Name / R. Kirkland Smith / Session / 1
Company / Eaton / Cutler-Hammer / Block / 1
Address / 200 Westinghouse Circle / Question n° / Paper No. 5 - ECH_Smith_A1
Horseheads, NY 14845 USA / Language used on the floor / English
Phone / 607 – 796 – 3370 / Accompanying visuals on file ? / YES
e-mail /
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.
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 Generator-Source 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.
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