Current Transformers:Current Transformers:

A Tester Survival GuideA Tester Survival Guide

Bryan ShannonBryan ShannonABB Inc.ABB Inc.Coral Springs, FLCoral Springs, FL

Current is Stepped Down [by primary and secondary windings]

Exciting Impedance is the primary source of error. [By design Zo, Zi or Xm is very high to reduce Ie as much as possible]

During Open CT Condition. A very high voltage is developed at CT terminals. [all secondary current flows through Xm]

Current Transformer: Current Transformer: Equivalent CircuitEquivalent Circuit

Why Relay Accuracy CTs?Why Relay Accuracy CTs?

Under short circuit conditions, a Under short circuit conditions, a small core that might be small core that might be adequate for metering adequate for metering applications would “fill up” and applications would “fill up” and be unable to carry the magnetic be unable to carry the magnetic flux required to transform the flux required to transform the current. current.

When this happens, the current When this happens, the current transformer might be unable to transformer might be unable to induce a voltage high enough to induce a voltage high enough to maintain proper current flow in maintain proper current flow in the secondary side. This could the secondary side. This could result in very serious errors.result in very serious errors.

Protection CTs require linearity Protection CTs require linearity in a wide range of currentsin a wide range of currents

Calculated or Tested AccuracyCalculated or Tested Accuracy

According to IEEE standards (C57.13), current According to IEEE standards (C57.13), current transformers are given an accuracy rating with a “C” or transformers are given an accuracy rating with a “C” or a “T” a “T”

““C” indicates calculated accuracy and “T” indicates C” indicates calculated accuracy and “T” indicates tested accuracy. tested accuracy.

Using this system, current transformer with rating T100 Using this system, current transformer with rating T100 would have to be tested to verify that it could sustain a would have to be tested to verify that it could sustain a voltage of 100 volts within normal accuracy limits.voltage of 100 volts within normal accuracy limits.

““C” type are transformers which are constructed so that C” type are transformers which are constructed so that the effects of leakage fluxes on its performance are the effects of leakage fluxes on its performance are negligible, such as bushing current transformers with negligible, such as bushing current transformers with uniformly distributed windings.uniformly distributed windings.

Maximum BurdenMaximum Burden

Current transformers for relay applications are rated in the terms Current transformers for relay applications are rated in the terms of the maximum secondary volts that can be induced on a twenty of the maximum secondary volts that can be induced on a twenty times short circuit rating – with error of current transformation times short circuit rating – with error of current transformation limited to 10%. limited to 10%.

For example, a current transformer rated 5 amperes secondary For example, a current transformer rated 5 amperes secondary current, might be given a relay accuracy rating at 200 volts current, might be given a relay accuracy rating at 200 volts (C200).(C200).

This would indicate that the current transformer could This would indicate that the current transformer could

sustain relaying accuracy – at 100 amps – as long as the sustain relaying accuracy – at 100 amps – as long as the secondary voltage IZ did not exceed 200 volts. secondary voltage IZ did not exceed 200 volts.

In this case, the maximum burden that could be used would be In this case, the maximum burden that could be used would be calculated as follows:calculated as follows: Z (Burden)= E/I ( voltage rating / 20 times rated current Z (Burden)= E/I ( voltage rating / 20 times rated current of 5 amps)of 5 amps)

= 200 volts / 100 amps = 2 ohms (Includes wiring and = 200 volts / 100 amps = 2 ohms (Includes wiring and relays)relays)

Benefits of Test SwitchesBenefits of Test Switches

Test switches provide a quick and safe means of Test switches provide a quick and safe means of testing relays. testing relays.

Test switches are especially important wherever Test switches are especially important wherever secondary current transformer circuits may need to be secondary current transformer circuits may need to be temporarily reconfigured to facilitate testing or where temporarily reconfigured to facilitate testing or where the relay must be temporarily disconnected from the relay must be temporarily disconnected from service. service.

Accidental opening of a CT secondary circuit can Accidental opening of a CT secondary circuit can result in extremely high voltage and arcing, creating a result in extremely high voltage and arcing, creating a dangerous hazard. dangerous hazard.

Test switches eliminate this possibility by diverting the Test switches eliminate this possibility by diverting the secondary current to an alternate path before opening secondary current to an alternate path before opening the connection to the relay. the connection to the relay.

This sequenced operation is inherent to the design of This sequenced operation is inherent to the design of the current-shorting poles of a test switch. the current-shorting poles of a test switch.

Circuit Design for TestingCircuit Design for Testing

Facilitate Simpler and Quicker Facilitate Simpler and Quicker TestingTesting

•Allows Access to Connections from Front of panel

•Assures correct testing procedure sequence (Make before break of CTs)

But, an Open CT can still occur.

Reasons for In-Service TestingReasons for In-Service Testing

Competitive Electric Utility MarketCompetitive Electric Utility Market– More power wheeling/power needsMore power wheeling/power needs– Control of supply chain resourcesControl of supply chain resources– Requires reliable power deliveryRequires reliable power delivery– Equipment availabilityEquipment availability

Deregulation of Electric PowerDeregulation of Electric Power– GENCO to TRANSCO separationGENCO to TRANSCO separation– ISO activity requires meteringISO activity requires metering– Need to use existing ITsNeed to use existing ITs

Bottom-line FocusedBottom-line Focused– Billing and current swingsBilling and current swings– Must verify performance of ITsMust verify performance of ITs

In Service Measurements – What In Service Measurements – What could go wrongcould go wrong

Accidental Opening of Accidental Opening of current transformer due current transformer due to:to:

-Blown Fuse-Blown Fuse

-Accidentally disconnected -Accidentally disconnected leads leads

-Defective leads-Defective leads

-Defective equipment -Defective equipment

-Incorrect Connections-Incorrect Connections

-Incorrect meter mode -Incorrect meter mode selected selected

Open CT Open CT

•Current goes to zero until condition is restored

•Every half cycle high voltage transients occur

Open Circuit VoltagesOpen Circuit Voltages

Because the open circuit voltage is limited by saturation Because the open circuit voltage is limited by saturation of the core, of the core, the RMS value measured by a voltmeter may the RMS value measured by a voltmeter may not appear to be dangerousnot appear to be dangerous. As the current cyclically . As the current cyclically passes through zero, the rate of change of flux at current passes through zero, the rate of change of flux at current zero is not limited by saturation, and is very high indeed. zero is not limited by saturation, and is very high indeed. This induces extremely high peaks or pulses of voltage. This induces extremely high peaks or pulses of voltage. Voltage transients up to 15Kv could be possible.Voltage transients up to 15Kv could be possible.

These high peaks of voltage may not register on the These high peaks of voltage may not register on the conventional voltmeter, but they can break down conventional voltmeter, but they can break down insulation and are dangerous to personnel. Current insulation and are dangerous to personnel. Current transformers are insulated to withstand, for emergency transformers are insulated to withstand, for emergency operation, secondary peak voltages up to 3500 voltsoperation, secondary peak voltages up to 3500 volts

Current DangersCurrent Dangers

Human Reaction (at 60Hz)Human Reaction (at 60Hz) Current (milliamperes)Current (milliamperes)

Perception ThresholdPerception Threshold 1 mA1 mA

Painful SensationPainful Sensation 3-10 mA3-10 mA

Can’t Let GO (paralysis or Can’t Let GO (paralysis or arms)arms)

10 mA10 mA

Can’t Breathe (Paralysis of Can’t Breathe (Paralysis of chest Muscles)chest Muscles)

30 mA30 mA

Fibrillation Threshold (Affects Fibrillation Threshold (Affects Heart- Could be fatal)Heart- Could be fatal)

65 mA65 mA

Heart ParalysisHeart Paralysis 4A4A

Tissue Burning.Tissue Burning. 5A5A

A value of 1500 ohms is commonly used as the resistance from arm to arm of the human body.

What Level is a Dangerous What Level is a Dangerous Voltage?Voltage?

Human Reaction (at 60Hz)Human Reaction (at 60Hz) Voltage Required (Arm to Voltage Required (Arm to Arm)Arm)

Perception ThresholdPerception Threshold 1 mA 1 mA

Painful SensationPainful Sensation 3-10 mA 3-10 mA

Can’t Let GO (paralysis or Can’t Let GO (paralysis or arms)arms)

10 mA 10 mA

Can’t Breathe (Paralysis of Can’t Breathe (Paralysis of chest Muscles)chest Muscles)

30 mA 30 mA 45V45V

Fibrillation Threshold (Affects Fibrillation Threshold (Affects Heart- Could be fatal)Heart- Could be fatal)

65 mA 65 mA 97.5V97.5V

Heart ParalysisHeart Paralysis 4A 4A

Tissue Burning.Tissue Burning. 5A5AOSHA says 50Vac is a hazard [1910.333(a)(1) and 1910.269(I)(1)]

IEC (International Electrotechnical Commision): 30Vac Rms, 42Vac Peak, or 60Vdc

Consequences of Open CTConsequences of Open CT

High voltage and shock High voltage and shock hazardhazard

Signal discontinuity to relay Signal discontinuity to relay and controls could lead to and controls could lead to incorrect operations and incorrect operations and outages.outages.

Potential magnetization of Potential magnetization of the CT leads to incorrect the CT leads to incorrect secondary output. This can secondary output. This can cause error in readings, cause error in readings, revenue metering revenue metering calculations, incorrect calculations, incorrect operations operations long after the long after the open CT condition is open CT condition is resolved.resolved.

Typical In-Service TestingTypical In-Service Testing

Test Switch

Solution to existing problemsSolution to existing problems

Prevents Shock Hazards [Created by human error, and other causes]

Prevent Blackouts [Created by human error, and other causes]

Prevents CT erroneous reading (Avoid Magnetization)

Test Plug with Open CT Protection

Test Switch

How it works?How it works?

•Detects rate of change of voltage and internal circuitry shorts CT

•LED would indicate open CT condition present

•Maintains signal to relay/IED

•Eliminates possibility of false trip

Test Plug with Open CT Protection

Test Switch

Open CT ProtectionOpen CT Protection

•Current signal integrity to relay maintained. [Prevents incorrect operations]

•Voltage spikes limited to first occurrence

•Voltage spikes limited to 35 Volts [Eliminates Shock Hazards]

Current waveform in Open CT with Current waveform in Open CT with New Test PlugNew Test Plug

Minimizes distortion or signal discontinuity to protective devices, eliminating false trips and potential blackouts

User Exposure Comparison

Maintains voltage to safe levels and reduces user exposure, thus increasing safety and eliminating equipment damage

ConclusionsConclusions

Open CT conditions are a very likely event as more Open CT conditions are a very likely event as more in-service testing is requiredin-service testing is required

Open CT condition leads to Shock Hazard, OutagesOpen CT condition leads to Shock Hazard, Outages Open CT could lead to incorrect readings long after Open CT could lead to incorrect readings long after

condition is eliminated due to magnetizationcondition is eliminated due to magnetization Test Switches are recommended for installation Test Switches are recommended for installation

with Current Transformerswith Current Transformers Proposed test plug device eliminate risks due to Proposed test plug device eliminate risks due to

operator error, incorrect equipment or risks arising operator error, incorrect equipment or risks arising due to normal testing practices and procedures.due to normal testing practices and procedures.

Questions?Questions?