Number Three

Banes Associates
P.O. Box 1177
San Clemente, CA 92674
Phone: 949-361-0700
FAX: 949-361-2072
Sales@BanesReps.com

Current Transformer Testing Simplified- Why and How

Why

Current transformers are used when there is a need to reduce the line current to a smaller value (secondary current) in order to operate metering or protective relays. In the case of circuit breakers and transformers that secondary current is generally 1 to 5 amps.

CT's perform an extremely valuable function in circuit breakers, for example, because they are connected to relays that can be programmed to trip the breaker at a predetermined fault level. Since a circuit breaker's function is so important, and a CT is such a critical element in that function, not only testing the relays but also testing the testing CT's is also vitally important.

The voltage produced from the secondary windings is what relay and metering engineers are looking at. A CT is ordered according to its 'C' class rating. For example a C800 rating is a maximum of 800 volts the CT will handle properly. Before a CT is energized testing will determine if the CT performs to its class rating, designed or nameplate ratio, and at what point (voltage and current) the transformer core is magnetically saturated (hysteresis curve) and output no longer reflects input and therefore inaccurate. The "knee point" is that point when the CT becomes saturated. Through basic Ohms Law calculations, the relay department can determine the size or class of the CT that is needed for a particular application:

The following is a simplified calculation for determining proper sizing of a current transformer. Let's assume that a relay engineer has a breaker with a fault current limit of 50,000 amps, a CT with a ratio of 2,000:5 amps, and a burden of 5 ohms.

Calculation:
2000/5 (CT ratio) equals 400 to 1.
Divide 50,000 fault current by 400 = 125
125 amps is the maximum the CT will see.
Ohm law: V = I x R
I (125) x R (5 ohms) = 625 volts

The 'C' class rating of the CT must be above 625.
Therefore, a C800 is needed for this application.
The relay must be programmed to trip below the voltage and current at the "knee point."

In order to perform the calculation, the burden (resistance) or load on the secondary side of the current transformer must also be determined. The burden is the sum of resistance of the relays, meters, secondary wiring, etc.

How

Critical tests performed on a CT before it is energized in equipment are ratio, polarity, phase angle, saturation level, and saturation knee point. This is accomplished by injecting AC voltage and current into the current transformer windings. The more voltage and current points recorded during the test the more accurate the test is. A hysteresis curve is hand plotted from all this data and the saturation point (knee) is determined and a tangent (knee) is drawn with a straight edge. This is tedious and time consuming.

The Vanguard EZCT is an excellent piece of test equipment because it performs all of these tests automatically in about two minutes. Here is what it does:

• Perform and record turns ratio
  
• Display winding polarity as a + or - sign and phase in degrees
  
• Perform saturation test on 22 to 32 voltages
  
• Record and print excitation current and voltage
  
• Plot excitation /saturation curves
  
• Find knee point voltage in 3 standard methods:
  
  • 10%-50%
    
  • IEEE-30
    
  • IEEE-45
• Record up to 10 excitation curves into memory

Here is an actual printout from its thermal printer

Here are the saturation curves as automatically plotted for two methods.

For a product demonstration call Banes Associates at (949) 361-0700.

Vanguard InstrumentsCompany is located in Ontario,Ca. They also make TTR's,breaker timers,and micro-ohmmeters. www.vanguard-instruments.com

Specifications

TYPE EZCT Portable Current-Transformer Test Set
SIZE 16.8"W x 12.6"H x 12"L; Weight: Less than 35 lbs.
INPUT POWER 115 V ac or 230 V ac (Selectable), 50/60 Hz
OUTPUT TEST VOLTAGE 0 to 50 V ac @ 2A max0 to 250 V ac @ 2A max0 to 1,200 V ac @ 1.5 A max (5 min on, 10 min off)
VOLTAGE READING RANGE 0 to 1,250 V, Accuracy: ± 1.0% of reading, ± 0.5 volt
CURRENT READING RANGE 0 to 2 A, Accuracy: ± 1.0% of reading, ± 0.002 A
TURNS-RATIO RANGE 0.8 to 99, Accuracy: ± 0.5%100 to 999, Accuracy: ± 1.0%1,000 to 2,000, Accuracy: ± 2.0%
CURRENT ACCURACY ± 2% of Reading (± 1 Digit)
PHASE-ANGLE MEASUREMENT 0 - 360 Degrees, Accuracy: ± 1.0 degree
PHASE-ANGLE ACCURACY ± 1 Degree (± 1 Digit)
DISPLAY Backlit LCD, 4 lines by 20 characters
HARD-COPY PRINTOUT 4.5 inch Thermal Printer
TEST STORAGE 64 Test records (of 10 curves)
COMPUTER INTERFACE RS-232C, 19,200 Baud
ENVIRONMENT Operating: -10°C to 50°C (15° to +122° F)Storage: -30° C to 70° C (-22° to +158° F)
CABLES One 20' X cable setOne 35' H cable setPower cordOne cable carrying duffel bag included
INCLUDED Shipping case, foam lined aluminum case for airline shipping
WARRANTY One-Year on Parts and Labor