Georgia Power commissioned the Laurens County 87-MVAR static var compensator in June 2005 to address voltage problems in Dublin, Georgia, U.S., and to defer construction of a new 230-kV transmission line. The Laurens County SVC, manufactured by Mitsubishi Electric Power Products Inc. (MEPPI), is installed adjacent to the North Dublin Substation and is the first of its kind on the Southern Company Transmission system. It provides voltage regulation, fast voltage recovery and heightened system reliability under normal and contingency conditions.

SEVERE VOLTAGE PROBLEMS

Dublin, the seat of Laurens County, is located in Middle Georgia. The town is served by two 230-kV/115-kV substations (North Dublin and Achord Road) located 7 miles (11 km) apart, and two very long 230-kV transmission lines (North Dublin-Wadley and Achord Road-Eastman-Plant Hatch). A 115-kV network connects the North Dublin Substation and the Achord Road Substation with the distribution substations in the area (Fig. 1).

Laurens County is predominantly agricultural. However, there are two large industrial customers in the area: a textile plant with about 6 MW of load and a paper mill that includes on-site generation. Both are served from the 115-kV transmission system. The paper mill's net load can fluctuate from 0 MW (0 MVAR), when all on-site generation is on-line, to 75 MW (45 MVAR), when all on-site generation is off-line. Typically, the paper mill's net load is about 52 MW (42 MVAR), which represents 28% of the MW (44% of the MVAR) of the total peak load projected for 2005 in the Dublin area.

Preliminary studies conducted in 2003 by Southern Company Transmission Planning indicated severe voltage problems in the Dublin area including:

• Low postcontingency voltages (< 0.90 p.u.) for an outage of either 230-kV line (the North Dublin-Wadley 230-kV line or the Achord Road-Eastman-Plant Hatch 230-kV line) starting in 2005

• Prolonged voltage recovery following a three-phase fault at North Dublin and subsequent clearing of the North Dublin-Wadley 230-kV transmission line that could trigger voltage collapse starting in 2005

• Low precontingency voltages (< 0.95 p.u.) in later years (about 2012).

The traditional solution to these problems would be to build a new 230-kV transmission line (about 32 miles [51 km] long) between North Dublin and Gordon (Fig. 1). This line would provide an additional feed to the North Dublin Substation — the strongest source in the area — during normal system operation and a backup source of power during contingency events.

Georgia Power decided to pursue a more cost-effective solution that included the installation of an SVC device at the North Dublin Substation for voltage support in the Dublin area. The preliminary steady-state and dynamic system studies estimated the SVC size as 80 MVAR. Additionally, it was clear that the low steady-state postcontingency voltage problems in 2005 and low precontingency voltage problems in later years could not be addressed with a single system improvement. Consequently, Georgia Power decided to install an SVC in 2005 and build a new North Dublin-Gordon 230-kV transmission line when an outage of the SVC would become the critical contingency, roughly in 2012.

SVC DESIGN

MEPPI designed and installed the SVC adjacent to the North Dublin 230-kV/115-kV Substation. As requested by Georgia Power, MEPPI initially submitted SVC proposals for both 230-kV and 115-kV injection points at the North Dublin 230-kV/115-kV Substation. Georgia Power selected the 115-kV injection point for the SVC because the study results suggested it would require a smaller size SVC than the 230-kV injection point. An SVC connected to the 115-kV bus at the North Dublin Substation would be electrically closer to the largest load in the area, the paper mill, and therefore more effective in providing voltage support to the area.

The SVC was rated and designed to meet Southern Company Transmission's voltage-support requirements and performance criteria under both 2012 peak and light system conditions, assuming the typical net load of 52 MW and a wide range of contingency events:

• Steady-state voltages in the area should be between 0.95 p.u. and 1.05 p.u. under both N-0 and N-1 conditions

• Following a disturbance, system voltages should recover and cross their steady-state postcontingency voltage magnitudes in less than 3 sec

• The system load margin should be at least 5%.

Additional requirements for the SVC were to maintain a virtually constant voltage of 1.01 p.u. at the point of interconnection (North Dublin 115-kV bus) and to operate two 115-kV mechanically switched capacitor (MSC) banks in the area — a 30-MVAR MSC at North Dublin Substation and a 30-MVAR MSC at the Dublin Substation — under the following conditions:

• The steady-state voltage deviation following capacitor switching should be equal to or less than 2.5%

• Following a contingency, voltage deviation from the precontingency steady-state voltage value should be less than 5% for nonregulated transmission buses and less than 8% for regulated transmission buses. In the Dublin area, all transmission buses are regulated except for the paper mill bus and the textile plant bus.

SVC INSTALLATION AND CONTROL

The SVC site layout and design are shown in Figs. 2 and 3. The SVC is connected to the 115-kV transmission system via three 115-kV/11.15-kV single-phase power transformers (plus one spare unit). The SVC consists of one 87 MVAR thyristor-switched reactor (TCR) with direct light-triggered thyristor (LTT) valves, and two harmonic filters rated 43.5 MVAR each and tuned for the 5th- and 7th-order harmonic filtering. About 10% margin was built in the SVC size to enhance system performance with the maximum net load of 75 MW at the paper mill.

The SVC is controlled and monitored locally from a human machine interface (HMI) and local control panel, and remotely from Georgia Power's supervisory control and data acquisition system via the HMI and remote thermal unit. When working in the automatic control mode, the SVC regulates voltage at the North Dublin 115-kV to 1.01 p.u voltage. In addition, the SVC maintains adequate reactive capacity for dynamic events by switching in the MSCs in the area when capacitive power demand monitored by the SVC exceeds 30 MVAR for a predetermined time. In the manual operation mode, the SVC output can be set manually regardless of the voltage error at the interconnection point.

The SVC has been designed to meet 99.5% availability criteria for forced outages whereby no more than two forced outages and no more than two deferred outages are allowed per year. To increase the SVC availability, the thyristor valve cooling system was designed to include dual water pumps and redundant fans in the heat exchanger. Redundancy was also built in the TCR valve system (one extra valve per six valves in the series) and battery supply system.

The total cost of the SVC project, which included the cost of the SVC itself and the cost of the SVC site and grading, was less than half of the estimated cost for a new North Dublin-Gordon 230-kV line.

EXPERIENCE WITH THE SVC

Since it was commissioned, the SVC has been quietly doing its job. A few minor problems occurred during the SVC installation and commissioning including:

• Problems with the cooling system alarms related to a flow gauge

• Problems with the ac supply throw-over controller

• Loss-of-sync alarm between the HMI and intelligent switching unit due to the New Year's “leap second.”

All of these problems were easily resolved.

During commissioning, the SVC was tested on an outage of the North Dublin-Wadley 230-kV line (the most critical outage in the Dublin area) and it successfully passed the test. Georgia Power denied MEPPI's request for a three-phase fault test at North Dublin.

AN UNPLANNED TEST

The SVC was tested on a phase-to-phase fault on the 230-kV system by an incident on June 16, 2005, during the commissioning test. The main fault occurred at 16:11:54 hours CDT, with precursor events beginning at 16:11:44 hours. The table shows the SVC MVAR output and voltage at the 115-kV injection point during the event, while Fig. 4 shows the SVC high-voltage three-phase current obtained from the HMI unit. The sequence of events on June 16 was as follows:

During that day, the paper mill's on-site generation was running and the 115-kV MSCs at the North Dublin Substation and the Dublin Substation were both switched off. The SVC operated around 0 MVAR at the time.

In the afternoon, a farmer was burning his fields directly under the Vidalia-Wadley 230-kV line, in the vicinity of the Wadley Substation (located in the upper righthand corner of Fig. 1). This caused the air surrounding the line to become laden with ionized combustion products, resulting in a precursor event, likely a partial short, which cleared itself without any breaker operation. The SVC saw the event and increased output from 6 MVAR at the time of the event to 21 MVAR during the event, in order to keep the voltage at the North Dublin 115-kV transmission bus at 1.01 p.u.

After a short time, the initial event propagated into a phase-to-phase fault on the Vidalia-Wadley 230-kV line and was cleared in 3.5 cycles. The SVC responded to the fault by providing its full MVAR output of 87 MVAR. Initially following the fault, the voltage at the North Dublin 115-kV bus dropped and then started recovering as the SVC increased reactive power supplied to the area. The voltage at North Dublin stabilized at its precontingency value of 1.01 p.u. within 2 sec (note that the HMI sampling time is 2 sec) and the SVC output returned to 0 MVAR. The SVC supported the system voltage during the fault as expected and passed the test.

A POSITIVE RESULT

The Laurens County SVC offered an economical means for meeting reactive power needs in the Dublin area and postponed the need for a costly new 230-kV transmission line for about seven years. The SVC has demonstrated it will provide rapid steady-state voltage control to the area under various load and contingency conditions and improve the fault-induced delayed voltage recovery during transients. The SVC control strategy involves coordination of local and remote MSC banks with the objective of ensuring that sufficient dynamic VAR capacity remains available for major local system disturbances.

Based on the operational experience to date, the Laurens County SVC has met Southern Company Transmission's expectations with regard to its performance and reliability.

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Borka Milošević joined Southern Company Transmission as a planning engineer in 2002. She earned her BSEE degree from the University of Belgrade, Serbia in 1992, and her MS degree and PhD in electrical and computer engineering from the Georgia Institute of Technology in 1998 and 2002, respectively. She worked as a research engineer at the Electrical Engineering Institute “Nikola Tesla” in Belgrade, Serbia from 1994 to 1997. bmilosev@southernco.com

Robert J. Beck is a senior engineer in Southern Company Transmission Planning. He has worked for Southern Company for more than 30 years. He earned his BSEE degree from the Georgia Institute of Technology and his MBA degree from Georgia State University. He is a professional engineer in Georgia. rjbeck@southernco.com

The SVC MVAR output and voltage at a 115-kV injection point.

Time (CDT)	SVC Output (MVAR)	SVC Primary Voltage (kV)
16:11:44	6	117
16:11:46	21	117
16:11:54	11	117
16:11:56	87	111
16:11:58	0	117
16:12:00	4	117
16:12:04	0	117