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Current Transformers
Copyright 2003 Kilowatt Classroom, LLC.
CT Saturation Curve Testing
CT9
CT Saturation Curve Tester
Designed by Vail Gilliland
Sheet 1 of 2
Purpose
This circuit is used to plot the Saturation Curve of an Instrument Current Transformer. The test results are
compared with the manufacturer’s published data (see sample curves on Sheet 6 of the Current Transformer
article published last month). A transformer with shorted secondary turns or a one-turn primary short due to
improper mounting will result in a test plot which varies from the published curve. This test is performed only
on de-energized, out-of-service equipment. The CT under test need not be removed from the equipment provided
the primary is first de-energized and isolated and the secondary is then disconnected. See Circuit Description
and Test Procedure on Sheet 10.
WARNING !
This test set-up develops high voltage.
The test procedure is intended for use by experienced electrical personnel only and requires the use of
established safety procedures and proper Personal Protective Equipment (PPE).
This test is performed only on de-energized, out-of-service equipment, and requires that the CT primary
be de-energized and isolated, and then the CT secondary must be disconnected from its burden (load).
Be certain to properly reconnect the current transformer at the conclusion of the test -
CT can develop a dangerously-high voltage; an incorrectly connected CT may not trip the protective relay!
an open-circuited
AUTOTRANSFORMER FUSE
A
DIGITAL AMMETER 0 - 10 AMPS
SEE CIRCUIT DESCRIPTION ON
THE FOLLOWING SHEET.
ADJUSTABLE
AUTOTRANSFORMER
2000 VA, 0 - 135 VAC
SAFETY GROUND ON
TRANSFORMER CASES
120 VAC HOT
120 VAC NEUTRAL
AUTOTRANSFORMER SWITCH
Test Set Schematic
Sheet 10
Test Methods
Current Transformers
Copyright 2003 Kilowatt Classroom, LLC.
CT Saturation Curve Testing
CT10
CT Saturation Curve Tester
Sheet 2 of 2
Test Method Overview
The circuit on the preceding page (Sheet 9) is used to provide an adjustable 0 - 1000 VAC which is injected on
the secondary winding of the current transformer being tested. Using the adjustable autotransformer, the secondary
excitation voltage and current applied to the CT are
and current readings are taken. A plot of the CT secondary voltage and current is made on log - log
(logarithmic) scale engineering graph paper at each step of the test. The constructed plot is then compared
with the manufacturers published curves (see Sheet 6 ); a deviation from these curves indicates either a primary
one-turn short circuit due to improper mounting or shorted secondary turns.
gradually increased from zero while incremental voltage
Circuit Description
Two 480 - 120 volt control transformers are back-fed with the 120 volt windings connected in parallel and the
480 volt windings connected in series. (The kVA rating of these transformers must be large enough to supply
5 amps of current on a momentary basis to get above the “knee” of the saturation curve.) An adjustable 0 - 135
VAC is supplied by the autotransformer which feeds the parallel connected 120 volt transformer windings.
To achieve accurate test results, both the CT secondary excitation voltage and excitation current need to be
accurately
The voltmeter must have a 1000 VAC range.
One method of measuring the excitation current is to series a Digital Multi-Meter (DMM) with the test circuit
output and measure the current directly using the AC amps function. Care needs to be taken not to exceed the
internal current rating of the instrument (10 amps on most DMM’s).
An alternate approach, for making the current measurement, is to use a low-current clamp -on adapter such as
the TPI A254 (see picture on Sheet 8) which has the ability to read AC currents as low as 10 milliamps. Several
wraps of the conductor through this current probe will extend the low-end accuracy; the meter reading is
then divided by the number of turns used.
measured.
Test Procedure
WARNING! This test set-up develops high voltage - see precautions on previous sheet.
·
Verify that the adjustable autotransformer is un-plugged, turned off, and set at zero.
·
the secondary leads of the current-transformer-under-test.
Connect the test equipment as shown in the diagram on the preceding page and connect the output leads to
·
Apply the 120 VAC power to the autotransformer input.
·
(0.010 amps) is a good first step. Read the voltage at this step and plot the voltage and current readings on
the log-log graph paper.
Gradually increase the autotransformer setting until a small output current is measured. Ten milliamps
·
at each step and plotting the results on the graph paper. Watch for the development of the “knee” of
the curve and make very careful adjustments in this voltage and current range. The current will increase
in much larger increments at this point for a given amount of voltage increase, so use care to prevent
blowing a meter fuse or autotransformer fuse.
Continue to increase the autotransformer setting in a series of small steps, taking voltage and current readings
·
the system.
Sheet 11
At the conclusion of the test, reduce the autotransformer output voltage to zero and remove power from
Current Injection Testing
Copyright 2003 Kilowatt Classroom, LLC.
Test Methods
HCT1
Purpose
Electrical equipment such as circuit breakers, protective relays, and meters are routinely tested to verify proper
operation of current sensing elements. This testing is performed using high-current, low-voltage test equipment
that provides a means of adjusting the value of current and also of measuring the operating time of the device under
test. The output waveform of the test current is critical and must be sinusoidal; testing with equipment that
produces a non-sinusoidal waveform - such as SCR’s - will not produce accurate results.
CAUTION! Current injection testing is performed on de-energized, out-of-service equipment only!
Types of High-Current Testing
·
current is injected in the Current Transformer (CT) primary winding and the resulting secondary current is
measured in each of the CT secondary devices such as meters and relays. This test is primarily conducted
during commissioning of new equipment or after a major circuit modification to insure that the equipment is
correctly connected. The polarity of the current may also be critical and other equipment, such as a Phase
Angle Meter, may be used in conjunction with the high-current test source.
Primary Injection Testing is used to test the overall operation of a current circuit. In this type of test, a high
·
verify the accuracy and proper operation of the equipment. These devices receive their input current from the
CT secondary winding so these tests will be at a much lower level of current than that used for primary injection.
Proper operation of the current-sensing protective equipment can be verified by comparing the device
operating characteristics with the manufacturers published time-current characteristic curves.
Secondary Injection Testing is periodically performed on the individual devices such as relays and meters to
Frequency of Tests
The frequency of these preventive maintenance current tests depend up the importance of the protection: high voltage
equipment will often be tested annually; medium-voltage equipment is often tested and calibrated every-other
year, and a three- or four-year interval for 480 volt equipment may be considered adequate.
Testing Thermal Devices
·
are not usually tested as the test current can damage the element. (Critical applications should be protected by
a more reliable device such as an electromechanical or electronic relay.)
Thermal overload relays for 480 volt and lower voltage equipment - either bimetallic or melting alloy type -
·
If successive tests are made the device must have time to cool-down between tests for accurate results to be
obtained.
Large thermal circuit breakers are sometimes periodically tested for proper operation by current injection.
·
breakers, the device must have time to cool down between successive tests.
Electromechanical thermal relays must be tested within the instrument case for proper results. As with thermal
Testing Instantaneous Elements
·
should be adjusted as quickly as possible using the test set MOMENTARY FUNCTION to prevent damage
to the equipment-under-test.
Because of the high current involved when testing magnetic trip elements in circuit breakers or relays, the current
·
breaker is usually 10 times the thermal element value. Testing the magnetic element may result in damage to
the thermal element (which is in series with the magnetic trip coil) if the test current is prolonged. Motor
Circuit Protectors (MCP’s) have a magnetic trip element only and can be safely tested.
The maximum trip point setting of the instantaneous magnetic trip element of a thermal/magnetic circuit
·
Sheet 12
Protective relay instantaneous elements are tested at either the engineered setting or the “as -found” setting.
High Current Testing
Copyright 2003 Kilowatt Classroom, LLC.
CT Primary Injection
Test Methods
HCT2
Field Test on 34.5 kV Recloser
Kilowatt Classroom Photo
Test Set-Up
In the photo below a high current test set is used to check the trip and reclose timing on a three-phase 35.4 kV
substation vacuum recloser. The relaying scheme is tested one-phase-at-a-time. The high current leads are
attached to the phase being tested. The test set timer start/stop leads are attached to a different phase using this
pole as a “dry” set of contacts. See Sheet 7 for a photo of the recloser internal bushing current transformers.
This recloser uses 130 VDC station battery for trip and close power.
CAUTION ! This procedure cannot be used on electronic controlled reclosers that have a high-voltage
closing coil or on hydraulic reclosers that have a high-voltage series trip coil and a high-voltage closing
coil.
High Current Test Set
Vacuum Recloser
Test set high current leads attached to A -Phase bushings.
Control Cabinet
Recloser is out-of-service.
Note open disconnect ( 6 total ).
Timer start/stop leads attached to C-Phase bushings.
ADJ
Test Set-Up Schematic
Recloser shown top view.
Red Dashed Lines = CT secondary leads to
multi-function protective relay. Relay “looks”
through the breaker. When the relay trips the
breaker, the fault current is interrupted and the
protective relay will reset.
Load-Side Bushings Line-Side Bushings
Recloser Main Contacts
Red Circles = Internal Bushing CT’s
High Current Test Set
Blue = High Current Leads
Green = Timer Start / Stop Leads
T A
T = Timer
A = AC Ammeter
ADJ = Amps Adjust Control

my dreams

Friday, March 25, 2011

Insulation Class

CT Saturation Curve

LT CT manufacture

Saturday, January 29, 2011

kat

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