Energy and Power Engineering, 2013, 5, 1288-1292
doi:10.4236/epe.2013.54B244 Published Online July 2013 (http://www.scirp.org/journal/epe)
Busbar Differential Relaying Method Based on Combined
Amplitude and Phase Information of High Frequency
Transient Currents
Xiao Wu, Zhengyou He, Xiaopeng Li
School of Electrical Engineering, Southwest Jiaotong University, Chengdu, China
Received February, 2013
ABSTRACT
Busbar differential relaying method based on combined amplitude and phase information of high frequency transient
currents is put forward in this paper for the speed and reliability problems of busbar protection based on fundamental
frequency. Under the analysis of features of bus high frequency differential currents, complex wavelet analysis is used
to extract the amplitude and phase features of 1/4 period high frequency differential currents, and amplitude and phase
information are used to form the polar coordinates. Bus fault is identified intuitively and precisely according to polar
locus differences. This polar coordinates represented busbar differential protection scheme based on high frequency
transient signals can not only avoid TA saturation, realizing quick protection, lots of PSCAD/EMTDC simulations also
show that this busbar differential protection scheme works well under different fault conditions.
Keywords: Busbar Protection; High Frequency Transient Currents; Complex Wavelet Analysis; Polar Coordinate
1. Introduction
Busbar is important to electrical power system. In UHV,
busbar fault will not only cause blackout of the compo-
nents connected to busbar, but also destroy system stabil-
ity [1]. So, research of quick and reliable bus protection
is essential to safe operation of power grid.
Traditional busbar differential protection based on
fundamental frequency can hardly meet the speed require-
ment, and has low capacity to avoid the saturation of
current transformer (TA). References [2-4] put forward
methods avoiding TA saturation, but it is not completely
settled. Reference [5] analyzes busbar differential currents
with complex wavelet analysis, and polar coordinate is
used to represent the results. But the speed property
needs to be improved.
In fact, TA changes from normal condition to saturation
condition needs at least 3-5 ms [6,7]. The transient fault
information can be used to avoid the saturation effect
radically and to improve the speed and sensitivity.
References [8-11] compare the polarities of wavefront,
but the dependency on capture of wavefront affect the
reliability.
Based on the idea of reference [5], this paper put for-
ward the busbar deferential relaying algorithm based on
combined amplitude and phase information of busbar
high frequency differential currents. Differ from it, this
paper firstly analyze the features of bus high frequency
differential currents inside and outside the bus fault, one
quarter period of bus differential currents is analyzed by
complex wavelet analysis to recognize the bus fault. This
algorithm not only avoids the effect of TA saturation, but
also realizes quick busbar protection. Lots of EMTDC
simulations and analysis show that the polar coordinate
method represented amplitude and phase information by
complex wavelet analysis can recognize bus fault intuit-
tively and can adapt to different fault conditions well.
2. Feature Analysis of Bus high Frequency
Differential Currents
The fault network is equivalent to the superposition of
non-fault network and fault attached network (passive
system), and fault component is the response of passive
system to attached power supply, containing abundant
fault information [1].
Transient fault components transmit along the lines,
and reflect or refract at the node of fault point or buses.
Lines, transformers and other components are connected
to bus, forming the bus equivalent capacitance. Study
shows that the bus system equivalent capacitance of 500
kV transformer substation varies between 6000 pf ~ 0.1
uf, enhancing the reflection of 50 kHz~100 kHz high
*Project Supported by National High-tech R&D Program(863 Program)
(2012AA050208).

X. WU ET AL. 1289
frequency transient currents [12].
Define that bus high frequency differential current is
the sum of the line high frequency transient currents
connected to bus. Figure 2 is the fault attached network
when Bus I fault in Figure 1.
In Figure 2, when busbar fault, the power supply gen-
erates will fault transient currents with the same polarity
along lines. Given that positive direction denotes to lines
from buses, the polarities of high frequency transient
currents of each line are positive. Bus high frequency
differential current is the sum of line high frequency
transient current with the same polarity.
Figure 3 is fault attached network when L3 fault.
When L3 fault, the high frequency transient currents
generated on fault lines transmit to bus, then refract to other
none-fault lines through bus, which owns the opposite
polarity, forming the relatively small bus high frequency
differential currents.
Above all, the bus high frequency transient differential
currents when buses fault are larger than that when lines
fault. Bus fault can be recognized by the features of bus
high frequency differential currents.
Figure 1. Sketch of power grid topology.
Figure 2. Sketch of superimposed components when Bus1
fault.
Figure 3. Sketch of superimposed components when L3fault.
3. Busbar Deferential Relaying Method
Based on High Frequency Transient
Currents
3.1. Criterion for Busbar Differential Protection
Define
f
i
I is the current fault component of line i
connected to bus, line number is ， N
1
N
f
i
i
I

 is the 1/4
period bus fault differential current. Complex Gaussian
wavelet analysis is applied in this paper to analyze
1
N
f
i
i
I


. Complex Wavelet Analysis (CWT) can reflect the
similarities of wavelet-amplitude and wavelet- phase
simultaneously, avoiding noise influence and announcing
signal features more precisely. Complex wavelet
coefficients under low scale are used to represent the
amplitude information l
M
WT and phase information
of high frequency differential currents.
l
For the reason that polar coordinates can represent
amplitude and phase information simultaneously, polar
coordinate is applied to represent l
QWT
M
WT and l.
Bus fault is recognized by locus diagrams of polar coord-
inates. If the locus diagram ll
QWT
M
WT QWT exceeds the
threshold
K
, it is concluded as bus fault, otherwise it is
line fault. The polar coordinates represented method can
not only show amplitude and phase information, but also
show the features of fault information intuitively.
For three phase lines, phase-model transformation
should be used to transform the coupled current phasors
into independent modulus, and appropriate modulus is
chosen according to fault type. This paper applies Clarke
transformation. Due to the reason that fault phase selec-
tion is not needed for bus protection, to ensure reliable
action under different fault type, judgments of α and β
are used to form protection criterion simultaneously.
Busbar protection act when one modulus denotes bus
fault.
3.2. Confirmation of Threshold
Threshold is essential to recognize bus fault. Tran-
sient currents caused by bus capacity are smaller than
that caused by fault. The current maximum of bus capac-
ity is defined as:
K
max maxc
I jwCU

(1)
In which, max is variation maximum of bus voltage.
Considered the effects of wavelet transform, model
transform and others, threshold
U
K
is defined as:
maxabcc
KkKKI (2)
where, a is impact factor of modulus, k2
a
k

; is
impact factor of wavelet transform, ; is
safety factor,
b
k
c
k0.125
b
k
1.4
c
k

.

X. WU ET AL.
1290
3.3. Protection Flow
Full phase subtraction is applied to gain fault current
components. Above all, the busbar differential protection
flow based on high frequency transient currents is con-
cluded as follow: (Figure 4)
4. Performance Analysis
4.1. Influence of Fault Resistance
Fault resistance when bus fault is less than dozens of
ohm [8], but line resistance in 500 kV grid is up to 300 Ω
[13]. For the reason that resistance decays transient cur-
rents more drastically when line fault, the influence of
fault resistance can be ignored.
4.2. Influence of Bus Structure
Protections act accurately when more than two transmis-
sion lines connected to bus. When just one transmission
line connected to bus, features of line fault and that of
bus fault are the same, the protection scheme is invalid.
In fact, it is rare that just one transmission line connect-
ing to bus in real grid [14].
5. Simulation Verification
5.1. Simulation Model
Establish 500kV grid model in PSCAD/EMTDC. The
basic parameters are: E1 = 50090°kV, E2 = 50080
°kV, E3 = 50075°kV, E4 = 50075°kV, ideal
source. All bus equivalent capacitances are 0.01 uF. L1 =
360 km, L2 = 345 km, L3= 330 km, L4 = 380 km, fre-
quency-related mod- els. Sample frequency fs= 200 kHz,
K=200.
Figure 4. Flow chart of busbar deferential relaying method
based on combined amplitude and phase information.
Figure 5. Locus diagram of α mode when bus fault.
Figure 6. Locus diagram of β mode when bus fault.
5.2. Exponential Analysis
Define fault distance be the distance between fault point
and bus. Fault attached network of BusI fault is shown in
Figure 2. Suppose that single-phase ground fault hap-
pened, fault phase is , fault resistance is 80 Ω. Polar
diagram is used to present the wavelet coefficients under
second scale. Wavelet coefficients locus of α modulus is
shown as Figure 5, locus of β modulus is shown in Fig-
ure 6.
90
Figures 5 and 6 announce that the wavelet coefficients
locus of α is within the threshold K, which denotes bus
fault.
Polar diagram presented busbar differential protection
based on high frequency transient currents can recognize
bus fault clearly and intuitively.
5.3. Adaptions Analysis
To confirm the adoptions to different fault conditions,
Tables 1-4 show judgments when L3 in grid Figure 1
faulted under different fault distances(100 Ω, 90°, sin-
gle- phase grounding fault), different fault type(155 km,
100 Ω, 90°), different fault resistance(155 km, 90°,
single-phase grounding fault), different fault phase(155

X. WU ET AL. 1291
km, 100 Ω, single-phase grounding fault), Tables 5-7
show judg- ments when BusI faulted under different fault
type(20 Ω, 90°), different fault resistance(90°, sin-
gle-phase grounding fault), different fault phase(20 Ω,
single-phase grounding fault). Concluded from the judg-
ment results, the proposed busbar differential protection
scheme adapts to different fault conditions well.
Although the bus differential protection scheme based
on high frequency transient currents is invalid under zero
voltage fault, the probability of zero fault is so rare that
the influence can be ignored.
Table 1. Judgment for different distance on L3.
Fault
distance
Maximum
of α locus
Maximum
of β locus Judgment result
1 km 88.94 0.0213 lucoses of α and β within K
line fault
75 km 69.01 0.0221 lucoses of α and β within K
line fault
155 km 45.35 0.0224 lucoses of α and β within K
line fault
255 km 38.57 0.02236 lucoses of α and β within K
line fault
329 km 23.13 0.0216 lucoses of α and β within K
line fault
Table 2. Judgment for different fault type on L3.
Fault
type
Maximum of
α locus
Maximum of
β locus Judgment result
Ag 45.35 0.0224
lucoses of α and β within K
line fault
BCg 20.82 12.53
lucoses of α and β within K
line fault
BC 0.0097 15.11
lucoses of α and β within K
line fault
ABC 96.73 16.23
lucoses of α and β within K
line fault
ABCg 74.7 12.53
lucoses of α and β within K
line fault
Table 3. Judgment for different fault resistance on L3.
Fault
resistance
Maximum
of α locus
Maximum
of β locus Judgment result
0 Ω 65.84 0.0224
lucoses of α and β within K
line fault
50 Ω 53.71 0.0224
lucoses of α and β within K
line fault
120 Ω 42.7 0.0223
lucoses of α and β within K
line fault
220 Ω 33.02 0.0223
lucoses of α and β within K
line fault
300 Ω 27.95 0.0223
lucoses of α and β within K
line fault
Table 4. Judgment for different fault phase on L3.
Fault
phase
Maximum
of α locus
Maximum
of β locusjudgment result
90°45.35 0.0224 lucoses of α and β within K line fault
60°40.81 0.0224 lucoses of α and β within K line fault
30°26.69 0.0223 lucoses of α and β within K line fault
0°0.01 0.0223 lucoses of α and β within K line fault
Table 5. Judgment for different fault type on BusⅠ.
Fault
type
Maximum
of α locus
Maximum
of β locusjudgment result
Ag 796.8 0.0224 lucos of α exceeds K busbar fault
BCg324.8 7626 locuses of α and β exceed K bus fault
BC 0.0097 8778 locuses of α and β exceed K bus fault
ABC3020 9233 locuses of α and β exceed K bus fault
Table 6. Judgment for different fault resistance on Bus I.
Fault
resistance
Maximum
of α locus
Maximum
of β locus Judgment result
0Ω 1209 0.0224 lucos of α exceeds K busbar fault
20Ω 796.8 0.0224 lucos of α exceeds K busbar fault
40Ω 622.7 0.0224 lucos of α exceeds K busbar fault
60Ω 547.6 0.0224 lucos of α exceeds K busbar fault
80Ω 489.8 0.0224 lucos of α exceeds K busbar fault
Table 7. Judgment for different fault phase on Bus I.
Fault
phase
Maximum
of α locus
Maximum
of β locusjudgment result
90°796.8 0.0224 lucos of α exceeds K busbar fault
60°622.6 0.0224 lucos of α exceeds K busbar fault
30°330.4 0.0224 lucos of α exceeds K busbar fault
0°0.0103 0.0224
both locuses of α and β within K
line fault
6. Conclusions
Bus differential protection scheme based on the features
of bus high frequency differential currents is put forward
in this paper, conclusions are summarized as following:
1) High frequency differential currents when bus fault
are larger than that when line fault.
2) Complex wavelet analysis can avoid noise influence,
extracting amplitude and phase features more precisely;

X. WU ET AL.
1292
Polar presented protection scheme can recognize bus
fault more intuitively.
3) Bus differential protection scheme combined am-
plitude and phase information is not affected by TA sa-
turation, realizing quick protection. EMTP simulations
show that this protection scheme adapts to different fault
conditions well.
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