Current Transformers - A Tester Survival Guide.ppt

Current Transformers:

A Tester Survival GuideBryan ShannonABB Inc.Coral 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: Equivalent Circuit

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

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

Protection CTs require linearity in a wide range of currents

Calculated or Tested AccuracyAccording to IEEE standards (C57.13), current transformers are given an accuracy rating with a C or a T C indicates calculated accuracy and T indicates tested accuracy. Using this system, current transformer with rating T100 would have to be tested to verify that it could sustain a voltage of 100 volts within normal accuracy limits.C type are transformers which are constructed so that the effects of leakage fluxes on its performance are negligible, such as bushing current transformers with uniformly distributed windings.

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

For example, a current transformer rated 5 amperes secondary current, might be given a relay accuracy rating at 200 volts (C200). This would indicate that the current transformer could sustain relaying accuracy at 100 amps as long as the secondary voltage IZ did not exceed 200 volts.

In this case, the maximum burden that could be used would be calculated as follows: Z (Burden)= E/I ( voltage rating / 20 times rated current of 5 amps)= 200 volts / 100 amps = 2 ohms (Includes wiring and relays)

Benefits of Test SwitchesTest switches provide a quick and safe means of testing relays. Test switches are especially important wherever secondary current transformer circuits may need to be temporarily reconfigured to facilitate testing or where the relay must be temporarily disconnected from service. Accidental opening of a CT secondary circuit can result in extremely high voltage and arcing, creating a dangerous hazard. Test switches eliminate this possibility by diverting the secondary current to an alternate path before opening the connection to the relay. This sequenced operation is inherent to the design of the current-shorting poles of a test switch.

Circuit Design for Testing

Facilitate Simpler and Quicker TestingAllows Access to Connections from Front of panelAssures correct testing procedure sequence (Make before break of CTs)But, an Open CT can still occur.

Reasons for In-Service TestingCompetitive Electric Utility MarketMore power wheeling/power needsControl of supply chain resourcesRequires reliable power deliveryEquipment availabilityDeregulation of Electric PowerGENCO to TRANSCO separationISO activity requires meteringNeed to use existing ITsBottom-line FocusedBilling and current swingsMust verify performance of ITs

In Service Measurements What could go wrongAccidental Opening of current transformer due to:-Blown Fuse-Accidentally disconnected leads-Defective leads-Defective equipment -Incorrect Connections-Incorrect meter mode selected

Open CT Current goes to zero until condition is restoredEvery half cycle high voltage transients occur

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

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

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

Human Reaction (at 60Hz)Current (milliamperes)Perception Threshold1 mAPainful Sensation3-10 mACant Let GO (paralysis or arms)10 mACant Breathe (Paralysis of chest Muscles)30 mAFibrillation Threshold (Affects Heart- Could be fatal)65 mAHeart Paralysis4ATissue Burning.5A

What Level is a Dangerous Voltage?OSHA says 50Vac is a hazard [1910.333(a)(1) and 1910.269(I)(1)]IEC (International Electrotechnical Commision): 30Vac Rms, 42Vac Peak, or 60Vdc

Human Reaction (at 60Hz)Voltage Required (Arm to Arm)Perception Threshold1 mA Painful Sensation3-10 mA Cant Let GO (paralysis or arms)10 mA Cant Breathe (Paralysis of chest Muscles)30 mA 45VFibrillation Threshold (Affects Heart- Could be fatal)65 mA 97.5VHeart Paralysis4A Tissue Burning.5A

Consequences of Open CTHigh voltage and shock hazardSignal discontinuity to relay and controls could lead to incorrect operations and outages.Potential magnetization of the CT leads to incorrect secondary output. This can cause error in readings, revenue metering calculations, incorrect operations long after the open CT condition is resolved.

Typical In-Service TestingTest Switch

Solution to existing problemsPrevents 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 ProtectionTest Switch

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 tripTest Plug with Open CT ProtectionTest Switch

Open CT ProtectionCurrent signal integrity to relay maintained. [Prevents incorrect operations]Voltage spikes limited to first occurrenceVoltage spikes limited to 35 Volts [Eliminates Shock Hazards]

Current waveform in Open CT with New Test PlugMinimizes 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

ConclusionsOpen CT conditions are a very likely event as more in-service testing is requiredOpen CT condition leads to Shock Hazard, OutagesOpen CT could lead to incorrect readings long after condition is eliminated due to magnetizationTest Switches are recommended for installation with Current TransformersProposed test plug device eliminate risks due to operator error, incorrect equipment or risks arising due to normal testing practices and procedures.

Questions?

*In-service testing of current transformers does avoid an outage, but also introduces additional hazards that must be mitigated. This begins with proper selection of the current transformer and design of the application, as well as using the correct tools during testing. *A typical transformer and its equivalent circuit is shown in Figure 1. The leakage flux is shown entering the outer part of the core and is represented by reactance X. The reactance develops voltage applied to the exciting branch Zo, also referred to as XM, magnetizing reactance, which represents the outer side of the core. The series impedance, RP + RS + j (XP + X), is responsible for the loss of voltage in transformation. The loss of current in transformation is due to current drawn by the magnetizing branch. Current transformers are specially designed to keep these by-pass exciting impedances as high as possible. *Two mains types of CTs, both metering accuracy and relaying CT, in this paper we will focus on relaying CTs as these are widely used for protection.**A popular rule-of-thumb suggests that the C-rating be twice the excitation voltage developed by the maximum available fault current.0.01 ohms for microprocessor relay burden*Easy facilitate in-service or out of service testing without user exposure to the rear of the panel..etc**Traditional EM relays had the test switches built in, but this is a convenience that modern solid state and microprocessor relays do not have. So in this case, separate test switches like the knife blade style shown here have become a quick, simple, safe, and effective method of testing ITs and protective relays*Higher reliability required, utilities strive to improve their SAIFI, SIADI, and MAIFI **If this relay we are showing above, it part of a differential scheme, this may cause a false trip and possibly a blackout due to the open ct event. The voltage transients are also dangerous to personnel and equipment, especially aging ITs************