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Practical Application of
the Railway Static Power Conditioner (RPC) for Conventional Railways
Koichi Shishime
Voltage fluctuation, Load fluctuation, AC feeding
1. Preface
In AC electric railroad system, the Scott connec- tion transformer or modified Woodbridge transformer is used to receive three-phase medium or high voltage power and convert it into single-phase AC power, which is supplied to the feeder line. When such trans- formers are used, three phases at the power supply side will balance if the loads connected to M output and T output of the secondary side balance. However, in an AC electric railroad system, because the loads connected to M output and T output on the secondary side are different and change abruptly, the imbalance between both outputs is frequent, which causes a volt- age imbalance and fluctuations on the primary side.
The RPC equipment is used to reduce such imbal- ances or fluctuations via an active power exchange between M output and T output and by compensation of reactive power(1).
We supplied some power compensation equip- ment for Shinkansen lines but the RPC supplied to the Aomori-nishi Substation was the first one installed on conventional railways, and on-site demonstrative testing was conducted. This paper introduces the details of the equipment and the demonstrative on-site testing.
2. Equipment of Aomori-nishi Substation At the Aomori-nishi Substation, two 66kV 3-phase power lines are received and 44kV power is fed to the Auto-Transformers (ATs) on the Ouu Line and the Tsugaru Line. On these two lines, trains with different power factor characteristics are operating. This substa- tion also supplies power to the premises of Aomori
Station and Aomori Rail Yard after step down 44kV to 22kV, and accordingly, one of the windings is subject to a nearly fixed load, which is a different condition from normal substations. Fig. 1 shows a single-line connection diagram for the Aomori-nishi Substation.
3. Equipment Configuration and Specifications for RPC
Fig. 2 shows basic configuration of the RPC. The basic functions of the RPC are compensation of the Abstract
Meidensha Corporation has been supplying the Railway static Power Conditioner (RPC) and Single phase Feeding power Conditioner (SFC) for Shinkansen lines as a measure for voltage fluctuations, imbalance of three phases, and for the effective use of transformer capacity at the power receiving point. Recently, we supplied the RPC to Aomori-nishi Substation as our first application of RPC to a conventional railroad line in Japan. Considering that Aomori-nishi Substation is designed to feed electric power for the station and load the rail yard where impedance of the load fluctuates from one of the respective windings and that the back power for the receiving circuit is small compared with that of the Shinkansen line, thorough demonstrative tests were conducted to verify the effect on the power supply system as compensation before the start of actual operations.
〈Case Study〉
Premises of Aomori Station
Aomori rail yard Ouu line STr
44/22kV DTr 500kVA 66/6.9kV
FTr 20MVA
66/44kV RPC
INV INV INV Tr 4MVA 44kV/1800V 66kV 50Hz 66kV 50Hz
Tsugaru line Fig. 1 Single Line Connection Diagram of
Aomori-nishi Substation
This diagram shows the circuits that convert 66kV 3-phase power received to two systems of 44kV single phase power using the Scott connection transformer and interchange and compensation of power between phases connecting each phase with the RPC.
MEIDEN REVIEW Series No.156 2012 No.3
(39) reactive power in each phase and the interchange of active power between M output and T output. The reactive power of a phase is compensated by inverter control of the respective outputs. With respect to active power, when there is a difference between M output and T output, active power equal to one-half of such difference is interchanged via the DC bus of the RPC from a less loaded output to a more loaded output in order to balance the loads on the secondary side of the Scott connection transformer, which will reduce volt- age fluctuations and the imbalance ratio of the power receiving point. Table 1 shows the specifications for the RPC.
The RPC consists of the circuit-breakers, inter- connected transformer groups, and inverter devices on the respective feeder lines of M output and T output that are interconnected by the DC bus. Antifreeze solu- tion is used instead of demineralized water as the cooling medium of the cooling device for the inverter because Aomori-nishi Substation is located in a cold area. Figs. 3 and 4 show the installed conditions of the interconnected transformer group and the inverter panel and its cooling pump unit, respectively, and Fig. 5 shows the equipment layout in the substation.
4. On-Site Verification Test
Aomori-nishi Substation, where the RPC was first installed for conventional railways, differs from the substation for the Shinkansen system, such as power receiving from the system with small back power, and it feeds power to Aomori Station and the rail yard. A significant change in the impedance of the feeding system is expected. Accordingly, simulations and stud- ies were conducted in the design stage to avoid reso- nance with the system and transient instabilities(2). The actual energizing tests were conducted to verify that there were no problems due to resonance or instability due to operation for short periods of time in a time zone when the load is small in order to restrict the adverse effect caused by possible problems.
4.1 Results of Compensation Test
Fig. 6 shows the waveforms at various points of the circuit before and after operation of the RPC. The results show that equipment operation was free of abnormal waveforms or resonance.
Fig. 7 shows the feeder load current and FTr secondary side current fluctuation extracted from the test results when the RPC was in operation. The graph shows that loads were balanced between both phases, excluding the no-load operation loss during M output INV M
output
INV T output PM− PC PT＋ PC
QM QT
PM PT
PC
PC PC
QM QT
Fig. 2 Basic Configuration of RPC
The RPC connects the inverters installed in each phase of the Scott connection transformer with the DC bus to interchange power between outputs.
Table 1 RPC Specifications
This table shows the rated capacity and principal specifications of the RPC.
Rated capacity 4000kVA × 2 100% continuous Rated voltage
Rating
44kV 50Hz
Cooling method Demineralized water circulation and forced air cooling
Active power interchange control Reactive power compensation control Rated frequency
Function
Fig. 3 Interconnected Transformer Group
The photo shows outside view of the 4MVA, 44kV/1800V inter- connected transformer group.
Fig. 4 Inverter Panel and Cooling Pump
The photo shows outside view of the inverter panel and the cooling pump for antifreeze solution.
MEIDEN REVIEW Series No.156 2012 No.3
(40) with active power effectively interchanged from T output to M output via the DC bus of the RPC even when the load current of T output is absent.
Fig. 8 shows fluctuations in the reactive power of
the load, reactive power of the inverter, and reactive power of FTr secondary side extracted from the test results, which shows that reactive power of the load is compen- sated by the RPC.
4.2 Reduction of System Voltage Fluctuation
Figs. 9 and 10 show the comparison of voltage fluctuations between the states when the RPC was in operation and not in operation. Due to the small short circuit capacity of the substation, voltage fluctua- tions due to load by the train were signifi- cant. As voltage fluctuation reduces to less than a half of the value when the RPC was in operation, the graph shows that the RPC functioned effectively.
5. Postscript
We installed the RPC at Aomori-nishi Substation as an application for conven- tional railways, and its effectiveness was confirmed. We anticipated potential prob- lems arising from the possible resonance phenomenon between the ground facilities and trains or transient instability due to the complex feeding system for a conventional railway and ongoing trains of various power factor characteristics in operation. However, the supplied RPC operated well. It was confirmed that the RPC operated without
230
Fence
Goods entrance Forced
air cooling
device Goods entrance
Step-in floor
Entrance
Lavatory Staircase
Feeding C-GIS Control panel room
AT413
INV.Tr INV.Tr 22kv/
6kv CH
INV.Tr INV.Tr
AT413 Cooling INV
device
Forced air cooling device
Feeding C-GIS
Fig. 5 Equipment Layout of Aomori-nishi Substation
Layout of the substation effectively utilizes the narrow strip of land along the track of the Tsugaru-kaikyo Line.
Receiving voltage R-S 0.12MV/1DIV Receiving voltage S-T 0.12MV/1DIV Receiving voltage T-R 0.12MV/1DIV Receiving current R 0.3kA/1DIV Receiving current S 0.3kA/1DIV Receiving current T 0.3kA/1DIV M output bus voltage 0.08MV/1DIV T output bus voltage 0.08MV/1DIV FTr secondary M output current 0.16kA/1DIV
RPC not operating RPC operating
Trigger: 2010-07-21 01：55：19 Sampling period: 200us (50ms/DIV)
RPC operation
FTr secondary T output current 0.16kA/1DIV
Fig. 6 Specification of Voltage and Current Waveforms before and after RPC Operation
This figure shows waveforms of receiving voltage, receiving current, and feeding voltage and feeding current when RPC is operating and not operating.
MEIDEN REVIEW Series No.156 2012 No.3
(41) problems against inrush current when the train passed through sections peculiar to conventional railways.
Going forward, we expect more demands for qual- ity traction power in various aspects. The use of the RPC in the conventional railroad system will increase.
We will continue technical development that meets market needs.
(1) Uzuka T, et al.: IEEJ Transactions on Industry Applications, Vol. 115, No. 12, 1995
(2) Kikuchi S. et al.: 2010 Railway Electric Power Tech- nical Forum, Japan Railway Electrical Engineering Association – Electricity 7 (to be published)
Load current
M output load current
T output load current
M output current
T output current 30
25 20 15 10 5
0 200 240 280 320 360 400
Time (s) 160 120 80 40
Current
( A
)Current
( A
)
0
Tr secondary current 18
16 14 12 10 8 6 4 2
0 200 240 280 320 360 400
Time (s) 160 120 80 40 0
Fig. 7 Active Power Trading Effect by the RPC The figure shows typical fluctuation of M output and T output current when RPC was in operation.
RPC not operating 1.4
1.2 1.0 0.8 0.6 0.4 0.2 0 0
550 1100 1650 2200 2750 3300 3850 4400 4950 5500 6050 6600 7150 7700 8250 8800 9350 9900 10450 11000 11550 12100 12650 13200 13750 14300
0 550 1100 1650 2200 2750 3300 3850 4400 4950 5500 6050 6600 7150 7700 8250 8800 9350 9900 10450 11000 11550 12100 12650 13200 13750 14300
Time (s)
Voltage fluctuation
( % )Voltage fluctuation
( %
) RPC operating
1.4 1.2 1.0 0.8 0.6 0.4 0.2 0
Time (s)
Fig. 9 Effect in Reduction of Voltage Fluctuation 1 This graph shows changes in the voltage fluctuation ratio at the power receiving point with time when the RPC is operating and not operating.
It shows that voltage fluctuation ratio decreases when the RPC is operating.
T output load Reactive power
T output Reactive power
T output inverter reactive power 4500
3500 2500 1500 500
−500
−1500
−2500
40 35 30 25 20 15 10 5 0
Time (s)
Reactive power
( kVar
)
Fig. 8 Reactive Power Compensating Effect by RPC This example shows reactive power compensation of T output by phase control of the T output inverter.
RPC not operating
RPC operating 1.2
1.0 0.8 0.6 0.4 0.2
00 50 100 150 200 250
Load current (A)
Voltage fluctuation
( %
)
Fig. 10 Effect in Reduction of Voltage Fluctuation 2 This graph shows that the voltage fluctuation ratio at the power receiv- ing point was large due to the increase in the load when the RPC was not in operation, but voltage fluctuations due to the increase in the load were reduced when the RPC was in operation.
《References》
