power S
In real AC circuits, there is a mix of active power and reactive power. This behavior occurs if there are lagging or leading loads in the utility grid. The shift between current and voltage is denoted by the displacement power factor cos φ.
Reactive Power Compensation and Control with Sunny Tripower
Compensating for Reactive Power Demand with Q on Demand 24/7:
Leading or lagging loads (e.g. cables, transformers) require reactive power. Transporting reactive power from the power plant to the load places a burden on the utility grid. It is therefore expedient to install a compensation system that provides reactive power at sites where there are many leading or lagging loads. In order to ensure the stability of the utility grid, grid operators demand that energy producers participate in reactive power compensation. A PV system can complement or replace such a compensation system.
Due to their design, large PV farms have a certain demand for reactive power that can be compensated for with Sunny Tripower inverters. At the same time, Sunny Tripower inverters can make reactive power available for the utility grid. Compensation can take place both during and outside of feed-in operation via the “Q on Demand 24/7” function. Via this function, the Sunny Tripower inverter provides reactive power for the PV farm equipment without placing an appreciable burden on the utility grid.
Controlling the Q(V) Characteristic Curve with Integrated Plant Control:
The Sunny Tripower inverter can provide reactive power to the utility grid with the “Integrated Plant Control” function. The grid operator specifies via which process the inverter is to provide reactive power to the utility grid. In many cases, the grid operator will request control in accordance with a Q(V) characteristic curve.
SMA inverters with “Integrated Plant Control” are capable of reproducing this Q(V) characteristic curve without performing any measurement at the grid-connection point. The inverter can automatically compensate for equipment installed between the inverter and the grid-connection point. The function “Integrated Plant Control” is not capable of compensating for irregular or fluctuating reactive power demands due to, for example, connected machinery, if the machinery is connected between the inverters and the gridconnection point. If the machinery is connected directly at the grid-connection point, it is possible to dynamically determine the additional reactive power demand of the machines using additional measurement equipment and then to provide this value as an offset to the Q(V) control.