2017 2nd International Seminar on Applied Physics, Optoelectronics and Photonics (APOP 2017) ISBN: 978-1-60595-522-3

Investigation on Power Supply Capability and Power Quality of Dazhun Railway

Zhi-xiao WANG, Fu-qiang TIAN

ABSTRACT

In order to investigate the power supply capability, the harmonics, the power factor and the three-phase voltage unbalance factor of the Dazhun Railway, seven substations of the Dazhun Railway were measured and analyzed. The test data show that the current in the contact network is far less than 1500A, but the maximum voltage of the power supply arms are close to or exceed 29kV; the third, fifth, seventh, ninth, eleventh, thirteenth low order harmonics and 31th, 33th, 35th, 37th, 39th, 41th high order harmonics of the traction substations are beyond the Chinese national standards; the power factor of β phase voltage in Dahongcheng is 0.837, the power factor of α phase in Fengou is 0.251, the power factor of β phase in Shudigou is 0.521, and the power factor of other substations is higher than 0.9; the three-phase voltage unbalance factor is higher than 2% in all substations. The analysis shows that the current load condition of Dazhun Railway has reached or approached the limit of power supply capacity. The harmonic content and the three-phase voltage unbalance factor in power quality are beyond the national standard, so it is necessary to reform the traction power supply system of Dazhun Railway.

INTRODUCTION The construction of Dazhun Railway started in July 1990, when it was planned as a part of the Zhungeer Energy project which includes coal, power plant and railroad. It was arranged to be connected with Fengzhen Station of - Railway, called Fengzhun Line1. But because of the saturated transport capacity of Beijing-Baotou railway at that time, the only way to continue the project was to construct the Fengzhun railway and to catch connection joint with other railways at the same time. Finally, Dazhun Railway met the national railway at East Railway Station of as a westward extension of the existing "west east coal" channel - Datong line2. This paper evaluated the power supply capability of Dazhun Railway through field test and data analysis. Meanwhile, analyzed the statistics of the power quality, which includes harmonic, power factor and the three-phase voltage unbalance factor of Dazhun Railway. Based on them, the paper puts forward reasonable and targeted solutions to provide technical guidance for the future transformation of traction power supply system.

Zhi-XIao WANG1 Fu-Qiang TIAN 2 First author, Zhi-Xiao WANG, School of Electrical Engineering, Beijing Jiaotong University, Beijing, Second author, Fu-Qiang TIAN, School of Electrical Engineering, Beijing Jiaotong University, Beijing, China. Corresponding author: [email protected]

264 PRINCIPLE AND EQUIPMENT OF THE TEST In this paper, 7 substations along Dazhun Railway were measured. The 264km-long Dazhun Railway is a single-wire electrified railway with 7 traction substations. The traction power supply mode is direct supply with reflux. After the expansion from 2005 to 2011, the installation capacities of 7 traction substations are: Diandaigou(DDG) V / v16 + 20MAV, Dahongcheng(DHC )V / v20 + 25MAV, Fenggou(FG)V / v20 + 20MVA, Liangcheng(LC) V/v25 + 25MVA, Shudigou(SDG) V / v20 + 20MVA, Waixigou(WXG )V / v25 + 25MVA, and YaoGou(YG )V / v16 + 25MAV3. Traction substations were continuously tested with the sampling frequency set to 20 kHz and the recording gap set to 3 seconds. The electric comprehensive test device independently developed by Beijing Jiaotong University was adopted as the test equipment. Tested by China Electric Power Research Institute in May 2008, the device met the requirements of this test mission since it was confirmed to be accurate enough for its harmonic is less than 2%, in line with the national A-class power quality test standard.

ANALYSIS OF POWER SUPPLY CAPABILITY OF DAZHUN RAILWAY A continuous test, not less than 24 hours, was carried out in the 7 substations in order to understand the power supply capability of the DaZhun railway. The statistical results of the test are shown in Table 1. TABLE 1. STATISTICAL RESULTS OF POWER SUPPLY CAPABILITY TEST.

Maximum Minimum Maximum Minimum Maximum Maximum Traction voltage of α voltage of α voltage of β voltage of β current of current of β substation phase(V) phase(V) phase(V) phase(V) α phase(A) phase(A) DDG 28519 23733 28927 23821 461 438 DHC 29374 24986 29431 23571 885 1165 FG 29683 26824 29933 25530 972 1329 LC 29141 26801 28900 26661 696 614 SDG 28952 27501 29365 27479 573 582 WXG 28906 22957 29051 21560 991 1240 YG 29404 26434 29261 26343 232 1227 It shows that the current in the catenary of all power supply arms is much less than 1500A, indicating that there is no overload problem of current in the Dazhun Railway. Also, it shows that the lowest voltage of substation is not lower than the minimum voltage (20kV).But the minimum voltage of Waixigou substation is 21560V.Considering that supply arms are terminal parallel, the voltage of pantograph is slightly lower than that of terminal of contact line .What’s more, if it comes to situation where many trains run in the same supply arm at the same time or trains start to move, lower voltage is more likely to appearance, even be under 20KV,which will directly affect the power factor of the train. However, the maximum voltage of the top of each power supply arm is almost higher than the maximum allowable voltage (29kV) of the locomotive, which indicates that the current Dazhun Railway catenary voltage level can no longer be simply improved by adjusting the traction transformer tap. The current load condition of Dazhun Railway has reached or approached the limit of power supply capacity. Catenary voltage level has become the main factor limiting the power supply capacity of Dazhun Railway4. In order to reach a higher volume of railway freight, following solutions should be took into fact: optimize the existing organization of train operation, employ AC drive electric locomotive with higher power factor, and start an extension remodel on electrical supply equipment.

265 ANALYSIS OF POWER QUALITY OF DAZHUN RAILWAY

Analysis of Harmonic Due to the more obvious three-phase asymmetry under the harmonics, a three-phase model must be adopted for the power system. At present, all traction substations in China are two-phase. The secondary side two-phase system needs to be equivalently transformed into the primary three-phase system by means of the transformation relationship of the transformer port electrical quantities. Traction substations are the hubs for connecting external power and traction networks. The traction transformer is the key equipment of traction substation. The measured traction transformers are V / v transformers. The V / v transformer is actually combination of two single-phase transformers, the secondary winding neutral point out and connect the rail (earth), the remaining two ports were connected to two power supply arms. The V / v traction transformer circuit diagram shown as figure 15.

• •

I A I B

•

IC

• •

Iα Iβ

β phase α phase

Rail

Figure 1. The V / v traction transformer circuit diagram.

In order to facilitate the analysis, we assume that no load will act on the secondary side of the V / v transformer and will not generate a zero sequence voltage on its primary side i

(i.e. U 0 = 0). We make the port wiring angle is -30 °. The V / v transformer port transformation relationship can be obtained with figure 1. i  i  U U A  1 2 1  0  i  1   i  U  =1 − 1 1 U  (1) B 3K   α i  1− 1 − 2  i  U    U  C  β  where K is the ratio of the secondary side port voltage and the primary A phase voltage. Measured voltage data of α, β power supply arms of Dazhun Railway substations. The data were transformed to obtain 110 kV side three-phase voltage data by transformer port electricity conversion. After FFT analysis of the three-phase voltage data, the amplitude value of arbitrary order harmonic voltage was obtained. The amplitude value of arbitrary order harmonics voltage in seven substations is shown as figure 2. It can be seen from the figure 2 that the harmonic condition of the seven traction substations is approximately the

266 same, and has certain regularity. Mainly the 3, 5, 7, 9, 11, 13 and other low order harmonics and 31, 33, 35, 37, 39, 41 and other higher harmonics.

Figure 2. The amplitude value of arbitrary order harmonic voltage in seven substations.

The 95% probability maximum is used to evaluate the traction load harmonics. Referring to GB14549-93, harmonic voltage content (Uh) and total harmonic distortion (THD) were calculated6. The results are shown in Table 2. TABLE 2. HARMONIC CONTENT OF SEVEN TRACTION SUBSTATIONS.

Harmonic Traction substation order DDG DHC FG LC SDG WXG YG 3 1.72 2.65 1.76 2.31 2.27 3.14 3.08 5 2.96 4.51 3.90 1.87 2.32 2.56 3.55 7 1.70 3.38 2.60 1.98 0.50 3.28 2.11 9 1.61 2.32 1.81 1.12 0.76 2.16 1.48 11 1.70 2.61 1.36 1.19 0.68 2.75 1.29 13 1.57 2.71 1.42 0.94 0.67 3.17 1.11 31 1.88 1.80 2.89 2.31 1.63 1.56 2.50 33 1.80 1.51 1.41 1.22 1.06 1.35 1.51 35 2.94 2.04 3.93 2.99 2.23 1.73 3.28 37 1.18 1.06 1.84 1.60 1.16 0.65 2.22 39 2.05 1.01 1.63 2.20 1.10 0.86 2.27 41 1.89 1.01 1.30 3.00 1.80 1.89 3.66 THD 7.88 9.40 9.00 7.91 5.94 9.38 10.17

As the electrified railway traction load even harmonic content is very small, it has little effect on the total harmonic distortion rate, and as the length limit, Table 2 lists only the partial odd harmonics and total harmonic distortion. It can be seen from Table 2 that the total harmonic distortion of 110KV side at seven traction substations of Dazhun Railway greatly exceeds the limit (2%) of the national standard. The 3rd, 5th, 31st and 35th harmonic voltage of all traction substations in Dazhun Railway exceed the national standard. The 7th harmonic voltage of six traction substations in Dazhun Railway exceeds the national standard. The 9th, 39th and 41st harmonic voltage of four traction substations in Dazhun Railway exceeds the national standard. The 11th, 13th, 33rd and 35th harmonic voltage of three traction substations in Dazhun Railway exceeds the

267 national standard. We should pay close attention to the influence of these harmonics on the surrounding protective relays, transformers, capacitors and so on, and filter them.

Analysis of Power Factor If the power factor of the load is too low, the capacity of the power supply will not be effectively utilized and will increase the power loss and voltage loss on the transmission line, which will increase the operating costs of power users and the grid, which is not conducive to reducing production cost7. Power supply capacity is generally expressed as apparent power S = UI, but the power output of the active power is described as P = UIcosφ. When the power factor (cosφ) is too low, the active power output reduced correspondingly. Then the power effective usage of capacity is greatly reduced. According to the measured data, the power factor of each substation is calculated, as shown in Table 3. TABLE 3. POWER FACTOR OF SUBSTATIONS.

Traction DDG DHC FG LC SDG WXG YG substation power factor of 0.979 0.996 0.251 0.969 0.978 0.986 0.972 α phase power factor of 0.979 0.837 0.996 0.966 0.521 0.956 0.934 β phase

As shown in Table 3, the power factor of DDG substation’s α phase and β phase, the power factor of DHC substation’s α phase, the power factor of FG substation’s β phase, the power factor of LC substation’s α phase and β phase, the power factor of SDG substation’s α phase, the power factor of WXG substation’s α phase and β phase, the power factor of YG substation’s α phase and β phase are all greater than 0.9, which meet the requirements of the power grid. The power factor of DHC substation’s β phase is 0.837, which is less than 0.9, and reactive power compensation is needed. The synchronous compensator, synchronous motor and shunt capacitor can be used to compensate the reactive power. The power factor of FG substation’s α phase is 0.251, while the power supply arm may not through the locomotive, only reactive power compensation. The power factor of SDG substation’s β phase is 0.521, and the locomotive on the β phase power supply arm is feeding back energy. Overall, the seven substations have high power factor and conform to the power grid standard.

Analysis of Three-Phase Voltage Unbalance Factor Due to the characteristics of traction load of single-phase AC electrified railway, when three-phase power system supplies power to it, it will cause negative sequence current in power system. The calculation results of the three-phase voltage unbalance factor on the 110kV side are shown in Table 4. TABLE 4. THREE PHASE VOLTAGE UNBALANCE OF SUBSTATIONS.

Traction substation DDG DHC FG LC SDG WXG YG Three phase voltage 5.65 4.63 4.62 4.71 5.68 5.71 5.68 unbalance (%)

As shown in Table 4, the three-phase voltage unbalance factor of each substation exceeds the requirement of the national standard (2%). Negative sequence component

268 will cause bad effects in power system, such as additional occupation of system and its equipment capacity, resulting in additional network loss, resulting in system voltage error, reducing generator and motor output, etc. In order to make the power system run economically and improve the power quality, it is necessary to reduce the negative sequence component as much as possible. The practical measures of reducing negative sequence component are shown as follows: adopt special transformer (usually the Scott traction transformer); parallel compensation device is adopted; adopt traction substation commutating connect. A great deal of practice has proved that the commutation connection of traction substation is very effective to reduce the negative sequence influence of electrified railway on power system.

CONCLUSION With the increase of traffic volume, the load of traction substation increases year by year, which puts forward higher requirements for the power supply capability and power quality of traction power supply system. Based on the analysis of traction power supply system theory and the existing power supply facilities of Dazhun Railway, this paper combines the theoretical analysis and practical testing, analyses the power supply capacity and power quality of Dazhun Railway, and puts forward the pertinence transformation proposal.

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