Voltage support is the provision or absorption of reactive power to the grid to maintain acceptable voltage. Transmission and distribution operators must inject appropriate amounts of reactive power into the grid due to resistive losses along transmission and distribution lines and due to consumption of reactive power by consumers. Voltage support is required on the bulk electric system so that acceptable voltage is provided at each distribution substation connecting to the transmission system. It is also required on the distribution system so that the distribution utility can maintain acceptable voltage to customers.
On the bulk electric system, voltage support is commonly provided by generators that have the capability to adjust the ratio of reactive power to real power provided to the grid. Many types of generators have this capability. Voltage support for the bulk electric system can also be provided by synchronous condensors, capacitor banks, and grid-scale batteries.
On distribution grids, voltage support is provided by a variety of utility assets. These include capacitors, resistors, and batteries at distribution substations and line regulators and switched capacitor banks on distribution lines. Voltage is also managed using transformers including transformers with tap changes (although use of transformers utilizes a different principle of electricity than provision of reactive power). Voltage support can also be provided by third-party or customer-owned assets such as batteries and smart inverters.
In vertically integrated wholesale markets, voltage support for the bulk power system is provided by utility resources under regulated cost-based tariffs. In competitive markets, the bulk system operator (the Independent System Operator or Transmission System Operator) contracts for voltage support services and the costs are charged out to loads. Voltage support on the distribution grid is managed by the distribution utility, typically using utility assets. In the future distribution utilities may also make use of third-party or customer-owned assets.