Progress Evaluates Capacitor Switching

Mar 1, 2006 12:00 PM
by Charles Haahr, Progress Energy

IN 2003, PROGRESS ENERGY FLORIDA EMBARKED ON A RESEARCH AND DEVELOPMENT PROJECT TO REVIEW EQUIPMENT used for switching existing capacitor-bank installations to ensure that reliability of the subtransmission remained at a high level.

Progress Energy Florida's retail service area spans about 20,000 sq miles (51,800 sq km) in Central Florida, U.S., including metropolitan St. Petersburg, Clearwater and portions of the greater Orlando area. In Florida, the company maintains more than 43,600 miles (70,167 km) of distribution and transmission lines, serving 1.5 million customers.

Progress Energy Florida, a subsidiary of Progress Energy, has many capacitor banks installed in the subtransmission voltage system. Since many of the installations were made in the 1970s and 1980s and have been operated 100 to 250 times per year, the circuit switchers are approaching or have exceeded their expected life. Experience has proven that the devices do wear out at about 5000 operations. Therefore, it is not a question of if they will wear out, but rather how and when the switches will need to be upgraded or replaced.

The technology used in the past for switching capacitor banks has performed exceptionally well. However, an examination of alternative switching methods was warranted due to new technologies that have come into the market in recent years. We have recently completed a review of capacitor switching technologies that are available in the market.

APPLICATION CONSIDERATIONS

When a capacitor bank is energized, there is an immediate drop in system voltage toward zero, followed by a fast voltage recovery that is superimposed on the system 60-Hz fundamental waveform. This recovery voltage can reach a peak of 2 per unit at frequencies between 300 Hz and 1000 Hz (Fig. 1).

While these transients are not typically harmful to utility equipment, they may be troublesome to some customers' sensitive equipment. The transients often show up a significant distance from the capacitor bus as the high-frequency transients pass through transformers and are magnified by capacitor banks located on the distribution system or at the customer's location. The resulting overvoltages can cause nuisance tripping of adjustable speed drives, computer network problems as well as customer equipment damage or failure.

Generally, transients maintained below about 1.2 per unit will not impact the customer. However, some form of control is necessary to achieve this performance.

When capacitor banks are installed in a back-to-back arrangement (two or more capacitors close to each other), the energization of the second bank looks like a short circuit to the first bank. This causes the first capacitor bank to discharge into the second capacitor bank resulting in high in-rush currents (Fig. 1). These in-rush currents can reduce the life of the capacitor-switching device. On grounded capacitor banks, transient currents may flow in the ground mat causing potential problems with electronic equipment in the substation because of induced voltage in the control voltage supply.

Progress Energy's work practice does not allow crews to work on a switching device without having an open point in the line feeding the capacitor-switching device. The circuit switchers with pre-insertion inductors have an integral disconnect that is part of the pre-insertion process. In most cases, the next closest disconnect is located at the substation entrance. Because of these station configurations, crews must take down and re-energize stations or build “boxes” around stations to work on or replace the circuit switchers.

For existing 69-kV installations that have circuit switchers with pre-insertion inductors, it was determined that the solution would need to include a separate disconnect switch and fit in the space currently occupied by the existing device.

PROJECT OBJECTIVES

The first objective of this project was that voltage transients during capacitor switching had to be 1.2 per unit or less. The second project objective was to reduce in-rush current during capacitor switching to a manageable level. The third objective was to install a separate line-side disconnect switch when replacing the capacitor-switching device at 69 kV to accomodate work practice considerations.

After an initial review of various capacitor-switching devices, we selected three technologies for a more in-depth review: pre-insertion inductors, zero-voltage closing breakers and pre-insertion resistors. These technologies were evaluated based on their effectiveness at limiting transients, installed cost (purchase price, and labor and equipment required for installation) and other distinguishing features.

TRANSIENT LIMITING TECHNOLOGIES

The pre-insertion inductor reduces the initial transient when energizing the capacitor bank. The in-rush events are balanced between this initial inrush and the inrush from bypassing the inductor. In this system, a two-stage switching device is used to momentarily introduce an inductance into the circuit. A circuit switcher disconnect makes the initial circuit through the pre-insertion inductor. The disconnect continues to close, eventually bypassing the pre-insertion inductor (Fig. 2). The inductor is in the circuit for 7-12 cycles (117 mS to 200 mS).

For the zero-voltage closing breaker, modern control systems attempt to repetitively control with precision the instant at which the switching contacts come together. Under ideal circumstances, if the poles close at the point of zero voltage, there will be no current transients created. This requires precise timing and control of the three individual poles. Any drift in the control must be compensated for or else the system reverts to the first case described previously. A 1-msec closing error (worst-case acceptable error for design) is chosen for simulation purposes only and is not meant to be indicative of the actual zero-voltage closing breaker performance of any specific breaker or installation (Fig. 3).

The pre-insertion resistor limits transients by the momentary insertion of a resistive device into the circuit before full energization of the capacitor bank. The insertion of the resistor is a two-step process. The initial circuit is made through the pre-insertion resistor in an SF₆ environment. The resistor is then shunted as the main contacts close. The resistor is typically in the circuit for about 5 msec to 15 msec before the main contacts close.

For similar levels of transient suppression, the pre-insertion resistor can be physically smaller than the equivalent pre-insertion inductor. Various values of pre-insertion resistors are available. Pre-insertion resistors have been used in combination with circuit breakers and circuit switchers, as well as in a new device designed specifically for switching capacitors. Worst-case transients occur when the initial switch closing occurs at a voltage peak and when the bypassing of the inserted device occurs at a current peak. Simulations were performed using this timing. Results for both options are included in Fig. 4.

As can be seen in Table 1, all three technologies examined by Progress Energy provided the effective limitation of transients.

INSTALLED COST

At 69 kV, the circuit switcher with pre-insertion inductor and the capacitor switch with pre-insertion resistors were evaluated. Zero-voltage closing breakers are cost prohibitive at 69 kV, so they were not evaluated. Table 2 indicates the approximate costs for installation of the various proposals received. Note that total installed costs comprise all costs associated with the work, including clearances, switching and working. These costs are typical, but each location is slightly different.

A significant factor in the lower labor costs for the 69-kV capacitor switch with pre-insertion resistors is that it is shipped in two pieces: the support pedestal and the balance of the switch. The pedestal also can be modified to match existing foundations, reducing the need to replace foundations when a new circuit switcher is installed.

ADDITIONAL CONSIDERATIONS

Operational life is an additional consideration that Progress Energy explored. The Southern States CapSwitcher and the zero-voltage closing breaker are rated for 10,000 operations, twice that of traditional circuit switchers with pre-insertion inductors. Recommended inspection interval for the breaker contacts is 2000 operations. The interval for the CapSwitcher is 5000 operations.

Other operational considerations Progress Energy kept in mind was that the blade of the circuit switcher inserts into the inductor. This takes place in an open-air environment and may result in increased maintenance caused by arcing between the blade and inductor contact arm.

The effectiveness of the zero-voltage closing breaker is dependant on the ability of the synchronous controller to analyze the environment and to close the mechanism to ensure effective transient mitigation. Factors that must be interpreted and factored in include operating history, control voltage, ambient temperature and any other factor that would impact the ultimate closing speed of the mechanism. One significant benefit of the breaker design is its ability to mount bushing CTs and its availability with a 63-kA interrupting rating (Table 3).

The Southern States capacitor switch with pre-insertion resistors has the fewest moving parts and is affected the least by the external environment. Due to its compact design, the switch may be used in retrofit applications more easily than other choices. One note of concern is that the pre-insertion resistor cannot easily be field replaced. If damaged, the manufacturer's current recommendation is to replace the pole unit and return the damaged pole to the factory for replacement or refurbishment.

ACTUAL PERFORMANCE RESULTS

At 69 kV, based on its lower total installed cost and effectiveness in limiting transients, Progress Energy selected Southern States' CapSwitcher capacitor-switching device. The compact design of the device also made it possible to retrofit existing capacitor bank switching applications and add the required disconnect with minimal modifications to the substation.

Since the capacitor-switching device chosen was a relatively new product on the market, we requested a recorder be placed on the system to see if the performance matched the claims of the manufacturer. Figure 5 shows the tracing from that 69-kV CapSwitcher installation. As you can see, the peak overvoltage was 1.05 per unit, well within our desired performance criteria.

Charles Haahr was a systems engineer for Progress Energy (Lake Mary, Florida, U.S.) at the time he wrote this article. His duties with the company included substation system analysis, reliability upgrade evaluations and strategic improvement planning. Haahr is currently employed with Sega Inc. (Stilwell, Kansas, U.S.). He is a licensed professional engineer in Florida and Kansas.
chaahr@segainc.com

Fig. 1. Voltage and current transients — no transient mitigation.
	Peak Current	Frequency	Peak Voltage
Bank 1 Energization	2172 A	943 Hz	182 kV (1.93 per unit)
Bank 2 Energization	14,038 A	16,400 Hz	137 kV (1.46 per unit)

Note: All graphs and performance simulation data in this article provided courtesy of Tri-Axis Engineering.

Fig. 2. Voltage and current transients — pre-insertion inductor.
Pre-Insertion Inductor	Peak Current	Frequency	Peak Voltage
Bank 1 Energization (Inductor inserted)	526 A	351 Hz	114 kV (1.21 per unit)
Bank 2 Energization (Inductor inserted)	365 A	1030 Hz	107 kV (1.14 per unit)
Bank 2 Transient	1620 A	16,130 Hz	98.5 kV (1.05 per unit)

Fig. 3. Voltage and current transients — zero-voltage closing breaker.
Zero-Voltage Closing Breaker	Peak Current	Frequency	Peak Voltage
Bank 1 Energization	942 A	944 Hz	120 kV (1.28 per unit)
Bank 2 Energization	5228 A	16,667 Hz	108 kV (1.15 per unit)
Bank 2 Transient	5021 A	16,807 Hz	108 kV (1.15 per unit)

Fig. 4. Voltage and current transients — pre-insertion resistor.
Pre-Insertion Resistor	Peak Current	Frequency	Peak Voltage
Bank 1 Energization (Resistor inserted)	835 A	948 Hz	97 kV (1.03 per unit)
Bank 2 Energization (Resistor inserted)	1100 A	809 Hz	114 kV (1.21 per unit)
Bank 2 Transient	1820 A	16,529 Hz

Table 1. Summary of transient mitigation approaches.
Simulation Case	Single-Bank Peak Current	Single-Bank Frequency	Back-to-Back Peak Current	Back-to-Back Frequency	Peak Voltage
No Transient Mitigation	2172 A	943 Hz	14,038 A	16,400 Hz	1.93 per unit
Enhanced Pre-Insertion Inductor	526 A	351 Hz	1620 A	16,130 Hz	1.21 per unit
Zero-Voltage Closing Breaker with 1-msec Error	942 A	944 Hz	5021 A	16,807 Hz	1.28 per unit
81-Ω Pre-Insertion Resistor	835 A	948 Hz	1820 A	16,529 Hz	1.21 per unit

Table 2. Cost comparisons at 69 kV.
	Purchase Cost*	Labor to Install (hr)	Total Installed Costs
Refurbish Existing Circuit Switcher with Pre-Insertion Inductor **	Varies	Varies	$70,000 to $90,000 (no switch)
Circuit Switcher with Pre-Insertion Inductor **	$55,000	$10,000	$120,000 (with switch)
Capacitor Switch with Pre-Insertion Resistor	$38,000	$5000	$90,000 (with switch)
* For existing installations, the cost of adding the required disconect in series with a noncandlestick-type circuit switcher is often not possible because of space limitations.
** The cost of adding the required disconnect switch is not included in the purchase cost.

Table 3. Interrupting and fault-close capability.
	Interrupting Rating	Interrupting Time	Fault-Close Rating
Circuit Switcher with Pre-Insertion Inductor*	31.5 kA	Three Cycles	30 kA
Capacitor Switch with Pre-Insertion Resistor	18 kA	Five Cycles	40 kA
Zero-Voltage Closing Breaker Circuit	40 kA	Three Cycles	40 kA
* The cost of adding the required disconnect switch is not included in the purchase cost.

115-KV APPLICATIONS

For 115-kV capacitor-switching technologies, Progress Energy Florida decided to compare the zero-voltage closing circuit breaker with the capacitor switch with pre-insertion resistors. Based on the lower installed cost and compact design, Progress Energy has utilized the Southern States CapSwitcher for replacement applications at 115 kV. Each new application will be evaluated to determine which switching device will be used.

Capacitor-Switching Technology	Purchase Cost	Rated Number of Operations
Zero-Voltage Closing Circuit Breaker	$145,000	10,000
Capacitor Switch with Pre-Insertion Resistor	$60,000 + $20,000*	10,000
* $20,000 is the approximate cost to install freestanding CTs for new installations that require overcurrent protection.