How Electric Operators Maintain Acceptable Voltage
by Bob Shively, Enerdynamics President and Lead Facilitator
A key factor in reliable electric grid operations is to keep voltages within acceptable tolerances. Failure to do so may result in damaged equipment, customer complaints, and/or outages. As the grid evolves to integrate more distributed resources, it is important to understand basic principles on how voltage is managed.
Unlike frequency which is the same throughout the system, voltage varies across the grid. At each key interface, voltage must also be maintained within acceptable values. Each point along the grid will have a desired voltage called V scheduled, and a maximum and minimum allowed voltage called V max and V min, respectively.
Often a tolerance of 5% from scheduled voltage is considered acceptable. On the transmission system, greater variances are sometimes acceptable. If voltage is too high, protective breakers will open to prevent damage to equipment, causing portions of the grid to lose power. If voltage is too low, distribution utilities may be unable to maintain voltage to their customers, and customer equipment will not operate properly and/or lines will drop offline causing outages.
Voltage changes along a transmission or distribution line based on uncontrollable factors including impedance of the line, line loading, and reactive power consumption by consumers connected to the line. Typically, voltage drops are based on distance from a substation or other source of power.
Line voltage may be controlled through two key methods:
- Devices that inject or absorb reactive energy
Transformers are placed in substations and at key locations along electric lines. Many transformers provide static voltage changes, but the amount of change can be adjusted by installation of devices called tap changers. Tap changers can be adjusted manually or automatically. Transformers are often used on long lines as voltage regulators.
Voltage on lines is also managed using various sources of reactive power including capacitors, static VAR compensators (SVC), and synchronous condensors.VAR compensators and synchronous condensors can typically be continuously adjusted. Capacitors are controlled by being switched in or switched out. This can be done manually or automatically. In a region where generation exists, generation sources can often be managed to produce more or less reactive power.
When distributed energy resources (DERs) are installed on distribution lines, they may change the expected voltage profile by injecting current at locations that historically only consumed electricity. This requires distribution engineers to redesign voltage regulation on the circuit. In some cases, DERs can assist in voltage regulation by providing control of reactive power output through smart inverters. An additional important factor associated with DERs is how they behave during temporary drops in voltage. It is now recommended that DERs include low-voltage ride-through capability so that DERs stay online during short voltage incursions. Failure to do so may cause additional system issues as DERs suddenly go offline.
While voltage can be a confusing issue, utility professionals must have a basic understanding of the principles of voltage regulation as the utility grid becomes increasing distributed. System operators and designers must learn new techniques for managing voltage, and customers installing DERs will need to understand why interconnection rules include requirements associated with voltage support.