Current Transformers in Electrical Panel
Electricity supply from the public grid enters every household through a main electrical switch board installed at the periphery of each home. From the board, the electricity reaches individual appliances through the wiring network. The electrical panel board is a critical component in the power supply system as it helps to isolate the grid power from the household electrical system.
Similarly, in industrial environments, especially with automated manufacturing processes, electrical control panels contain devices that direct the operations of various equipment and machines. The electrical panel performs a similar function as in household electrical installations – that of isolation from the main power grid and protection of equipment, in case of a fault.
A Protection type Current Transformer is installed as part of every electrical board to aid this critical function.
Industrial electrical control panels typically consist of the power supply, circuit breakers, disconnection switch, terminal blocks, protective relays, contractor switches, fuses, motor drives and the protection current transformer.
Domestic electrical panels also house many of the above mentioned components, along with the transformer. To explain the functions of an electrical panel in simpler terms…the main breaker panel turns the power on or off in all the branch circuits within the house. Electrical current travels through wires from the circuit breakers to power the various electrical devices.
Components such as circuit breakers, disconnect switch, fuses and overload relays play a protective role in the system. Their primary function is to protect the motor and other devices by opening / breaking the circuit when there is excess current flow from the supply. Another function is to shut off power to the control panel during repair and maintenance apart from emergency shutdown.
The Protection Current Transformer is used to provide the currents to protection Relay. This is achieved by producing proportional currents in the unit’s secondary windings that remain isolated from the main circuit. This helps to supply power to instruments and devices that cannot be directly connected to a power supply.
These current transformers have their primary winding connected in series with the conductor which carries the current to be measured or controlled. The secondary winding is insulated from the high voltage and is connected to low-voltage metering or distribution circuits. The replica current is used as an input to a protective relay, which automatically isolates part of the power circuit, when a fault occurs. In doing so, the protection current transformer allows the unaffected areas of the circuit to continue in operation.
Rated primary current – Defined by standards. Such as 10 – 12.5 – 15 – 20 – 25 – 30 – 40 – 50 – 60 – 75 A and their decimal multiples.
Rated secondary current – either 1A or 5 A.
Rated accuracy power – apparent power supplied to the secondary circuit for the given secondary current and accuracy load. Measured in VA.
Accuracy class – guaranteed error range on the CT ratio and on the phase shift in known power and current conditions.
Accuracy limit factor – ratio between nominal overcurrent and rated current (In). A protection CT must saturate sufficiently high, so that an accurate measurement of the fault current by the protection is possible. Its operating threshold can be very high, so the Accuracy Limit Factor (ALF) of the CT is also usually high. The Current Transformer is generally installed in conjunction with a protection relay. The associated relay is also able to withstand high over-currents.
Knee Point Voltage – voltage at which a 10% increase in voltage of the CT secondary results in a 50% increase in secondary current. The burden of a protection CT is quite high when compared to a metering class CT, therefore the voltage drop across the burden will be high. Therefore, the knee point voltage of a protection class CT must be more than the voltage drop across the burden and load to maintain the CT core in its linear zone.
Construction and Mounting – Protection CTs are generally designed with different construction and mounting styles, depending on the space availability and the installation location (home, factory, etc). Common construction styles include:
Example case of a Protection CT:
Rated primary current: 200 A, Rated secondary current: 5 A
Accuracy load: P = 15 VA, Accuracy Limit Factor, ALF = 10
For I0 = ALF. In, its accuracy is 5% (5P). That is, the Composite error is less than 5% at 10 Ip, load with 15 VA burden.
KS Instruments is a leading player in the design and manufacturing of high accuracy, Low Tension Current Transformers for measuring and protection applications. KSI Current Transformers products come in tape wound, resin cast and ABS enclosure constructions.
KSI has a wide range of catalogue products to suit all needs. These products have been validated by our clients for high efficiency, robust performance and long service life. KS Instruments has an expert design engineering team that can design and manufacture custom components for specific Current Transformer applications.
KSI is approved and widely used for metering applications at various State Government power supply companies such as BESCOM, HESCOM, CHESCOM, and MESCOM. KSI Current Transformers are tested and certified at the reputed CPRI Bengaluru, India (NABL).
These CTs are used to activate the protective relay in case of a fault current and isolate a part of or the full system from the main supply. Apart from the standard ranges, we can design and manufacture for your specific requirements.
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This series of ring type (also called as window type) allows bus bars or cables to be passed through the CT and act as a Primary for the CT.
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These Summation Protective CTs are used to sum secondary currents of several main CTs and supply to a single meter or relay.
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KS Instruments is a leading player in the design and manufacturing of high accuracy Current transformers for measuring and protection applications. KSI CT products come in tape wound, resin cast and ABS enclosure constructions. These products have been validated by our clients for high efficiency, robust performance and long service life. KSI has a wide range of catalogue products to suit all needs, check out here.