Energy Saving Performance Analysis of An Inverter-based Regenerative Power Re-utilization Device for Urban Rail Transit

The inverter-based regenerative braking power utilization devices can re-utilize the regenerative energy, thus reduce the energy consumption of urban rail transit. In this paper the power absorption principle of the inverter-based device is introduced, then the key influencing factors of energy saving performance are analyzed based on the absorption model. The field operation data verified that the control DC voltage plays an important role and lower control DC voltage yields more energy saving. Also, the one year energy saving performance data of an inverter-based re-utilization device located in NanJing S8 line is provided, and more than 1.2 million kWh energy is recovered in the one year operation.

. In this paper, the inverter-based device is analyzed in terms of working principle, power absorption model, energy-saving performance.

Working principle and modeling
2.1. The principle of regenerative power utilization As shown in the Figure 1, the inverter-based device is interfaced with the DC traction network and the medium voltage AC grid, and is parallel with the traction rectifier device. When the train is braking, a part of the regenerative power can be absorbed by the adjacent accelerating trains, and the rest of the power is fed back to the AC grid by the inverter and utilized by other loads.  Figure 1. The system structure of substation with inverter-based utilization device As shown in Figure 2 the control objectives of the inverter-based utilization device are to control the overhead line DC voltage and regulate the current injected into the AC grid. Seen from the DC traction network the inverter-based device behaves as a DC voltage source, while seen from the AC grid, the device behaves as a AC current source.  Figure 2. the control strategy of inverter-based device In some cases, the on-board resistor-based devices as well as the inverter-based devices are equipped in the metro line. Since the DC control threshold voltage of the inverter-based device V th_inverter is set be lower than that of the resistor-based device V th_resistor , the inverter-based device has the priority to absorb the power. Figure 3 shows the typical power absorption process waveform, at t0 instant, the inverter-based device starts to absorb the power. During the t0-t1 interval, the regenerative power is within the rated power of the inverter-based device, so the device can absorb all the regenerative energy and control the DC line voltage independently. at t1 instant, the regenerative power exceed the rated power of the inverter-based device, then the inverter-based device keeps outputting its rated power, and the exceeding power boosts the the DC overhead line voltage and reaches V th_resistor and the 3 1234567890 ''"" resistor-based device start to absorb the excessive power and stabilize the overhead line DC voltage. During t2-t3 interval, the regenerative power drops within the rated power of the inverter-based device, then the resistor-based device quits and all of the power is absorbed by the inverter-based device.

Modeling of DC line regenerative power absorption model
As stated in the last section, the inverter-based utilization device is controlled as a DC voltage source, and the braking train can be modeled as a current injection into the overhead DC line. Since there are resistance between the braking train and the inverter-based utilization device located substation, the injection current could incur a voltage drop across the overhead DC line and rail track. Figure 4 shows the flowing path of the regenerative current I cl . As shown, the control DC voltage of the inverter-based device located substation is U dc , and the DC voltage of the braking train U cl can be expressed in equation (1), where µ a and µ b are the resistance rate of the overhead line and the rail track respectively where µ a is 0.007Ω/km，µ b is 0.009Ω/km, and d is the the distance between the braking train and the substation.  It is assumed that the maximum allowable DC voltage for the regenerative braking train is 1800V, and the maximum absorption power of inverter-based device can be derived as equation (3).
There are two key factors influencing the maximum absorption regenerative power, one is the control DC voltage of the inverter-based utilization device, the other one is the distance between the braking train and the substation.

System configuration
A inverter-based utilization device of 2.5MW power rating is installed in a substation of Nanjing S8 line which is also equipped with the on-board resistor-based utilization device. As shown in Figure 6, the 2.5MW inverter-based device is installed in station B, and within the 5kM distance there are three other stations, namely StationA、StationC、StationD. As stated in the last section, the inverter-based device can recover the regenerative power from the braking train not only at the station B but also the adjacent stations. Table 1 shows the recovered energy with different control DC voltages, and the lower control DC voltage yields more saved energy. Considering there should be enough margin to ensure the control DC voltage is higher than the floating voltage of diode rectifier device, so the control DC voltage is set to be 1730V for the field operation.

The DC line voltage stabilization
The inverter-based utilization device can effectively reduce the fluctuation of the overhead DC line voltage. As shown in Figure 7   In one year operation the inverter-based utilization device with the control DC voltage of 1730V recovered more than 1.2 million kwh, and more than 100000 kwh per month averagely.The monthly and daily energy saving data are shown in Figure 8.

Conclusion
The inverter-based regenerative power utilization device can effectively stabilize the overhead line DC voltage and recover the regenerative energy. Several key influencing factors like control DC voltage, braking distance are analyzed based on the absorption model, and the field data verified the lower control DC voltage can yield more saving energy. The field operation data of energy saving performance and the overheard DC voltage stabilization effect of an inverter-based power utilization device installed in NanJing S8 line are given, in the one year operation the device recovered more than 1.2 million kWh energy , at the same time the overhead DC voltage maximum value is constrained and reduced from 1800V to 1745V.