Instrument Transformers 2009-7

7/28/2019 Instrument Transformers 2009-7

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Instrument Transformers

For currents greater than 100A and

voltages higher than 500V, it is difficult to

construct ammeters and current coils of

wattmeters, energy meters and relays

carrying alternating currents greater than

100A.

Specially designed transformers knownas instrument transformers are used

for this purpose.


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Protection or metering devices have to

receive data on electrical values (current or

voltage) from the equipment to be protected.

For technical, economic and safety reasons,this data cannot be obtained directly on the

equipments. Intermediary sensors have to be

used. Current transformers and Voltage

transformers.

.


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For currents greater than 100A and

voltages higher than 500V, it is difficult to

construct ammeters and current coils of

wattmeters, energy meters and relays

carrying alternating currents greater than

100A.

Specially designed transformers knownas instrument transformers are used

for this purpose.


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These devices carry out the functions of:

Reducing the size of value to be measured

Providing galvanic separation

Supplying the power needed to process thedata, or even for the protection device to work.


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Instrument Transformers-

As transformers, they areelectromagnetic devices

By using instrument transformers,

electrical instruments have beenstandardized to operate on 110V and 5Aor 1A.

They are essential parts of manyelectrical systems, and are used for

Measuring (metering) and

Monitoring (relaying) devices.


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Instrument Transformers (DO)

The quality of instrument transformers

will affect directly the overall accuracy

and performance of these metering

and monitoring systems.

Instrument transformer performance is

critical in protective relaying, since the

relays can only be as good as theinstrument transformers.


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Basic Function of Instrument

Transformers

To change the magnitude (but not the

nature) of the quality (voltage or current)

being measured to a suitable level for

use with standard instruments (protectiverelays, metering equipment, etc).

To provide insulation between primary

and secondary circuit for equipment andpersonnel safety


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Types of Instrument

Transformers

Instrument transformers are oftwo

types,depending upon whether it is used

to excite the current or voltage coil of the

measuring instrument

Current Transformers- CTs

Voltage Transformers VTs (also

referred to as Potential Transformers,

PTs).


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Types of Instrument

Transformers (Contd)

Both of these types act as insulators betweenhigh-voltage primary and low-voltagesecondary.

The primary of the VT is connected either line-to-line-to-neutral, and the current that flowsthrough its winding produces a flux in the core.

The ratio of primary to secondary voltage is in

proportion to the turns of ratio and will usuallyproduce 110-120V at the secondary terminalswith rated primary voltage applied.


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Accuracy of Instrument

Transformers

To be a useful part of a measurement system,

instrument transformers must change the

magnitude of the quantity being measured

without introducing any excessive unknownerrors.

The accuracy of an instrument transformer

must either be of a known value, so that errors

may be allowed for, or the accuracy must besufficiently high that errors introduced by the

instrument transformer may be ignored.


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Factors Affecting Accuracy of


Design of the instrument transformer

Circuit conditions such as voltage,

current and frequency

Burden connected to the secondary

circuit of the transformer


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Burden of Instrument

Transformers

In instrument transformer operations, the

primary quantities are reduced by the

turns ratio to provide a secondary current

or voltage to energize protective relaysand other equipment.

The totality of the impedances of the

loads connected to current or voltagetransformers are referred to as burden.


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Transformers (Do)

The burden consists of the impedances

of the following:

Secondary winding of the instrument

transformer

Interconnecting leads

Relay and/or other connected devices.


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Transformer (Do).

For the devices, the burdens are usually

expressed in volt-amperes at a specified

current or voltage.

Thus for CTs or VTs, if Zb is the total

connected burden impedance and is the

volt-ampere burden, then the following

burdens are obtained for CTs and VTs.


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Rated Characteristics of CTs

Rated primary current.

Rated short time current (primary)

Rated secondary current. Rated exciting current.

Rated burden.

Insulation level (primary).


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CTS Characteristics ( Do)

Current error or ratio error.

Phase angle error.

Composite error. Accuracy class.

Over current factor.


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Example 1: Sample Calculation


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Example 1: Sample Calculation

of CT Burden

A current transformer rated 30VA has a

secondary rated current of 5A and

impedance of 0.211 ohms.

If this CT supplies a relay through as

lead of resistance 0.4 ohm, calculate the

relay burden.

Solution 1: Calculation of CT


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Solution 1: Calculation of CT

Burden

The Permissible burden of the CT isZB=VA = 30 = 1.2

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ZB= relay burden+ lead resistance + CTsecondary impedance

Relay burden = ZBlead resistance-CT secondary

impedances= 1.2-0.4-0.-0.211

= 0.589


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CTs For Relay Applications

Current transformers, which step down

primary currents to lower, safer,

measurable values, are required for

Indicating and graphic ammeters

Energy meters and wattmeter (kWh and

kW meters)

Telemetering

Protective relays.


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CTs For Relay Application- (Do)

A CT has a high-current primary winding

which is connected in series with the line

or load whose current is to be metered,

Whilst its secondary winding isconnected in series with the current coil

of the meter (e.g. ammeter, wattmeter,

energy meter, relay, etc).


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CTs For Relay Applications- (Do)

The primary current rating should be

selected from standardized values.

The value of the rated secondary current

shall be either 1A or 5A.


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Some Standard CT Radios

CT Ratio CT Ratio CT Ratio

50:5 300:5 800:5

100:5 400:5 900:5

150:5 450:5 1000:5

200:5 500:5 1200:5

250:5 600:5


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Selection Criterion For CT Ratio

The major criterion for the selection is almost

invariably the maximum load current

In other words, the CT ratio should be selected

such that the CT secondary current atmaximum load should NOT exceed the

continuous current rating or the thermal limits

of the connected relay and equipment.

This is particularly applicable to phase-typerelays where the load current flows through

the relays.


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VT F R l A li ti


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VTs For Relay Applications

Voltage transformers, which step down

system voltages to sufficiently low, safer,

measurable values, are required for

Indication of the voltage conditions. Energy meters and watt meters (kWh

and kW meters)

Protective relays

Synchronizing

Types of VTs for Protective


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Types of VTs for Protective

Relaying.

Voltage transformers have wound

primaries that are

Either connected directly to the power

systems (VTs)

Or across a selection of capacitor string

connected between phase and ground,

that is, coupling-capacitor voltagetransformers (CCVTs)

P i t T N t Ab t VT


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Points To Note About VTs

VTs are used at all power system voltages,and

are usually connected to the bus.

Usually the CCTVs are connected to the line,

rather than to the bus, because the couplingcapacitor devices may also be used to couple

radio frequencies to the line for use in pilot

relaying

At about 115kV, the CCVT types becomesapplicable and generally more economical

than VTs at the higher voltages.


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TERMINAL DESIGNATION OF CTS


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TERMINAL DESIGNATION OF CTS


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Single Ratio


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Single Ratio

D bl ti ith it hi th i


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Double ratio with switching on the primary

D bl ti ith t th d i di


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Double ratio with tap on the secondary winding


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Instruments transformers


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locations

Double bus bar station


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Double bus bar station

Transfer bus bar station


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Transfer bus bar station

Double breaker arrangement


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Double breaker arrangement


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Documents

Instrument Transformers 2009-7