Learn Some Basic Knowledge about Capacitor Voltage Transformer



Warm hints: The word in this article is about 2500 and  reading time is about 12 minutes.

Summary

In the power system, in addition to the traditional harmonic sources, such as electric arc furnace and frequency converter, the nonlinear loads such as new energy access and charging pile may produce a lot of harmonics. In order to prevent harmonic from further affecting the power grid, accurate monitoring and timely treatment of harmonic level in power grid is a necessary step. The correct measurement of harmonic content in power grid is the basis of monitoring and governance. This paper mainly introduces some basic  knowledge about capacitor voltage transformer including the capacitor voltage transformer symbol; testing; working principle; capacitive voltage transformer VS inductive voltage transformer and etc.

Article corecapacitor voltage transformerAbbreviationCVT
English namecapacitor voltage transformerCategoryPower
SubjectPower engineercomposeCapacitive voltage divider and medium voltage transformer
FieldEnergy

 


Catalogs

CatalogsI. What is Capacitor Voltage Transformer( CVT)3.1 Insulation resistance measurement
1.1 The composition of CVT3.2 Capacitance Measurement
1.2 CTV Judgment of common anomalies3.3 Pressure swing ratio test
1.3 CVT equivalent circuit model3.4 Polar measurement
II.Terminal sign of capacitive voltage transformerIV.Working principle of capacitive voltage transformer (CVT)
III.Capacitor voltage transformer testingV.Capacitive Voltage Transformer VS Inductive Voltage Transformer



Introduction

I.What is Capacitor Voltage Transformer( CVT)

1.1 The composition of CVT

一、The capacitive voltage transformer is mainly composed of a capacitor voltage divider and a medium voltage transformer. The capacitor divider is made up of porcelain bushing and series capacitors installed in it. The porcelain bushing is filled with insulating oil that keeps 0.1MPa positive pressure, and steel bellows are used to balance different environments to maintain oil pressure. The capacitor divider can be used as a coupling capacitor to connect the carrier device. The medium voltage transformer is composed of a transformer, a compensating reactor, a lightning arrester and a damping device installed in a sealed tank, and the space on the top of the tank is filled with nitrogen. The primary windings are divided into main windings and fine tuning windings, and a low loss reactor is connected in series between one side and one winding. Due to the capacitance and the inherent nonlinear impedance of the capacitor voltage transformer sometimes cause Ferroresonance in capacitor voltage transformer, thus suppressing resonant damping device, damping device is composed of a resistor and reactor, connected across the two windings, normally the damping device has very high impedance, when iron magnetic resonance caused by overvoltage in medium voltage transformer affected before the reactor is saturated only resistive load, the oscillation energy will soon be reduced.

1.2 CTV Judgment of common anomalies

  • (1)The secondary voltage fluctuation. The two connection is loose, the distributor is not grounded or the carrier coil is not connected. If the damper is a fast saturable reactor, it may be improper parameter matching.

  • (2)The Secondary voltage is low. Its connection is bad and the electromagnetic unit failure or the capacitor unit C2 is damaged.

  • (3)The secondary voltage is high.The capacitance unit C1 is damaged and the ground end of the partial voltage capacitor is ungrounded.

  • (4)The oil level of the electromagnetic unit is too high. The next capacitance unit is leaking oil or electromagnetic unit into the water.

  • (5)There is a different sound in the transportation. Bolt loosening of reactor or medium pressure rheostat in electromagnetic unit.

1.3 CVT equivalent circuit model

Under the condition of steady state, the whole CVT equivalent circuit can be regarded as a linear system, which compensates the stray capacitance of reactor C. The influence of the primary stray capacitance C: of the intermediate transformer at the high frequency can not be ignored. CVT intermediate transformer core can be regarded as linear segments in the magnetization curve, ignoring the core magnetizing inductance, one or two intermediate transformer side leakage resistance reduction to compensation reactor.

II.Terminal sign of capacitive voltage transformer

· A single phase transformer with a two - time winding

A single phase transformer with a two - time winding--Learn Some Basic Knowledge about Capacitor Voltage Transformer 

It represents a single-phase transformer with two times winding. A represents the primary winding terminal of capacitive voltage transformer, and N represents the primary winding grounding terminal of voltage transformer. A represents the two winding terminal terminal of a capacitive voltage transformer, and N represents the first winding grounding terminal of the voltage transformer.

· Single phase transformer with two two times windings

Single phase transformer with two two times windings--Learn Some Basic Knowledge about Capacitor Voltage Transformer 

It represents a single-phase transformer with two two times windings, A represents the primary winding terminals of capacitive voltage transformers, and N represents the primary winding grounding terminals of voltage transformers. 1A and 2A represent the two winding terminals of the capacitive voltage transformer, and 1n and 2n represent the primary winding grounding terminals of the voltage transformer.

· A single phase transformer with two two windings with a tap

 A single phase transformer with two two windings with a tap--Learn Some Basic Knowledge about Capacitor Voltage Transformer

A single phase transformer with two taps and two winding is represented. A represents the primary winding terminal of capacitive voltage transformer, and N represents the primary winding grounding terminal of voltage transformer. 1A1, 1A2, 2a and 2A2 respectively represent the two winding terminals of the capacitive voltage transformer, and 1n and 2n represent the primary winding grounding terminals of the voltage transformer.

· A single phase transformer with a residual voltage winding and two two times windings

 A single phase transformer with a residual voltage winding and two two times windings--Learn Some Basic Knowledge about Capacitor Voltage Transformer 

It represents a single-phase transformer with a residual voltage winding and two two winding. The A represents the primary winding terminal of capacitive voltage transformer, and N represents the primary winding grounding terminal of voltage transformer. 1A1, 1A2, 2a and 2A2 respectively represent the two winding terminals of the capacitive voltage transformer, and 1n and 2n represent the primary winding grounding terminals of the voltage transformer. Da and DN represent the residual voltage terminal.

 


Detail

III.Capacitor voltage transformer testing

3.1 Insulation resistance measurement

The insulation resistance should be measured by the main capacitor, the partial voltage capacitor and the one or two winding insulation resistance of the intermediate transformer.

3.2 Capacitance Measurement

The purpose of the test is to determine whether the capacitance of the voltage divider has a change, and the capacitor is insulated without water and dampness.

Learn Some Basic Knowledge about Capacitor Voltage Transformer 

3.3 Pressure swing ratio test

The test transformer exerts high voltage as far as possible. Due to the rise effect in the test, the high voltage voltage must be measured at the high voltage end. The voltage transformer used must be level 0.1 or above to ensure the accuracy of the test results. After the voltage is applied at the high voltage side, the voltage of the low voltage side is measured in turn on the two side and in the auxiliary side, and the voltage ratio is compared with the pressure ratio of the nameplate.

3.4 Polar measurement

The purpose of polar measurement is to check the mark of the nameplate.

Test method: using DC method, CAR instantaneous addition of 1.5V battery power "+", "-" with "N", respectively, with a multimeter or mA DC or mV meter, pay attention to the polarity put right, A1, A0 "+" X1, XD "-" pointer in the power supply to the deflection of the "+" direction; open to "-" deflection. The test of polarity and pressure variable ratio of windings is usually done in hand over and after overhaul.

IV.Working principle of capacitive voltage transformer (CVT)

There is a video about CVT:

 

This vidoe explained How Capacitor Voltage Transformer CVT works.Capacitor potential transformer concept is explained.What is high voltage measurement using capacitor type voltage transformer is explained. How to measure high voltage? Educational tutorial on electrical engineering 126 by G K Agrawal.

The basic part of the capacitive voltage transformer is the capacitor voltage divider, and it also includes the electromagnetic parts such as the intermediate transformer, the compensating reactor, the damper and so on. Its principle connection is shown in the following picture,the picture shows capacitor voltage transformer wiring diagram capacitance divider is composed of main capacitor C1 and voltage divider capacitor C2 series. Without considering the electromagnetic part, the voltage is divided by capacitance. The voltage on C2 is the following formula: K is the partial voltage ratio. When the two ends of the C2 are connected to the two load, due to C1, C2

The basic part of the capacitive voltage transformer is the capacitor voltage divider, and it also includes the electromagnetic parts such as the intermediate transformer, the compensating reactor, the damper and so on. Its principle connection is given below.

Learn Some Basic Knowledge about Capacitor Voltage Transformer 

The capacitor voltage divider is composed of the main capacitor C1 and the partial voltage capacitor C2 in series, without considering the electromagnetic part, then the voltage is divided according to the capacitance inverse ratio, and the voltage on the C2 is:

 

In this formula,K is the ratio of partial pressure

When the two ends of C2 are connected to two loads, the larger capacitance internal impedance is due to the existence of C1 and C2, which makes UC2 smaller than the capacitance partial voltage. The larger the load current is, the greater the error is. In order to reduce the capacitance internal impedance, a compensatory reactor L can be connected in series, and the UC2 is not related to the load as much as possible. In fact, because the capacitor has loss, the reactor also has resistance, so that the internal impedance can not be zero, so when the load changes, there will always be error. In order to further reduce the effect of load current, the measuring instrument is connected to the divider after the intermediate transformer TV is boosted.

When the two side transformer short circuit occurs, the resistance in the circuit and the total reactance reactor L after compensation are very small, several times the short-circuit current may reach the rated current, will produce a very high voltage resonance in L and C2, in order to prevent overvoltage caused by the breakdown of insulation in capacitor C2 parallel at both ends of the discharge gap F1.

Capacitor voltage transformer with capacitance and nonlinear inductance (e.g. TV magnetizing inductance etc.), when the transformer side suddenly close or receive two side and eliminate the impact of sudden short circuit, overvoltage in the transient process may cause nonlinear inductor saturation, which excite ferroresonance overvoltage, such as harmonic 1/3 resonant.  Because the resistance is very small, the resonance will last for a long time, which will cause damage to voltage transformers, instruments and relays, and may lead to incorrect operation of the protective devices. Therefore, the damping resistance RD or damper is often installed on the two side of the capacitive voltage transformer to consume the resonant energy as soon as possible to suppress the ferroresonance. For a common capacitive voltage transformer, a resonant damper is used. It is the capacitance and the inductor in parallel and then added to the damping resistance. In UHV power grid, a capacitive voltage transformer often uses a fast saturation type damper, which is composed of a fast saturation reactance and a damping resistor.

 

 


Analysis

V.Capacitive Voltage Transformer vs Inductive Voltage Transformer

Inductive Voltage Transformers (IVT), are used for voltage metering and protection in high voltage network systems. They transform the high voltage into low voltage adequate to be processed in measuring and protection instruments secondary equipment, such as relays and recorders). A 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. It’s a standard transformer available in the market for step-up or step-down voltages. The advantage is it can be used for high load current and provides isolation.

Learn Some Basic Knowledge about Capacitor Voltage Transformer 

However,as we mentioned in the above,capacitor voltage transformer is a specialized circuit whose purpose is to convert a high voltage AC signal to lower voltage, usually used with very high input voltages, and a large ratio between input and output voltage. It's usually only used in cases where you're trying to extract a very small amount of power from a high-power circuit, usually for monitoring the high-power circuit. Its advatage is economical but there is no galvanic isolation.

Learn Some Basic Knowledge about Capacitor Voltage Transformer 

 


Book Recommendation

· American National Standard for Requirements for Power-Line Carrier Coupling Capacitors and Coupling Capacitor Voltage Transformers (CCVTs) (ANSI/NEMA C93.1-1999)

--The Association of Electrical Equipment and Medical Imaging Manufacturers (Author)

 

· Calibration of Voltage Transformers and High-Voltage Capacitors at Nist (Classic Reprint)

In the past nist had used a comparative null method to calibrate voltage transformers. The un known transformer was compared to a nist refer ence transformer using a voltage comparator consisting of a variable resistive divider and a mu tual inductor. Reference transformers were avail able with ratios ranging from up to Measurement uncertainties in the comparison of the unknown transformer with the reference trans former were to.ol% for ratio and t0.3 minutes for phase angle. The ratio and phase angle of the reference transformers were known to about the same accuracy. There are several disadvantages to this approach. Since the comparator has a limited range, several reference transformers must be available to cover the anticipated users' needs. The ratio and phase angles of each one of these trans formers must be carefully determined over the secondary voltage range of interest. These trans formers then have to be rechecked at regular inter vals to determine if the ratios and phase angles have changed.If a direct measurement method were available that was sufficiently accurate and straightforward to make the calibration of these reference trans formers a simple task, then that method could be used to measure the client's transformer directly. At nist, the direct null method in use origi nally involved balancing the secondary of the ref erence transformer against the output of a resistive divider used in conjunction with a variable mutual inductor to provide phase angle balance. Such a measurement was difficult because the resistive di vider ratio changed with heating. Since the late 19605 a direct null method has been available that is straightforward and accurate and is now used at nist in place of comparative methods us ing reference transformers.

--William E. Anderson (Author)

 

 


Relevant information about "Learn Some Basic Knowledge about Capacitor Voltage Transformer"

About the article "Learn Some Basic Knowledge about Capacitor Voltage Transformer", If you have better ideas, don't hesitate to  write your thoughts in the following comment area. You also can find more articles about electronic semiconductor through Google search engine, or refer to the following related articles.


· Power Transformers Encyclopedia


HOW TO BUY ELECTRONIC COMPONENTS
How to buy
Search
Inquiry
Order
Track
 
Delivery
FedEx
UPS
DHL
TNT
 
Payment Terms
By PayPal
By Credit Card
By Wire Transfer
By Western Union
 
After-sales Service
Quality Control
Guarantee
Return & Replacement
 
 
About us
Company Profile
Our History
Corporate Culture
Contact us
Join us
© 2008-2018 kynix.com all rights reserved.
Tel:00852-81928838    Email:info@kynix.com