CAPACITIVE VOLTAGE TRANSFORMER

BOGHANI KAUSHAL B. (130760109002)

CAPACITIVE VOLTAGE TRANSFORMERoA capacitor voltage transformer (CVT or CCVT), is a transformer used in power systems to step down extra high voltage signals and provide a low voltage signal, for metering or operating a protective relay.

INTRODUCTIONoCapacitor Voltage Transformers (CVT), are used for voltage metering and protection in high voltage network systems. oThey transform the high voltage into low voltage adequate to be processed in measuring and protection instruments secondary equipment, such as relays and recorders).oA Voltage Transformer (VT) isolates the measuring instruments from the high voltage of the monitored circuit. VTs are commonly used for metering and protection in the electrical power industry.oA voltage transformer comprising a capacitor divider unit and an electromagnetic unit so designed and interconnected that the secondary voltage of the electromagnetic unit is substantially proportional to and in phase with the primary voltage applied to the capacitor divider unit.

WORKINGoIn its most basic form, the device consists of three parts: two capacitors across which the transmission line signal is split, an inductive element to tune the device to the line frequency, and a voltage transformer to isolate and further step down the voltage for metering devices or protective relay.oThe tuning of the divider to the line frequency makes the overall division ratio less sensitive to changes in the burden of the connected metering or protection devices.o The device has at least four terminals: a terminal for connection to the high voltage signal, a ground terminal, and two secondary terminals which connect to the instrumentation or protective relay.

oIn practice, capacitor C1 is often constructed as a stack of smaller capacitors connected in series.

o This provides a large voltage drop across C1 and a relatively small voltage drop across C2. As the majority of the voltage drop is on C1, this reduces the required insulation level of the voltage transformer.oThe voltage across the larger capacitance C2 will be V2 = V*C1/(C1+C2) oThis makes CVTs more economical than the wound voltage transformers under high voltage (over 100kV), as the latter one requires more winding and materials.

APPLICATIONoThe CVT is also useful in communication systems. CVTs in combination with wave traps are used for filtering high-frequency communication signals from power frequency. This forms a carrier communication network throughout the transmission network.oVoltage Measuring: They accurately transform transmission voltages down to useable levels for revenue metering, protection and control purposesoInsulation: They guarantee the insulation between HV network and LV circuits ensuring safety condition to control room operatorsoHF Transmissions: They can be used for Power Line Carrier (PLC) coupling oTransient Recovery Voltage: When installed in close proximity to HV/EHV Circuit Breakers, CVT’s own High Capacitance enhance C/B short line fault / TRV performance

Difference between CVT and PTCVT PT

where as CVT is capacitive voltage transformer consisting of stack of series connected capacitor the voltage across the capacitor is used to measure the voltage. It also serves the purpose of power line carrier communication.

Potential Transformer is Inductive step down transformer used for measurement of voltage and protection.

CVT is rated for extremely high voltage levels above 230KV.

while PT's aren't designed for such large values. upto 12KV

 CVT's offer the advantage that the voltage divider capacitor, being itself relatively smaller and lighter, configuration makes the transformer's iron core much smaller in size, and hence more economical, versus what it would be if a pure magnetic transformer would be used.

Core loss is too much high and so it is less economical

  Also the CVT's can be tuned to the fundamental frequency of the line, and the capacitance prevents the inductive "fire-back" of the coils in the transformer when a breaker trips.

PT's can't provide such advantage. 

WHAT IS SATURATION IN CURRENT TRANSFORMER?oThe term saturation of the CT can be defined as a condition in which the CT is no longer able to reproduce the secondary current in proportion to its primary current . oThe basic principle on which the CT reproduces the secondary current is based on the well known Transformer principle where current flowing through the primary winding produces a flux in the core of the transformer which is normally proportional to the current in the primary.o This flux is the basic reason for the current in the Secondary winding . Now if the core is not able to reproduce a flux in proportion to the primary current , then the secondary current will not be in proportion to the primary current and hence the measurement of the current become wrong . oThe phenomena of the CT core unable to produce the flux in proportion to the primary current is called saturation of CT or CT core.

REASON OF SATURATION IN CT.

o With constant VA applied in the CT secondary circuit the CT goes to saturation if the primary current is increased beyond the designed value.oWith constant current in the primary and if the burden is increased beyond the designed value , still we will find the same phenomena of CT saturation.oThe amount of the DC component available in the primary fault current etc.

EFFECT OF SATURATION ON METERING AND PROTECTIVE CURRENT TRANSFORMERoOnce the CT is saturated then it will not be able to reproduce the secondary current in proportion to its primary current and hence the relays connected to this CT circuit may malfunction indiscriminately and trips the circuit without coordination with other relays in the network or it may completely fail to operate for a fault both the condition to be avoided.oThis will give wrong readings.oDue to this huge loss to cost will take place.

REMEDIESoIt is the duty of the protection design engineer to do a CT sizing calculation so that , the CT will not saturate for the worst case through fault . There are number of factors to be considered before a CT is designed for protection purposes which included the maximum symmetrical and asymmetrical short circuit current and the system X/R ratio.oThe symmetrical and asymmetrical short circuit current can be calculated by doing a short circuit study on the system under consideration . For small systems manual calculation of the short circuit study is possible . However for large systems the application of software are generally required , otherwise the calculation become more tedious

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