Voltage Control Using Inverter Reactive Power Control

This is the fifth of five articles in the series “Reactive Power in Utility-Scale Solar PV Applications.” Check out the previous articles linked below.

1. What Are Reactive Power and VArs?
2. Reactive Power and Transmission & Distribution Operations
3. Inverter-Based Resources Reactive Power Capabilities
4. Active Power Priority vs. Reactive Power Priority

Key points

• Modern smart inverters can be configured in one of four reactive power control modes to help manage voltage on the grid
• The power system operator — not the solar owner — specifies which control mode and settings to use
• Constant power factor mode (power factor = 1.0) is the default per IEEE 1547-2018
• Volt-VAr mode is the most effective for active voltage control but requires engineering studies before use
• Understanding reactive power requirements before commissioning is critical — voltage control can have significant impact on grid operation

In the previous four posts in this series, we discussed what reactive power is and where it comes from, its impact on T&D systems, and inverter-based resources’ capabilities for reactive power injection and absorption.

As mentioned in Blog #2 of this series, Distributed Energy Resources (DERs) impact voltage on feeders, no matter how small they are.  For small DER penetration, the voltage impacts are relatively small.  For higher penetrations and utility-scale DER, the potential for adverse voltage impacts becomes greater.  The capability of DER to help control these voltage changes on the power system becomes important.

In this post, we’ll look at four reactive power control modes that can be selected in modern smart inverters to control inverter reactive power production (or absorption) and subsequently voltage where the plant connects to the system.

1. Constant power factor mode
2. Voltage-reactive power (“Volt-VAr”) mode
3. Active power-reactive power (“Watt-VAr”) mode
4. Constant reactive power mode

It is appropriate to note that the distribution or transmission operator of the system to which the DER is connected will specify which control mode, if any, and the settings of that control mode. 

1. Constant power factor mode

In constant power factor mode, the inverter changes its reactive power injection (or absorption) in proportion to the inverter’s real power such that power factor remains constant.   At higher real power production the inverter produces (or absorbs) higher reactive power, with the converse at lower real power production.

The power factor setting of many smart inverters is adjustable from + 0.8 to 1.0.  According to IEEE 1547-2018, constant power factor mode with 1.0 power factor is the default reactive power control mode.

2. Voltage-reactive power (“Volt-VAr”) mode

In this mode, the solar PV system adjusts its reactive power injection (or absorption) based on the actual voltage, if the actual voltage is outside of a specified dead band.

A typical control characteristic of Volt-VAr mode is shown in the figure.  The points on the characteristic diagram are all configurable through settings and are specified by the power system operator.

Basically, if the voltage is too high and outside of the dead band, the inverter absorbs reactive power.  This has the impact of reducing the voltage.

If the voltage is too low and outside of the dead band the inverter injects reactive power, like a shunt capacitor on the system.  This has the impact of raising the voltage.

This capability is quite effective, but engineering studies are usually performed by the power system operator to ensure that Volt-VAr control does not interfere with other voltage control equipment on the system.

3. Active power-reactive power (“Watt-VAr”) mode

In this mode, the inverter adjusts its reactive power based on the real power output.  An example characteristic is shown in the figure.

For the characteristic in the figure, during relatively high levels of real power output, the inverter will absorb reactive power, which has a downward influence on the voltage.

During relatively low levels of real power output, the inverter operates at zero reactive power (or power factor equal to 1.0), and the power system must neither supply or absorb reactive power from the inverter.

4. Constant reactive power mode

In this mode, the inverter either injects or absorbs a constant amount of reactive power, independent of real power output, depending upon its setting.  The inverter appears to be either a shunt capacitor (injection) or a shunt inductor (absorption).  The inverter therefore appears to the power system as the  static capacitor or inductor equipment which engineers are familiar with.

Summary

There are many other details involved in specifying these reactive power control modes.  These may include default values and ranges of allowable settings, as specified by the power system operator. 

It is very important that utility-scale solar PV owners/operators/engineers are aware of the requirements that the power system operator has for reactive power control before enabling it, as voltage control can have significant impact on power system operation.

Tim Taylor is the founder of Electric Distribution Academy, and all his courses are hosted exclusively on HeatSpring.  Tim is the instructor for several courses focused on utility distribution, including “Interconnection of Utility-Scale Solar PV to Distribution” and “Understanding IEEE 1547-2018 – Interconnection Standard for DER on Distribution.

Frequently asked questions about inverter reactive control

What is reactive power control in a solar inverter?

Reactive power control in a solar inverter refers to the inverter’s ability to inject or absorb reactive power in addition to producing real power. This capability allows utility-scale solar PV systems to help manage voltage levels on the grid where the plant connects.

Who decides which reactive power control mode is used?

The distribution or transmission operator of the system to which the solar plant connects specifies which reactive power control mode to use, if any, as well as the settings for that mode. Solar owners and operators must understand these requirements before enabling any voltage control functions.

What is Volt-VAr mode?

Volt-VAr mode is a reactive power control mode in which the inverter adjusts its reactive power output based on actual voltage at the connection point. If voltage rises above a set threshold the inverter absorbs reactive power to bring it down. If voltage drops below a threshold the inverter injects reactive power to raise it.

What is the default reactive power control mode per IEEE 1547-2018?

Per IEEE 1547-2018, constant power factor mode with a power factor of 1.0 is the default reactive power control mode for inverter-based resources connected to distribution systems.

What is the difference between Volt-VAr and Watt-VAr mode?

In Volt-VAr mode, the inverter’s reactive power output is determined by the actual voltage at the connection point. In Watt-VAr mode, reactive power output is determined by the inverter’s real power output level, independent of voltage. Both modes adjust reactive power automatically, but they respond to different inputs.