Q1) Why does opening a CT's secondary cause dangerously high secondary voltage?

A1) With the flow of both primary and secondary currents the transformer's exciting current is very low. The secondary current mmf serves to keep the magnetizing flux in check! If the secondary now opens the primary current mmf produces an exciting current 'orders of magnitude' greater than normal. And, the resultant large increase in flux density produces an extremely high voltage in the secondary.

Q2) (Paraphrasing ) Why don't power transformers exhibit the same phenomena?

A2) While current transformer (CT) and power transformer (PwrT) theory is about the same the salient difference is their Volt-Ampere characteristic. The PwrT is a constant-voltage (shunt-loaded) device whose primary and secondary VAs are equal. Conversely, the CT is constant-current (series-loaded) device. That is, the input VA is insignificant, having no influence upon the primary circuit. Its inter-related variables like flux, volts, current, and burden are not constant but more likely non-linear.

Simply transformer action. The turns ratios of most power transformers are low so the voltages are low also. In a CT the primary is 1 turn so the turns ratio is high. Normally the primary voltage drop is low because of the reflected impedance of the load which is usually a very low resistance like a current shunt. When you remove the load, the primary drops appreciable voltage according to it's now higher inductive reactance and the voltage is multiplied by the high turns ratio and you have sometimes very high voltages. The biggest danger is when you open the load under power as the voltage rises quickly enough to maintain an arc. If the transformer or something connected to it is not insulated for high voltages you can get a fire. If Jumper Joe is pulling the wire off, he can become the easiest path to ground. Tsk, tsk... I too appreciate not-to-technical answers... but, they should be technically correct:

A) Turns-ratio is not responsible for overvoltage. Even CTs with lower ratios, like 5:1 or 1:1, have the cautionary notice (at least in the USA!) Furthermore primary voltage is a fraction of Volts. So, using turns-ratio as the primary to secondary multiplier, secondary volts would be in the order of volts to tens of volts.

B) Overvoltage occurs when the secondary open-circuits and the demagnetizing effect of the secondary emf is lost. Flux density quickly increases, limited only by core saturation.

C) As core saturation progresses, waveshape changes. The usual sinewave changes to a peaked wave. Such a waveshape has an extremely high dV/dT characteristic. It is very likely that insulation flashover and/or fire results from dV/dT... not from a high amplitude sine wave!Yes, but it is technically correct. You are also correct in that there is di/dt action going on. The theoretically instantanious collapse of the field is going to cause theoretically infinite voltage. And that field is generated by primary current. You will find that within physical limits, the change in flux density affects both windings. Actually a better model than "normal" transformer action might be the common Kettering ignition coil. And I don't recall stating that the voltage would be sinusoidal although you might get lucky and interrupt the sinusoidal current at a zero crossing. In any case, it's a bad idea. The non-

technical explanation is you are going to get a big spike to light an arc and greater than normal voltage to help keep it going. Basic electrical theory (which I am not going to go into) - an open CT present itself as a large inductance to the primary current source and develops a large voltage on it primary winding. The secondary winding will then develop a voltage that is N times larger than the primary (because the transformer ratio is 1:N - Primary:Secondary).The CT is designed to lower current to a safe and measureable level. To accomplish this, the voltage level on the secondary is raised. P=IE, if P is constant, I and E are then inversely proportional. The windings ratio determines the secondary voltage levels. While the secondary is connected to a measuring device, it is essentially shorted. Opening the secondary circuit immediately raises the voltage to levels pre-determined by the primary voltage and the windings ratio. There are shunted CT's available that greatly reduce potential hazards typically associated with standard design CT's. Always remove power and install shorting bars prior to working on CT circuits.when you think about power transformer then voltage is aprox. constant and current in primary is depend on secondary load. so if you short power transformer then current in primary become very huge and may (must) be damage to winding and insulation.

But about currant transformer current passing in primary is depend (=same) on busbas or cable where current transformer mount (which is not dependent on load of secondary of CT) & voltage of primary is depend on secondary load. if your secondary of CT is open than as per magnetic coupling very high voltage generated (because of secondary current = 0 ) at secondary. And insulation will be fail. A few off-list questions have prompted revisiting this topic, as well as two related ones, namely, "Current Transformer" (#1026172605) and "Current Transformer Failures" (#1026113301). From the questions asked it is obvious the topic is still in a state of flux (pun intended!)

Some commenters expressed a 'very strong' belief that overvoltage is due to CT turns-ratio. Others, myself included, believe that while ratio is present, it is not the reason for the high voltage. A new approach is presented here to resolve the issue.

Consider the example submitted by Pagnair... a CT whose current ratings are is 2,000/1/5. But, ratio alone does not complete the specification. So, for calculation purposes, I will assume it's capacity is 10 VA (volt-amperes) for the 1-Amp tap.

Tagging primary and secondary parameters with subscripts p and s, respectively, letting the winding turns-ratio = Np / Ns, then:

A) Primary Parameters. The pri / sec current-ratio (Ap / As) is 2,000:1. Primary voltage, Vp, is determined by the simple calculation VA / Ap, or 10 / 2,000 = 5.0 mV. Primary ampere-turns, ATp, are 2,000 x 1.0 = 2,000 AT.

B) Secondary Parameters. Secondary voltage, Vs, is VA / As = 10 Volts. Secondary ampere-turns, ATs, are As x Ns / Np = 2,000 AT!

C) Ampere-Turns Impact on Core-Flux. Note, from above, the primary and secondary ampere-turns are equal. The direction of their flux flows are in opposition. But, a small core-flux is still required to cover both iron-core and winding losses. This net core-flux, ATe, is quite small, as low as 1/2% to 2% of the rated ampere-turns. (An aside, it is this value that establishes the CT's accuracy!)

D) Net Core-Flux Produces Output. Using a 1% value, the exciting ATe is, in this case, 20-AT. This inturn (pun not intended) results in an output voltage of 0.5 Volt per ampere-turn.

E) Effect of Opening Secondary. Opening the secondary results in the full primary ampere-turns increasing the core-flux, i.e., from 20 to 2,000 ampere-turns. This of course produces 2,000 AT x 0.5 V / AT volts, or 1,000 Volts at the secondary.

F) Conclusion. CT-ratio is not the cause of overvoltage. Instead it is the substitution of an overwhelming primary ampere-turns for the very small excitation ampere-turns! Note, the calculations above are based solely on an rms-treatment of both flux and voltage. Core-saturation effects were not considered!

You know I'm a stickler (some say obsessed) for Technical Correctness. Several contributors to this and related current transformer topics madeseveral errors:

1) When transferring the secondary impedance, Zs, into the primary as Zs', the correct expression is,

Zs' = Zs x (Np/Ns)^2, where,

Np and Ns are the primary and secondary winding turns, respectively.

For the CT example cited the secondary impedance reflected into the primary is,

Zs' = 10 x (1/2,000)^2 = 2.5E-6.

2) This low value illustrates that secondary impedance has no influence on the primary circuit... contrary to what some stated.

3) Some presented the formula for the development of an extreme secondary voltage as Es = Ep x current-ratio! Again using the CT cited, for an open circuit condition the secondary voltage would have been 5 mV x 2,000 or 10 Volts. Clearly not the extreme voltage observed!

If any of you disagree, then present a point of view free of obfuscation!I have read with great interest the various comments expressed and I feel that a number of questionable interpretations have been presented.Under open circuit conditions the total primary current will be equal to the total excitation force of Ampereturns/metre, thus inducing maximum

induction (B of 1.9-2.0 tesla for normal materials).

The voltage generated in the secondary winding under open circuit will be directly proportional to:Number of secondary turnsCross sectional area coreFrequency

Extremely large voltages can be generated in Cts that have large turn ratios, i.e. 5000/1, and havelarge CSA as typically used in protection class cts for differential protection.

My experience in laboratory tests shows that in CT designs with AT/m up to 1500, the relationship of generated voltage is linear and proportional to the items listed above. I have developed curves to estimate open circuit voltages and would greatly appreciate to have some comparisons if anyone has similar information.I have a problem - maybe you can point me in the right direction.

I have an application where my customer is requiring FS(instrument security factor)>=5.

We were unable to meet this with the BCT design - so we suggested to use a secondary protector (current shunt) that would short circuit when the voltage across the burden would reach a certain voltage (due to over current during a short circuit event).

This was agreed to - but when I went to the vendor (ITI/GE) to see if they could make up a secondary protector (similar to their OCP line of open circuit protectors) with a lower activation voltage than their current designs- they said that they are unable to design any new devices for several reasons - which aren't really important.

SOOO - for my question - do you have any information on who could manufacture such a secondary protector (activation voltage is 60V)?

Any help you (or anyone else) could give would be very much appreciated.we made an experiment.A 3000/1 Current transformer was gradually energized in steps with secondary open right from beginning.

We observed that at 10% rated current in primary. after about 10 seconds, the CT failed. The secondary when tested was, found to have shorted, also confirmed by significant reduction of winding resistance indicating interlayer shorts. Since we did not open secondary while primary was energised, it is not a Ldi/dt voltage failure.

Also, when tested earlier, this CT showed a knee point voltage of 750 volts at the secondary, which is well below the insulation level of the enamel wire used for secondary, i.e. 2 kV.

Thus even when open, the voltage developed may have exceeded the knee voltage to a saturation level of the order of at most 1 kV.

Principle of operation

A current transformer is defined as "as an instrument transformer in which the secondary current is substantially proportional to the primary current (under normal conditions of operation) and differs in phase from it by an angle which is approximately zero for an appropriate direction of the connections." This highlights the accuracy requirement of the current transformer but also important is the isolating function, which means no matter what the system voltage the secondary circuit need be insulated only for a low voltage.

The current transformer works on the principle of variable flux. In the "ideal" current transformer, secondary current would be exactly equal (when multiplied by the turns ratio) and opposite to the primary current. But, as in the voltage transformer, some of the primary current or the primary ampere-turns is utilized for magnetizing the core, thus leaving less than the actual primary ampere turns to be "transformed" into the secondary ampere-turns. This naturally introduces an error in the transformation. The error is classified into two-the current or ratio error and the phase error.

Kappa CT s are designed to minimise the errors using the best quality electrical steels for the core of the transformer. Both toroidal (round) and rectangular CT s are manufactured.

Rated primary current: The value of current which is to be transformed to a lower value. In CT parlance, the "load" of the CT refers to the primary current.

Rated secondary current: The current in the secondary circuit and on which the performance of the CT is based. Typical values of secondary current are 1 A or 5 A. In the case of transformer differential protection, secondary currents of 1/ root 3 A and 5/ root 3 A are also specified.

Rated burden: The apparent power of the secondary circuit in Volt-amperes expressed at the rated secondary current and at a specific power factor (0.8 for almost all standards)

Accuracy class: In the case of metering CT s, accuracy class is typically, 0.2, 0.5, 1 or 3. This means that the errors have to be within the limits specified in the standards for that particular accuracy class. The metering CT has to be accurate from 5% to 120% of the rated primary current, at 25% and 100% of the rated burden at the specified power factor. In the case of protection CT s, the CT s should pass both the ratio and phase errors at the specified accuracy class, usually 5P or 10P, as well as composite error at the accuracy limit factor of the CT.

Composite error: The rms value of the difference between the instantaneous primary current and the instantaneous secondary current multiplied by the turns ratio, under steady state conditions.

Accuracy limit factor: The value of primary current upto which the CT complies with composite error requirements. This is typically 5, 10 or 15, which means that the composite error of the CT has to be within specified limits at 5, 10 or 15 times the rated primary current.

Short time rating: The value of primary current (in kA) that the CT should be able to withstand both thermally and dynamically without damage to the windings, with the secondary circuit being short-circuited. The time specified is usually 1 or 3 seconds.

Instrument security factor (factor of security): This typically takes a value of less than 5 or less than 10 though it could be much higher if the ratio is very low. If the factor of security of the CT is 5, it means that the composite error of the metering CT at 5 times the rated primary current is equal to or greater than 10%. This means that heavy currents on the primary are not passed on to the secondary circuit and instruments are therefore protected. In the case of double ratio CT's, FS is applicable for the lowest ratio only.

Class PS/ X CT: In balance systems of protection, CT s with a high degree of similarity in their characteristics are required. These requirements are met by Class PS (X) CT s. Their performance is defined in terms of a knee-point voltage (KPV), the magnetizing current (Imag) at the knee point voltage or 1/2 or 1/4 the knee-point voltage, and the resistance of the CT secondary winding corrected to 75C. Accuracy is defined in terms of the turns ratio.

Knee point voltage: That point on the magnetizing curve where an increase of 10% in the flux density (voltage) causes an increase of 50% in the magnetizing force (current).

Summation CT: When the currents in a number of feeders need not be individually metered but summated to a single meter or instrument, a summation current transformer can be used. The summation CT consists of two or more primary windings which are connected to the feeders to be summated, and a single secondary winding, which feeds a current proportional to the summated primary current. A typical ratio would be 5+5+5/ 5A, which means that three primary feeders of 5 are to be summated to a single 5A meter.

Core balance CT (CBCT): The CBCT, also known as a zero sequence CT, is used for earth leakage and earth fault protection. The concept is similar to the RVT. In the CBCT, the three core cable or three single cores of a three phase system pass through the inner diameter of the CT. When the system is fault free, no current flows in the secondary of the CBCT. When there is an earth fault, the residual current (zero phase sequence current) of the system flows through the secondary of the CBCT and this operates the relay. In order to design the CBCT, the inner diameter of the CT, the relay type, the relay setting and the primary operating current need to be furnished.

Interposing CT's (ICT's) : Interposing CT's are used when the ratio of transformation is very high. It is also used to correct for phase displacement for differential protection of transformers.

The Indian and international standard references for CT s are as given in the table below:

Standard Standard Number Year

Indian IS 2705 1992

British BS 3938 1973

British BS 7626 1993

InternationalElectro technicalCommission (IEC)

IEC 60044-1 1996

Australian AS 1675 1986

American ANSI C.57.13 1978

Although the definitions given above are based on IS/BS and IEC standards, Kappa manufactures CT s to any of the above standards. Our designs are backed up by extensive up by extensive type testing at national and international laboratories.

A number of routine and type tests have to be conducted on CT s before they can meet the standards specified above. The tests can be classified as :

a. Accuracy tests to determine whether the errors of the CT are within specified limits. b. Dielectric insulation tests such as power frequency withstand voltage test on primary

and secondary windings for one minute, inter-turn insulation test at power frequency voltage, impulse tests with 1.2u/50 wave, and partial discharge tests (for voltage >=6.6kv) to determine whether the discharge is below the specified limits.

c. Temperature rise tests. d. Short time current tests. e. Verification of terminal markings and polarity.

Kappa conducts routine tests on each and every CT produced and all designs are type tested.

Typical specification for a 11 kV CT

System voltage:11 kVInsulation level voltage (ILV) : 12/28/75 kVRatio: 200/1 - 1 - 0.577 ACore 1: 1A, metering, 15 VA/class 1, ISF<10Core 2: 1 A, protection, 15 VA/5P10Core 3: 0.577 A, Class PS, KPV>= 150 V, Imag at Vk/2 <=30 mA, RCT at 75 C<=2 ohmsShort time rating:20 kA for 1 second

Instrument transformers - A reference manual

Kappa has published a handy reference manual for instrument transformers. The manual runs to about 160 pages and further details on all of the above topics as well as on much more, including Australian and ANSI standards can be found in the manual. The table of contents of the manual is reproduced below:

Principle of operation

The standards define a voltage transformer as one in which "the secondary voltage is substantially proportional to the primary voltage and differs in phase from it by an angle which is approximately zero for an appropriate direction of the connections."

This, in essence, means that the voltage transformer has to be as close as possible to the "ideal" transformer. In an "ideal" transformer, the secondary voltage vector is exactly opposite and equal to the primary voltage vector, when multiplied by the turns ratio.

In a "practical" transformer, errors are introduced because some current is drawn for the magnetization of the core and because of drops in the primary and secondary windings due to leakage reactance and winding resistance. One can thus talk of a voltage error,which is the amount by which the voltage is less than the applied primary voltage ,and the phase error, which is the phase angle by which the reversed secondary voltage vector is displaced from the primary voltage vector.

Kappa designs its VT's so that the resistance and reactance drops are minimized. It also uses the best grades of cold rolled grain oriented electrical steels which enables operation at optimum levels of magnetic induction, thus reducing both the size and the cost of the VT.

Typical terms used for specifying a voltage transformer (VT)

a. Rated primary voltage: This is the rated voltage of the system whose voltage is required to be stepped down for measurement and protective purposes.

b. Rated secondary voltage: This is the voltage at which the meters and protective devices connected to the secondary circuit of the voltage transformer operate.

c. Rated burden: This is the load in terms of volt-amperes (VA) posed by the devices in the secondary circuit on the VT. This includes the burden imposed by the connecting leads. The VT is required to be accurate at both the rated burden and 25% of the rated burden.

d. Accuracy class required: The transformation errors that are permissible, including voltage (ratio) error and phase angle error. Phase error is specified in minutes. Typical accuracy classes are Class 0.5, Class 1 and Class 3. Both metering and protection classes of accuracy are specified. In a metering VT, the VT is required to be within the specified errors from 80% to 120% of the rated voltage. In a protection VT, the VT is required to be accurate from 5% upto the rated voltage factor times the rated voltage.

e. Rated voltage factor: Depending on the system in which the VT is to be used, the rated voltage factors to be specified are different. The table below is adopted from Indian and International standards.

Rated voltage factor

Rated time Method of connecting primarywinding in system

1.2 Continuous Between phases in any network

Between transformer star-point and earth in any network

1.21.5

Continuousfor 30 seconds

Between phase and earth in an effectively earthed neutral system

1.21.9

Continuousfor 30 seconds

Between phase and earth in a non-effectively earthed neutral system with automatic fault tripping

1.21.9

Continuous for 8 hours

Between phase and earth in an isolated neutral system without automatic fault tripping or in a resonant earthed system without automatic fault tripping

f. Temperature class of insulation: The permissible temperature rise over the specified ambient temperature. Typically, classes E, B and F.

g. Residual voltage transformer (RVT): RVTs are used for residual earth fault protection and for discharging capacitor banks. The secondary residual voltage winding is connected in open delta. Under normal conditions of operation, there is no voltage output across the residual voltage winding. When there is an earth fault, a voltage is developed across the open delta winding which activates the relay. When using a three phase RVT, the primary neutral should be earthed, as otherwise third harmonic voltages will appear across the residual winding. 3 phase RVTs typically have 5 limb construction.

h. Metering Units:11kV metering units consist of one 3 phase VT and 2 CT's connected together in a single housing. This can be used for three phase monitoring of energy parameters. It is used with trivector meters and energy meters.

i. The Indian and international standard references for VT s are as given in the table below:

Standard Standard Number Year

Indian IS 3156 1992

British BS 3841 1973

British BS 7625 1993

British BS 7729 1994

InternationalElectro technicalCommission (IEC)

IEC 186 1987

Australian AS 1243 1982

American ANSI C.57.13 1978

j. Kappa manufactures VT s to all these standards. Our designs are backed up by extensive type testing at national and international laboratories.

A number of routine and type tests have to be conducted on VT s before they can meet the standards specified above. The tests can be classified as:

a. Accuracy tests to determine whether the errors of the VT are within specified limits

b. Dielectric insulation tests such as power frequency withstand voltage test on primary and secondary windings for one minute, induced over-voltage test , impuse tests with 1.2u/50u wave, and partial discharge tests (for voltage>=6.6 kV) to determine whether the discharge is below the specified limits.

c. Temperature rise tests d. Short circuit tests e. Verification of terminal markings and polarity

Kappa conducts routine tests on each and every VT produced and all designs are type tested.

System voltage: 11 kVInsulation level voltage (ILV) : 12 /28/75 kVNumber of phases: ThreeVector Group: Star / StarRatio: 11 kV/ 110 VBurden: 100 VAAccuracy: Class 0.5Voltage Factor: 1.2 continuous and 1.5 for 30 seconds With provision for fuse

Double and triple secondary VT s are also manufactured.

CTs are used for the measurement of high currents as a primary sensor. The output of the CT is normally connected to the relevant ammeter which has very low impendence and secondary remains almost short circuited. The secondary current has no control over primary current in a CT. If secondary is open circuited two things will happen :1. Due to absence of secondary current no counter magnetic field is produced and flux in core of CT rises to saturation level which can change the charateristics of CT and damage it permanently 2. since number of turns in secondary is very high (1: 1000) for , a very high voltage may be induced in secondary that can harm the prsons working on the instruments panel.

The CT secondary should not be left opened secondary due to following reasons:

1)Zero current in secondary leads zero back emf.So primary current becomes very high as the primary resistance is very low.Large heat generated may burn the winding and core.If the transformer sustain this heat, even then its operation becomes improper as explained below.

2)CT is bascially a step up transformer.So high voltage induced in secondary may saturate the core (core becomes permanently damaged).So the operating point advances in non linear region,which makes the transformer operation improper(secomdary current will not replicate the primary according to the turn ratio).

3)High voltage spikes(order of kV) produced in secondary side may endanger the life of personnel working on the secondary side.

4)High voltage spikes(order of kV) produced in secondary side is sufficient to break down the transformer insulation.