04A- Notes_1_CTs

CURRENT TRANSFORMERS – STEADY STATE BEHAVIOUR Current transformers are among the most commonly used items of electrical apparatus and yet, surprisingly, there seems to be a general lack of even the most elementary knowledge concerning their characteristics, performance and limitations among those engineers who are continually using them. The importance of current transformers in the transmission and distribution of electrical energy cannot be over emphasised because it is upon the efficiency of current transformers, and the associated voltage transformers, that the accurate metering and effective protection of those distribution circuits and plant depend. Current and voltage transformers insulate the secondary (relay, instrument and meter) circuits from primary (power) circuit and provide quantities in the secondary which are proportional to those in the primary. The role of a current transformer in protective relaying is not as readily defined as that for metering and instrumentation. Whereas the essential role of a measuring transformer is to deliver from its secondary winding a quantity accurately representative of that which is applied to the primary side, a protective transformer varies in its role according to the type of protective gear it serves. Failure of a protective system to perform its function correctly is often due to incorrect selection of the associated current transformer. Hence, current and voltage transformers must be regarded as constituting part of the protective system and carefully matched with the relays to fulfil the essential requirements of the protection system. There are two basic groups of current transformer, the requirements of which are often radically different. It is true in some cases the same transformer may serve both purposes but in modern practice this is the exception rather than the rule: 1. Measurement CT`s - The measuring current transformer is required to retain a specified

accuracy over the normal range of load currents. 2. Protection CT`s - The protective current transformer must be capable of providing an

adequate output over a wide range of fault conditions, from a fraction of full load to many times full load.

Therefore they generally have different characteristics. CURRENT TRANSFORMER STANDARDS Various international standards are available. Such standards give information on the classification, selection, error and operation of current transformers. They are a valuable source of reference and can be used in conjunction with the relay manufacturer guide when selecting the appropriate CT. The list below gives some examples: IEC IEC 185:1987 CTs IEC 44-6:1992 CTs IEC 186:1987 VTs EUROPEAN BS 7625 VTs BS 7626 CTs BS 7628 CT+VT BRITISH BS 3938:1973 CTs BS 3941:1975 VTs AMERICAN ANSI C51.13.1978 CTs and VTs CANADIAN CSA CAN3-C13-M83 CTs and VTs AUSTRALIAN AS 1675-1986 CTs

Please note that the above are the applicable standards at the time of print of this document and therefore they may vary. CURRENT TRANSFORMER CONSTRUCTION A current transformer consists essentially of an iron core with two windings. One winding is connected in the circuit whose current is to be measured and is called the primary and the other winding is connected to burden, and called the secondary. Two of the most basic construction of current transformers are the bar type and wound type: 1. Bar Type – Sometimes referred to as ‘Bushing Type’. Such current transformers normally

have a single concentrically placed primary conductor, sometimes permanently built into the CT and provided with the necessary primary insulation, but very often the bushing of a circuit breaker or power transformer. At low primary current ratings it may be difficult to obtain sufficient output at the desired accuracy because a large core section is needed to provide enough flux to induce the secondary emf in the small number of turns.

PRIMARY

SECONDARY

2. Wound Type – With this device it is possible to change the number of primary turns, thus increasing the CT output voltage with altering the turns ratio. Therefore, for the same output the wound CT is smaller in CSA than the bar type.

SECONDARY

PRIMARY

CURRENT TRANSFORMER POLARITY There is no official standard when it comes to defining the polarity of current transformers. However, most Engineers will use P1 and P2 to define the primary winding and S1 and S2 to define the secondary winding. Generally speaking when P1 goes high S1 goes high. Therefore when current flows from P1 to P2 it is transferred and flows through the external circuit from S1 to S2. Typically P2/S2 is towards the item of plant being protected.

P2P1

S2S1

Ip

CURRENT TRANSFORMER THEORY The flow current in the primary winding produces an alternating flux in the core and this flux induces an e.m.f. in the secondary winding which results in the flow of secondary current when this winding is connected to an external closed circuit. The magnetic effect of the secondary current, in accordance with fundamental principles, is in opposition to that of the primary and the value of the secondary current automatically adjusts itself to such a value, that the resultant magnetic effect of the primary and secondary currents, produces a flux required to induce the e.m.f. necessary to drive the secondary current through the impedance of the secondary. In an ideal transformer, the primary ampere-turns are always exactly equal to the secondary ampere-turns and the secondary current is, therefore, always proportional to the primary current. In an actual current transformer, however, this is never the case. All core materials, so far discovered, require a certain number of ampere-turns to induce the magnetic flux required to induce the necessary voltage. The most accurate current transformer is one in which the exciting ampere-turns are least in proportion to the secondary ampere-turns. Exciting ampere-turns may be reduced in three principle ways: 1. By improving the quality of the magnetic material

Cold rolled grain oriented silicon steel (C.R.O.S.S.) has a magnetisation characteristic with a knee point at 1.6 tesla. Nickel steel (Proprietary name Mumetal) has a knee point of 0.7 tesla.

2. By decreasing the mean magnetic path of the core. 3. By reducing the flux density in the core.

CURRENT TRANSFORMERS BASIC FORMULAE

Protective relays are designed to operate from secondary quantities supplied from current transformers and from voltage (or potential) transformers. The secondary output of these devices is the information used by the relays to determine the conditions existing in the plan being protected. It is necessary, therefore, that the secondary output of current and voltage present a true picture to the relays of the conditions in the primary circuit during faults as well as during normal loads. Or, alternatively, that their performance be known under extreme conditions so that any error in reproduction in the secondary circuit can be partially or completely compensated for in the setting and characteristics of the relay.

In many applications, core saturation will almost inevitably occur during the transient phase of a

heavy short circuit. The performance of the associated instrument transformers during faults is, therefore, an important consideration in providing an effective relaying scheme. The relays and their associated current transformers must be considered as a unit in determining the overall performance of the protective scheme. Consequently, the characteristic of the current and potential transformers at high currents and low voltage respectively, must be known. In any current transformer the first consideration is the highest secondary winding voltage possible prior to core saturation. This may be calculated from :

Ek = 4⋅44 x B A f N volts Where : Ek = secondary induced volts (rms value, known as the knee-point voltage) N = number of secondary turns f = system frequency in hertz A = net core cross-sectional area in square meters.

This induced voltage causes the maximum current to flow through the external burden whilst still

maintaining a virtually sinusoidal secondary current. Any higher value of primary current demanding further increase in secondary current would, due to core saturation, tend to produce a distorted secondary current.

The relevant circuit voltage required is typically :

Es = Is (ZB + ZS + ZL) ……Equation 1 Where :

Is = secondary current of ct in amps (assume nominal value, usually 1A or 5A) ZB = the connected external burden in ohms ZS = the ct secondary winding impedance in ohms ZL = the resistance of any associated connecting leads

In any given case, several of these quantities are known or can usually be estimated in order to

predict the performance of the transformers. From the ac magnetisation characteristic, commonly plotted in secondary volts versus exciting current, Es can be determined for a minimum exciting current. The equation for the relevant circuit voltage given above then indicates whether the voltage required is adequate.

EXAMPLE Assume that a bar primary type 2000/5A (CROSS core) current transformer having a core csa

area of 20 square cm's is available with a secondary resistance of 0⋅31 ohm. The maximum current up to which the transformer must maintain its current ratio is 40,000 amperes. It is required to determine the maximum secondary burden permissible if core saturation is to be avoided. Assume that the current transformer core will start to saturate at 1⋅6 tesla.

From the data given : N = 2000/5 = 400 turns f = 50 Hz Secondary current (Is) with a primary current of 40,000A is given by :

Ιs = 40,000 x 52000

= 100 amps

Knee point voltage Ek is given as follows :

E = 4.44 x 1.6 x 20 x 50 x 40010

k 4

= 284 volts

Maximum burden permissible (including ct secondary resistance and lead burden) is equal to

284 / 100 = 2⋅84 ohms Consequently, the connected burden including that of the pilots can be as high as 2.84 - 0⋅31 = 2

⋅53 ohms for negligible saturation in the core. Thus it may be seen that the secondary burden and the maximum available fault current are two important criteria in determining the performance of a given current transformer.

A current transformer may operate satisfactorily :

a) At a high primary current where the connected secondary burden is low

b) At a lower primary current where the secondary burden is high. CURRENT TRANSFORMER MAGNETISATION CURVE The primary current contains two components. These are respectively the secondary current which is transformed in the inverse ratio of the turns ratio and an exciting current, which supplies the eddy and hysteresis losses and magnetises the core. This latter current flows in the primary winding only and therefore, is the cause of the transformer errors. It is, therefore, not sufficient to assume a value of secondary current and to work backwards to determine the value of primary current by invoking the constant ampere-turns rule, since this approach does not take into account the exciting current. From this observation it may be concluded that certain values of secondary current could never be produced whatever the value of primary current and this is of course, the case when the core saturates and a disproportionate amount of primary current is required to magnetise the core.

The amount of exciting current drawn by a current transformer depends upon the core material and the amount of flux which must be developed in the core to satisfy the burden requirements of the current transformer. The appropriate current may be obtained directly from the exciting characteristic of the transformer since the secondary e.m.f. and therefore the flux developed is proportional to the product of secondary current and burden impedance. The general shape of the exciting characteristic for a typical grade of CROSS (cold rolled grain orientated silicon steel) is shown. The characteristic is divided into three regions, defined by ‘ankle-point’ and the ‘knee-point’. The working range of a protective current transformer extends over the full range between the ‘ankle-point’ and the ‘knee-point’ and beyond, while a measuring current transformer usually only operates in the region of the ‘ankle-point’. The difference in working ranges between metering and protective current transformers stems from the radical difference in their functions. Metering current transformers work over the range 10% to 120% full load and it is even an advantage if the current transformer saturates for currents above this range in order to provide thermal protection for the instruments. Protection current transformers on the other hand are required to operate correctly at many times rated current.

MMF ampere-turns per metre

Flux

Den

sity

tesl

as

ankle point

knee point

KNEE-POINT The knee-point of the excitation characteristic is defined as the point at which a 10% increase in secondary voltage produces a 50% increase in exciting current. It may, therefore, be regarded as practical limit beyond which a specified current ratio may be maintained.

The current transformer magnetisation curve, is usually expressed in terms of Kv and Ki which when multiplied by the flux density in teslas and ampere-turns per cm respectively gives corresponding volts and amperes : Es = 4⋅44 f B A N volts.

In this equation, the flux density B is in teslas and the core cross-sectional area is in square meters.

When the flux density B is in teslas and the cross-sectional area is in square centimetres :

E = 4.44 x 50 x B x A10

x N

= 222 B A N 10

But K = EB

K = A N45

s 4

-4

vs

v

The exciting current Ie in amps can be obtained from the MMF using the relationship; Ie = Ki x MMF

∴ K = MMFi

eΙ

The units of Ki will depend on the units of MMF. If the MMF is in ampere-turns per meter;

K = LN

where L is in metres

If the MMF is in ampere - turns per cm;

K = LN

where L is in cms

i

i

L = mean magnetic path

EXAMPLE

Consider the case of a current transformer ratio 100/5A connected to an earth fault relay. Relay burden at minimum tap setting of 10% of rated current is given as 2 VA. Calculate the required values of Kv and Ki to provide the necessary output up to 10 times the plug setting, with : i) A bar primary type current transformer and with ii) A wound primary (5 turns current transformer). Assume the use of a CROSS core; B = 1⋅6 tesla.

i) Ring Type Current Transformer (Bar Primary) Relay current setting = 0⋅5 ampere : ie 10% of 5A.

Volts required to operate relay = 20.5

= 4 volts

Volts required at 10 times the = 4 x 10 = 40 volts ignoring lead the plug setting burden and CT secondary winding resistance Therefore, 40 volts must correspond to the knee-point of the saturation curve which

represents a flux density of 1⋅6 tesla. With a bar primary, secondary number of turns = 20 Ek = 4⋅44 f B A N 40 = 4⋅44 x 50 x 1⋅6 x A x 20 x 10-4. (A in cm²)

A = 400.71

= 56.3 cm2

Assume stacking factor = 0⋅92 ∴ Gross CSA = 56⋅3/0⋅92 = 61⋅2 cm² Assuming : I.D. = 18 cms O.D. = 30 cms Depth = 10⋅2 cms

18 cm10.2 cm

30 cm

K = A N

45 = 56.3 x 20

45 = 25

K = LN

= 2420

= 3.77 cm / turn

v

iπ

ii) Wound Primary CT Assume current transformer is wound with 5 primary turns :

Secondary turns = 5 x 1005

= 100

40 = 4⋅44 x 50 x 1⋅6 x 100 x A. 10-4 (A in cm²)

22 cm 12.24 = 0.92

11.26 = csa ie, cm 11.26 = 3.5540 =A

Assuming : I.D. = 18 cm O.D. = 30 cm Depth = 2⋅04 cm

18 cm

2.04 cm30 cm

K = A N

45 = 11.26 x 100

45 = 25

K = LN

= 24100

= 0.754 cm / turn

v

iπ

OPEN CIRCUITED SECONDARY WINDING The secondary circuit of a current transformer should never be left open-circuited whilst primary continues to flow. In these circumstances only the primary winding is effective and thus the current transformer behaves as a highly saturated choke (induction) to the flow of primary winding current. Thus a peaky and relatively high value of voltage appears at the secondary output of terminals, endangering life, not to mention the possible resulting breakdown of secondary circuit insulation. In those cases where current transformers are associated with the “high impedance type” earth fault relay the secondary circuit burden may have ohmic values up to several thousands of ohms.

EQUIVALENT CIRCUIT The errors of a current transformer may be considered as due to the whole of the primary current not being transformed, a component thereof being required to excite the core. Alternatively, we may consider that the whole of the primary current is transformed without loss, but that the secondary current is shunted by a parallel circuit the impedance of which is such that the equivalent of the exciting current flows there in. The circuit shown is the equivalent circuit of the current transformer. The primary current is assumed to be transformed perfectly, with no ratio or phase single error, to a current IP/N which is often called 'the primary current referred to the secondary'. A part of the current may be considered consumed in exciting the core and this current Ie is called the secondary excitation current. The remainder Is is a true secondary current. It will be evident that the excitation current is a function of the secondary excitation voltage Es and the secondary excitation impedance Ze. It will also be evident that the secondary current is a function of Es and the total impedance in the secondary circuit. This total impedance consists of the effective resistance (and any leakage reactance) of the secondary winding and the impedance of the burden.

Ip = primary current in amperesN = current transformer ratio (primary to secondary amperes)Zb = burden impedance of relays in ohms (r + jx)Zs = current transformer secondary winding impedance in ohms (r + jx)Ze = secondary excitation impedance in ohms (jx)Ie = secondary excitation current in amperesIs = secondary current in amperesEs = secondary excitation voltage in voltsVt = secondary terminal voltage in volts across the current transformer terminals

(input to the relay or burden)

N

N

ZsIs

IpIp Ie EsZbZe

Vt

SATURATION Beyond the knee-point the current transformer is said to enter saturation. In this region the major part of the primary current is utilised to maintain the core flux and since the shunt admittance is not linear, both the exciting and secondary currents depart from a sine wave. For example, in the case of a wholly resistive burden, correct transformation takes place until saturation flux density is reached. The secondary volts and current then collapse instantly to zero, where they

remain until next primary current zero is reached. This process is repeated each half cycle and results in a pulse waveform as shown.

Ampere-Turns Per Core Length

Mean Magnetic Path

Assumed Magnetisation

Curve

Flux Density

Time

Flux Density Secondary Current and Voltage

Primary Current

Flux Density

CURRENT TRANSFORMER ERRORS The error associated with current transformers are the current error and the phase error. The errors for metering and protection current transformers are quite different, those for protection transformers being less exacting, as might be expected. For Classes 0.1 to 1 the current error and phase displacement at rated frequency shall not exceed the values given in Table 1 when the secondary burden is any value from 25% to 100% of the rated burden. For Class 3 and Class 5, the current error at rated frequency shall not exceed the values given in Table 2 when the secondary burden is any value from 50% to 100% of the rated burden. The secondary burden used for test purposes shall have a power factor of 0⋅8 lagging, except where a burden is less than 5VA a power factor of 1 shall be used. In no case shall the test burden be less than 1VA.

TABLE 1 Limits of Error for Accuracy Classes 0.1 to 1

± % RATIO ERROR AT PERCENTAGE OF RATED

± PHASE DISPLACEMENT AT PERCENTAGE OF RATED CURRENT SHOWN BELOW

CLASS

CURRENT SHOWN BELOW

MINUTES CENTIRADIANS

10 UPTO BUT NOT

INCL 20

20 UPTO BUS NOT

INCL 100

100 UPT

O 120

10 UPTO BUT NOT

INCL 20

20 UPTO BUS NOT INCL 100

100 UPT

O 120

10 UPTO BUT NOT

INCL. 20

20 UPTO BUS NOT INCL 100

100 UPT

O 120

0.1 0.25 0.2 0.1 10 8 5 0.3 0.24 0.15 0.2 0.5 0.35 0.2 20 15 10 0.6 0.45 0.3 0.5 1.0 0.75 0.5 60 45 30 1.8 1.35 0.9 1 2.0 1.5 1.0 120 90 60 3.6 2.7 1.8

Table 2 Limits of Error for Accuracy Classes 3 and 5 CLASS ± % RATIO ERROR AT

PERCENTAGE OF RATED CURRENT SHOWN BELOW

50 120 3 3 3 5 5 5 LIMITS OF ERROR FOR PROTECTION CT`s - At rated frequency and with rated burden connected the current error, phase displacement and composite error shall not exceed the values given in Table 3. For test purposes, when determining the current error and phase displacement, the burden shall have a power factor of 0.8 inductive except where the burden is less than 5VA a power factor of 1.0 is permissible. TABLE 3 Limits of Error for Accuracy Class 5P and Class 10P ACCURACY CLASS

CURRENT ERROR AT RATED PRIMARY

PHASE DISPLACEMENT AT RATED PRIMARY CURRENT

COMPOSITE ERROR AT RATED

ACCURACY CURRENT (%)

MINUTES

CENTIRADIANS

LIMIT PRIMARY CURRENT (%)

5P ± 1 ± 60 ± 1.8 5 10P ± 3 10

Considering the excitation impedance (Ze) as a constant, the vectorial relationships between Ip and Is' is Ie. Ic constitutes the current error and Iq the component of Ie in quadrature with Is which results in the phase difference. If Ze were in fact a constant impedance, the vectorial error Ie of the diagram would be the composite error, but in practice the magnetising impedance Ze is not constant with the result that the exciting current Ie contains some harmonics of the fundamental frequency which increases its rms value and thus increases the composite error. This effect is most noticeable in the region approaching saturation of the core when the wave-forms of the primary, secondary and exciting currents would be somewhat as shown.

Zb

Zs

Es

Is

Ie

Ze

Ip

Φ

IpIq

IeIcI's

Ep Es RATIO ERROR (CURRENT ERROR) The ratio error is defined as the error in the secondary current due to the incorrect ratio and is expressed as a percentage, by the expression :

(Kn s - p) 100p

Ι ΙΙ

Where :

Kn is the nominal ratio (rated primarycurrent/ratedsecondarycurrent) Is is the actual secondary current Ip is the actual primary current The ratio is considered positive when the actual secondary current of the transformer is

larger than the rated current PHASE ANGLE ERROR The phase angle error is the angle by which the secondary current vector, when reversed, differs in phase from the primary current. This angle is considered as positive if the reversed secondary current vector leads the primary current.

On very low burden power factors the phase angle error may be negative. Only on rare occasions is it necessary to determine the phase error of a CT used for relaying. These occasions occur when very low circuit settings ie 1% - 5% of rated current, are used in relays which are directionalised by voltage transformer inputs. For example, sensitive reverse power relays may require taking phase error into account in order to ensure correct directional operation where very low power factor primary currents are involved eg 1% - 3% power when certain types of prime movers are being motored eg, steam turbines and hydro sets. CURRENT TRANSFORMER RATINGS Current Transformer Burden - All c.t. accuracy considerations require knowledge of the c.t. burden, which is the load applied to the secondary of the c.t. and should preferably be expressed in terms of the impedance of the load and its resistance and reactance components. In practice it is usual to quote the relay burdens, in the first place, in terms of V.A. ( volt-amperes ) and

power factor. A burden of 12.5VA at 5A would have an ohmic value of :- ohm-0.5 5

12.52 =

CT burdens are usually given in preferred values, such as, 2.5, 5, 7.5, 10, 15, 30VA Continuous Rated Current - This is the maximum current the current transformer can carry continuously. It is usually the rated primary current. Short Time Rated Current – This is the amount of current which can flow for a given time period without any harmful effects. This is usually specified for 0.5, 1, 2 or 3 seconds and with the secondary short circuited. Rated Secondary Current – This is the maximum continuous current the secondary is rated to carry. It is usually 1 or 5A. Accuracy Limit Factor (ALF) - A current transformer is designed to maintain its ratio within specified limits up to a certain value of primary current, expressed as a multiple of its rated primary current. This multiple is termed its rated accuracy limit factor. In determining the accuracy limit factor it is necessary to consider the maximum value of primary current up to which the current transformer is required to maintain its ratio. (The ratio of accuracy limit primary current to the rated primary current). CHOICE OF C.T. PRIMARY RATING The c.t. primary rating is usually chosen to be equal to or greater than the normal full load current of the protected circuit. Standard primary ratings are given in B.S. 3938:1973. Generally speaking, the maximum ratio of c.t.s is usually limited to about 3000/1. This is due to ( i ) limitation of size of c.t. and more importantly ( ii ) the fact that the open circuit volts would be dangerously high for large c.t primary ratings, such as those encountered on large turbo alternators, e.g. 5,000 amperes. It is standard practice in such applications to use a cascade arrangement of say 5,000/20A together with 20/1A interposing auxiliary c.t.

CHOICE OF C.T. SECONDARY The total secondary burden of a current transformer includes not only the internal impedance of the secondary winding, the impedance of the instruments and relays which are connected to it, but also of the secondary leads. In modern outdoor switchgear, the distance between the current transformers and the relay panels may be considerable and with a rated secondary current of 5 amperes, the impedance of these leads constitutes a considerable burden. Because the losses vary as the square of the current, they are reduced to 1/25 at 1 ampere and to 1/100 at 0.5A. In most installations, the use of 1 ampere secondaries is sufficient to keep the pilot losses within reasonable limits and 0.5 ampere should be used only in very special cases. N.B. The same pilot cable size ( 7/029 or 2.5mm2 ) is used for both 1A and 5A CT’s Generally speaking, the usual value of rated secondary current is 5 amperes provided that the length of the pilots between the current transformers and the connected apparatus does not exceed about 25 yards. Up to this length the additional burden to the resistance of the pilots is reasonably small in relation to the total output of the transformer. CURRENT TRANSFORMER DESIGNATION Current transformers are usually designated as either Class `P` or Class `X`: Class P – Usually specified in terms or:

• Rated Burden • Class (5P or 10P) • Accuracy Limit Factor

Consider the following example: 15VA 10P20 i.e with an external secondary burden of 15VA the composite error will be 10% or less for primary currents up to 20 times rated current. To convert from VA and ‘accuracy limit factor’ (ALF) into volts, we can use the expression

ALF. VA VN

K Ι=

or when the internal voltage drop in the ct needs to be taken into account

⎟⎟⎠

⎞⎜⎜⎝

⎛Ι

+Ι=N

CTNKVA R (ALF) V

Class X – The performance of Class X current transformers of the low (secondary) reactance type shall be specified in terms of each of the following characteristics : ( i ) Rated primary current ( ii ) Turns ratio. (The error in turns ratio shall not exceed ± 0.25%)

( iii ) Knee-point voltage ( iv ) Exciting current at the knee-point voltage and/or at a stated percentage thereof. ( v ) Resistance of secondary winding CHOICE OF CURRENT TRANSFORMER CURRENT TRANSFORMERS SHOULD ALWAYS BE SPECIFIED IN ACCORDANCE WITH THE PROTECTION RELAY MANUFACTURERS RECOMMENDATIONS. Instantaneous Overcurrent Protection - Class P method of specification will suffice. A secondary accuracy limit current greatly in excess of the value to cause relay operation serves no useful purpose and a rated accuracy limit of 5 will usually be adequate. When such relays are set to operate at high values of overcurrent, say from 5 to 15 times the rated current of the transformer, the accuracy limit factor must be at least as high as the value of the setting current used in order to ensure fast relay operation. Rated outputs higher than 15VA and rated accuracy limit factors higher than 10 are not recommended for general purposes. It is possible, however, to combine a higher rated accuracy limit factor with lower rated output and vice versa. But when the product of these two exceeds 150 the resulting current transformer may be uneconomical, and/or of unduly large dimensions. Relays With Inverse & Definite Minimum Time Lag Characteristic - In general, for both directional and non-directional relays class 10P current transformers should be used. However, where time grading is tight it may be beneficial to use 5P. Differential Protection - Class ‘X’ specification are generally applicable to differential/unit systems where balancing of outputs from each end of the protected plant is vital. This balance, or stability during through fault conditions, is essentially of a transient nature and thus the extent of the unsaturated (or linear) zone is of paramount importance. Hence a statement of kneepoint voltage is the parameter of prime importance and it is normal to derive, from heavy current test results, a formula stating the lowest permissible value of VK if stable operation is to be guaranteed, e.g. VK = KIN (RCT + 2RL + R0) Where K - is a constant found by realistic heavy current tests IN - rated current of C.T. and relay RCT - secondary winding resistance of the line current transformers RL - lead burden (route length) in ohms R0 - any other resistance (or impedance) in circuit

Rogowski Coils Invented in 1912, this system may be regarded as a poorly coupled transformer since there is no magnetic circuit to intensify the flux. Applying a primary current (Ip) gives rise to a magnetising force creating a flux, which couples with the secondary winding producing a voltage. The voltage is proportional to the rate at which the flux changes; however, since the flux is set-up by the current we can say E=M.dIp/dt. The core can be solid or flexible and of a non-magnetic material, its purpose is to support the secondary windings. Unlike traditional magnetic CT`s, Rogowski coils are not susceptible to saturation and remenance. Since there is no inductance they are able to reproduce the primary signal with less distortion than a traditional ct. The inductance in a traditional ct creates a back emf which can distort the waveform being transformed. However, they only produce a small output, which in not a problem for modern relays but was in the past. Since they differentiate the signal the protective device needs to integrate the signal before it can be used. Any harmonics on the system are amplified by the harmonic number. They are also susceptible to noise on the system. Optical CT`s Optical ct`(OCT) were first used for high voltage current measurement in the late 1960`s. The main difference between OCT`s and traditional ct`s is that the power output from OCT`s is typically a few micro watts as oppose to several watts from a traditional CT. OCT`s detect rotation of the plane of polarisation of linearly polarised light in proportion to a magnetic field through the material. They have a number of advantages over traditional ct`s, such as: • Light weight • Smaller support structure • Noise Immunity • Safe • No Saturation However, they do have a number of disadvantages: • Detectors can be expensive • Affected by changes in environmental conditions • Delicate VOLTAGE TRANSFORMERS – STEADY STATE BEHAVIOUR The voltage transformer (v.t.) is a device which will provide isolation from the high voltages on the system and can transform (in the case of protection applications) reasonably accurately, the h.v. system voltage to which it is connected, to a value which is more convenient to handle, typically 110 / 63.5V. The accuracy requirement for measuring v.t.’s is, of course, far more demanding than that necessary for protection v.t.’s but this aspect is outside the scope of the lecture.

The diagramillustrate where, on the B – H curve, the v.t. is situated. Because the v.t.’s main role is the representation of system voltage, and accuracy is relevant, it can be seen that the flux density employed is somewhat lower than that used for c.t.’s when at their maximum output.

Magnetising Force AT/m

1000 2000 3000

‘H’Protection C.T. (at full load)

V.T.’s

Saturation

Cross C.T.’s & Power Transformers

Flux Density ‘B’

1.6

1.0

Tesla PROTECTION APPLICATIONS Protection v.t.’s are of two main types :- Electromagnetic and capacitor :- Electromagnetic V.T.’s The equivalent circuit of this type of transformer is shown. The secondary output voltage Vs is required to be an accurate replica of the input primary voltage Vp in both magnitude and phase over a specified range of output. Winding impedances are kept small and the magnetising current is kept low over the required range of operating voltages and burdens, by ensuring that the normal operating flux density in the core is well below the saturation level.

ZB (Burden)

LS

IS

RS

VSICReLM

EP = ES IM

LP

IP

RP

VP

NP / NS

= Kn

Voltage Transformer Error Specification Ratio (or voltage) error is defined as :-

100% x Vp

Vp)- Vs. (Kn where Kn = nominal Vp / Vs ratio

The turns ratio will not necessarily be equal to Kn due to the use of compensating turns to spread the errors over the range of burdens (i.e. –ve errors at low burdens and +ve errors at high burdens). The phase error is the angle θ between the primary and secondary voltages. It is positive when the secondary voltage leads the primary voltage. For the operation of meters and instruments the accuracy of a v.t. is usually only important at or about the normal system voltage, so that with a given burden the errors are practically constant. For protection, however, whilst the accuracy requirements may not be very exacting, the errors are required to be within certain limits over the wide range of voltage possible under system fault conditions. This range may be from 5% to 150% or 190% of rated primary voltage for v.t.’s connected between line and earth. This requires a corresponding range of core flux density and thus appreciable change in the value of the exciting impedance Ze. over this range. This results in a change in the no-load errors which may increase considerably unless attention is given to this problem at the design stage. Protective v.t.’s must meet the requirements of Table 7, BS 3941, detailed below :-

Phase Displacement Accuracy Class Percentage Voltage (ratio) error Minutes Centiradians

3P 6P

+ or – 3.0 + or – 6.0

+ or – 120 + or – 240

+ or – 3.5 + or – 7.0

Note - If the v.t. is also required for a measurement function it must, additionally, comply with BS 3941 : 1975 Table 6 and conform to one of the accuracy classes in it. For protective v.t.’s, the voltage error and phase displacement, at rated frequency, shall not exceed the values given in Table 7 at 5% rated voltage, and at rated voltage multiplied by the rated voltage factor (1.2, 1.5 or 1.9) with burdens of between 25% and 100% of rated burden at a power factor of 0.8 lagging. At 2% of rated voltage, the limits of error and phase displacement with burdens of between 25% and 100% rated burden at a power factor of 0.8 lagging shall be twice as high as those given in table 7.

Voltage factors. The voltage factor Vf is determined by the maximum operating voltage, which in turn is dependent on the system and v.t. earthing conditions, together with the permissible duration of the maximum operating voltage, as given in the table below :-

Earthing Conditions Voltage Factor Vf Duration v.t. Primary Windings

System

1.1 Not limited Non-earthed Eff or non-eff earthed 1.5 30 sec Earthed Effectively earthed 1.9 30 sec or 8 hr Earthed Non-eff earthed Residual voltages. For applications such as directional earth-fault protection residual voltages are required and are usually provided by windings connected in open delta.

dnda

Under normal conditions, the three phase-to-earth voltages are of equal magnitude and 120 degrees apart and the residual voltage (of system frequency) is zero, but under earth-fault conditions the voltage to earth of one phase collapses, either totally or partially depending on the location of the fault, the voltage applied to the other two phase-to-earth windings is increased and changed in phase, by amounts dependent on the method of earthing the system neutral, and a residual voltage Vr appears. Note that the third harmonic voltages add up in an open delta connected winding, and if present in the applied voltages will appear at the terminals of that winding. CAPACITOR V.T.’s At 132kV and high voltages, capacitor v.t.’s may be more economic than electromagnetic types, particularly when the h.v. capacitors can also be employed for carrier-current coupling. In fact, the main difference between an E.M. and a CAP. V.T. is the addition of a capacitor divider to the “front end” of an electromagnetic v.t., which virtually converts it into a capacitor v.t.This diagram shows the basic circuit usually employed.

VS

T

L

Vi VC2

VP C2

C1

ZB

The line to ground voltage Vp is applied across the capacitor voltage divider comprising C1 and C2 and the intermediate voltage VC2 is fed to the primary winding of an electromagnetic transformer T via a tuning inductance L which resonates approximately with C1 + C2 at the system frequency. The transformer T steps down to the secondary voltage Vs. There are numerous versions of this basic circuit. The inductance L may be a separate unit or it may be incorporated in the form of leakage inductance in the transformer T. Because the capacitors C1 and C2 cannot conveniently be made to close tolerances it is necessary to provide adjustments of ratio by means of tappings either on the transformer T or on a separate auto-transformer in the secondary circuit. Adjustments of the tuning inductance L is also necessary and this may be effected variously by tappings, by a separate tapped inductor in the secondary circuit, by adjustment of gaps in iron cores, or by shunting with a variable capacitor. As for electromagnetic v.t.’s, certain types of protection capacitor v.t.’s must have magnitude and phase errors within prescribed limits over a wide range of primary voltage such as from 5 to 150 or 190% of rated voltages. Because the core of the intermediate transformer is usually operated at a low flux density at normal voltage, little difficulty is experienced in maintaining the required accuracy at voltages above the nominal value. At very low system voltages, however, the core flux density falls to a level at which the permeability of the core material is relatively low and thus the exciting impedance Ze (Fig. 9) is reduced. Because the series reactance in the intermediate voltage circuit, through which the exciting current Ie must flow, is usually capacitive (a part of the inductance being in the transformer T and in the secondary circuit) and a positive phase error (output voltage leading). These increases in errors at 5% of rated voltage may be as great as +5% in amplitude and +5 degrees in phase in capacitor v.t.’s of relatively poor basic accuracy, but in the transformers now being supplied, which permit variation of the burden from 25% to the rated value without adjustment (full range transformers), the increase in errors, due to the smaller series reactance, may be no more than +1.5% and +60 minutes.

04A- Notes_1_CTs

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