Application of New Regenerative Braking Energy Absorption Device in Subway

Feng Ji

(Mechinery and Electrical Department,Wuhan Railway Technicians College,Wuhan,Hubei 430064)

Abstract:The braking of rail transit vehicles often produces huge energy,which will be better absorbed and utilized when the new subway lines are put into operation,which is also one of the main energy-saving and emission reduction measures.Through the analysis and comparison of the application of regenerative energy absorption devices in the power supply system of Beijing,Shanghai,Urumqi,Guiyang and other subway operating lines,the characteristics of several kinds of absorption devices are explained,and the configuration Suggestionsareput forward.

Keywords:regenerativebraking;medium-voltageenergy fed

Direct current power supply is mostly adopted in subway,and power diodes are adopted in rectifying.The energy flows from the AC side to the DC side in a one-way way.When the train speeds down braking,the speed is greater than 8 km/h,and generally uses regenerative braking,the traction electric motor state will transform into a power generation condition,absorb mechanical kinetic energy and take into electricity.The energy can be consumed by the auxiliary power equipment of the train itself,the remaining will be back to the drawing online.In general,there will be about 60%of the a train kinetic energy transformed into electric energy feedback to DC net.The following figure is the comparison diagram of braking energy consumption and traction energy consumption monitored in some stations of Shanghai metro lines 3 and 4(shown asfigure 1).

Within the same power supply section,if there are other trains are pulling in this period of time,the energy of the DCtraction net can be absorbed,but if the trains are at low density as the obligatory(spare time),there is no other trains absorb energy at the same power supply section,or no other trains can"t completely absorbed renewable energy,as a result,the net pressure appears soared,when more than limit value(1900V),it will launch the protection of the vehicle equipment(inverter)closure,so it must havetheability toabsorb theequipment.

At present,the train regenerative energy absorption devices are mainly resistance energy dissipation type,energy storage type and inverter feedback type.

Figure 1 Energy consumption diagram the upper bar isthe energy consumption of absorbablebraking the following isthe traction energy consumption

Resistance energy dissipation type.Early represented by Chongqing metro design was set up 64D regenerative braking absorption equipment near the traction substation,consisted by chopper and resistance,the change of the DC bus voltage state can open the electronic switch conduction,when the net pressure is high,the braking resistance is put in,and the excess braking energy is converted into heat energy to be consumed,thus changing the absorbed power,which will maintain a constant DC voltage over a set value,which will allow the continuous stabilization of the regenerative current emitted by the vehicle to maximize the use of the electric braking function.Compared with the control device,the principle of this scheme is simple and reliable,which has been applied in the early stage.However,the main disadvantage of this scheme is that the electrical energy is converted into heat energy in vain,and the heat dissipation of the resistance will lead to the increase of the ambient temperature,which will cause waste.In general,the substation will install the corresponding ventilation deviceand placeit in thewell-ventilated section.

Energy storage type.The energy storage regenerative energy absorber is more expensive and can be divided into capacitance type and flywheel typein principle.

Capacitor energy-storage type regenerative energy absorption device uses large capacitance or battery,and uses ICBTto control the input.After the input,the regenerative braking energy of the train is absorbed into the large-capacity capacitor bank[1].If the network voltage is low(1400V)and there is a need for the train to take the current,the capacitor will feed back to the power grid to release the stored energy and raise the voltage.The complete set of SITRAS SES produced by Germany Siemens is adopted in Beijing metro line 5,which is mainly located in 4 traction substations with long traction power supply interval and large slope interval[2].In the early stage of equipment input,due to the performance of capacitors,the energy storage capacity is relatively small,most of the renewable energy is not absorbed,and the absorbed energy is very limited.Capacitors are bulky and require a lot of space.After frequent charging and discharging,capacitorsusually haveashort servicelife.

The basic principle of the flywheel energy storage type is that the energy storage element is pressed into the energy storage element by the network,but the energy storage element becomes the flywheel motor.When the motor is electrified,it will drive the flywheel to rotate at a high speed.Obviously,electrical energy is changed into kinetic energy and stored.When there is no regenerative energy,the flywheel drives the generator by inertia to generate electricity.At present,the flywheel device has no mature products in China,and the price of foreign products is higher.In addition,due to the extremely long running time of high-speed rotating flywheel causes large rotary inertia,and with high requirements for the mechanism,serious problem of friction and energy consumption,the flywheel working life is short,the whole system is complex,and space demand is large,it is in the stage of technical research and laboratory verification.

When using energy-dissipating and energystoring devices,the AC side is not considered,and only the DC traction network pressure of subway is considered.In this way,the control logic is relatively simple and the voltage stabilizing effect is good.In addition to the investment economy,the existing energy storage device is difficult to meet the requirements of subway operation in terms of service life, safety performance and maintenance convenience,but it can be regarded as an ideal application typetobestudied in thefuture.

Inverter feedback type.This device feeds energy back into the subway power supply system.The direct current shall be inversely converted into alternating current and classified according to the AC side voltage,which can be divided into three levels:

Low voltage feedback:400V

Mediumvoltage feedback:1180V

High voltage feedback:10kV,35kV(33kV)

Inverter feedback type regenerative energy absorption device is used to transmit energy back to the power grid,mainly using three-phase inverter circuit,which does not need to be equipped with large- capacity energy storage elements or resistance[3].When the network voltage is raised,the energy is inverted from the DC bus into AC through the inverter,and feed back to the 0.4kv low-voltage AC power grid of this station or the 35kv mediumvoltage network of the entire subway.

Capacity to low voltage 0.4 kV inverter device can meet the low voltage side load and dynamic change of sent,at the same time it should be taken into account that the 0.4 kV is stable and reliable.Therefore,due to the capacity problem,it is often used in combination with the resistance,resulting in limited energy saving effect.In the operation of Beijing subway line 9,but because of capacity constraints,therearerenewablefailurecondition.

One kind of regenerative braking energy utilization device of medium voltage energy-fed type is to feed energy back to the side of 1180V,that is,the auxiliary side of the rectifier transformer.The inverter energy is returned through the rectifier transformer.At the same time,the switch cabinet on the AC side can also protect this device.When designing device capacity,it is necessary to consider thesidecapacity of rectifier transformer.

Medium voltage energy- fed regenerative braking energy utilization device is most widely used in the way of feeding energy to the medium voltage power grid independently.It has less conditions and is convenient to measure.However,an extra switch cabinet should be installed.In addition, the harmonic and reactive power compensation of the whole AC medium voltage grid is considered in the design.Typical application schemes are as follows:

Figure2 Typical Applications

This regenerative braking energy utilization deviceismainly composed of converter cabinet,low-voltage switch cabinet,feedback transformer,35kV switch cabinet,DCswitch cabinet and negative pole isolation cabinet,which are six parts in total.This method requires the addition of a high-power inverter[4].The inverter changes the DC on the net into AC and returns it to the alternating side.In order to meet the actual capacity, safety,maintenance demands,is usually changed the rectifier cabinet one-way rectifier unit into a twoway converter.This is to change the power level into a two-way converter,which constitutes a two-way converter unit with transformers,switchgear,etc.Or add a converter,and then with the original diode rectifier unit in parallel.In this way,the two-way converter cabinet contains 2 sets of two-way converter,two-way converter is swithed through the size and phase control by the network pressure,and then transmit activepower and reactivepower.

When ac current is controlled,the amplitude and phase of the output voltage Ua,Ub,Uc of the inverter are controlled to realize the control of AC current ia,ib,ic.Thus the inverter output and the grid voltage synchronization.As shown in the figure below:

Figure3 CH1 isthe voltage waveform CH2 isthecurrent waveform

From the above analysis,it can be seen that the current mainstream solution is to use medium voltage inverter feedback device,which can meet the requirements of energy saving and control network voltage of each company.In addition,the current waveform of the feedback network is good,and the power factor can also be improved.However,the high cost of equipment also puts forward a further demand for the configuration of ventilation and heat dissipation,and these problems need tobefurther solved.

Additional Thoughts on This Paper.In addition to flywheel energy storage regenerative energy absorption device,other methods are applied in the subway.Medium voltage inverter feedback type equipment investment should be as early as possible,early in the metro companies under the condition of traffic volume is not big,adjacent train absorption is not big,the medium voltage inverter feedback device can absorb more feedback,recovery cost,and the bidirectional converter can act as the backup of the rectifier,just the device cost is high,but the control strategy has been quite mature,with the improvement of device,the focus of the application must bein thefuture.