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Voltage Regulation: Tapping Distributed Energy Resources

Voltage Regulation
Fortnightly Magazine - September 2004

What Feeder Design and Control Is Needed?

From the point of view of voltage stability, existing feeders could be retrofitted to provide local voltage regulation with DER by using a microprocessor that is provided with the needed information. The local microprocessor needs only a small, selected set of data. A relatively simple local microprocessor could be used if a hierarchical control system were used with a group of distributed system managers geographically spread over a wide area. A study by Hydro Quebec of a decentralized approach found that a completely peer-to-peer distributed architecture could be applied without changing the system dynamic operation.6 In fact, improved damping to contingencies was achieved with distributed control. For one contingency, global control using a hierarchical/decentralized architecture was the only scheme capable of keeping the system stable. One of the significant benefits was the enhanced voltage profile at remote weak buses. All this can be accomplished using conventional processor and SCADA technology. The difference is in the placement of the control authority.

New feeders can be designed to provide DER voltage control with and without a hierarchical/decentralized architecture as described above. Obviously, with a hierarchical/decentralized architecture, there are significant advantages in system operation. However, local voltage regulation still could be provided to some degree without distributed control. The challenges, again, will be protection, coordination and voltage stability.

Local control will not require a major replacement of infrastructure. A few sensor locations with communications (such as radio) distributed along the feeder would be needed to advise the central control authority of voltage conditions, but no more locations than are presently used.

Developing Markets for Reactive Power

The New York ISO provides reactive power services at embedded cost-based prices. Generators and synchronous condensers are paid for reactive support based on a calculation involving an annual fixed rate, current capital investment, and operation and maintenance expenses. In addition, if the ISO dispatches a generator to increase reactive power generation and as a result the generator must reduce active power output, the generator receives a Lost Opportunity Cost (LOC) payment for the amount of revenue it loses from the lost generation.

In California, the ISO purchases reactive power from reliability must-run (RMR) units on long-term contracts. The short-term requirement is determined on a day-ahead basis. Daily voltage schedules are issued to contracted generators. For reactive power absorption or generation beyond the limits of 0.9 lagging and 0.95 leading, the generators are compensated, with an additional payment if they are required to reduce their active power output.

In the PJM RTO, voltage control services are paid for with a two-part tariff. In the first part, for reactive power within rated capacity, the customer pays a charge proportional to the generation owner's total revenue requirement and the amount of monthly use of the network. For the second part, generators are paid when they are required to reduce active power production in order to produce reactive power.

Dynamic Reactive Power As A Regulated T&D Resource

There may be several advantages to providing voltage regulation from DER technologies as a regulated T&D resource. These benefits