FIELD OF THE INVENTION
The invention relates to a 24kV PR Class Current Transformer for AC and
DC Filter Circuit of HVDC project.
BACKGROUND OF THE INVENTION
The main function of a current transformer is to transform and reproduce
the primary current in the secondary circuit with a minimum distortion in
ratio and phase angle relationship and in current wave shape. For any CT
under steady state conditions, the major considerations in evaluating the
performance are its ratio and phase angle errors which have to be kept
within the required accuracy classes for specified burdens and primary
currents.
On the other hand the operational requirements of protective current
transformers are different and their performance during transient
conditions (after the inception of a short circuit) is of prime importance.
The increase of fault power in the present system requires the operating
time for the protective relays to be constantly reduced to minimize the
effects of heavy fault current. Because of the expansions in the system
network, the time constant of the system becomes larger. With a long time
constant, the transient component of the fault current may persist for long
periods and the relay may act during the time when transient conditions
still exist. The protective current transformers subjected to such fault
current should have transient characteristic adequate to transmit the
sudden variation of primary current to the secondary side without
distortion for the required length of time.
The transient performance of current transformers (CT) is influenced by a
number of factors with most notable the exponentially decaying dc
component of the primary current. Its presence influences the build-up of

core flux, a phenomenon which is likely to cause saturation and
subsequently substantial errors in the magnitude and phase angle of the
generated signals. The core flux is composed of an alternating and an
unidirectional component corresponding to the ac and dc components of
the primary fault current. The transient flux swing generated by the dc
component of the primary fault current can be quite large compared to the
one created by the ac component.
The current transformer core may also retain an unknown amount of flux,
because of the ferromagnetic character and whether or not the current
transformer has antiremanence air gaps. This remanent flux will either aid
or oppose the build-up of core flux and could contribute to CT saturation,
during subsequent faults in the power system such as high-speed auto
reclosing into a permanent fault, depending on the relative polarities of the
primary dc component and the remanent flux. Furthermore, after primary
fault interruption, the current transformer could still produce a unipolar
decaying current due to the stored magnetic energy.
The presence of residual flux left in the core can affect the saturation time,
reducing then available flux swing before saturation. If a fault containing a
DC transient is interrupted within the first few cycles, the core may be at a
high flux density, perhaps even saturated, and will decay from this level,
leaving considerable residual magnetism interruption will again occur at
fixed points on the flux way forms and maximum remanence will be left in
a current transformer with a resistive burden.
Main Features of 24kV PR Class Current Transformer:
For outdoor use
Remanence factor (Kr) =: 100 x Φr/Φs shall not exceed 10%
Minimum Current Time Area before Saturation= 20 As
Maximum continuos DC current in primary winding = 10 mA
Maximum Transient Current (crest value): 30.3kA

Time to Crest = μS
Rated Continuous Thermal Current including harmonics 182 A (CT ratio:
2000/l)
Major harmonic Current, rms value n/A
2nd /119.0
12th /90.3
6th /66.1
3rd /50.9
48th /28.5
24th /23.9
54th /19.6
42nd /18.7
45th/ 17.8
51st/16.3
Invention for Control of Residual Flax by Air Gap:
To overcome the problems of AC and DC effect on CT without saturation
during operations, CT with air gap core is developed. The most practical
way to control residual flux is to put an air gap into the magnetic circuit
which will reduce the possible residual flux. Larger air gaps may cause
higher magnetizing current which will cause larger ratio error. So the
means for controlling residual flux must not ruin the performance over the
normal current range.
Remanence Factor,

Where,
Φs: Saturation flux
Φr : Remanent flux
Ψs = Saturation Flux, Peak value of the flux which would exist in a core in

the transition from the non saturated to the fully saturated condition and
deemed to be that point on the B-H characteristic for the core concerned at
which a 10% increase in B causes H to be increased by 50%.
Ψr = Value of the flux which would remain in the core three minutes after
the interruption of the exciting current of sufficient magnitude as to induce
the saturation flux Ys defined above
Remanence factor (Kr) = 100 x Ψs /Ys expressed as a percentage (%). For
PR class CT, The remanence factor (Kr) shall not exceed 10%.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig-1 shows Hysteresis or B-H Curve of Core Material (CRGO).
Fig. 2 shows air gap in a CT.
Fig. 3 shows Opposition of fields in Core & air gap.
Fig. 4 shows Determination of remanence by intersection of air gap shear
line & BH curve of magnetic core
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT:
The operational requirement of protective Current Transformer is to operate
without saturation at rated current of AC and DC components and under
transient conditions, including A.C and D.C. components and also any
remanent flux density Br, if present, must not exceed the saturation
fluxdensity Bs, i.e.,

Using the AC and DC component, the above condition becomes after
rearrangement,


Current Transformer is designed on the basis of air gap and the air gap is
introduced after precise calculation as per to get the desired Remanence
factor. Amperes' law, i.e., H.dl = NI and to find remanence I=0 is considered
(flux in core = flux in air gap).
Demagnetization after every cycle is achieved in anti remanence CT design
by introducing small air gaps in core of transformer. Considering the
Current Transformer or even a Transformer core with following parameters:
Because of small air gap length, flux fringing is negligible, consequently:
Flux in magnetic iron core = flux in air gap.
flux density in magnetic iron core = flux density in air gap
Field in air gap Hg = B/μ0 (1)
FieldincoreHj = B/( μ0 μr)
Symbols,
s = Saturation Flux = Bs (Tesla, or, Weber/m2 or Volt-Second/m2 )
r= Remanent flux = Br do
Ψs = Saturation Flux, Peak value of the flux, which would exist in a core in
the transition from the non - saturated to the fully saturated condition and
deemed to be that point on the B-H characteristic for the core concerned at
which a 10% increase in B causes H to be increased by 50%.
Ψr = Value of the flux, which would remain in the core three minutes after
the interruption of the exciting current of sufficient magnitude as to induce
the saturation flux Qs defined above.
Hj = Mean magnetic path, mmp in metre

Hg = Introduced small air gap length, metre ( If more than one air gap then
it is the total sum of gap lengths in metre
Hj = Magnetic field intensity in the magnetic core in A/m
Hg = Magnetic field intensity in air gap in A/m
μ0 = Permeability of free space or vacuum in H/m or Newton.Amp-2
From Ampere’s law:
Ampère’s law is generally stated formally in the language of calculus: the
line integral of the magnetic field around an arbitrarily chosen path is
proportional to the net electric current enclosed by the path

= Total enclosed current in Amps
Remanence is the flux after current removal. To find, we
have to consider current I = 0

Thus, Hj. Lj + Hs. 6„ = I)
Or,
Hi ■ LJ = " Hs ' 6* - (2)
Therefore, Hj and Hg are in opposite direction
Consequently Hg causes demagnetisation
Multiplying equation (2) by μ0,


Fig. 3 Determination of remanence by intersection of
air gap shear line & BH curve of magnetic core

Now, equation 3 gives the shear line due to small air gap & its intersection
with BH curve of the CT magnetic core determines the remanent point.
Taking advantage of the fact that for a small amount of remanence the
slope of the BH curve is almost vertical, we get:

(4)
Where, Hc is the coercivity or coercive field strength.
For CRGO (M4 & M5), Br is 0.2 Tesla. Putting this value in equation
6
-£ = 0.19 xlO"3
h
(4), 5
-! = 0.19 xi(T3
LJ
Therefore, for Kr = 0.1, total air gap will be 0.2 mm per metre length of
mean magnetic path (mmp) of CRGO ring core. This value of total air gap is
the sum of the air gap lengths, if these are more than one.
Nevertheless, for the anti-remanent design ring core of outdoor CT it is
essential to have only TWO numbers of air gaps .

Similarly, if we need Kr of any other value say from 0.01 to 0.2, this
relationship can be conveniently used to find the desired value of air-gap
length.
The location of these two air gaps in CT Core relative to outdoor hairpin
CT primary is also important for the correct performance of anti-remanent
design.

We claim:-
1. A method of improving operation of a current transformer (CT) under transit
conditions by controlling remanent flux in which an air gap is introduced into a
magnetic circuit of the CT.
2. The method as claimed in claim 1, in which the air gap in chosen to suit the
specification of the CT.
3. The method of improving operations of a current transformer (CT) under
transit conditions as substantially descried and illustrated herein with reference to
the accompanying drawings.