Dead Tank Circuit Breaker
A circuit breaker is said to be a dead tank if the switching or interruption occurs within a tank at ground potential. In other words, the tank in which the opening and closing of contacts occur is at ground potential. The incoming and outgoing conductors are taken at the insulated bushings as shown in Figure 1. Due to the construction of a dead tank circuit breaker, it is possible to install current transformers (CTs) within the bushings for measuring current for protection and metering applications. It is relatively expensive compared to live tank circuit breakers of similar ratings.
The voltage range applications for a dead tank circuit breaker differ based on the manufacturer’s capability. For instance, GE offers dead tank breakers that support applications up to 550 kV and can carry continuous current up to 5000 A.
Live Tank Circuit Breaker
This type of circuit breaker has the interruption or switching (the opening/closing of contacts) occurring in an insulator (bushing) that is at potential or voltage above ground. Live tank breakers may seem simple (in terms of structure or parts) in construction but could have a higher voltage rating range compared with dead tank circuit breakers. They are comparably cheaper and require less space relative to dead tank circuit breakers. If a project’s economics and space availability are constraints, then live tank circuit breakers are ideal for such a project. GE offers live tank breakers with a voltage from 72.5 kV to 800 kV. Some applications could go as far as 1,100 kV if needed. Figure 2 shows a live tank circuit breaker.
Factors to Consider when Selecting a High Voltage Circuit Breaker
Dielectric Medium
The opening and closing of circuit breaker contacts result in arcs during the interruption. The arcs would be even higher at high voltages. It is therefore important to quench this arc using a dielectric medium. There are many kinds of dielectric mediums such as sulfur hexafluoride (SF6), vacuum, oil, air blast, etc.
Frequency
A system frequency of 50 Hz or 60 Hz should be specified. This depends on the region or country of use. In most parts of the world (about 80%), the grid operates on 50 Hz while in the United States, Canada, and some other countries use 60 Hz. This is a list of countries with either 50Hz or 60Hz frequencies. Check where the breaker is going to the installed and select it appropriately.
Rated Voltage
Specify the line or system voltage under which the circuit breaker will be used. For instance, if the system voltage is 800 kV, then such voltage should be specified as the rated voltage of the circuit breaker.
Short Circuit Current
The short circuit ratings of equipment are usually selected based on a short circuit study that indicates the maximum amount of fault (bolted fault current) contribution towards each device in a circuit. For high voltage systems, this current will even be less. Manufacturers have specified short circuit currents at high or (any) voltages that could be selected during equipment ordering.
Minimum Temperature
Temperature is a key factor that affects the normal operating conditions of devices and circuit breakers are no exception. Typically, higher temperatures tend to derate the current carrying capacity of equipment due to added heat. Heat is usually energy lost and contributes to starting fires. More importantly, cold weathers tend to increase the amount of voltage an equipment will handle. Increasing voltages beyond rated voltage will cause damage to the equipment insulation thereby leading to short circuit faults. The weather conditions of the location where the circuit breaker will be installed should be checked and considered during the selection of breakers.
Altitude
At higher altitudes, the amount of cooling air circulation decreases and the dielectric strength of the quenching medium is affected as well. Circuit breakers would derate in rated continuous current and voltage at altitudes greater than 6600 feet or 2000 m. Find altitude derating values and their correction factors by Schneider Electric.
Applications
Dead tank (DT) circuit breakers
Pros
- They come compact with embedded current transformers (CTs) which reduces the cost of purchasing separate CTs.
- Can operate in extreme temperatures of -60 up to +60 ͦC.
- Can operate in high-polluting conditions and/or corrosive environments
- They have over 10,000 mechanical operations guaranteed typically be manufacturers
- Less emission rate of about 0.5% per year
- More resilient to earthquakes or vibrations compared to live tank breakers
Cons
- Dead tank circuit breakers occupy more space due to their construction
- Uses more dielectric medium SF6 gas as an arc-quenching material
- SF6 is a greenhouse gas and still would contribute to global warming despite small quantities of release per annum.
- Expensive relative to the similar rating of a live tank breaker
- Less voltage range capability as shown in Figure 3.
Live tank (DT) circuit breakers
Based on the voltage and other specifications listed above, live tank breakers have several application ranges and advantages.
Pros
- They are easy to install due to fewer parts or components to assemble
- A long period of service before inspections are due. Siemens has periods of up to 25 years
- Enormous switching operations are guaranteed up to 10,000
- They have a long life span of up to 50 years in operations.
- Occupies less space
- Less amount of dielectric medium (SF6) filled in the interruption chamber relative to the dead tank breaker.
- An emission rate of less than 0.1% per annum
- Wide temperature range (extreme cold to high, -60 up to +60 ͦC) applications depending on manufacturer’s product capabilities.
- Wide voltage range capability as shown in Figure 4.
Cons
- SF6 is a greenhouse gas and still would contribute to global warming despite small quantities of release per annum.
- Separate current transformers are typically needed for measurement applications
- Less resilient towards natural events such as earthquakes.
Top 10 Manufacturers of Circuit Breakers
- General Electric, GE
- ABB Limited
- Siemens AG
- Alstom
- Eaton Corporation
- Schneider Electric, SE
- Mitsubishi Electric Power Products, Inc
- G & W Electric Co
- Camsco Electric Co. Ltd
- Toshiba Corporation
Others include:
- Kirloskar Electric Co. Ltd
- Powell Industries Inc
- Schurter Holding AG
- Larsen & Toubro Limited
- Sensata Technologies Inc
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