Mid-American Power, LLC has bought a 53-Mw, coal-fired generating plant, put it on the power grid, and plans to convert the facility into a 300-Mw, gas-fired, combined-cycle plant. Mid-American...
Voltage Regulation: Tapping Distributed Energy Resources
Reactive power is the key to an efficient and reliable grid.
Traditionally, the role of the distribution system is to provide the interconnection between the generation and transmission system and industrial, commercial, and residential load centers. The distribution systems generally can be considered to be passive networks- that is, they do little to dynamically regulate voltage. In contrast, the transmission system operator must deal with voltage problems that arise from a number of power-system events such as lost load, line or cable outages, dropped generation, capacitor bank outages, heavy power transfers, parallel flows, or unusually high or low load demands. The primary voltage control methods available to the operator include increasing or decreasing generation and adding or adjusting sinks or sources of reactive power in the system. In the future, real-time regulation of voltage at the customer's own buses may be best performed using local sources of active and reactive power, or distributed energy resources (DER). Local regulation is much more efficient with local sources, and the DER can supply precisely the level of regulation needed. In some areas it may be most economical for the distribution utility to supply only a nominal level of reliability, and the reliability would be elevated to the customer's requirements with DER.1 Within the customer's distribution system, some buses may be designated for critical or sensitive loads, and some may be for loads that could be reduced or shed if needed to maintain correct voltage at the critical loads. This concept already is occurring in some parts of the country.
What about the reliability of power from local sources? Lambda is an index used to assess reliability by measuring the frequency of sustained outages. In a recent discussion of the attributes of distributed resources, the best frequency of interruption data, or lambda, for a primary distribution system is 0.515 based on Best In Class utility data.2 Surprisingly, the subtransmission and transmission systems' contributions are quite low, 0.115, but the distribution primary contribution is 0.4, or 78 percent of the total frequency of interruption of 0.515. Breaking the distribution primary contribution down, 88 percent of the outages are due to the overhead-line component. The other two components, underground line and substations, have comparatively few failures. Typically, local power parks do not contain overhead lines. They are composed of cables in conduit and direct burial. Using the values for these components, and an availability of 0.95 for an internal combustion engine, outages at local power parks should be about four times better than the average T&D performance.
Dynamic reactive power reserves from generation increase in effectiveness as voltage decays, and they also are the most reliable means for voltage stability enhancement.
Reserves provided from local generation reduce reactive losses resulting from increased active power transfer. Simulations found cases where a 15 percent increase in pure megawatts at a single bus can trigger a voltage collapse. This type of flow-triggered voltage collapse played a role in the Aug. 14 blackout. This is because the increased active power flow aggravates reactive losses in the occupied transmission paths. Dynamic