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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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Instrument Transformers
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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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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Instrument Transformers
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
Instrument Transformers
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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Burden of Instrument
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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Burden of Instrument
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
12 52
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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