Power Electronics Provide Control and Flexibility for Series Compensation
By Ronald A. Hedin, Siemens Energy & Automation, and Duane R. Torgerson, Western Area Power Administration
With the onset of competition, utilities are becoming more and more concerned about transmission line construction expenses. Consequently, utilities are operating existing transmission networks closer to their limits in order to avoid costly investments in new or upgraded lines. Western Area Power Administration (Western) is demonstrating a technology that promises to improve power system performance while allowing reliable operation of interconnected systems at higher limits. The demonstration, known as the Kayenta Project, validates a feasible and practical solid-state power electronics application which provides continuous, fast and automatic control with series compensation systems. The technology can benefit future, as well as existing transmission systems, by utilizing transmission lines to their full thermal capacity.
A 330 MVAR first-of-a-kind advanced series compensation (ASC) system installed at the Kayenta Substation is the project`s backbone. The project demonstrates potential benefits of on-line automatic control of series compensation systems by combining conventional series capacitors with power electronic technology. Using three-phase, thyristor-controlled reactors as a Flexible AC Transmission System (FACTS) element, the ASC`s ability to provide direct control of the transmission line compensation level allows dynamic regulation of power flow across the line, as well as improved capacitor bank protection, subsynchronous resonance (SSR) mitigation and network power swing damping.
In the early 1990s, Western faced a challenge at its Kayenta Substation in northeastern Arizona. The substation, originally constructed in 1964 as part of the Colorado River Storage project, is located in the middle of the 200-mile, 230 kV Glen Canyon-to-Shiprock transmission line. It serves the community of Kayenta, Ariz., with a 69 kV link to Western`s transmission system between Shiprock, N.M., and Glen Canyon Dam in Arizona. Designed with an initial transfer capability of 300 MW, the line`s effectiveness to carry scheduled power was diminished in the late 1960s due to construction of parallel 345 kV and 500 kV lines in the area. In 1977, a 230 kV phase-shifting transformer was installed at the Glen Canyon Substation to re-establish the 300 MW path to Shiprock.
Load growth on the interconnected network approached the transmission system`s ability to reliably serve the increasing loads and, with restrictions on building new transmission lines, the economic benefits of adding series compensation became an attractive method to eliminate the transmission line “bottleneck.” The addition of 330 MVAR conventional series capacitors to the transmission line provides 72 percent compensation and increases the power scheduling capability by 100 MW, allowing it to be used at its full thermal rating. The ASC system allows the transmission line to be 100 percent compensated and provides the opportunity to evaluate new FACTS technology applications for meeting the challenges and trends of power transmission systems in the future.
Impedance Control and Harmonics
With the Kayenta Substation operated remotely from Western`s power operation center in Montrose, Colo. (180 miles away), extensive commissioning tests verified the ASC scheme`s operating theory and confirmed the ASC`s ability to smoothly control the 15 ASC impedance as a function of thyristor-controlled reactor conduction angle.
Power systems` harmonics, resulting from static var compensators (SVC) and high-voltage direct current (HVDC) power electronics applications, are well documented and require studies and specially designed filters to ensure harmonics remain within required limits. For this reason, harmonic currents present in the 15 ASC capacitor and their impact on the alternating current (AC) system were considered during the ASC system design. Commissioning studies and tests confirmed that the 15 ASC capacitor provides the filtering necessary to contain ASC harmonic currents and minimize the individual harmonic current components propagating into the AC system to less than 0.5 percent.
Studies of SSR interaction caused by the Kayenta series compensation installation have shown no credible operating condition which would require SSR countermeasures. On the contrary, when the AC network was purposely tuned to a SSR condition, studies indicated that the 15 ASC segment could detune the SSR resonant condition and provide positive SSR damping using impedance modulation. Investigations are under way to develop the modulation strategies necessary for SSR mitigation using active control to produce positive SSR damping.
The ASC project was jointly developed by Western and Siemens Power Transmission and Distributio (Siemens). Siemens is responsible for researching, designing and supplying equipment. Facilities installation and commissioning was performed by Western.
The series capacitor banks are split into three segments, each rated at 1,000 A: two conventional series capacitors (55 /phase and 40 /phase) and one ASC segment (15 /phase). Flexibility and impedance control is provided by the thyristor-controlled reactors in parallel with the 15 ASC segment. By controlling the thyristor conduction angle, a wide range of capacitive impedance (60 Hz) is achieved from the parallel reactor-capacitor combination. A net reactive impedance can also be realized by having the bidirectional thyristor valves conduct for 180 degrees with the reactor effectively bypassing the 15 ASC segment. These combined features allow the 15 ASC segment to perform the following functions:
vernier control of series capacitive impedance (15 to 60 );
line current regulation;
protective functions for both the capacitor and the system;
eliminate protective gaps and reduce metal-oxide varistor arrester requirements; and
The thyristor valve assemblies for the Kayenta ASC are mounted in an outdoor weatherproof enclosure on the series capacitor platforms with cooling provided by a low conductivity water/glycol cooling system. Each single-phase assembly has 11 bidirectional pairs forming individual levels, including one thyristor valve redundant level which allows the valve to provide normal service in the event of a single thyristor level failure. The main features of the ASC thyristor valves include:
100 mm, 3.5 kA, 5.5 kV thyristors;
fiber-optic firing; and
on-line monitoring at ground level.
Although the thyristor-controlled reactor ASC scheme represents a new application for solid-state power electronic devices, the use of thyristors for SVCs and HVDC applications is a widely accepted technology in the utility field. Design and operational tests on a thyristor valve module of the type used for the ASC were successfully completed in 1988 for a SVC application. With built-in monitoring of ASC segment operating parameters, operation of the scheme is allowed within the electrical and thermal limits of the thyristor valves and associated capacitor elements.
Ronald A. Hedin is Siemens Energy & Automation`s manager of system analysis and simulation. He received his bachelor`s of science degree in electrical engineering from Illinois Institute of Technology and a master`s of science degree in electrical engineering from the University of Wisconsin. Hedin joined Siemens Power Transmission and Distribution (then Siemens Allis) in 1978 during the formation of the jointly owned electrical equipment company. His main activity is to perform electrical power application studies involving electric utility high-voltage thyristor systems.
Duane R. Torgerson is an electrical engineer at Western Area Power Administration`s substation design division, Golden, Colo. He received a bachelor`s of science degree in electronic engineering from California Polytechnic University and a master`s of science degree in electrical engineering from Purdue University. Torgerson is responsible for substation applications involving solid-state power technology. He is a member of the International Conference on Large High-Voltage Electric Systems (CIGRE), a senior member of the Institute of Electrical and Electronic Engineers (IEEE), and participates in various IEEE, CIGRE and International Electrotechnical Commission working groups.
Kayenta Substation, pictured here, was selected for the unique demonstration project because of its location in the middle of the 200-mile transmission line.
The 69 kV link to Western`s transmission system between Shiprock, N.M., pictured here, and Glen Canyon Dam in Arizona serves the community of Kayenta, Ariz.