A Methodology For Implementing Fault Current Limiting Reactors (CLRS) On Feeders With Minimal Constant Power Losses
A Methodology For Implementing Fault Current Limiting Reactors (CLRS) On Feeders With Minimal Constant Power Losses
A Methodology For Implementing Fault Current Limiting Reactors (CLRS) On Feeders With Minimal Constant Power Losses
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Victor Ogboh
Nnamdi Azikiwe University, Awka
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Abstract
This paper discusses the deficiency in the use of series current limiting reactors (CLRs) on feeders. It also suggests
circuit breaker design modifications that can mitigate this problem and as well reduce the operating mechanism energy
requirement of power circuit breakers.
Keywords: reactors, operating mechanism, fixed and moving contacts, current limiting reactor, short-circuit currents,
blast pressure.
1.0 Introduction
Short circuit incidents increase every day. The problem of this increase in short circuit currents is being experienced
by many utilities in the world. Though current limiting reactor (CLR) is the most practical technique for reducing short
circuit currents to levels within the rating of the equipment on the load side of the reactor, their use is limited to some
critical feeders due to the constant power losses caused by them since they are connected in series with the feeders and as
such carry the full load current [2, 3, 4]. Circuit breakers (CBs) of high capacity and reliability may be expensive but they
help to prevent the losses caused by feeder CLR.
Solving the problems of constant power losses when CLRs are used on feeders as well as reducing the cost of
feeder CBs shall be achieved by modifications in the following parts of a feeder CB interrupter unit:
(1) The CB terminals
(2) The CB fixed contact
(3) The CB moving contact
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G.J. E.D.T.,Vol.4(5):1-7 (September-October, 2015) ISSN: 2319 – 7293
value.
Figure 2: CLR connection during (a) healthy condition and (b)/(c) fault condition.
In figure 2, R is the CLR; C is the CLR bypass bar; IR is the current through the CLR; IC is the current through the CLR
bypass bar; IL is the normal load current; If is the fault current.
As seen in figure 2 above, (a) represents the proposed connection during normal condition, while (b) represents the
proposed connection (open circuited C) during fault. At normal condition, IC >> IR (or IR ≈ 0 and IC ≈ IL), meaning
infinitesimal losses across R. open circuiting C at fault threshold would bring R in series with the feeder as shown in
figure 2(c) (i.e. figure 2(b) redrawn).
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G.J. E.D.T.,Vol.4(5):1-7 (September-October, 2015) ISSN: 2319 – 7293
Figure 3: Post Insulator/Interrupter unit of a CB showing (a) existing and (b) recommended modification.
(B) THE FIXED CONTACT:
The fixed contact shall be split into two and separated for connecting: (1) the CLR and (2) the CLR bypass bar. This is
shown in figure 4.
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G.J. E.D.T.,Vol.4(5):1-7 (September-October, 2015) ISSN: 2319 – 7293
The prototype connection that gives the desired result is shown in figure 6.
In figure 6, R is the CLR; C is the CLR bypass bar; IR is the current through the CLR; IC is the current through the CLR
bypass bar.
5.0 Effect of Rated Short Circuit Breaking Current (SCBC) on the Operating Mechanism
Energy
The rated short circuit breaking current, ‘I’, exercises its influence on the required operating mechanism energy
mainly through the blast pressure, ∆P, which is necessary to warrant reliable arc-quenching under short line fault
conditions. From experimental results,
Ia ∆P
N-a/n ------- (1)
Where
N = number of breaks
I = rated short circuit breaking current.
The constant ‘a’, assumes a value between 1.1 and 1.42 depending on the filling pressure and the values for ‘n’ are
between 1 and 7 [5].
Taking a value of 1.4 for ‘a’ and a value of 5 for ‘n’ in equation (1), the following results:
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G.J. E.D.T.,Vol.4(5):1-7 (September-October, 2015) ISSN: 2319 – 7293
Equation (2) implies that even a little reduction in ‘I’ reduces ∆P greatly as seen in figure 7, for the values of ‘I’
between 35.7MVA and 98.7MVA and for number of breaks N = 2.
100
90
80
Fault level(MVA)
70
60
50
40
30
20
100 150 200 250 300 350 400 450 500 550
BLAST PRESSURE[kg/cm2.g]
Figure 7: graph of blast pressure against fault current
Assuming a linear relationship as a rough approximation, ∆P relates to the compression work, W COMP thus:
WCOMP N.A. ∆P ------- (3)
But
I
A ----- (4)
And
P = P0 + ∆P ----- (5)
Where
P = pressure
A = piston area
∆P = blast pressure
P0 = filling pressure
A I
(I .N-0.28)1/2
1.4
OR
A I0.3.N0.14 ------- (6)
WCOMP N.I0.3.N0.14.I1.4.N-0.28
OR
WCOMP N0.86.I1.7 ------ (7)
Equation (7) means that a little reduction in the short circuit breaking current, I, would reduce WCOMP significantly
as seen in figure 8, for the values of I between 35.7MVA and 98.7MVA and for number of breaks N = 2.
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G.J. E.D.T.,Vol.4(5):1-7 (September-October, 2015) ISSN: 2319 – 7293
100
90
80
Fault level(MVA)
70
60
50
40
30
0.5 1 1.5 2 2.5 3 3.5 4 4.5
COMPRESSION WORK[kilo Newtons]
Figure 8: Graph of compression work against fault current
The use of CLR implies reduced short circuit breaking current, ‘I’. Reduced ‘I’, leads to reduced kinetic energy,
‘WKIN‘, requirements of a CB. What this means is that the accelerating force which stress the moving contact rod is also
reduced in proportion to the energy, WKIN. The cross-sectional area and hence the mass of this moving contact rod must
therefore be reduced. Hence the kinetic energy, ‘WKIN’, thus reduces again.
From kinetic energy, WKIN = ½ M.V2, if the speed, ‘V’, is maintained constant, it is obvious that the kinetic energy,
‘WKIN’, has a proportional relationship with the mass, ‘M’, of the moving contact rod.
6.0 Discussion
6.1 CLR connection
From figures 2(a) and 6, (CB in closed position), the reactance of the CLR, ‘X R’, is parallel to the reactance of the
CLR bypass bar, ‘XC’, (XR // XC). With XR >> XC, the equivalent reactance, ‘Xeq’, of the circuit shall be less than XC.
This implies that no power losses are encountered under parallel connection of R and C in the circuit. Also, with X R >>
XC, the CLR could be shorted out by the bypass bar ‘C’, under parallel arrangement of ‘R’ and ‘C’ in the circuit. This
again is a desirable result as no constant power losses shall be recorded at normal system condition.
6.2 BRINGING THE CLR IN SERIES WITH THE FEEDER DURING CB OPENING/CLOSING
During CB opening, the shorter prong of the moving contact rod breaks contact with the CLR bypass bar while the
longer prong is yet to break contact with the CLR. Instantaneously, there shall be current reversal in the CLR/CLR
bypass bar junction, thereby bringing the CLR in series with the feeder just before the breaker contacts open.
During CB closing, the CLR is serially connected in the circuit while the shorter prong is yet to make contact with
the CLR bypass bar. As soon as the CB closing operation is completed, by current reversal again at the CLR/CLR bypass
bar junction, the CLR becomes shorted out by the bypass conductor.
Based on the above discussion, it is seen that the CB always opens (whether during fault or not) with the CLR in
series but the CLR is shorted out when CB is closed. This is a desired result. Also, should the CB be closed on fault,
there shall be no cause for alarm even if the protection is so fast that it opens the CB before it is completely closed.
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G.J. E.D.T.,Vol.4(5):1-7 (September-October, 2015) ISSN: 2319 – 7293
100
90
80
Fault level(MVA) 70
60
50
40
30
5.5 6 6.5 7 7.5 8
AREA[cm2]
Figure 9: Graph of the moving contact area against the fault current
7.0 Conclusion
This paper presented the deficiency in implementing fault current limiting reactors on feeders. It also gave possible
ways to mitigate it.
References
[1] H. Seyedi, B. Tabei, “Appropriate Placement of Fault Current Limiting Reactors in Different HV Substation
Arrangements,”Scientific Research, Circuits and Systems, 2012, 3, 252 – 262.
[2] www.trenchgroup.com, “Current Limiting & Power Flow Control Reactors – Air Core Reactors,” Trench Group, 2014.
[3] en.wikipedia.org/wiki/current_limiting_reactor, “Current Limiting Reactor – Wikipedia,” The Free Encyclopedia.
[4] B.R. Gupta, “Generation of Electrical Energy,” Eurasia Publishing House (P) Ltd, 7361, Ram Nagar, New Delhi – 110 055, 2012,
pp. 275-280.
[5] F. Bachofen, P. Steinegger, R. Glauser, “A Novel Solution for High Speed SF6 Puffer Type Power Circuit Breakers of High Rated
Capability with Low Operating Mechanism Energy,” International Conference on Large High Voltage Electric Systems, Cigre WG 13-
07, 1982.
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