PROPOSAL OF AN ADAPTIVE OVERCURRENT RELAY
FOR 110 KV NETWORK09
IONUȚ CIPRIAN BORĂSCU 1
Key words: Network topology, Relays, Fault currents.
Connection of many distributed generations (DG) has a major impact to the setting of
numerical overcurrent relays since the power produced by wind power plants or
photovoltaic power plants varies considerably. Therefore, in order to not exceed the
limits of transportable power in certain sections, in the case of unavailability of a 110 kV
line, territorial power dispatcher imposes operating regimes that have consequences to
numerical overcurrent protections. Since power system is a dynamic system, changing
the source impedance ratio (SIR), affects sensitivity, selectivity or in some cases can
lead to the impossibility of starting the numerical overcurrent relay. Hence, in order to
improve the functionality of a numerical overcurrent protection, under constant modification
of the source behind the relay, the paper is proposing an adaptive overcurrent algorithm
based on the phase to phase differential currents (PDC). As a result, the relay is sensitive to
all types of phase to phase short circuits and the protection area is improved through the
recognition of equivalent impedance source.
1. INTRODUCTION
In 110 kV network, the overcurrent protection is used as backup protection in
most of the cases. In the case of unavailability of the distance protection, the
overcurrent protection can become a primary protection for a certain period of time [1].
Latest reviews in the field shows that it can remain as basic protection up to a few
hours because it is dependent on the network topology changes and it cannot detect
short circuits with high resistance fault.
As is known, establishing the current setting threshold for conventional
overcurrent protection cannot be achieved so easily because it raises a number of
issues, like: number of operating schemes, radial or loop scheme, providing
transportable power, setting the load current without affecting the sensitivity criterion,
selective trip of the local overcurrent protection, fault resistance, etc.. Analysis of
data provided by multi-function transient recorders for events that have occurred in
the 110 kV network, have shown that, the changes in the source’s impedance and
“Politehnica” University of Bucharest, 313 Splaiul Independenței, 060042, Bucharest,
Romania, E-mail: borascu.ciprian@yahoo.com1
Rev. Roum. Sci. Techn. – Électrotechn. et Énerg., 60, 4, p. 376–386, Bucarest, 2015
2
Proposal of an adaptive overcurrent relay for 110 kV network
377
the limitation of the transmitted power, are two of the main reasons for which the
setting of the overcurrent protection in 110 kV loop network is not used.
The protection relay domain has made significant progress, mainly in order
not to limit the transmitted power. Unlike the phase- to- phase overcurrent relays,
the negative sequence overcurrent elements are not affected by the load current and
it can be set below the load in order to operate faster and more sensitive [2]. On the
other hand, the negative sequence overcurrent does not trip in the case of three
phase short circuits because the only one sequence that occurs is the positive one.
To prevent the changing of settings for numerical overcurrent protections
every time a new power source appears in the 110 kV network, or to ensure the
adequate protection zone, even when the dispatcher imposes modification of
operating schemes due to the unavailability or revisions of the power system
equipment, is necessary to use adaptive settings [3–6]. With the capabilities of the
microprocessor-based devices, the operating parameters, in order to maintain
optimal performance, are changing according to network conditions.
The adaptive overcurrent proposed detects phase faults by measuring the
phase to phase differential currents ( I AB , I BC , I CA ). The prospective short circuit
current is detected in any conditions of the system. As a result this eliminates the
need to establish a short circuit scenario in order to meet the maximum conditions [7].
Another advantage of this adaptive overcurrent logic is that it does not limit
the transportable power, so it can be set under the load current level.
2. CHARACTERISTICS AND OPERATION OF THE NUMERICAL
OVERCURRENT RELAYS BASED OF PHASE TO PHASE
DIFFERENTIAL CURRENTS
The three phase currents that flow through protective relay are I A , I B , I C ,
and the PDC and their amplitudes are defined as [8–10]:
I AB = I A − I B
I AB = I AB
I BC = I B − I C
I BC = I BC .
I CA = I C − I A
I CA = I CA
(1)
For two-phase fault, the equivalent circuit diagram of a transmission line with
a single power source, with load conditions, is represented in Fig. 1. The overview
diagram with load conditions is represented in Fig. 2. The sequence impedances
from bus S to fault location are Zl1 = Z12 = Zl 3 , Z 'L = Z L − Zl . The balanced load
impedances are Z 'load = Z 'load1 = Z 'load2 . E S represents the phase potential of the
equivalent source.
Ionuț Ciprian Borăscu
378
3
Fig. 1– Sequence impedances for double phase short circuit, considering a transmission line
with a single power source, with load conditions.
Fig. 2 – A 110 kV transmission line with a single power source, with load conditions.
IAB =
ES ⎡⎛ 3 3 ⎞ 3 Z 'L + Z 'load ⎤ ,
⋅ ⎢⎜ + ⋅ j ⎟ − ⋅
⎥
ZS + ZL ⎢⎣⎜⎝ 2 2 ⎟⎠ 2 ZS + Zl + Zload ⎥⎦
⎛
I BC = ⎜ − j ⋅ 3 ⋅
⎝
I AC =
ES ⎞
⎟,
Z S + Zl ⎠
ES ⎡⎛ 3 3 ⎞ 3 Z 'L + Z 'load ⎤ ,
⋅ ⎢⎜ − + ⋅ j ⎟ + ⋅
⎥
ZS + Zl ⎢⎣⎜⎝ 2 2 ⎟⎠ 2 ZS + Zl + Zload ⎥⎦
where
Zload = Z' L + Z'load .
(2)
(3)
(4)
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Proposal of an adaptive overcurrent relay for 110 kV network
379
Suppose that Z s = j ⋅ X S , Z L = RL + j ⋅ X L , Z load = j ⋅ X load .
Equation (5) can be expressed as follows:
I AB < 3 ⋅ I K
(3)
; I BC
= 3 ⋅ IK
(3)
; I AC
< 3 ⋅ IK
(3)
.
(5)
Technical regulations and the data provided by multi-function transient
recorders from numerical relays confirm that in the case of two phase short circuits,
for example, between phases B and C, the phase B current is equal and opposite to
the phase C current and the current from phase A tends to 0 amp. Also, the ratio
3
between three phase short circuit and two phase short circuit is
. In order to
2
identify the phase faults, in a 110 kV transmission line with a single source with
load, the paper proposes the equation (6). The logic is to calculate the maximum
absolute value of the phasors I AB , I BC , I CA , then it is checked that the absolute
value of the phasor identified as maximum, is less or equal than 3 ⋅ I K (3) .
max( I AB , I BC , I CA ) ≤ 3 ⋅ I K (3) ,
(6)
where I K (3) = three phase short circuit current.
Analyzing the equation (6) it can be deduced that the overcurrent detects
phase faults by measuring the maximum of PDC and the trip command can be
released when the maximum exceeds the setting. Therefore, the trip logic is:
I op = K ⋅
E Sph
ZS + ZL
,
max( I AB , I BC , I CA ) ≥ I op ,
(7)
(8)
where: K = reliability coefficient; E Sph = phase to phase potential of the equivalent
source; Z S = equivalent source impedance; Z L = positive sequence impedance of
the line.
3. ADAPTIVE IMPEDANCE RECOGNITION
In order to extend the zone of protection, selectivity increased, trip criterion and
coordination simplified, the adaptive impedance recognition represents a solution.
Consider that before the fault the phase currents and the phase to phase
voltages are: I AB (0) , I BC (0) , I AC (0) respectively U AB (t ) , U BC (t ) , U CA (t ) . The
source impedance can be calculated as follows [8–10]:
– for three phase faults
ZS = −
U BC (t ) − U BC (0)
U (t ) − U CA (0)
U (t ) − U AB (0)
= − AB
= − CA
;
I BC (t ) − I BC (0)
I AB (t ) − I AB (0)
I CA (t ) − I CA (0)
(9)
Ionuț Ciprian Borăscu
380
5
– for phase to phase short circuit faults
ZS = −
U PPf (t ) − U PPf (0)
I PPf (t ) − I PPf (0)
,
(10)
where: U PPf = phase to phase voltage between the faulted phases; I PPf = phase to
phase differential current between the faulted phases.
In comparison with the three PDC, the highest magnitude is associated with
the differential current between faulted phases and therefore the faulted loop can be
selected.
For a phase to phase short circuit (which involves phases A and B) the phases
A and B are selected as faulted phase base, on operation criterion
max( I AB , I BC , I CA ) = I AB .
ESph = E AB (t ) = U AB (t ) + I AB (t ) ⋅ Z S
ZS = −
U AB (t ) − U AB (0)
I AB (t ) − I AB (0)
.
(11)
In these conditions, the expression for I op can be expressed as [8–10]:
I op = K ⋅
I op = K ⋅
E Sph
ZS + ZL
U
AB
( t ) ⋅ ( I A B ( t ) − I A B (0 ) ) - I A B ( t ) ⋅ (U A B ( t ) − U A B (0 ) )
.
− (U A B ( t ) − U A B (0 ) ) + Z L ⋅ ( I A B ( t ) − I A B (0 ))
(12)
In order to avoid the malfunction of the adaptive overcurrent relay in the case
of disconnection of power transformer (PT), the relay uses the fault currents or
both fault currents and fault voltages.
When the PT is disconnected, the protection scheme will use only the fault
currents automatically.
4. PROTECTION ZONE OF THE NUMERICAL ADAPTIVE
OVERCURRENT RELAY BASED ON PDC
4.1. WITHOUT REAL-TIME CALCULATION
OF THE EQUIVALENT SOURCE IMPEDANCE
For a classic instantaneous overcurrent relay, the I op is [8–10]:
I op = K ⋅
ES
,
Z S min + Z L
(13)
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Proposal of an adaptive overcurrent relay for 110 kV network
381
where: I op represents the value of the pickup current. K represents the reliability
coefficient.
When the fault occurs on the position of m ⋅ Z L , the fault current is:
IF =
K f ⋅ ES
ZS + m ⋅ ZL
,
(14)
where I F represents the value of the fault current, K f represents the coefficient of
the fault type.
The zone of protection can be expressed as follows:
m=
K f ⋅ (Z S min + Z L ) − K ⋅ Z S
K ⋅ ZL
.
(15)
In the case that the real-time calculation of the equivalent source impedance
is not possible, the equivalent source impedance is constant, I op' = I F ' , the zone of
protection can be calculated from (16):
Z
+ ZL − K ⋅ ZS
m' = S min
.
K ⋅ ZL
(16)
Z S min + Z L − K ⋅ Z S
m'
=
.
m K ⋅ (Z S min + Z L ) − K ⋅ Z S
(17)
Then
In equation (17) the coefficient of fault type can take the following values:
K f = 1 – for three phase short circuit, then
Kf =
m'
=1;
m
3
m'
2
– for phase to phase short circuit, then
.
>
2
m
3
So, to all types of faults there is m' ≥ m .
4.2. WITH REAL-TIME CALCULATION OF THE EQUIVALENT SOURCE
IMPEDANCE
With real-time calculation of the equivalent impedance, I op'' = I F '' and the
zone of protection can be calculated as follows:
Ionuț Ciprian Borăscu
382
Z + (1 − K) ⋅ Z S
.
m'' = L
K ⋅ ZL
7
(18)
From equation (15) and (18) it can be derived:
Z L + (1 − K) ⋅ Z S
m''
.
=
m (K f ⋅ Z S min − K ⋅ Z S ) + K f ⋅ Z L
(19)
In equation (19), K f can take two values:
– K f = 1 for three phase short circuit, then
Z L + (1 − K) ⋅ Z S
m''
=
> 1;
m (Z S min − K ⋅ Z S ) + Z L
– Kf =
(20)
3
for three phase short circuit, then
2
Z L + (1 − K) ⋅ Z S
2
m''
=
>
.
m ⎛ 3
⎞
3
3
⎜
⋅ Z S min − K ⋅ Z S ⎟ +
⋅ ZL
⎜ 2
⎟ 2
⎝
⎠
(21)
From equations (20) and (21) it can be concluded that m'' ≥ m to all types of
faults.
m'
< 1 denotes that the zone of protection of the numerical
The ratio
m''
adaptive overcurrent relay based on the PDC is larger than the traditional one.
5. SIMULATION
In this paper I did a study in order to show the protection zones of the
overcurrent relay based on the PDC and of classic instantaneous overcurrent relay
(Figs. 3, 4).
The phase to phase short circuits and three phase short circuits, were made on
100 % of a 110 kV radial transmission line ( Z L =1.88+3.96 ⋅ j [Ω]), considering
different short circuit powers-source impedances, K and Kf.
8
Proposal of an adaptive overcurrent relay for 110 kV network
Fig. 3 – The zone of protection considering K f = 3 / 2, 1 and K=1.1 .
383
384
Ionuț Ciprian Borăscu
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Fig. 4 – The zone of protection considering K f = 3 / 2, 1 and K=1.5.
6. CONCLUSION
Short circuits from established scenarios are an important reference for
protection engineers. In reality the variable nature of the source impedance determined
mainly by commissioning respectively by out of service of significant production
capacity, significantly influence the selectivity and the speed of the overcurrent
protections. Therefore, in order to eliminate errors generated by the changes of SIR
or operating regimes, the paper proposes a numerical adaptive overcurrent relay
based on phase to phase differential currents using real-time calculation of the
equivalent impedance.
10
Proposal of an adaptive overcurrent relay for 110 kV network
385
Simulations that have been made show that the zone of protection of the
numerical overcurrent relay, for all types of short circuit, it is improved
significantly, using the adaptive impedance recognition. On the other hand, even if
the communication path is not available, the proposed logic for numerical
overcurrent relays is more sensitive in the case of phase to phase short circuits than
the traditional one.
Overall, compared with the classical overcurrent protection, the proposed scheme
is sensitive to all types of short circuits and not influenced by the load current.
The adaptive scheme provided in this paper can contribute to develop new
adaptive algorithms for overcurrent relays. Important future approaches can treat
adaptive algorithms for phase overcurrent protection within 110 kV loop network,
so as for phase faults located on the protected line to allow the start of automatic
reclosing from this protection.
ACKNOWLEDGMENTS
This work was co-funded by the European Social Fund through the
Operational Programme Human Resources Development 2007-2013 (Contract
number POSDRU/159/1.5/S/132395).
Received on September 17, 2014
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