Wide-area Feedback/Response-based Controls

A promising alternative or complement to local controls or to SPS is wide-area feedback/response-based controls. Two types of these controls are continuous feedback control, and discontinuous control, which take actions similar to those taken by SPSs. Compared to local controls, wide-area controls provide greater observability and controllability. Positive sequence, synchronized phasor measurements are the preferred sensors for control inputs. High-speed digital/ optical communications are required.

Continuous Wide-area Control

Continuous wide-area control is being studied by many utilities, vendors, and universities. Perhaps the most serious work is that by Hydro Quebec for power system stabilization (oscillation damping improvement) through generator excitation control, and through the use of static var compensators and other power electronic devices.

Wide-area Discontinuous Feedback Control

Wide-area discontinuous feedback control is based on power system response to disturbances rather than on direct detection of only certain outages, as in most SPSs. Control action occurs for outages anywhere in the interconnection that causes a threatening response. Notable is the Wide-Area Stability and Voltage Control System (WACS) in development at BPA (Taylor et al., 2005).

Figure G.1 shows a block diagram of power system stability controls. The SPS path is feedforward. The continuous feedback controls are normally local and mainly at generators, but could be wide area. The feedback (response-based) discontinuous controls are often wide area, but could be local (e.g., underfrequency or undervoltage load shedding).

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FIGURE G.1 Power system stability controls.

Sophisticated Control Algorithms

Sophisticated control algorithms use various techniques such as adaptive or “Intelligent” control as part of digital control and communication capabilities. Integration with the energy management system (EMS) functions, such as dynamic security assessment, is possible to adapt control to present operating conditions. The description of wide-area controls above focuses on actions to prevent instability and controlled or uncontrolled separations and islanding. If these actions fail, controlled separations could be initiated. This is relatively easy for well-defined inter-ties between areas, but more difficult in a highly meshed system. Adaptive islanding is a research area. Some aspects of this concept have been demonstrated recently in simulation on a large, realistic test system (Yang et al., 2006).

Example of Impact of Voltage/Reactive Power Practice

An example of the impact of voltage/reactive power practices on system performance from the August 14, 2003, blackout is presented (U.S.-Canada Power System Outage Task Force, 2004). The initial outage of the Eastlake 5 generator on August 14 was related to excitation equipment problems during production of high reactive power. (The outage likely would have been avoided with modern equipment.) As an example of poor voltage/reactive power practice, Figure G.2 and Figure G.3 show conditions on August 14, 2003. Figure G.2 shows the 345 kV voltage profile that many engineers would regard as terrible, especially considering that the load was less than 80 percent of peak summer load and that the



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