Data processing: generic control systems or specific application – Specific application – apparatus or process – Electrical power generation or distribution system
Reexamination Certificate
1998-05-15
2001-04-17
Lee, Thomas (Department: 2182)
Data processing: generic control systems or specific application
Specific application, apparatus or process
Electrical power generation or distribution system
C700S295000
Reexamination Certificate
active
06219591
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to power systems and protective relays employed therein, and more particularly to methods and apparatus for adaptively shedding load to improve stability in the power system.
BACKGROUND OF THE INVENTION
Voltage instability is closely related to the notion of maximum loadability of a transmission network. In present-day power systems, this may take place as a precursor to the traditional frequency instability problem (see Proceedings of Bulk Power System Voltage Phenomena-III: Voltage Stability, Security and Control, Davos, Switzerland, August 1994; and K. Vu, et al., “Voltage Instability: Mechanisms and Control Strategies,” Proc. of IEEE, Special Issue on Nonlinear Phenomena in Power Systems, pp. 1442-1455, November 1995). It is critical for the utility company to track how close the transmission system is to its maximum loading. If the loading is high enough, actions have to be taken to relieve the transmission system.
A problem associated with tracking the maximum loading of the transmission system is that such maximum loading is not a fixed quantity, but rather depends on the network topology, generation and load patterns, and the availability of VAR resources. All of these factors can vary with time due to scheduled maintenance, unexpected disturbances, etc.
Despite the fact that voltage instability is a system problem, there is still a need for relays that process only local measurement. These relays are to be counted upon when other controls cannot mitigate the aggravating situation; they also form the fall-back position for any global protection scheme when communication channels fail. Controls that use only local data provide an attractive approach because they are low cost and simple to build. The most common form is to shed load based on voltage level—undervoltage load shedding. This scheme has been attempted on the Pacific Northwest system, as reported by C. W. Taylor, Power System Voltage Stability, McGraw Hill, 1994. However, for many other systems, the difficulty with choosing the setpoint poses a challenge. In fact, voltage is often a poor indicator of instability, and a fixed setpoint may result in unnecessary shedding or failure to recognize an instability. Some systems may ride through voltages much below the setpoint of the relay but, for others, the voltage can appear normal even though the grid is on the verge of instability. The idea of using an adjustable voltage setpoint has been known, as reported in IEEE Power Systems Relaying, Committee, Working Group K12, “Voltage Collapse Mitigation,” 1995.
The true goal of a local relay should be to determine whether the load connected to the substation is excessive. A fundamental issue here is whether the transmission system's strength can be “sensed” from local measurements. It has been well known that conventional, local quantities such as voltage level and reactive reserve are poor indicators of voltage instability, and therefore advanced methods are needed. For example, the use of artificial intelligence on local measurements is disclosed in K. Yabe, et al., “Conceptual Designs of AI-based Systems for Local Prediction of Voltage Collapse,” IEEE 95 WM 181-8 PWRS. The idea is to simulate a range of system conditions to generate patterns in local observations. In the real environment, true measurements are then compared against known patterns, from which the proximity to collapse is inferred.
SUMMARY OF THE INVENTION
In contrast to the above-described methods, the present invention provides a method that is quite simple and does not require off-line simulation and training. Based on local measurements (voltage and current), it produces an estimation of the strength/weakness of the transmission system connected to the bus, and compares that with the local demand. The closer the local demand is to the estimated transmission capacity, the more imminent is the voltage instability. This information can be used for load shedding as well as other applications.
The present invention may be embodied or implemented in a device referred to as a Voltage Instability Predictor, or VIP. Such a VIP in accordance with the present invention comprises the steps of, or means for, measuring current and voltage waveforms at the bus, and deriving current and voltage phasors therefrom; based on the current and voltage phasors, determining an apparent impedance ({overscore (Z)}
app
) associated with the load and a Thevenin impedance ({overscore (Z)}
Thev
) associated with the source; comparing the Thevenin impedance and apparent impedances; and deciding whether to initiate a prescribed action based on the relationship of the apparent impedance to the Thevenin impedance.
The Thevenin impedance is tracked by a curve-fitting technique. In one example of such a technique, the Thevenin impedance is estimated based on the following equation:
{overscore (E)}={overscore (V)}+{overscore (Z)}
Thev
{overscore (I)},
where {overscore (E)}=E
r
+jE
i
is the Thevenin source voltage, {overscore (V)}=u+jw is the measured voltage phasor at the bus, and {overscore (I)}=g+jh is the measured current phasor at the bus. The quantities g, h, u and w are available from the measurements at the bus.
Other features of the present invention are disclosed below.
REFERENCES:
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patent: 5642000 (1997-06-01), Jean-Jumeau et al.
patent: 5773980 (1998-06-01), Yang
patent: 5839093 (1998-11-01), Novosel et al.
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Taylor, C.W., Power System Voltage Stability, McGraw Hill, 1994.
Tuan, T. et al., “Emergency Load Shedding to Avoid Risks of Voltage Instability Using Indicators”,IEEE Trans. PWRS, Feb. 1994, 9(1), 341-351.
Vu, K. et al., “Voltage Instability: Mechanisms and Control Strategies”,Proc. of IEEE, Nov. 1995, 83(11), 1442-1455.
Vu, K. et al., “Grids Get Smart Protection and Control”,IEEE Comp. Appl. Power, 1997, 40-44.
Vu, K. et al., “Use of Local Measurements to Estimate Voltage-Stability Margin”,IEEE, 1997, 318-323.
Yabe, K. et al., “Conceptual Designs of AI-based Systems for Local Prediction of Voltage Collapse”,IEEE PWRS, Feb. 1996, 11(1), 181-188.
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Use of local Measurement to Estimate Voltage-Stability Margin By Khoi Vu, Miroslav M. Begovic, Damir Novosel and Murari Mohan Saha, Jan. 1997.
Novosel Damir
Vu Khoi Tien
ABB Power T&D Company Inc.
Lee Thomas
Patel Nitin
Woodcock Washburn Kurtz Mackiewicz & Norris LLP
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