The traditional polarity comparison based travelling wave protection, using the initial wave information, is affected by initial fault angle, bus structure, and external fault. And the relationship between the magnitude and polarity of travelling wave is ignored. Because of the protection tripping and malfunction, the further application of this protection principle is affected. Therefore, this paper presents an ultra-high-speed travelling wave protection using integral based polarity comparison principle. After empirical mode decomposition of the original travelling wave, the first-order intrinsic mode function is used as protection object. Based on the relationship between the magnitude and polarity of travelling wave, this paper demonstrates the feasibility of using travelling wave magnitude which contains polar information as direction criterion. And the paper integrates the direction criterion in a period after fault to avoid wave head detection failure. Through PSCAD simulation with the typical 500 kV transmission system, the reliability and sensitivity of travelling wave protection were verified under different factors’ affection.
According to the protection principle, travelling wave protection methods include travelling wave differential protection, travelling wave distance protection, travelling wave amplitude comparison protection, and travelling wave polarity comparison protection [
Travelling wave differential protection principle is simple and clear. But travelling wave has attenuation characteristic. There may be large unbalance current in the transmission line to cause wrong operation. And it is also affected by the bus structure [
The traditional travelling wave protection principle’s application is limited by the transformer technology. The traditional current transformer (CT) and voltage transformer (VT) cannot transfer the travelling wave signal correctly. Currently, Rogowski based electronic current transformer (R-ECT) and capacitive divider electronic voltage transformer (C-EVT) have been able to transfer current travelling wave and voltage travelling wave accurately. And the output of C-EVT and R-ECT is the differential signal of the input. By integration circuit, the original signal can be regained exactly. So there is no transformer technology limit in the travelling wave protection principle anymore [
This paper compares the polarity relationship between voltage travelling wave and current travelling wave with different fault directions. Combined with empirical mode decomposition algorithm (EMD), the new travelling wave polarity comparison protection principle based on the integration of amplitude is derived. It not only uses the initial travelling wave but also uses the travelling wave after fault happens. So it is a reliable protection principle with obvious direction discrimination. By the way, this new protection principle is not affected by different initial angles, different grounding resistance, different bus structures, and different fault locations. To verify the characteristics of the new principle, a simulation based on PSCAD/EMTDC is carried on. And the PSCAD simulation proved that this new protection principle has the characteristics mentioned above indeed.
The high operation speed is a very important advantage for travelling wave protection. Considering the different parts of the new travelling wave’s operation time, the new protection principle can determine if the fault is internal or external in 5 ms. Then it can send the signal to breaker to operate. So, it can be called an ultra-high-speed travelling wave protection.
The direction element of the protection principle is a polarity comparison relay. It detects the initial voltage travelling wave and current travelling wave as comparison objects. When the voltage travelling wave and current travelling wave have opposite polarities of both sides, the internal fault can be determined. When the voltage travelling wave and current travelling wave have same polarities of any side, the external fault can be determined. The schematic of protection is shown in Table
Analysis of travelling wave polarity comparison protection.
Fault location | Superimposed voltage polarity | M side | N side | Schematic of fault superimposed state circuit | ||
---|---|---|---|---|---|---|
Voltage | Current | Voltage | Current | |||
Internal fault | + |
+ |
− |
+ |
− |
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External fault | + |
+ |
− |
+ |
+ |
|
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External fault | + |
+ |
+ |
+ |
− |
|
The empirical mode decomposition algorithm can distinguish the different scale fluctuations or trends in the signal gradually. And the result of EMD is a series of different characteristic scales data called intrinsic mode functions (IMF) [
The result of EMD can be described as
Intrinsic mode function is a single component signal and it must meet the following two conditions: (1) difference between the number of extreme points and zero crossing points is not more than one over the entire length of the signal; (2) the envelope of IMF is symmetry about time axis.
The processes of EMD are described as the following steps.
(1) Find all the maxima of the original signal
(2) Let
Then check if the following condition can be met:
If (
If (
(3) Let original signal be minus
Repeat steps (1) to (3) to get another intrinsic mode function
Based on EMD, the first IMF of voltage travelling wave and current travelling wave can be calculated as Figure
The comparison of travelling wave and travelling wave after EMD.
As shown in Figure
Equivalent circuit of single phase line.
Now the magnetic flux around
Because the voltage on capacitance cannot change suddenly and
Take (
As we can see, the ratio of voltage and current is a constant value called wave impedance.
Define voltage amplitude conditioning factor
And the value of
Considering the initial polarity of voltage and current travelling wave, define a factor
The discretization of (
Considering different fault directions, the value of
(1) If the fault happens as Figure
Now the amplitude conditioning factor can be calculated:
Because it is a forward direction fault for R1, the reverse voltage and current travelling wave have different polarities. So (
Take (
As we can see, the value of
(2) If the fault happens as Figure
Now the amplitude conditioning factor can be calculated:
Because it is a reverse direction fault for R2 and also a forward direction fault for R1, the reverse voltage and current travelling wave have different polarities. And the forward direction of R2 is opposite to R1. So (
Take (
As we can see, the value of
Taking a variety of errors in the actual system into account, the fault direction discrimination schematic is shown in Figure
Fault direction discrimination schematic.
The protection scheme is shown in Figure
Flow chart of travelling wave protection.
The 500 kV power transmission system is constructed in PSCAD/EMTDC as shown in Figure
Model of 500 kV power transmission system.
The transmission line uses frequency-dependent model and has uniform transposition. The transmission line parameters for per km length are shown in Table
Transmission line parameters.
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|
|
| |
---|---|---|---|---|
(Ω/km) | (Ω/km) | (S/km) | (S/km) | |
Positive sequence | 0.01798 | 0.29278 | 1 × 108 | 3.93905 × 10−6 |
Negative sequence | 0.01798 | 0.29278 | 1 × 108 | 3.93905 × 10−6 |
Zero sequences | 0.28662 | 1.08210 | 1 × 108 | 2.43767 × 10−6 |
In order to verify the protection principle’s operating characteristics, A phase to ground fault is set located at F3. The initial fault angle is
Take
Comparison chart of
Take
Comparison chart of
As we can see, the fault discrimination results of R1 and R2 are corrected. Taking the protection scheme of Figure
Based on some different fault locations at F1 and F2, the fault discrimination factor
Table
Simulation results for different fault locations.
Fault location | Fault distance/km | M side | N side | Results | ||
---|---|---|---|---|---|---|
Direction | Direction | |||||
F1 | 10 | −0.8160 | Forward | −0.5732 | Forward | Internal |
100 | −0.8763 | Forward | −0.4071 | Forward | Internal | |
190 | −0.9378 | Forward | −0.7965 | Forward | Internal | |
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F2 | 10 | 0.9972 | Reverse | −0.9603 | Forward | External |
50 | 0.9980 | Reverse | −0.3843 | Forward | External | |
90 | 0.9972 | Reverse | −0.9603 | Forward | External |
Based on some different grounding resistance at F1 (100 km away from the bus of M side) and F2 (10 km away from the bus of M side), the fault discrimination factor
Table
Simulation results for different grounding resistance.
Fault location | Grounding resistance/Ω | M side | N side | Results | ||
---|---|---|---|---|---|---|
Direction | Direction | |||||
F1 | 1 | −0.4944 | Forward | −0.8818 | Forward | Internal |
100 | −0.4829 | Forward | −0.8802 | Forward | Internal | |
300 | −0.4984 | Forward | −0.8839 | Forward | Internal | |
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F2 | 1 | 0.9978 | Reverse | −0.9438 | Forward | External |
100 | 0.9963 | Reverse | −0.8730 | Forward | External | |
300 | 0.8522 | Reverse | −0.9033 | Forward | External |
Based on some different fault initial angel at F1 (100 km away from the bus of M side) and F2 (10 km away from the bus of M side), the fault discrimination factor
Table
Simulation results for different fault angles.
Fault location | Initial angle/° | M side | N side | Results | ||
---|---|---|---|---|---|---|
Direction | Direction | |||||
F1 | 1 | −0.3884 | Forward | −0.3256 | Forward | Internal |
45 | −0.4291 | Forward | −0.8732 | Forward | Internal | |
90 | −0.4944 | Forward | −0.8818 | Forward | Internal | |
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F2 | 1 | 0.9972 | Reverse | −0.2356 | Forward | External |
45 | 0.9997 | Reverse | −0.9687 | Forward | External | |
90 | 0.9997 | Reverse | −0.9700 | Forward | External |
Based on some different fault types at F1 (100 km away from the bus of M side) and F2 (10 km away from the bus of M side), the fault discrimination factor
Table
Simulation results for different fault types.
Fault location | Fault type | M side | N side | Results | ||
---|---|---|---|---|---|---|
Direction | Direction | |||||
F1 | AG | −0.4944 | Forward | −0.8818 | Forward | Internal |
AC | −0.9600 | Forward | −0.9602 | Forward | Internal | |
ABG | −0.4471 | Forward | −0.8804 | Forward | Internal | |
ABCG | −0.9664 | Forward | −0.7452 | Forward | Internal | |
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F2 | AG | 0.9972 | Reverse | −0.9700 | Forward | External |
AC | 0.9991 | Reverse | −0.6022 | Forward | External | |
ABG | 0.8275 | Reverse | −0.4659 | Forward | External | |
ABCG | 0.9997 | Reverse | −0.9586 | Forward | External |
Based on some different sampling rate and AG fault at F1 (100 km away from the bus of M side) and F2 (10 km away from the bus of M side), the fault discrimination factor
Table
Simulation results for different sampling rate.
Fault location | Sampling rate/Hz | M side | N side | Results | ||
---|---|---|---|---|---|---|
Direction | Direction | |||||
F1 | 100 k | −0.9794 | Forward | −0.6816 | Forward | Internal |
500 k | −0.7345 | Forward | −0.9637 | Forward | Internal | |
1 M | −0.5018 | Forward | −0.8825 | Forward | Internal | |
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F2 | 100 k | 0.9751 | Reverse | −0.9807 | Forward | External |
500 k | 0.9994 | Reverse | −0.9055 | Forward | External | |
1 M | 0.9979 | Reverse | −0.9684 | Forward | External |
Traditional travelling wave protection principle is affected by the number of transmission lines connected to the bus. To verify the new EMD based protection principle, a new power transmission system is constructed in PSCAD as Figure
Model of 500 kV power transmission system with different bus structures.
Table
Simulation results for different fault locations.
Fault location | M side | N side | Results | ||
---|---|---|---|---|---|
Direction | Direction | ||||
F1 | −0.9731 | Forward | −0.9603 | Forward | Internal |
F2 | 0.9980 | Reverse | −0.9779 | Forward | External |
F3 | −0.9718 | Forward | 0.9992 | Reverse | External |
The operation time of the travelling wave protection using polarity comparison principle based on EMD includes three parts: algorithm time, detection time, and propagation time. This new travelling wave protection principle can determine if the fault is inside or outside of the protection region in 5 ms. Then it can send the signal to breaker to operate. So it can be called ultra-high-speed travelling wave protection. The following is the introduction and analysis of the three parts.
Algorithm time includes two parts: the integration time and calculation time of the principle. In this paper, integration time length (the factor
Detection time is the time difference of two sides’ travelling wave arrival point. As we can see, the fault may happen everywhere in the transmission line. Then the arrival times of two sides are different, except that the fault happens in the middle of the line. As a protection principle which needs two sides’ information to decide the operation of breaker, the time difference will delay the operation time. As shown in Figure
Schematic diagram of detection time.
And
Because the transmission line is generally several hundred kilometers, this time is obviously not longer than 2 ms.
After the direction discrimination of one side, as shown in Figure
Schematic diagram of propagation time.
Comparing with the traditional polarity comparison travelling wave protection, the new travelling wave protection combines the relationship between amplitude and polarity. Based on empirical mode decomposition, the derivation of the direction criterion is finished. And this protection criterion not only uses the initial travelling wave front but also uses short time’s (0.1 ms in the paper) travelling wave information after the initial travelling wave front. Through the integration of travelling wave, it can avoid the failure of the travelling wave’s detection. So it can increase the reliability of the protection principle.
To verify the new protection principle, a simulation based on PSCAD is carried on. Taking the simulation results into account, this new protection principle is not affected by different fault locations, different fault types, different initial angels, different grounding resistance, and different bus structures. So it is a reliable travelling wave protection.
Operation speed is an important advantage for travelling wave protection. Because the new protection principle can send the operation signal to breaker in 5 ms, it can be called ultra-high-speed travelling wave protection.
The authors declare that there is no conflict of interests regarding the republication of this paper.
This work was supported in part by the National Natural Science Foundation of China (51177094, 51277114).