The 3rd National Graduate Conference (NatGrad2015), Universiti Tenaga Nasional, Putrajaya Campus, 8-9 April 2015.
Time Base Zero Crossing Directional Overcurrent
Protection Relay
Yan Soon Huat
College of Engineering study, University Tenaga Nasional Putrajaya Malaysia. Email:y.soonhuat@hotmail.com
Associate Professor Dr. Farrukh Hafiz Nagi
Department of Mechanical Engineering Universiti Tenaga Nasional Putrajaya Malaysia. Email:Farrukh@uniten.edu.my
Dr. Aidil Azwin Bin Zainul Abidin
Department of Electrical Communication Engineering Universiti Tenaga Nasional Putrajaya Malaysia.
Email:AidilAzwin@uniten.edu.my
Abstract— Conventional fast fourier transform directional
protection relay requires complex butterfly computation
overheads and memory buffers. In this work, the directional
function is realized by using time base zero crossing method. The
time base zero crossing directional method is simulated in a
configuration of a parallel medium voltage feeder circuit, and by
modeling fault current in the cable between the generation and
load. The zero crossing method requires less computation and
memory space, as a result it responds faster in comparison to fast
fourier transform method. Furthermore, it is also easier to
program in a real time hardware system. The result from the
simulation shows that both directional overcurrent and normal
inverse current relay coordinate successfully when fault current
is introduced to different length in the cable.
Keywords— directional protection relay; time base zero
crossing method; parallel medium voltage feeder circuit; relay
coordinate.
I.
INTRODUCTION
The medium voltage power network is growing in
complexity and dimension, and renewable energy source such
as wind farm and solar PV are connecting into the system, and
making the situation more complicated and difficult for power
protection system[1,2]. Therefore, a robust power system
protection relay is required to ensure the system is protected
and safeguarded against power interruption. The most
commonly used inverse definite minimum time (IDMT)
overcurrent protection relay insufficient to protect the system
and the equipment against the fault. This is because, IDMT
protection relay is based on the current/time tripping curve
characteristic which is pre installed in the relay and it operates
to isolate the fault when the fault current meet the pre-set pick
up current level of the relay; the greater the fault current enter
the relay, the faster the relay operate and trip. However, IDMT
protection relay will not able to determine the direction of the
fault current flow in the system. Therefore, a directional
overcurrent protection relay is more suitable for the
application to protect the network, as the directional
overcurrent protection relay has the ability to identify the
direction of fault current flow in the network and the
current/time tripping curve same as IDMT protection relay to
justify the tripping characteristic. With this features,
directional overcurrent relay is able to make the network more
reliable and sturdy.
Previously, there are several types of methods being
designed to determine the direction of the fault current flow in
the system, among all the techniques fast fourier transform is
the most common technique implemented[3]. Fast fourier
transform technique is base on digital sampling and use phase
estimation between the voltage and current for the forward or
reverse flow direction of the fault current. However, fast
fourier transform technique also have some drawback in
determining the direction of the fault current, such as during
the fault occurrence in the network, both voltage and current
signal are badly distorted and in addition to that the signal may
contain harmonic as well as decaying of dc component, which
cause the phases measurement error[4]. Moreover, fast fourier
transform technique requires stored data which is
accomplished by buffering the data which adds latency on
delay in detection of the fault.
Time base zero crossing technique is another potential
technique to implement on the directional overcurrent
protection relay, because this technique had already been
researched and implemented to other application and showing
good result, such as power system synchronization system[5].
Zero crossing technique is very useful not only for the process
to determine the signals for the power system protection and
control/monitoring, but it also useful for detecting the current
transformer saturation condition due to external faults
occuring in the transmission line[6]. Accurate and fast
respones time delay estimation (TDE) is require to resolve and
minimize RADAR and SONAR time delay estimation, where
the zero crossing technique is able to be updated whenever
there is a new zero crossing detected. Moreover, the time
delay estimation algorithm has a low computational
complexity as well as low latency which gives good benefit to
the TDE to respond more accurately and real time[7].
In this paper a time base zero crossing technique is
proposed for the directional overcurrent protection relay. This
zero crossing technique is simple and straight forward; all it
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does is obtaining both voltage and current signal in real-time,
and compare it to get time difference between current and
voltage crossing point, from there it determines either the fault
current flow is in forward or reverse direction of the protection
relay. Due to the simplicity of zero crossing technique, it
requires a less computation function and able to perform in
faster processing rate, as well as react intuitively and produce
real-time output[8,9]. This method is not restricted to number
of samples and no data is lost to give false tripping instruction.
The design of this time base zero crossing directional
overcurrent protection relay is constructed on the
MATLAB/Simulink platform. To confirm it is working
compatible to the real situation in the power network system,
it is configured in series to a IDMT overcurrent relay on a
medium voltage parallel feeder circuit. A fault is modeled
with different length of cables connected between both normal
inverse overcurrent and directional overcurrent protection
relay. The result from the simulation is very promising as the
tripping of the directional overcurrent relay and normal
inverse overcurrent relay are well coordinated even though the
fault current occur in difference length in the cable.
II.
NETWORK CONFIGURATION
There are few circuit configurations which are applicable
to directional overcurrent protection scheme for protection of
power network. This paper proposes and discusses a
directional overcurrent protection scheme for a medium
voltage parallel circuit feeder. Parallel circuit is commonly
found in the utilities and in the industries network; and cause
of simple directional overcurrent protection scheme. This kind
of circuit use combination of overcurrent protection relay and
directional overcurrent protection relay with discrimination
properties Figure. 1.
R3 at the source end shall use overcurrent protection relay and
grade them with both directional relay R2 and R4 at the
receiving end, to ensure correct discriminative operation of the
relays during line fault occurrence which can be achieved by
setting the directional relay R2 and R4 time setting and current
pick up less then overcurrent relay's R1 and R3.
III.
RELAY PROTECTION SYSTEM IN GENERAL
In general, the protection relays are installed closed to the
source and load side. From fig.2, when the fault occurs in the
first power line (L1), the current in this system will flow in
two directions. In the first case, the current will flow from
source to B1, CB1, R1 and to the F, the second path the
current start to flow from source to B1, CB3, R3, R4, CB4,
B2, CB2, R2 and lastly F. In this situation both directional
relay R2 and R4 detects the occurrence of the fault, but only
directional relay R2 will pickup and trip the circuit breaker
CB2 due to its directional characteristic. Directional relay R4
is set similar to R2, but R4 will not trip CB4 due to the current
flow in opposite direction for the directional setting in R4.
Overcurrent protection relay R3 is a back up protection for
directional relay R2, if the discriminative setting for these
relays is set correctly, relay R3 will not trip CB3, in case the
directional relay R2 trip CB2. After CB2 trip, the fault current
now only flow in the first path, and R1 will pickup and
eventually trip CB1 due to the overcurrent relay R1, the fault
is isolated and the receiving end still getting supply from the
sending end via the second power line (L2).
Fig.2 Fault in First Power Line and the Fault Flow
Fig. 1 Typical Parallel Circuit Feeder Configuration
The circuit shows protection relays R1 and R3 (
) as
non directional protection relay at the sending end and R2 and
R4 (
) at the receiving end as directional overcurrent
protection relay. These directional relay are checking the fault
direction in power line L1 and L2. Without these directional
relay, the circuit will not be able to continue supply to the load
when the fault occurs in one of the two power lines. This is
because the circuit only have one generating source, if all
these four protection relays are non directional overcurrent
relay then all the four non directional protection relay will trip
and isolate both lines completely and disconnecting the power
supply to the load. Therefore, this parallel circuit, relay R1 and
Same phenomena happens when the fault occurs at the L2
illustrated in fig.3. Directional relay R4 will pickup and trip
CB4, and followed by the overcurrent relay R3. Fault in L2
will isolate and disconnect from the circuit, but the source still
supply to the load from the L1. The operation of the
component in the network is summarizing in Table1.
Fig. 3 Fault in Second Power Line and the Fault Flow
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The 3rd National Graduate Conference (NatGrad2015), Universiti Tenaga Nasional, Putrajaya Campus, 8-9 April 2015.
In table 1 the fault occurrences and relays response as well
as the circuit breaker operation are summarized for the
proposed zero crossing directional protection relay system
Table 1 Fault Location and Operation of Component in the Network
IV.
The zero crossing in Fig.5a shows that the overlapping
area between the voltage and the current is more than the nonoverlapping, it also shows that this characteristic is for
forward direction of load flow. On the other hand, in fig.5b the
overlapping area between voltage and current is smaller
compared to fig.5a, this condition indicates the load flow in
the reverse direction. Nevertheless, this overlapping area is
generated by the voltage and current, which can be determined
by the phase angle difference between the voltage and current.
The characteristic of phase angle for forward or reverse
direction are as follows:
If ϴ be the angle between current and the voltage in the same
phase. Then for normal load flow (forward direction).
∴ 90°
90°
For reverse direction:∴ 90°
270°
RELAY PROTECTION SYSTEM IN GENERAL
The directional function propose in this paper is based on
the zero crossing method to determine the overlapping
difference interval between the voltage and the current to
justify the power flow in the system. However, zero crossing
detection method in this paper is improved by using few and
efficient use of the simulation blocks compared with old
method [10], and is shown in fig.4a and in fig.4b.
Fig.5a Phase Angle between Voltage and Current for the Normal
Condition
Figure 5b Phase Angle between Voltage and Current for Reverse
Directional
Fig.4a Proposed Current and Voltage Zero Crossing Simulation Blocks
Fig.4b Earlier Design of Zero Crossing Function Block [10]
To obtain the overlap between voltage and current in
MATLAB/Simulink platform, both sinusoidal voltage and
current signals are first converted to square ware. The square
wave signals are then input in product function block, the
results are then input to an integration function block. The
upper limit for the integrator is set as "0" to ensure that the
integrator returns less than zero, even under normal load flow
condition, refer to fig.4a for the MATLAB directional block
diagram. On the other hand, in reverse direction flow the
outputs of the integral starts to drop until the fault is isolated.
In fig.6 it is shown that both fault current and integrator
output, and the result of the integrator drop gradually due to
directional function indicating load flow is reversed due to
fault.
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The 3rd National Graduate Conference (NatGrad2015), Universiti Tenaga Nasional, Putrajaya Campus, 8-9 April 2015.
properly in the power network system, a fault is modeled at
different length on line L1 and L2 as show in fig.2 and fig.3.
The results from the simulation are very promising as the
tripping of the directional overcurrent relay and overcurrent
relay are well coordinated even though the fault current occurs
at different location in the circuit.
Fig.7 Block Diagram for Parallel Circuit Feeder
VI.
Fig.6 Output from the integral when experiencing revere power flow
If the direction function signals from the integration are less
than threshold -0.01, a logic high "1" signal will be selected,
indicating the fault in the line is in reverse direction to the
current flow in the system, the function block are as shown in
figure 6a. The logic high signal from the directional element
will then be multiplied with the phase current signal acting
like a switch to let the phase current to feed in to the
overcurrent relay, if the current exceeds the pickup current set
in the overcurrent relay, it will trigger the overcurrent relay
and trip the circuit breaker to isolate the fault in the line. If the
directional function issue a logic "0", the phase current will
not able to feed in to the relay and no action will be taken.
Fig. 6a Integrator and threshold switch function block for in Directional
Element.
V.
PROPOSED ZERO CROSSING DIRECTIONAL
OVERCURRENT RELAY NETWORK.
The proposed protection system is base on the directional
relay described in section 2.2. In fig.2 it can be observed that
both R1 and R3 overcurrent protection relay are at the source
end and two time base zero crossing directional protection
relay R2 and R4 at the receiving end. To verify it is working
ANALYSIS AND RESULT
The analysis and result for the proposed relay system
discussed in this section is based on Table 1. The fault is
modelled in line L1, L2 in Figure 1-3, the line are modelled
with ratio of 9 to 1Kilometers. The faults start after 2 second
of simulation this will sustain for 1 second only.
A. Fault occur in Power Line L1 and L2
It can be seen in figure 7 and Table 1 row 1, a fault occurs in
L1. In this system as shown in Fig.2, R2 Directional
overcurrent relay will trip first before R1 overcurrent relay
within the fault time and isolate the fault, and during this time
L2 is not interrupted by the fault and still supplies power to
the load. This is achieved because R2 have detected
directional element and reverse current flow in the system and
has a lower pickup current setting for the overcurrent element
compare to the R1 overcurrent relays. When the fault occur in
L1 the current and circuit breaker status is shown in appendix
1.1. The same tripping coordination will happen when in the
event of fault occurring in L2, the R4 directional relay will trip
before R3 overcurrent relay trip and isolate the fault in L2, the
supply from the source will keep supplying power to the load
without interruption from the fault at L2.
B. Fault occur at the load side
It can be seen in the result of Table 1 row 3, that only both R1
and R3 overcurrent relay pickup and trip CB1 and CB3, and
both R2 and R4 directional overcurrent relay are idle. This
happens because R2 and R4 only detecting the reverse
direction of current flow and the fault at load side is
considered as forward flow. Therefore, the directional element
for R2 and R4 did not detect the fault. However, both R1 and
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R3 overcurrent relay will trip the circuit breaker in case the
fault current exceeds the pickup current setting in the relay.
Once both overcurrent protection relays tripped, there will be
no supply to the load and the fault will be removed. The
circuit breaker and current status when fault occur at the load
side can refer to appendix 1.2.
C. Fault occur at the source side
In table 1, row 4 illustrates the fault occurrence at source side
and all circuit breaker are considered in close position when
fault occurs near the source. This is because all protection
relay are not operating due to the fault caused by the
generation impedance and short circuit current flow, when the
fault is removed after the 1 second period, the system resume
back normal operation. All 4 of the Circuit breakers and
current status are shown in appendix 1.3.
VII. CONCLUSION
The Time base zero crossing directional method proposed in
this paper gives promising result in determining the fault
direction occur in the medium voltage parallel feeder circuit in
the result. The time base zero crossing directional function
operate accordingly in event of fault occurrence at any of the
location in the parallel feeder circuit, as well as at different
distance from the directional overcurrent relay. The advantage
of this time base zero crossing technique is that it requires less
computation and memory space, yet it can respond faster in
giving result compare to other techniques. This is because, the
zero crossing technique is performed by comparing both the
instantaneous overlapping voltage and current signal to
determine the fault direction. However, to have a proper
discrimination between the overcurrent relay and the
directional overcurrent relay during fault isolation, a set of
pickup value and tripping level setting must follow the
overcurrent relay. For future work real time implementation
of the proposed system is recommended.
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The 3rd National Graduate Conference (NatGrad2015), Universiti Tenaga Nasional, Putrajaya Campus, 8-9 April 2015.
VIII. APPENDIX
Appendix 1.1 Status of Current and Circuit Breaker when Fault occurs in L1 Appendix 1.2 Status of Current and Circuit Breaker when Fault occurs at load side Appendix 1.3 Status of Current and Circuit Breaker when Fault occurs at Source side ISBN 978-967-5770-63-0
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