In Thailand, electrical energy consumption has been rapidly increasing, following economic and population growth. In order to supply constant power to consumers, reliability is an important factor the electric utility needs to consider. Common disturbances that cause severe damage to transmission and distribution systems are lightning and faults. The system operator must deal with these two phenomena with speed and accuracy. Thus, this study aims to investigate the differential behaviour of transmission systems when disturbance such as lightning strikes and faults occurs in a 115-kV transmission line. The methodology consists of using the ATP/EMTP program to model the transmission system by the 115-kV Electricity Generating Authority of Thailand (EGAT) and simulate both lightning and fault signals in the system. The discrete wavelet transforms are then applied to obtain the signals in order to evaluate the characteristics and behaviour of both signals in terms of high-frequency components. The obtained data will then be used to construct the fault and lightning classification algorithm based on DWT and travelling wave theory. The proposed algorithm shows the effectiveness in classifying the fault and lightning based on the transmission system that was modelled after the actual system in Thailand. Thus, it can further improve the protection scheme and devices in terms of accuracy and reduce the response time for an operator to address the disturbance and ensure the reliability of the system in the future.
Presently, technological development and economic growth make electricity important in both industry and daily life. Therefore, to cater to this demand and improve power systems, the Electricity Generating Authority of Thailand (EGAT) has started the innovation of a degenerative transmission system in the range of 115-230 kV. The project is expected to be completed in 2017 and will increase the capacity of substations, thus increasing the energy flow in the systems. After the completion of these projects, the growing demand of electricity usage will be met, and electricity will be accessible to large areas. However, the expanding power networks frequently cause power-transmission accidents. These accidents are divided into 2 types: accidents caused by natural phenomena (lightning) and those caused by the system structure (fault). The accidents caused by natural phenomena seriously damage life and property. However, these do not commonly occur in systems, whereas faults on the power line occur more often in complex than in normal networks.
The literature review on various researches and studies in the field of fault and lightning on power system has been done. There are three main objectives of lightning for the researcher to focus on: the electric field, the magnetic field, and the influence of lightning on equipment in power systems. Lin Li and Vladimir A. Rakov [
When faults occur in the power systems, the characteristic of voltage and current change. Thus, various methodologies that analysed the change in the signal have been used for the protection system. The method for fault location and classification on transmission line has been reviewed in K. Chen et al. research [
Following the literature reviews above, we concluded that numerous researchers focused on various impacts that occur in the system, such as pre- and postattribution of faults and the influence of faults on devices by using dissimilar methods: FT and DWT. Each method has an exclusive property. FT and DWT have a similarity in displaying the time and frequency domains. FT is unable to display time and frequency together, but DWT is able to do so. From the lightning and fault analyses, both parameters are significantly essential. Therefore, DWT is more appropriate than FT. Although there are numerous studies on lightning and faults, there are only a few that focused on the apparent coexistence in lightning and faults. Consequently, it is impossible to differentiate both forms. In addition, those case studies are inadequate. Therefore, the objectives of this research are analysing both lightning and the faults that occur in two terminal lines of transmission systems. The system under study is 115-kV transmission line that was modelled after Thailand’s transmission system. The ATP/EMTP software has been used to simulate lightning and fault signal on transmission line, and DWT based methodology has been used for the algorithm. The influence of inception angles, phase lines, and the position at which lightning and faults had occurred in the system has been taken into consideration to evaluate the performance of the proposed methodology in various case studies.
Wavelet transform is one form of signal processing by using mathematical model to describe the structure of the analysed signal in a practical form. The assumption is that signals are composed of a set of similar small functions called “Mother Wavelet” combined by scaling and shifting. The general equation of wavelet at scale ‘a’ and position ‘b’ can be illustrated in (
where
The discrete wavelet transforms (DWT) are analysed by shifting and scaling position of mother wavelet in discontinuous interval and can be calculated using (
where
The Daubechies Wavelets (db) have been selected for the proposed algorithm due to the effectiveness in the analysed transient signal in the power system, especially in case fault and lightning occur on the transmission system [
where
The simulating circuit consists of a sending substation (RYG), receiving substation (CTI), and a load of 150 MVA. The distance between RYG and CTI is 88.5 km, and the voltage is 115 kV. The one-line diagram for the simulation circuit is shown in Figure
Parameters of the multi-storey model,
Transmission Tower Parameter | |||||
---|---|---|---|---|---|
Z1 | 180 | R1 | 12.390927 | L1 | 0.0024947 |
Z2 | 180 | R2 | 13.892908 | L2 | 0.0027971 |
Z3 | 180 | R3 | 13.892908 | L3 | 0.0027971 |
Z4 | 140 | R4 | 31.248555 | L4 | 0.0062914 |
System for simulation. (a) One-line diagram of simulated circuit. (b) Multistorey transmission tower model.
The objective of this research was to observe the characteristic of currents when lightning and faults occur. The simulated lightning was based on 1.2/50
The simulation in this paper observed three main factors. The first is the position of the lightning and fault, which varied from 10% to 90% of the distance of the transmission line, with a step size of 10% of the line distance. The second factor is the inception angle, which varied from 0 to 150 degrees, where each observed angle increased by 30 degrees. The third is the phase line, which varied between phase A, phase B, and phase C. All factors were observed on 5 incidence cases, consisting of 1 lightning case and 4 fault cases. The fault cases were single-line, line-to-line, double-line-to-ground, and three-phase faults. In this paper, we show the result of the lightning case and the single-line-fault, which were the most frequent occurrences in the power system.
Case of lightning strike at phase A at 30% of transmission line (inception angle 0°). (a) All phases at sending substation (RYG). (b) All phases at receiving substation (CTI). (c) Positive sequence at sending substation (RYG). (d) Positive sequence at receiving substation (CTI).
From the sending substation, the three-phase currents suddenly increase at the time of the lightning strike. The amplitude of the lightning phase, phase A, is higher than that of the others, and the amplitude that increases is equal to the amplitude of the lightning current. The other phases, phase B and phase C, increase as well, because lightning induces a change in the magnetic field, while, in turn, it induces an increase in the current of the other phases. From the receiving substation, the behaviour of the three currents is similar to that at the sending substation.
In terms of direction, the current of phase A increases in the negative direction because of the lightning return stroke. It suddenly occurs in the range of 0.1-10 ms then slowly decreases and disappears at 100 ms. Phase B and phase C have the opposite changing direction to phase A, because these two phases are not affected by the return stroke. However, this simulation only focuses on the time of lightning occurrence in the range of 2-4 ms, so the lightning vanishes after 4 ms. At that time, the three-phase currents return to the normal condition.
The inception angle varied from 0° to 90°. The waveform of the three-phase currents is sinusoidal, which has amplitude in the positive and negative directions according to the angle; thus, as the inception angle changes, the amplitude of the current changes as well. Moreover, when the lightning location changes from 30% to 70% of the transmission line, it is found that the current characteristic also depends on the position of the lightning. The position is referenced from the sending substation. Changing the position from 30% to 70% of the transmission line means that the distance between the lightning point and the sending substation increases, while the distance between the lightning point and the receiving substation decreases. This distance directly impacts impedance inverse current. Thus, the location increase makes the amplitude of the current at the sending substation decrease and the amplitude of the current at receiving substation increase.
By considering the sending substation, the waveform of the wavelet suddenly increases in the negative direction at the time of the lightning strike. After that time, the waveform gradually returns to the initial conditions. From the receiving substation, the behaviour of the wavelet is similar to the waveform at the sending substation, which suddenly increases at the time of the lightning strikes, but the coefficient is lower than that at the sending substation because of the effect of impedance from the lightning position.
Likewise, as shown in Figure
Summary of DWT results when the location of lightning was varied (inception angle 0°).
Summary of DWT of positive-sequence current results when inception angles of lightning condition were varied (lightning location 30%). (a) Sending substation (RYG). (b) Receiving substation (CTI).
Case of phase-A-to-ground fault at 30% of transmission line (inception angle 0°). (a) All phases at sending substation (RYG). (b) All phases at receiving substation (CTI). (c) Positive sequence at sending substation (RYG). (d) Positive sequence at receiving substation (CTI).
From the sending substation, the current at the fault phase, phase A, increased in the positive direction. The direction of the change is according to the current waveform. At 2 ms, the time at which the fault occurred, the direction of the fault phase is positive, so the amplitude also increases in the positive direction. The amplitudes of the other two phases, in which the fault does not occur, slightly increase because the effect of the change in the magnetic field is less.
From the receiving substation, the characteristic of the three-phase currents is similar to that at the sending substation, where the amplitude of the current at phase A is higher than that at phase B and phase C. The direction of phase A also changes in the same direction as the sending substation due to the direction of the current waveform when the fault occurred. Phase B and phase C slightly change for the same reason mentioned at the sending substation.
In the wavelet graph in Figures
The results in Figure
Summary of DWT results when the location of single line fault condition was varied (inception angle 0°).
Summary of DWT of positive sequence current results when inception angles of single line fault condition were varied (Fault location 30%). (a) Sending substation (RYG). (b) Receiving substation (CTI).
At the sending substation, the characteristic of the positive current is slightly changed; thus, DWT is better than the positive current because this method can identify the high-frequency component. From the perspective of the wavelet signal, the coefficient of the wavelet is more apparent than that of the positive current method. The coefficient suddenly occurred at the time of a fault. From the receiving substation, the characteristic of both currents is similar to that at the sending substation, in that the changing amplitude of the positive current is less and the coefficient of wavelet is obvious.
Further observation was made for other faults, namely, line-to-line, double-line-to-ground, and three-phase faults, in which the angle and position condition were varied as in the previous section. Figures
Summary of DWT results when location of the fault was varied (inception angle 0°). (a) Line-to-line fault. (b) Double-line-to-ground fault case. (c) Three-phase fault.
Next, the inception angle was varied, and the fault position was constant. The observed conditions were the same as the previous. Figures
Summary of DWT results when the inception angle of the fault condition was varied (fault location 30%). (a) Line-to-line fault. (b) Double-line-to-ground fault. (c) Three-phase fault.
In short, the current attributes from the ATP simulation program are not comprehensive for the analysis of lightning and faults because the differentiation between these is not obvious. Next, a transformation will be used to make the current waveform more evident.
The previous section described the behaviour of wavelets when lightning and faults occur in the transmission system. This section will describe the discrimination algorithm between lightning and faults in transmission line. The important parameters used for classification consist of 3 parameters: the initial variables, comparison variables, and check variables. The initial variable is maximum DWT coefficient of three-phase current zero sequence. The comparison variables are maximum DWT coefficient of Phases A, B, and C. The check variables are the normalization of maximum DWT coefficient of each phase by dividing the comparison variables by the initial variable. The example of the algorithm process is illustrated as shown in Figure
Operation for discrimination between lightning and fault.
where
Figure
Flow chart for classification between lightning and fault.
Table
Variables at normal condition of transmission system.
Normal Condition | |||||
---|---|---|---|---|---|
Initial variable | |||||
Pos_S | Pos_R | Ia_S | Ib_S | Ic_S | Zero_S |
| |||||
3.1 x 10−5 | 9.8 x 10−5 | 2.6 x 10−5 | 9.5 x 10−5 | 2.6 x 10−5 | 2.4 x 10−5 |
| |||||
Compared Variable | |||||
S_A | S_B | S_C | |||
| |||||
1.0552 | 0.0385 | 1.0552 | |||
| |||||
Check Variable | |||||
S_Ph | S_Ph | ||||
| |||||
1.0552 | 0.0385 |
Lightning condition.
Initial variable | ||||||||
---|---|---|---|---|---|---|---|---|
Lightning location on transmission line | Voltage angle | Pos_S | Pos_R | Pos_min | Ia_S | Ib_S | Ic_S | Zero_S |
| ||||||||
30% | 0° | 336.9104 | 222.5332 | 222.5332 | 224.5902 | 56.1444 | 56.184 | 7.554 |
| ||||||||
Compared variable | ||||||||
| ||||||||
Fault location on transmission line | Voltage angle | S_A | S_B | S_C | ||||
| ||||||||
30% | 0° | 29.7314 | 7.4324 | 7.4377 | ||||
| ||||||||
Check variable | ||||||||
| ||||||||
Fault location on transmission line | Voltage angle | S_Ph | S_Ph | |||||
| ||||||||
30% | 0° | 29.7314 | 7.4324 | |||||
| ||||||||
Result | ||||||||
| ||||||||
Type | | Lightning | ||||||
| ||||||||
Phase | S_A | Phase A |
Single-line-to-ground fault condition.
Initial variable | ||||||||
---|---|---|---|---|---|---|---|---|
Lightning location on transmission line | Voltage angle | Pos_S | Pos_R | Pos_min | Ia_S | Ib_S | Ic_S | Zero_S |
| ||||||||
30% | 0° | 13.3318 | 8.8739 | 8.8739 | 8.8806 | 2.2321 | 2.2195 | 0.0893 |
| ||||||||
Compared variable | ||||||||
| ||||||||
Fault location on transmission line | Voltage angle | S_A | S_B | S_C | ||||
| ||||||||
30% | 0° | 99.4347 | 24.9921 | 24.8515 | ||||
| ||||||||
Check variable | ||||||||
| ||||||||
Fault location on transmission line | Voltage angle | S_Ph | S_Ph | |||||
| ||||||||
30% | 0° | 99.4347 | 24.8515 | |||||
| ||||||||
Result | ||||||||
| ||||||||
Type | | Fault | ||||||
| ||||||||
Phase | S_A | Phase A |
Table
Table
Figure
Accuracy of each type of fault when inception angle and phase were varied. (a) Lightning type. (b) Single-line fault. (c) Line-to-line fault. (d) Double-line-to-ground fault. (e) Three-phase fault.
This research aimed to analyse the behaviour and characteristics of transmission system when lightning and faults occur. Analyses were performed by obtaining the signal and high-frequency component from a wavelet transform. From the characteristic, the three-phase currents were found to increase to higher values than the normal state currents. The increasing currents of the lightning phase are higher than those without lightning, because this phase is directly affected by the lightning current, whereas the others are indirectly affected by the change in the magnetic fields. The fields induce an increase in current. The behaviour of faults is similar to that of lightning, in which the amplitude of the fault phase increases and the others slightly increase. The change in the faults is lower than that in lightning because the short-circuit current caused by lightning is higher due to the amplitude of lightning currents. However, the short-circuit current caused by faults is low because of the fault phase. The short-circuit currents depend on the fault phase, so a three-phase fault, which has the maximum fault phase, has the highest short-circuit currents. Simultaneously, a single-phase fault, which has the minimum fault phase, has the least short-circuit currents. The positive currents originate from three-phase currents separated into three elements: positive, negative, and zero sequences. The positive sequence was observed, and the direction of positive currents was found to depend on the direction of the lightning and fault phase. Based on the wavelet transform, it was shown that the coefficient of the wavelet depends on the inception angle and the position of the lightning and faults. Because the original current is sinusoidal, which has positive- and negative-amplitude currents varying in angles, the coefficient changes according to the angle. In addition, the positions also affect the coefficient because the distance between the reference and accident positions caused an impedance. This research referenced accident points from the sending substation; thus, as the distance increases, the coefficient of the wavelet detected at the sending substation decreases. Meanwhile, the trends at the receiving substation are the opposite.
The result from the proposed classification algorithm, the proposed method, classifies faults and lightning with 100% accuracy in all case study, while in case of fault it can achieve lower accuracy due to the slight error in detecting the high frequency component of DWT in different types of fault. One of the reasons is the current signal in some types of fault when applied DWT generated the similar coefficient value. This result, in the algorithm, identifies wrong types of faults. In case of lightning, the algorithm can correctly identify because the lightning signal has significant differentiate characteristic.
The details of 115 kV transmission line and tower from EGAT transmission system using in ATP/EMTP software simulation are shown in Table
Parameter of 115 kV transmission line and tower.
Tower characteristic | |
---|---|
Tower Name | DA1 |
| |
Conductor size | 795 MCM ACSR/GA |
| |
Ruling span | 330 m. (Distance from tower to tower) |
| |
Conductor characteristic | |
| |
Size | 795 MCM ACSR/GA |
| |
Diameter | 26.80 mm. |
| |
Number of conductors | 1 conductor per bundle |
| |
Current capacity | 845 A per conductor |
| |
Max. sag of conductor | 10.55 m. |
| |
Overhead ground wire characteristic | |
| |
Size | 3/8” (HS) Galvanized steel |
| |
Diameter | 9.144 mm. |
| |
Number of conductors | 1 conductor |
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.
The research presented in this paper is part of a research project sponsored by the Srinakharinwirot University Research Fund. The authors would like to gratefully acknowledge the financial support for this research.