"Clean" electric energy of electric locomotives

With the development of urban rail transit and intercity expressways, electric locomotives are fully showing their capabilities, and high-power electric and electronic devices are being extensively deployed. Power quality detection and analysis systems based on data acquisition platforms help with analytical measurement, hopefully eventually avoiding the pollution of power grids by high-power electric and electronic equipment and fully realizing:

With their increasingly extensive deployment, electric locomotives will generate a large amount of harmonic and negative sequence pollution. If not promptly handled in traction substations, the harmonic and negative sequence pollution will be transmitted to power grids, reducing power quality. With the development of urban rail transit and subways, this contradiction is becoming increasingly prominent. The preconditions of power quality improvement are reasonable monitoring and careful analysis of power quality under different conditions; however, most domestic power stations and power substations are not equipped with the relevant analytical instruments and equipment. To solve this problem, it is necessary to develop cost effective, multipurpose, portable power quality detection and analysis devices that have powerful analysis functions and are in line with the habits and requirements of railway work according to the characteristics of the electrified railways in China.

Opportunity to become "clean"

According to the definition of power quality, the first step is to perform analysis and measurement for parameters such as frequency, reactive power, negative-sequence current and harmonics. At the beginning of the design stage for power quality analysis systems of traction power supply systems, statistic analysis was done for the maximums, minimums, averages and 95-percent values of the traction load harmonics, negative sequence current and voltage quality of electrified railway passenger line AT stations, direct-supply substations, AT stations, section posts and sub-section posts over fixed periods of time.

Other load properties determined test requirements for the traction load harmonics and negative sequences of electrified railway passenger line AT stations, direct-supply substations, AT stations, section posts and sub-section posts:

1. In tests of power supply systems of railway passenger lines, particularly when the test periods during joint debugging are long and complete power failures of the power stations used may often occur, the test system should have the functions of virtual instruments, including immediate testing following starting up and shutting down upon reaching thresholds and at fixed times.
2. The requirements for harmonic analysis and THD calculations with different harmonic orders determined by the performance of the motor train units and electric locomotives and the performance of SVCs and resonance inhibiting filters, and the requirement for calculating or not calculating even harmonics.
3. Tests of the feeder currents of the power supply systems of railway passenger lines should be related to the relevant voltage calculations (power and phase difference).

Viewed in terms of hardware, both the voltages and the currents should reach certain standards, and both the sampling rate and accuracy of the system must be high. ADLINK data acquisition digitizers (PCI-9846), DVDI transformers and FLUKE i200s AC current clamps are used.

The software of the system is compiled based on a LabVIEW development platform. A power quality detection system based on virtual instruments consists of a detection segment and an analysis segment. The detection segment is used for changing the original voltage and current signals into digital signals, displaying the waveforms and the spectrums, and saving and outputting the data files. Its function modules include a data transmission module, a data analysis module, a graphical display module, a data file output module, and more. The analysis part is used for performing deep analysis for the data files output by the detection segment of the system, obtaining statistical results and change trends of the power quality parameters and printing power quality analysis reports for the traction power supply systems according to actual requirements.

Effect verification

According to the characteristics of the traction power supply systems, eight controls are designed with the help of a piece of LabVIEW software, including Parameter Set, Station Transformation Ratio, Real-time Waveform Data, Real-time Data Analysis, Historical Data, Historical Data Analysis, Current on the Traction Side and related Calculation and Counting.

The real-time harmonic data of the electrified railways were analyzed using the software phase locking technology-based FFT harmonic detection method and the combined wavelets-based harmonic detection method. The harmonic data were provided by a substation of the Beijing — Guangzhou Line in the Shijiazhuang power supply section of the Beijing Railway Bureau, a substation of the Shijiazhuang — Taiyuan Passenger Line, and a substation of the Shujiazhuang — Dezhou Line. There were 2 to 63 harmonic processing sets, and the sampling frequency was 256 points per cycle. The duration of data recording was 24 hours. While the loads of different locomotive models are different and only some of the power supply sections of the traction substations have locomotive loads, the voltage and current data of the traction power supply systems have different harmonic contents. This paper mainly analyzes the harmonics caused by different locomotive loads for the traction power supply systems.

(1) Test of a substation of the Beijing — Guangzhou Line
The substation uses a 110/27.5kV impendence-matched balancing transformer, a three-phase V/V connection traction transformer, a transformation ratio of 110kV/100V and 300/5A for the voltage transformer and the current transformer on the 110kV side respectively, and a transformation ratio of 27.5kV/100V and 1000/5A for the voltage transformer and the current transformer on the traction side respectively. The power supply section uses both a direct current power supply mode and a return current power supply mode. The locomotive loads of the power supply section include CRH locomotives, SS locomotives and "Hexie" goods locomotives.

(2) Test of a substation of the Shijiazhuang — Taiyuan Passenger Line
The substation uses a 110/27.5kV impendence-matched balancing transformer, a three-phase V/V connection traction transformer, a transformation ratio of 110kV/100V and 400/5A for the voltage transformer and the current transformer on the 110kV side respectively, and a transformation ratio of 27.5kV/100V and 1200/5A for the voltage transformer and the current transformer on the traction side respectively. The power supply section also uses both a direct current power supply mode and a return current power supply mode,. This traction substation only supplies power for the Shijiazhuang — Taiyuan Passenger Line, mainly using CRH EMUs.

The harmonic content results of analysis of the current-collecting waveform of CRH locomotives are shown in the column diagram in Fig. 3. According to Fig. 3, the current-collecting waveform of CRH locomotives is almost a sine waveform, and the current-collecting harmonic content mainly exists on the third-order harmonic.

(3) Test of a substation of the Shujiazhuang — Dezhou Line
The substation uses a 110/27.5kV impendence-matched balancing transformer, a three-phase V/V connection traction transformer, a transformation ratio of 110kV/100V and 400/5A for the voltage transformer and the current transformer on the 110kV side respectively, and a transformation ratio of 27.5kV/100V and 600/5A for the voltage transformer and the current transformer on the traction side respectively. The power supply section uses both a direct current power supply mode and a return current power supply mode. The main goods locomotives running on the power supply section of the Gaocheng substation are SS4 goods locomotives, and the main passenger locomotives running on it are diesel locomotives.

Fig. 5 is the column diagram of the current-collecting harmonic content of SS locomotives. The current-collecting harmonic of SS electric locomotives is mainly distributed on the 3rd, 5th, 7th, 9th and 11th orders. Furthermore, the harmonic content decreases with an increase in the harmonic frequency.

As is evidenced by these results, the power quality detection systems designed for traction power systems in line with the characteristic of the serious pollution of power grids by traction power systems can detect frequency, negative sequence current, reactive power, harmonics and other parameters. Viewed in terms of the hardware of the power quality detection systems, ADLINK high-speed data acquisition digitizers are used, which guarantees high speed and accuracy for data acquisition. To date these power quality detection systems have been used on multiple railway passenger lines and shown stable and reliable performance.

*LabVIEW™ is a registered trademark of National Instruments.

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