Download CIRED 2003 Barcelona

Name
Company
Address
Phone
e-mail
Contributor identification
R. Kirkland Smith
Eaton / Cutler-Hammer
200 Westinghouse Circle
Horseheads, NY 14845 USA
607 – 796 – 3370
[email protected]
Contribution identification
Session
1
Block
1
Question n°
Paper No. 5 - ECH_Smith_A1
Language used on the floor
English
Accompanying visuals on file ? YES
VACUUM INTERRUPTERS FOR GENERATOR CIRCUIT BREAKERS
THEY'RE NOT JUST FOR DISTRIBUTION CIRCUITS BREAKERS
ANYMORE
Vacuum interrupters, first widely used in distribution circuit breakers, are now also applied to
generator circuit breakers.
Vacuum interrupters are small, permanently sealed and provide a long, low maintenance life.
Vacuum circuit breakers are therefore compact and can now replace older, larger generator
breakers. Our new Vacuum Generator Circuit Breaker was successfully tested according to ANSI
C37.013a, the soon to be adopted amendment to the generator circuit breaker standard. This
amendment addresses the needs of smaller generators rated between 10 and 100 MVA.
Transverse Magnetic Field or TMF vacuum interrupters are well suited to generator circuit
conditions. The contacts of a TMF interrupter are comprised of petals. Current flowing through the
petals to and from the arc forms a loop that produces a magnetic force to drive the arc rapidly
around the contacts. The arc energy is thereby spread over the contacts and long arcing times are
tolerated. TMF interrupters also produce a significant arc voltage that is useful in this application.
The short circuit current interrupting capability of vacuum interrupters has steadily increased since
their introduction. Cutler-Hammer’s first vacuum interrupter in 1966 was 182 millimeters in diameter
and rated at 15 kV – 12 kA. With steady improvements to product design and manufacturing
methods, interrupters of this same size are now rated at 63 kA with 80 kA possible in the near
future. This interrupter also includes the ability to function in generator circuit breaker applications.
Generator circuit fault interrupting conditions are severe. These include:
High short circuit currents
Long DC time constants and High DC components
Fast Transient Recovery Voltages and
Delayed current zeros
Vacuum interrupters, especially TMF vacuum interrupters, are well suited to these conditions.
A generator circuit breaker is located between the generator and the transformer that connects to
the system. Two fault types are important.
In a System-Source Fault, shown at location “A”, current flows to the circuit breaker
from the system through the transformer.
In a Generator-Source Fault, shown at location “B”, current flows to the circuit
breaker from the generator.
We will first discuss the System-Source Fault.
SMITH_USA_Author_ALPHA1_BLOCK1_Paper No 5-ECH_Smith_A1
1/2
System-Source Faults produce the highest currents to establish the breaker rating. Generator circuit
connections are usually very short with low resistance to minimise losses while carrying high
continuous currents. In comparison to distribution circuits, generator circuits have
a high ratio of inductance to resistance resulting in a long DC time constant during
asymmetrical currents and a high DC component at contact part, and
a low added capacitance resulting in fast Transient Recovery Voltage Rate-of-Rise.
System-Source Faults combine a High DC component with a fast Transient Recovery Voltage.
The DC component at contact part of a System-Source Fault in a generator circuit is typically 75%
or more in contrast to 30% to 50% in a typical distribution circuit fault as shown in this figure.
The Transient Recovery Voltage Rate-of-Rise for System-Source Faults is very fast. The natural
frequency of large transformers is very high and the added capacitance of the connected circuit is
low resulting in a fast Transient Recovery Voltage. The Transient Recovery Voltage Rate-of-Rise is
about 10 times greater in generator circuits than for distribution circuits as shown in this figure.
The other important fault condition is the Generator-Source Fault.
Generator-Source Fault currents are only about half the magnitude of System-Source Fault currents.
However, the Transient Recovery Voltage Rate-of-Rise for Generator-Source Faults is still very fast
since the natural frequency of the generator is also very high. A unique feature for GeneratorSource Faults is that the DC component can be greater than 100%, shifting the current completely
above zero, and resulting in a substantial time delay to the first current zero. The delay of first
current zero requires that a long arcing time be tolerated.
In this example, the first current zero occurs at 5 cycles. For a circuit breaker whose contacts part in
about 2 cycles, the arcing time must then be nearly 3 cycles. This is very much longer than the
typical 1 cycle or shorter arcing time experienced when current zeros are not delayed.
The average arc voltage in a TMF vacuum interrupter is approximately 150 volts. In this example,
this is enough to double the overall circuit resistance and cut the DC time constant in half. In
contrast, the arc voltage from an Axial Magnetic Field or AMF vacuum interrupter is only about 50
volts producing only one third of the effect on the DC time constant.
The resistance of the arc voltage increases the DC component decay and advances the first current
zero. As shown in the same example, with arc voltage included, the first current zero occurs at least
1 cycle earlier, reducing the arcing time to less than 2 cycles.
The tests on our new breaker were based on a rating of 15 kV - 63 kA - 60 Hz – 3000 Amps
applicable to generators of about 80 MVA.
The Short Time Current Rating was demonstrated at 63 kA, rms - 3 seconds.
The System-source fault tests at 63 kA included a 79 % DC component and a rate of rise of
3.7 kV / microsecond.
The Generator-source fault tests at 31.5 kA, rms included a 135% DC component and a fast
TRV rate of rise to show the performance with delayed current zeros.
The Out-of-Phase Switching tests also at 31.5 kA, rms demonstrated performance with a
higher TRV peak of 45 kV as well as a fast TRV rate of rise.
These tests were performed in the CESI synthetic test laboratory in Milan, Italy.
As these tests illustrate, vacuum interrupters now provide the robust performance required for the
severe duty of generator circuit protection. With a high arc voltage and a great tolerance for long
arcing times, the TMF interrupter is particularly well suited for generator circuit breaker applications.
SMITH_USA_Author_ALPHA1_BLOCK1_Paper No 5-ECH_Smith_A1
2/2