Capacitor voltage transformerFrom Wikipedia, the free encyclopediaJump to: navigation, search

A capacitor voltage transformer (CVT), or capacitance coupled voltage transformer (CCVT) is a transformer used in power systems to step down extra high voltage signals and provide a low voltage signal, for measurement or to operate a protective relay. In 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 transformer to isolate and further step down the voltage for the instrumentation or protective relay. The 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. [1] 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. CVTs are typically single-phase devices used for measuring voltages in excess of one hundred kilovolts where the use of wound primary voltage transformers would be uneconomical. In practice, capacitor C1 is often constructed as a stack of smaller capacitors connected in series. This provides a large voltage drop across C1 and a relatively small voltage drop across C2.

The CVT is also useful in communication systems. CVTs in combination with wave traps are used for filtering high frequency communication signals from power frequency. [2] This forms a carrier communication network throughout the transmission network.

CCVT VS. CVT VS. VT

CVT means capacitive voltage transformer which is connected

between the lightening arrester and CT,which is used to measure the voltage in the line in higher ratings,PT is connected at the end of the bus bar system which is also used to measure the voltage.

The only difference is in its usage...

The VTs are used upto 132kV level the CVTs usage starts from 420kV level and above..CVT means capacitive voltage transformerPT means potential transformer both are same in view of votage measurment CVT is used in the line ckt while PT is used in the bus Ckt.for ur kind informtion CVT may be used as PT but PT can not be used as CVT as per .cvt is capacitive voltage transformer this is using from132KV level PT potential transformer this is using upto 132KV levelbeyond this PT is not economical. CVT and PT both r stepdown transformer. both r using measuring and protectionpurpose but CVT we can use communication purpose.in CVTcapacitor acting as a potential divider.CVT & PT both are protection & measuring devices. CVT is used for high voltages above 132 Kv while Pt is used below this. CVT has an added advantage that it can be used for carreir communication.

We have 2 types of VT in the power systems:

1- Inductive Voltage Transformer (VT or PT)2- Capacitive Voltage Transformer (CVT)

The main difference between VT and CVT is that the CVT isbased on capacitive voltage divider principle which stepsdown the high voltage to a lower voltage level that can bemeasured easier. This can make the VT much less bulky (savespace) and also less expensive. This is why that for voltagelevels up to 132 kV, it’s common to use conventional VTs butat voltage levels more than 132 kV we use CVTs.

Generally the PT or CVT are used to measure voltage.

The secondary of the PT will be 110V which will be connected for the voltage measurement. For high voltage applications like 132 or 230 or 400kV. Designing the PT for 132 to 110V will be very bulky and practically leads to very big size.

Hence for high voltage applications we use CVT. The CVT is constructed using serious of capacitors.

1. What is the difference between Capacitor Voltage Transformer, Coupling Capacitor Voltage Transformer and Voltage Transformer?

2. Why is DC Voltage used in Testing Electrical Equipments not AC Voltage?

a PT, potential transformer, can be thought of as a pure transformer with primary and secondary windings; PT's are sometimes referred to as magnetic transformers due to the fact that their mode of operation is purely magnetic. It is used to step-down the input voltage from a power line to a voltage level that can be processed by metering devices and protection relays in a substation. CVT or CCVT, capacitor-coupled voltage transformer, is made with two capacitor sets acting as a voltage divider that brings the line (actually the phase) voltage down to around 12Kv then this voltage is fed to a relatively small transformer for the voltage signal to be processed. CVT is rated for extremely high voltage levels above 230KV, while PT's aren't designed for such large values. 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. 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. some CVT's are also used to tune to PLCC, Programmable Logic Controller Carrier frequency, which is a signal transmitted over power lines providing inter-PLC communication.

Well a PT uses windings to drop voltage step down transformer high voltage to lower voltage just a transformer only !!! next CVT uses two capacitors in series to drop voltage or split ac like a filter circuit its a tuned 60 cycle resonant cap & coil circuit on primary side . and like a voltage divider circuit but uses capacitors . combined for protection relaying work fault trip zone portection function circuit its also for phase shift ,there really a coupling capacitor voltage splitter circuit centered tap to a reactor coil on primary side input of trans CCVT which is dropped to around 6kv pri input , the center tap of these series caps feed coil and coil feeds trans pri one cap is connected to ground one cap gos to hv line. But multi caps are used in series for the incoming hv line 165kv transmission line , not just two caps by example to make it simple and theres a iron choke coil called a reactor coil it cancels out the caps at 60 cycles for phase shift on output from primary to secondary of CVT transformer theres also a supression circuit on CVT secondary side thats what electricians call it but its just a old snubber circuit for discharge of a potential high voltage spike or back feed kick protection when a major fault occurs on hv transmision line voltage drops off sharply. But the whole circuit works in a delay of false tripping by capacitance stored energy in the caps to hold voltage on primary this controls the tripping time in seconds before a trip of deff relay coils circuits on secondary side which is 115 volts to supply differ relay coils as stated and to control all this with total CVT package circuit and must be included in explaining the difference of both PT and CVT . if you need a full detailed run down how it works just ask? best to ya Transfomer overfluxing can be result of1. Overvoltage2. Low system frequency

A transformer is designed to operate at or below a maximum magnetic flux density in the transformer core. Above this design limit the eddy current in the core and nearby conductive components cause overheating which within a very short time may cause severe damage.The magnetice flux in the core is proportional to the voltage applied to the winding divided by thhe impedence of the winding.The flux in the core increase with either increasing voltage or decreasing frequency.During starrtup or stardown of generator connected transformers, o following a loadrejection, the transformer may exeperience an excessive ratio of volts to hertz, that is, become overexcited.

What is overfluxing relay?

A transformer is designed to operate at or below a maximum

magnetic flux density in the transformer core. Above this design

limit the eddy currents in the core and nearby conductive

components cause overheating which within a very short time

may cause severe damage. The magnetic flux in the core is

proportional to the voltage applied to the winding divided by

the impedance of the winding. The flux in the core increases

with either increasing voltage or decreasing frequency. During

startup or shutdown of generator-connected transformers, or

following a load rejection, the transformer may experience an

excessive ratio of volts to hertz, that is, become overexcited.

When a transformer core is overexcited, the core is operating

in a non-linear magnetic region, and creates harmonic

components in the exciting current. A significant amount of

current at the 5th harmonic is characteristic of overexcitation.

difference between over current and over load protection?

Over current:The full load current capacity, if it exceeds the rated fullload current it is over current.Over Load Protection:Protection given to the electrical equipment to safeguard itfrom buring during the fault level or overloading.Byperforming tripping action.IN AN ELECTRICAL CIRCUIT IF THE CURRENT EXCEEDS THESPECIFIED LEVEL(FULL LOAD CURRENT) IS CALLED OVER CURRENT. TO PROTECT THE MOTOR OR ANY ELECTRICAL EQUIPMENT FROM OVERLOAD(OVER LOAD CAUSES WINDINGS MAY BURN OUT OPEN ETC.,)SOMECIRCUITS ARE PROVIDED TO PROTECT THE MOTOR THIS IS CALLEDOVER LOAD PROTECTION..eg., IN DOL STARTER OLR IS PROTECTTHE MOTOR FROM THE OVER LOAD  EMVT- for protection only(accuracy is very high)CVT- both metering and protectionP.T- both

P.T uses winding for step down the voltageCVT- uses two capacitor in series(capacitor coupling) to step down the voltage

CVT which is also called (CCVT) capacitor coupled voltagetransformer which is used to reduce the voltage from 400 KVand higher to 110 volts for the purpose of derivingmetering, relaying and protection signals. Here the highvoltage is applied across two capacitors in series with aninductor in parallel of these capacitors which form theprimary winding of CVT. The secondary winding terminals atdifferent tapping points are brought out to a small junctionbox mounted on the base of the equipment from where it iswired for different functions

What is the relationship between a current transformer and a voltage transformer?

A transformer is a device that steps up, or steps down voltage. During this process current is also stepped up or down:however, voltage and current are inversely proportional ( meaning an increase in voltage results in a decrease in current and vice versa ) As an example: A step up transformer of 10:1 ratio with 12 volts and 10 amper of current applied to the primary will have ten times the voltage ( 120 volts ) and ten times less current ( 1 amrere ) at the secondary...and a step down transformer with the same turns ratio with 120 volts and 1 ampere applied to the primary will have 12 volts and ten ampere available at the secondary. The electricity supplied into homes and business uses wires carrying very high voltage and low current over long distances, then uses step down transformers to step down the voltage and step up the current.

However, in power engineering and protective relaying applications, there are what are called "instrument transformers" which have the specific purpose of providing information to devices (such as relays or meters) about the voltages or currents in the power system. Therefore, there are some differences in construction and connectivity

between a Current Transformer (CT) and a Voltage (or Potential) Transformer (PT).

A CT will typically have a toroidal core and evenly distributed secondary windings so as to minimize leakage reactance. The primary is typically the main power line conductor, which passes directly through the toroidal core. This type of transformer is specifically for the purpose of measuring current values, and the secondary windings cannot be left open-circuited, or a large voltage will be produce, resulting in dielectric failure (and often an explosion). If a device is not connected to the CT, its secondary must be short-circuited.

A PT is connected between the main conductor and ground and can be either wound in the normal way, or the voltage can be taken from a subsection of a string of capacitors (this is called a Capacitive Voltage Tansformer or CVT, and is usually cheaper than the wound type, but is typically not as accurate). This type of transformer measures voltage values, and the secondary winding cannot be short-circuited, as this will produce excessively high currents, resulting in the failure of the PT or the wires it is connected to. A PT can be left open-circuited.