The electric-utility industry, the engine that drives the economies of countries around the world, has been the beneficiary of unending innovations in equipment design, theoretical analyses of system operations and unique applications of computer technology.

Technological advances from the start were modest, even primitive. For example, the rationalization of insulator design was hailed for its technical insight, as was the development of the three-phase ac system that was necessary for long-distance transmission. Transformers, circuit breakers and relays were developed to achieve some measure of reliability in power delivery. Year by year, new and sophisticated developments have enhanced utility operations.

Passing into a new millennium, a proliferation of electronic devices is eliminating the human factor in instantaneous system control. When one speculates about the future major building blocks, one could focus on innovations such as the use of superconducting windings for transformers; the coming of age of smart relays for system protection; the use of advanced communication systems for automating the distribution system; or the installation of various forms of distributed generation, such as fuel cells, microturbines, wind generators and photovoltaics.

One of the most important building blocks for the immediate future is a technology that ensures reliable power delivery over existing lines. That technology, Flexible Alternating Current Transmission Systems (FACTS), increases current-carrying capacity, improves system stability and results in a robust system.

Taking Control of Transmission

Regulatory constraints, environmental concerns and general public opposition present almost insurmountable hurdles for utilities building new transmission lines. To address the problem, utilities rely on existing generation and power export/import arrangements instead of trying to build. This is especially true with the uncertainty of being able to recoup transmission investment in a competitive era.

The ideal solution is to direct more power over underutilized lines while, at the same time, strengthening the power-transfer capabilities of the heavily loaded lines. This solution requires a review of power transmission principles and practices to create new concepts that allow the use of the existing system assets without reducing stability and security margins.

The power-electronics-based FACTS concept incorporates advanced microprocessor controls and powerful analytic tools. The technology was conceived by EPRI to enhance controlability and increase the power-transfer capabilities of the transmission system.

The New York Challenge

In New York, much of the available generating capacity is located in the northwestern part of the state with the greatest demand in the southeast. A major transmission bottleneck exists on the west-to-east corridor between the cities of Utica and Albany. The group of transmission lines in this corridor, known as the Central-East Interface, frequently operates at capacity limits imposed to avert the potential effects of voltage collapses or low-frequency oscillations during system contingencies.

Most power-system devices operate mechanically and, therefore, are relatively slow. These devices are useful in the steady-state operation of a power system, but their response time is too slow to react to contingencies or sudden changes in voltage and flow conditions. The FACTS concept of using high-power electronic solid-state switches instead of mechanically controlled devices leads to more efficient use of existing transmission resources while maintaining and improving power-system security.

Convertible Static Compensator

To take advantage of the new technology, the New York Power Authority (NYPA) installed a convertible static compensator (CSC) at its Marcy Substation near Utica. The CSC is the latest in a series of increasingly sophisticated FACTS controllers that have been developed by EPRI, Siemens Power Transmission & Distribution (Raleigh, North Carolina, U.S.) and several utilities including the NYPA.

The CSC project, scheduled for completion by the end of 2002, will be the world's most advanced and versatile device for controlling voltage and power flow on transmission lines.

Phase One, which was completed in April 2001, has demonstrated its potential value by improving voltage stability with strong voltage support at the critical Central-East Interface.

In addition, along with a 135 MVAR capacitor bank installed at a New York State Electric & Gas Corp. (NYSEG, Ithaca, New York, U.S.) substation, the CRC permitted increases of 60 MW in the power-transfer limit of Central-East lines. By allowing the New York Independent System Operator (NYISO, Schenectady, New York) to operate at higher transfer limits, the statewide system realized an increase of 114 MW. This was critical during the summer of 2001 when New York set peak demand records on three successive days.

Phase Two will include completion of the world's first Inter-Line Power Flow Controller (IPFC). This installation will allow operators to exchange power instantly between independent lines. In this circumstance, operators have the ability to transfer power from a heavily or fully loaded line to one with spare capacity, thereby maximizing the capacity and efficiency of the transmission system.

Phase Two, when completed in conjunction with a 200 MVAR capacitor bank at a Niagara Mohawk Power Corp. (NMPC, Syracuse, New York) substation, will permit an additional increase of 40 to 60 MW in the power-transfer limit of Central-East lines and a total increase of 200 MW or more on the statewide system. In addition to relieving congestion on the grid and enhancing voltage control, the CSC provides additional damping in response to system contingencies and enhances operational flexibility during system outages.

As the nation's largest state-owned electric utility, NYPA is taking the lead in demonstrating new technologies such as the CSC by investing US$39 million in the project whose total cost is estimated to reach $52 million. EPRI, Siemens, and more than 30 electric utilities and ISOs in the United States, Canada and New Zealand also are investing in the project.

FACTS — The 4^th Generation

FACTS has come a long way since the early 1970s, when the first concepts were developed for generating controllable reactive power through switching power converters. The new generation of FACTS devices, employing a voltage-sourced converter (VSC) approach and high-power semiconductor switches, is extremely flexible and can be applied to solve most ac power-transmission problems and to increase the use of the overall power system.

Phase One of NYPA's CSC project employs a ±200 MVAR STATCOM (two ±100 MVA converters) to regulate the Marcy bus voltage at the desired level. The CSC at NYPA's Marcy 345-kV substation will incorporate all features of a STATCOM and Unified Power Flow Controller (UPFC) while, for the first time, permitting simultaneous, real-time control of voltage and power flow on two or more lines in the same substation by introducing the IPFC concept.

How the CSC Works

The CSC is installed in a building at the Marcy Substation that is about the size of a high school gymnasium. The heart of the device consists of two VSCs made up of a series of small solid-state silicon valves (gate turnoff thyristors) controlled by microchips. The two converters are connected to the 345-kV Marcy bus through a shunt transformer, permitting voltage control at the bus. When the project is completed, they will connect through two series transformers to the two 345-kV transmission lines on which flow will be controlled for NYPA's line from Marcy to Coopers Corner and for NMPC's line from Marcy to New Scotland.

Completion of Phase One allows operation in shunt mode, in which two converters and the shunt transformer provide ±200 MVAR dynamic voltage support and control at the Marcy Substation. The present configuration allows operators to improve voltage profile through the rapid addition of reactive power. Phase Two will provide ±100 MVAR series compensation for the Marcy-Coopers Corners and the Marcy-New Scotland lines, creating the capability to simultaneously control flow on two lines and to transfer power from one line to the other.

Under the CSC concept, two ±100 MVA converters may be deployed in five different power-circuit configurations for strategic use in system compensation. The CSC's five power-circuit configurations can be further divided into 11 different modes of operation in response to system requirements. Selection of power-circuit configuration will initiate the operation of switchgear and will be permitted only when the equipment is de-energized. For each power-circuit configuration, there will be various modes of control. However, mode selections could be made at any time with control software providing a smooth transition from the prevailing mode to the newly selected mode. Figures 2 through 6 show the five main configurations:

• Shunt compensation (STATCOM)

• Series compensation (SSSC)

• Independent shunt and series compensation (STATCOM and SSSC)

• Coupled shunt and series compensation (UPFC)

• Coupled series and series compensation (IPFC).

Simplified representation of the algorithmic structure of the CSC real-time controls (Fig. 7) shows how the converters are controlled by appropriate algorithms dictated by the prevailing power-circuit and mode selection. Each control algorithm requires its own particular set of feedback signals representing voltages and currents in the power circuits, as well as the associated reference values for the controlled variables.

Operating Performance

During the first months of CSC Phase One operation, some recorded system disturbances occurred in which the response of the ±200 MVAR STATCOM could be seen from digital fault recorder records. Figure 8 shows the STATCOM response to change in voltage reference that prompted the STATCOM to go into a short-term overload for one second. The STATCOM increased transient ratings in both capacitive and inductive regions. The overload capability is 23% for one second in both regions.

Figure 9 depicts the STATCOM response to a remote fault in the transmission network that caused voltage sags at the Marcy bus. The depth of the sag at Marcy was about 5% on the most severely affected phase. The STATCOM injected capacitive-reactive power in an attempt to restore voltage. The permitted one-second overload to 123% rated current enabled 250 MVAR to be supplied at the Marcy bus. STATCOM response was very fast at both the onset of the voltage sag and at the time the fault was cleared, and the system voltage was restored.

As utilities respond to the continuing deregulation of the industry, the effective use of transmission and distribution assets will be a critical discriminator and a key component of their long-term viability. The need to adapt to changing and often unpredictable system conditions will pose unprecedented challenges. Capital investments will have to be made wisely and with consideration for their applicability to all possible system demands. The CSC will meet that objective while providing long-term economic and system-operation benefits, as well as unparalleled flexibility and capability to meet the changing demands on the system.

For the first time, a single controller will be able to not only control power flow and regulate voltage in a “convertible” mode on a single line, but also to provide that capability for two or more lines. The CSC will represent, in a single power-electronic system, the most versatile, easily configurable and complete power-management system yet developed. No other device is capable of such a broad and far-reaching utilization, or more able to provide for changing power-delivery requirements.

Already the CSC has demonstrated its effectiveness in helping to alleviate a major transmission bottleneck in New York state. Additional benefits will follow with the completion of Phase Two of the NYPA project. However, the CSC's vast potential will be realized only through further implementation of the technology by other transmission operators in the United States and beyond.

Shalom Zelingher, director of research and technology development at NYPA, holds the BSEE and MSEE degrees from Poly-technical Institute of New York. Prior to joining the NYPA staff in 1983, Zelingher worked at American Electric Power Service Corp.

Abdel-Aty Edris, technical leader of EPRI's Transmission and Substation Asset Utilization group, joined EPRI in 1992 after 12 years with ABB in Sweden and the United States. Edris holds the BS degree from Cairo University, the MS degree from Ain-Shams University in Egypt and the Ph.D. from Chalmers University of Technology in Sweden.

Leonard J. Kovalsky is manager of the FACTS Business Segment in Siemens PTD's High-Voltage Systems Division, Orlando, Florida. He holds the BSEE degree from the University of Pittsburgh and the MSEE degree from Cornell University.