As background, we describe frequently used feedforward wide-area discontinuous power system stability controls. Then we describe online demonstration of a new response-based (feedback) Wide-Area stability and voltage Control System (WACS). The control system uses powerful discontinuous actions for power system stabilization. The control system comprises phasor measurements at many substations, fiber-optic communications, real-time deterministic computers, and transfer trip output signals to circuit breakers at many other substations and power plants. Finally, we describe future development of WACS. WACS is developed as a flexible platform to prevent blackouts and facilitate electrical commerce.

WACS-Wide-Area Stability and Voltage Control System: R&D and Online Demonstration

Damping of slow oscillations with active and reactive power modulation of high-voltage direct current (HVDC)-links is analyzed with the aim of gaining a physical insight into the problem. The analysis shows that active power modulation is efficient when applied a short mass-scaled electrical distance from one of the swinging machines, and reactive power modulation is most efficient when a well-defined power flow direction exists and the modulation is made at a point close to the electrical midpoint between the swinging machines. It is shown that the intuitively appealing feedback signal frequency and derivative of the voltage are appropriate for active and reactive power modulation. The impact of the constraints imposed by the HVDC equations is analyzed, and it is determined when the implicit reactive power modulation resulting from constant gamma control may be detrimental for the damping. >

Utilizing HVDC to damp power oscillations

A superconducting magnetic energy storage (SMES) system has been built to damp power oscillations on the Western U.S. Power System, particularly on the Pacific AC Intertie that is used to transmit power from the Northwest to southern California. The 30-MJ superconducting inductor that stores energy for this purpose is contained in a nonconducting dewar and is supported by a helium refrigerator and a gas-handling system mounted on trailers. Energy flows in and out of the inductor at frequencies from 0.1 to 1.0 Hz with power amplitudes up to 11 MW. The principal oscillation to be damped has a characteristic frequency of 0.35 Hz. The superconducting coil maximum current is 5 kA with terminal voltages up to 2.2 kV. The coil interfaces with the Bonneville Power Administration 13.8-kV bus at the Tacoma Substation through a converter and transformers. The system can be operated with the converter either in parallel-bridge mode or for constant VAR control with the bridges in buck-boost mode. The program for the design, fabrication, installation, and the preliminary experimental operation of the system is reviewed.

30-MJ superconducting magnetic energy storage system for electric utility transmission stabilization

Low frequency synchronizing oscillations between the Pacific Northwest and Pacific Southwest have long been a characteristic of the western power system. Recently new low frequency oscillations have been observed. Simulation results and an analytical model indicate these oscillations are due to a swing mode involving the whole western loop. System measurements show the mode can become poorly damped.

Emergence of a New Swing Mode in the Western Power System

Superconducting magnetic energy storage (SMES) is unique among the technologies proposed for diurnal energy storage for the electric utilities in that there is no conversion of the electrical energy, which is stored directly as a circulating current in a large superconducting magnet, into another energy form such as mechanical, thermal, or chemical. Thus one advantage of SMES is the inherent high storage efficiency that is possible because energy conversion processes are avoided. The actual round-trip efficiency of a large unit is expected to be 90 percent or greater. The fast response (< 100 ms) of the system to power demand means that a diurnal storage unit can also function as a swing generator or provide system stabilization. The major components of a SMES system are a large superconducting coil cooled by liquid helium, an ac-to-dc convertor, and a refrigerator that maintains the temperature of the helium coolant. This paper describes the design and functions of these and other components of an engineering reference design for a 1-GWh SMES unit. Also included is a sketch of the historical development of superconductivity, which was first discovered in 1911, and SMES, which was first proposed as a method of diurnal storage in 1969.

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Superconducting magnetic energy storage

Implementation and operating experience with a control system which modulates power on the Pacific HVDC Intertie to damp oscillations on the parallel Pacific AC Intertie is described. This system significantly improves the damping of the Western interconnected power system, which has had a long history of periods of negatively damped oscillations. The successful operation of dc modulation was a key factor in permitting an increase in the rating of the Pacific AC Intertie from 2100 MW to 2500 MW. This uprating results in large economic and energy conservation benefits.

D. N. Scott

Papers

Operating Experience with Modulation of the Pacific HVDC Intertie