Download “Capacitor Switching and Capacitor Switching Devices” Thomas P

“Capacitor Switching and Capacitor Switching Devices”
Thomas P. Speas, Jr.
30 Georgia Ave., Atlanta, GA 30228
Phone: 770-946-4562
Fax: 770-946-4562
e-mail: [email protected]
Presented at the 45th ANNUAL
MINNESOTA POWER SYSTEMS CONFERENCE (MIPSYCON)
November 3-5, 2009
Abstract
Capacitors represent an effective and low cost alternative for improving the power flow of transmission
and distribution systems. Switching capacitor banks can produce transients resulting in damaged
equipment or nuisance outages. This paper presents an overview of capacitor switching options showing
the expected transient currents in single-bank and back-to-back switching.
Introduction
There is an increasing demand on utilities to improve the power quality due the sensitivity of electronics
employed in manufacturing and communication systems. Maintaining the power quality level has further
been complicated with the recent and increasing focus on renewable energy generation. Utilities also
strive to maximize the power transfer efficiency of lines by maintaining a power factor close to unity. The
application of shunt capacitor banks has become a proven and favored tool for adding reactive loads,
improving voltage control on heavily loaded transmission and distribution systems. An often overlooked
part of the shunt capacitor system is the capacitor switching device. When the load levels are high the
shunt capacitor would typically be connected to the transmission or distribution lines to counteract the
inductive loads on the system. But as the load drops, it is critical that the capacitors be disconnected from
the system to avoid overvoltages that could cause damage to equipment and unexpected outages.
As one might expect, it is not unusual for shunt capacitors to be switched daily as the system loads vary
during the day. Connecting and disconnecting shunt capacitor banks from the system presents some
unique challenges since the voltage across the capacitor cannot change instantaneously. This switching
causes unwanted, high frequency, voltage and current transients that may cause damage to equipment,
nuisance equipment operation, or disrupt customer processes. Several techniques have been developed to
mitigate these transients including application of arrestors, inrush reactors, controlled voltage switching
devices, pre-insertion resistors, and pre-insertion inductors.
Switching Shunt Capacitors
Interruption of capacitive currents is generally considered easily achievable for modern SF6 interrupting
devices due to the low magnitude of the currents (hundreds of amperes) being interrupted and the very
slow recovery voltage. This is also true for vacuum interrupters at medium voltage levels. There is a
probability though that there may be a re-ignition (resumption of current less than ¼ cycle after current
interruption) or restrike (resumption of current ¼ cycle after current interruption) during the interruption
which may lead to undesirable overvoltages or high frequency transients that may cause damage to the
switching device or capacitor cans and affect system power quality.
Restrikes or re-ignitions occur when the dielectric strength of the open gap, during contact parting, is not
great enough to withstand the recovery voltage across the open gap. With a capacitive load, the current
waveform leads the voltage waveform by 900 (see Figure 1)
Fig. 1. Capacitive load voltage and current waveforms
The current is interrupted close to the zero crossing when the voltage is at its maximum value. The
supply side voltage is relatively unaffected following the interruption, but the voltage of the capacitor
bank becomes trapped maintaining the peak voltage level (very low rate of decay) that was present at the
time of current interruption. The initial low rate of rise of the recovery voltage and the low level of
current being switched make it easy for the switching device to interrupt.
Since the level of the currents being switched are low, the switching device, which is often a general
purpose device such as a power circuit breaker or vacuum interrupter, may interrupt the current at a point
where the contact separation and parting speed is not sufficient to withstand the voltage difference across
the contacts leading to a restrike and the resumption of current flow. This restrike causes an arc that
reestablishes current flow. The switching device will attempt to interrupt the current at each current zero.
If the high frequency zero is interrupted, then voltage escalation can occur. It is then possible that a
second reignition will occur and another attempt at interruption will have to wait until the next current
zero. Figure 2 shows the worst case that may occur.
Fig. 2. Effect of multiple restrikes
An ideal capacitive switching device would be designed with a higher transient recovery voltage
capability than needed to eliminate or minimize the likelihood of a restrike or reignition occurring.
General purpose switching devices such as power circuit breakers and vacuum interrupters designed
mainly for switching of resistive loads may fail to meet the required level of performance when used as
stand-alone devices. Frequently, this is because damage to the interrupters is progressive with more
operations.
As discussed previously, the current waveform leads the voltage waveform by 900 (see Figure 1). If the
switching device was to close and energize the capacitive load at voltage zero, the current might be
expected to jump immediately to its maximum value. Since this “instantaneous” change is not possible,
there is a very fast rise in current that overshoots the maximum value. The peak current is determined by
the rated capacitor current, the strength of the system, and the available fault current. As the current
increases, there is a steep drop in system voltage toward zero, followed by a fast voltage recovery
(overshoot). The voltage and the current waveforms will oscillate at a high frequency eventually settling
down to their steady state value (see Figure 3). The high frequency voltage transient is superimposed on
the 60 Hz fundamental waveform and can reach 2.0 times the normal system voltage peak (per-unit)
under worst case conditions. The transient frequencies will typically fall in a range of 300 – 1000 Hz.
V
I
Fig. 3. Simulation – Energizing a Capacitor Bank
The transient overvoltages that occur are typically not a concern to the utility as the peak magnitudes fall
below that of surge protection devices. Due to their relatively low frequency, fast transient overvoltages
will be passed through system transformers and magnified due to capacitive coupling. Open circuited
lines can also contribute to the overvoltage conditions by reflecting the waveform back to the source,
where it can add to the standing voltage waveform. This system resonance can result in overvoltage
conditions reaching 2.0 to 4.0 per unit at customer facilities. This may result in customer equipment
damage and shut down of electronic equipment such as adjustable speed drives resulting in lost
production or downtime.
When two or more capacitor banks are located close to one another the transients become more severe.
This is due to the fact that when the second capacitor bank is energized, it appears to the system as a short
circuit. This will result in any capacitor bank nearby to discharge into the recently closed second bank. In
addition, the two capacitors in parallel create a larger capacitive load and result in drawing a much larger
inrush current that if there was only one bank. This is called back-to-back switching.
High inrush currents produce a large arc, during switching, which in general purpose SF6 interupting
devices can result in erosion of the contacts and pre-mature nozzle failure. In vacuum interrupters there is
a potential that excessive contact melting could occur which may result in welding of contacts. When the
device is opened the weld is broken leaving a rough contact surface that may be more prone to restrikes.
Common Methods Used to Control Transients
Devices currently utilized for switching capacitors:
•
•
•
•
•
General purpose power breakers and circuit switchers
Vacuum switches
Circuit Switchers with pre-insertion inductors
Synchronous (Zero Voltage) close power breakers or vacuum switches
Purpose built switching devices
The following is a brief summary of the pros and cons of each switching method.
A. General purpose power breakers and circuit switchers.
Power circuit breakers and circuit switchers are general purpose devices designed primarily for
line/bus switching and protection. They have been used for shunt capacitor switching for many
years. Circuit breakers will almost always interrupt the current at a forced current zero or the first
current zero following contact separation. At this point, the contact separation is usually
sufficient to withstand the recovery voltage imposed upon it. They have no inherent capability
for mitigating voltage or current transients that occur during energization of the capacitor bank.
The inrush current peak, especially in back-to-back capacitor switching applications, can be quite
high such that ANSI standards have recommended limiting this inrush current to 16kA peak at a
frequency of up to 4.2 kHz by applying series reactors in the circuit. Even with the application of
reactors, multiple events will eventually result in contact wear and nozzle punctures. The reason
for concern about this arcing damage is the fact that the effects are cumulative over time and
interrupter life is significantly reduced. Power tests in laboratories generally demonstrate
capability of new devices but are rarely done with hundreds of operations in power test labs on
72.5 kV and higher voltage rated equipment. This is principally because the cost of doing these
tests is generally quite expensive. Generally, reliance on actual field experience is the best
method but this does take time to expose weaknesses and verify good field performance. The use
of series reactors is a quite common solution for back to back switching of capacitor banks.
Given the available alternatives, this use of reactors was a major improvement over the previous
option of switching without them.
When adding a series reactor, it is also advisable to include a capacitor across the breaker
contacts, across the reactor or line-to-ground between the breaker and reactor to control the
transient recovery voltage during when a fault occurs on the line side of the reactor.
Fig. 4: Power Circuit Breaker
Fig. 5: Circuit Switcher
The circuit breaker or circuit switcher used as a capacitor switching device has the following advantages:
• Full interrupter ratings
• Bushing mounted current transformers (breaker only)
• Local visual gas system indicator
• Remote gas monitoring
• Making and breaking the circuit in SF6
• Same device used for protection of transformers
They also have the following disadvantages:
• Do not mitigate current transients
• Do not mitigate voltage transients
• Require series reactors to limit inrush
• Use of series reactors creates need for capacitors to protect contacts
• Inrush currents likely to damage interrupter contacts over time causing pre-mature failure or
increased maintenance
B. Vacuum Switches
Vacuum switches and breakers are general purpose devices that have been used for shunt
capacitor switching at medium voltage for many years. They have no inherent capability for
mitigating voltage or current transients that occur during energization of the capacitor bank.
The arcing experienced during energizing of a capacitor bank, especially when the second bank
closes on a back-to-back installation, will cause melting of the vacuum interrupter’s contact
material. With the contacts pressed together under high force, the melted contact material will
tend to weld the contacts. When the contacts are opened, this weld is torn apart, leaving rough
contact surfaces (see Fig. 6). This results in a higher voltage stress across the contacts and
increases the probability of restrikes.
Stationary Contact
Moving Contact
Pre-arc melts the copper on
the surface and the contacts
can close on molten copper
Localized heating due to Prearc or high currents can weld
the contacts
The welds that can form will tear on opening and leave
projections that can cause the dielectric strength to breakdown
on opening causing restrikes
Fig. 6: Restriking of Vacuum Switch Contacts
There is also a significant risk of contact welding with the switcher on the first bank. The high
frequency, high magnitude inrush current is likely to exceed the withstand rating of the vacuum
interrupter. This will result in welding of the contacts and lead to either pre-mature failure of the
device or restriking during operation.
Fig. 7: Vacuum Circuit Breaker
Fig. 8: Vacuum Switcher
The vacuum breaker or switcher used as a capacitor switching device has the following advantages:
• Full interrupter ratings (breaker only)
• Bushing mounted current transformers (breaker only)
• Lowest First Cost
• Capable of a high number of operations
• Vacuum Switch can mount in the rack at 38 kV and below
They also have the following disadvantages:
• Do not mitigate current transients
• Do not mitigate voltage transients
• Require series reactors to limit inrush
• Use of series reactors creates need for capacitors or arrestors to protect contacts
• Inrush currents likely to damage interrupter contacts over time causing pre-mature failure or
increased maintenance
• Typically limited to medium voltage applications
C. Circuit Switchers with pre-insertion inductors
One approach to mitigating transients is to briefly place an inductor
in series with the capacitor during energization of the capacitor
bank. This has been done using a circuit switcher where the
inductors (with resistance) are inserted into the capacitor closing
circuit for 7-12 cycles during the closing of the disconnect blade.
Insertion is accomplished through a sliding contact between the
blade and the inductor on each pole of the switch. This operation
introduces impedance that limits the initial inrush current and
reduces voltage transients. The impedance (inductor) is shorted out
(bypassed) a few cycles after the initial insertion transient damps
out. The insertion method utilized limits the ability of this solution
to handle closing in on faults.
The circuit switcher with pre-insertion inductor has the following
advantages:
• Inductor is in the circuit only during closing
• Limited Inductor and Resistance ratings
• Provides visible break
• Current Interrupted in SF6
They also have the following disadvantages:
• No fault close or fault interrupting capability
• Limited Inductor and Resistance ratings
• Relatively high cost
• Arcing in air during closing operation
• Many moving parts increase maintenance requirements
Fig. 9: Circuit Switcher with
pre-insertion inductor
E. Synchronous close power breakers or vacuum switches
A synchronous close device utilizes controlled switching as a method to
minimize or eliminate switching transients by energizing a capacitor at the
point when the voltage across the circuit breaker contacts is zero. This will
minimize or eliminate voltage and current transients.
A sophisticated control must be utilized to monitor many factors that
effect the probability of the device achieving a zero voltage close. Most
controls monitor mechanism speed, the voltage waveform, ambient
temperature, and control voltage. Complex algorithms are required to
utilize all of the available information to predict when the signal to close
should be given to insure a zero-voltage close operation. To achieve the
benefits of a zero-voltage close the controller must close with less than a
1.0 ms error.
While many utilities have had success with synchronous close circuit
breakers, there are others that have had difficulty maintaining a consistent
zero-voltage close operation. This is most likely due to the fact that the
mechanical systems have not yet caught up to the control technology in
their ability to consistently meet the response requirements.
Fig, 10: Zero voltage close
circuit breaker
Synchronous close devices have the following advantages:
• Theoretically the best way to mitigate transients
• Full interrupter ratings (breaker only)
• Bushing mounted current transformers (breaker only)
• High first cost
• High maintenance cost
They also have the following disadvantages:
• High first cost
• High maintenance cost
• Difficult to achieve repeatable performance
F. Purpose built switching device
A special purpose capacitor switcher was introduced into the market in 2003. The primary objective, of
the design, was to achieve a reliable long life solution for mitigating the transients that occur when
switching in a capacitor bank. The resulting product, capable of a high number of operations (10,000),
utilizes closing resistors to mitigate voltage transients and inrush current. By designing specifically for
this application, the manufacturer was able to include those features that optimize the reliability and
performance of the device. A side benefit was that by removing certain features inherent in general
purpose devices, the resulting device was also very cost competitive.
Fig, 11: 38 kV CapSwitcher®
In this design, the resistor, resistor contact
design, and resistor insertion method were the
keys to achieving a reliable long term solution
for mitigating the transients that occur when
switching in a capacitor bank. Additionally, the
contacts and nozzle were designed to minimize
or eliminate the possibility of restrikes during
the disconnection of the capacitor bank from
the system. Some models include interrupting
capability, although this was a lower priority in
the initial approach to the design. The contact
and nozzle design are the keys to insuring that
the device could survive high frequency inrush
currents and virtually eliminate restrikes.
Fig, 12: 138 kV CapSwitcher®
Fig, 13: Special purpose capacitor switch
interrupter diagram
The CapSwitcher purpose built switching device has the following advantages:
• Closing resistors for mitigation of voltage and current transients
• Resistor sizes to allow optimization for best results
• Simple, cost effective, mechanical design that provides repeatability
• Long Life (10,000 operations)
• Eliminates need for inrush reactors
• Interrupting capability on most designs
• Fault close ratings on all designs
• Making and breaking the circuit in SF6
• Design virtually eliminates restrikes
They also have the following disadvantages:
• Some ratings do not have fault interrupting capability
Comments on Standards covering capacitor switching devices
TM
In 2005, IEEE issued C37.09a titled “IEEE Standard Test Procedure for AC High-Voltage circuit
Breakers Rated on a Symmetrical Current Basis – Amendment 1: Capacitance Current Switching”. This
Amendment was the outcome of a joint IEEE/IEC task force assigned to revise and harmonize standards
for switching capacitance current. One of the key outcomes was the unbundling of tests previously
covered in C37.09 “IEEE Standard Test Procedure for AC High-Voltage circuit Breakers Rated on a
Symmetrical Current Basis”. The tests now allow a circuit breaker to have one or a variety of capacitive
current switching ratings including: overhead line switching (LC), cable switching (CS), Capacitor Bank
ratings (both single bank and back-to-back)(BC), or additional ratings which must be specified separately.
Three classes of circuit breaker were established based on the probability of restriking performance.
“Class C0” (one restrike per operation, unspecified probablility), “Class C1” (Low Probability), and Class
C2 (very low probability). A probability of restrike performance is established for each type of capacitive
current switching rating.
It is recommended that devices used to switch capacitor banks meet the requirements of “Class C1” or
“Class C2” for duty BC.
Conclusion
The increasing requirement for improved power quality and the desire to maximize power transfer
efficiency of lines has resulted in the need for improved voltage control. As a result, the application of
capacitor banks is increasing, which has exposed the weaknesses of existing switching device designs.
Switching shunt capacitors places stress on the system, the capacitors, and the switching device. A utility
should look for a device that mitigates voltage and current transients, is reliable, is capable of a high
number of operations, and is cost effective. Switching device designs that are based on general purpose
requirements may have performance requirements that may reduce the intrinsic performance when
switching capacitor banks. Significant improvement in product life and reliability may be achieved
through use of purpose built devices, with closing resistor technology, for switching shunt capacitors.
References
[1]
[2]
[3]
[4]
[5]
IEEE Standard C37.012, “Application Guide for Capacitance Current Switching for AC High-Voltage Circuit Breakers”
IEEE Standard C37.009a, “Standard Test Procedure for AC High-Voltage Circuit Breakers Rated on a Symmetrical Current Basis –
Amendment 1: Capacitance current Switching”
M. Beanland, T. Speas, and J. Rostron, “Pre-insertion Resistors in High Voltage Capacitor Bank Switching,” – Western Protective Relay
Conference October 2004.
T. Grebe, “Capacitor Switching and its Impact on Power Quality,” Cigre 36.06 / CIRED 2 CC02.
A Greenwood, “ELECTRICAL TRANSIENTS IN POWER SYSTEMS - second edition,
© 1991 John Wiley and Sons, ISBN 0471-62058-0