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Grading
Capacitor Requirements for Metal-Enclosed Capacitor
Banks Utilizing Vacuum Switches with Two or More Series
Connected Vacuum Bottles
Metal-Enclosed
Capacitor Banks and Harmonic Filter Banks applied
at the 34.5kV to 38kV voltage level often utilize vacuum
switches (as shown in Figure 1) equipped with two or
more series connected vacuum interrupters. These vacuum
switches, when applied within a metal enclosure, are
more likely to re-strike and result in equipment damage.
To reduce this possibility, NEPSI recommends a grading
capacitor be connected across each of the series connected
vacuum interrupters.
This
tech-note provides background information on the above
recommendation and is based upon actual analysis and
events experienced by NEPSI during the commissioning
of two 34.5kV, 18 MVAR, 3 Step 3 Stage Metal-Enclosed
Automatic Capacitor Banks purchased by the City of Burbank.
Definitions
Transient Recovery Voltage - Also referenced
as TRV, is defined as the voltage across the vacuum
contacts of a vacuum switch immediately following current
interruption. Capacitor de-energization provides a high
level of transient recovery voltage to vacuum switches
as peak positive or negative voltage is trapped on the
capacitor just following current interruption.
Restrike
- Is defined as the re-ignition of load current following
current interruption within a vacuum switch. It occurs
during the switch interruption process when the parting
contacts of a vacuum switch do not achieve adequate
dielectric recovery and the extinguished arc reignites
("restrikes"). The restrike can result in
capacitor and system transient over-voltages of 3, 5,
7, and even 9 per unit system line-to-neutral voltage.
Figure two shows a typical case rupture caused by vacuum
switch re-strike. In this particular restrike case,
five capacitors cans ruptured as shown.
Description of Problem
Vacuum switches that utilize two or more series connected
vacuum bottles work on the premise that each vacuum
bottle will share nearly equal percentages of the transient
recovery voltage during interruption. The application
of this premise allows vacuum switch manufacturers to
use lower voltage rated vacuum bottles to achieve load
current interruption at higher voltage levels. When
the transient recovery voltage is not equally shared,
however, restrike free load interruption may not take
place. Restrike results in the re-ignition of current
flow and is followed by a high voltage and current transient
similar to an overshoot experienced during capacitor
energization (see NEPSI tech-note on Capacitor Bank
Switching Transients at the following web address http://www.nepsi.com/transients.htm).
The transient however, is far worse and can result in
equipment damage as shown and experienced by our equipment
in Figure 2.
Figure
3
- Capacitor Voltage, Vacuum Bottle Voltage and Line
Voltage With
Cmech + Cstray =35Pf
Figure
3 illustrates the reason restrike is more probable in
vacuum switches that utilize two or more series connected
vacuum bottles. The figure shows a single-phase representation
of a vacuum switch utilizing two series connected vacuum
bottles, utility voltage source, 2 MVAR capacitor bank,
vacuum bottle capacitance, and capacitance connected
to the mid-point of the vacuum bottles. The mid-point
capacitance consists of Cstray and Cmech. Cmech exists
regardless of whether the bank is mounted in an enclosure
or outside on a rack. Cstray consists of capacitance
to all other grounded objects (in our case, the metal
enclosure).
The
mid-point capacitance causes unequal voltage division
between the two series vacuum bottles during interruption.
Ideally, as stated above, equal voltage division between
the vacuum bottles is desired. The unequal voltage division
causes the line-side vacuum bottle to carry a disproportionate
amount of the Transient Recovery Voltage (TRV) during
de-energization of the bank. Based on field measurements,
nearly 70% of the TRV can be distributed across the
line-side vacuum bottle (in a two vacuum bottle switch),
while the load side vacuum bottle experiences near 30%.
Figure 4 below illustrates how the voltage division
is developed during de-energization. In looking at the
figure and the equations, and assuming that C1 equals
C2, it is obvious that the sum of Cmech+Cstray causes
the unequal voltage division.
Figure
4
- Capacitor Voltage, Vacuum Bottle Voltage and Line
Voltage With
Cmech + Cstray =35Pf
EMTP
Results
Figure 5 and Figure 6 below show the waveform plots
as simulated in EMTP by NEPSI using field test data
of the actual system at the City of Burbank. The waveforms
show a disproportionate voltage across the line side
vacuum bottle. Nearly 70% of the TRV is dropped across
the line-side vacuum bottle and only 30% across the
load-side vacuum bottle. This unequal sharing resulted
in switch restrike.
Figure 5
- Capacitor Voltage, Vacuum Bottle Voltage and Line
Voltage With
Cmech + Cstray =35Pf
Key
to Figure 5
V[SW2A,NEUT1](1) = Capacitor Voltage to Ground
V[BK1A,SW1A](1) = Voltage Across Line-Side Vacuum Bottle
during interruption
V[SW1A,SW2A](1) = Voltage Across Load-Side Vacuum Bottle
during interruption
V[BK1A,gnd](1) = System Line-Ground Voltage at Capacitor
Bank Terminals
Figure 6
- Capacitor Current With
Cmech + Cstray =35Pf
Key
to Figure 6
i[SW2A,NEUT1](1) = Switch current or capacitor bank
current
Corrective
Measures
To ensure equal voltage division across the series connected
vacuum bottles, NEPSI recommends the following measures:
-
After Installation of the vacuum switch within the
enclosure, measure the capacitance across each vacuum
bottle when the switch is in the open position.
A high end, low capacitance meter such as the one
shown in figure 7 is required to perform this test.
If nearly equal values of capacitance are not measured,
install sufficiently sized grading capacitors to
equalize the voltage.
-
After
installation of the grading capacitors, re-check
the capacitance across each vacuum bottle to ensure
nearly equal values. If values are not nearly equal,
adjust the size of the grading capacitor.
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Figure
8
- Picture to left shows the 18 MVAR 3 Step
3 stage automatic capacitor bank being loaded
for shippment to the City of Burbank. The bank
was of all stainless steel construction and painted
per the customer's requested color.
After the installation of grading capacitors,
the vacuum switches operated free from restrikes
during de-energization.
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Northeast
Power Systems, Inc.
66 Carey Road
Queensbury, NY 12804
Phone:
518-792-4776
Fax: 518-792-5767
Website: www.nepsi.com
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