WO2021260361A1

WO2021260361A1 - Electric railway

Info

Publication number: WO2021260361A1
Application number: PCT/GB2021/051576
Authority: WO
Inventors: Pietro TRICOLI
Original assignee: The University Of Birmingham
Priority date: 2020-06-23
Filing date: 2021-06-22
Publication date: 2021-12-30
Also published as: GB202009582D0

Abstract

The disclosure relates to an electrified railway system, and in particular to systems, methods and apparatus for balancing a traction power supply with a three-phase electrical distribution grid. Examples disclosed include a method of balancing an electrical power supply of an electric railway system (200), the method comprising: providing an input AC supply from first and second phases (209, 208) of a three-phase electrical supply (201) to an input of a power electronics autotransformer (211), the power electronics transformer comprising first and second converters, an intermediate DC link and a controller connected to the first and second converters; converting the input AC supply to an intermediate DC supply with the first converter; converting the intermediate DC supply to an output AC supply with the second converter; and providing first and third phases (209, 207) of the three-phase electrical supply with the output AC supply from an output of the power electronics autotransformer (211), wherein the output AC supply is phase shifted by 120° relative to the input AC supply.

Description

ELECTRIC RAILWAY

Field of the Invention

The invention relates to an electrified railway system, and in particular to systems, methods and apparatus for balancing a traction power supply with a three-phase electrical distribution grid.

Background

Alternating current electrification based on a single phase 25kV supply is commonly used in electric railway systems worldwide, especially for high-traffic and high-speed rail. Electric power from a generating station is transmitted to railway feeder stations using a higher voltage three-phase distribution system. At each feeder station, a step- down transformer is connected across two of the three phases of the high-voltage supply. The transformer lowers the voltage to 25 kV which is supplied to a railway feeder station located beside the tracks. An example system 100 is illustrated in Figure 1. A high voltage three-phase AC transmission grid 101 supplies power to transformers 102, 103 connected to respective track sections 104, 105, each transformer 102, 103 being connected to two of the three phases of the transmission grid 101. Neutral sections 106 are required between track sections 104, 105 to ensure electrical separation between each section, which operate at different voltages. The running rails 107, which are earthed, are not connected to any phase but instead to the centre tap of the secondary winding of each transformer 102, 103. In each track section 104, 105 an overhead line

108 and negative feeder line 109 transmit power to the train 110. Autotransformers 111 are connected at regular intervals along each track section 104, 105, for example every few kilometres, and serve to divert the return current from the neutral rail 107, step it up, and send it along the negative feeder line 109. Due to low power losses and reasonable capital investment required, such a system, known as the 2x25 kV autotransformer system, represents an example of the current state of the art for AC railway electrification. A problem with such a system, however, is the need for neutral sections between feeding points and connecting such a system to a high-voltage three- phase electrical power network. Summary of the Invention

In accordance with a first aspect there is provided a method of balancing an electrical power supply of an electric railway system, the method comprising: providing an input AC supply from first and second phases of a three-phase electrical supply to an input of a power electronics autotransformer, the power electronics transformer comprising first and second converters, an intermediate DC link and a controller connected to the first and second converters; converting the input AC supply to an intermediate DC supply with the first converter; converting the intermediate DC supply to an output AC supply with the second converter; and providing first and third phases of the three-phase electrical supply with the output AC supply from an output of the power electronics autotransformer, wherein the output AC supply is phase shifted by 120° relative to the input AC supply.

The input AC supply may be provided from any pair or phases of the three-phase electrical supply, while the output AC supply is provided to any other different pair of phases. If, for example, the input AC supply is provided from first and second phases, the output AC supply may be provided to the first and third or second and third phases.

An advantage of the above method is that the power electronics autotransformer is able to balance the currents flowing through each of the three phases of the electrical supply by phase shifting the input of the first and second phases. The first converter functions to regulate the DC voltage at the intermediate DC supply and compensate for the reactive power of the first and second phases. The second converter functions to reduce or eliminate any imbalance by transferring active power from the third phase to the first and second phases and compensating for the reactive power of the third phase.

In some examples the power electronics autotransformer comprises: an input transformer arranged to convert the input AC supply to a transformed AC supply provided to the first converter; and an output transformer arranged to convert an AC output from the second converter to the output AC supply. In other examples, the power electronics autotransformer may not require input and output transformers but may include an input and output inductive filter.

The first and second phases of the three-phase electrical supply may be connected to an overhead line and running rails of the electric railway system, and the third phase connected to a negative feeder of the electric railway system.

In particular examples the method comprises: measuring a voltage and current of each phase of the three-phase electrical supply; determining an imbalance between the currents of the three-phase electrical supply; and providing switching signals to the first and second converters to reduce the imbalance.

Reducing the imbalance of the three-phase currents allows the three-phase electrical supply to remain in balance under changing loads being drawn by electric trains operating on the system.

The above steps may be performed by a controller connected to the first and second converter. The controller may provide switching signals to the first converter to cause the first converter to regulate a voltage at the intermediate DC link and compensate for reactive power of the first and second phases, and/or provide switching signals to the second converter to cause the second converter to reduce or eliminate any imbalance between the first, second and third phases by transferring active power from the third phase to the first and second phases and compensate for the reactive power of the third phase.

In accordance with a second aspect there is provided a power electronics autotransformer for balancing an electrical power supply of an electric railway system, the power electronics autotransformer comprising: first and second converters; an intermediate DC link; and a controller connected to the first and second converters, wherein: the first converter is arranged to receive an input AC supply from first and second phases of a three-phase electrical supply and convert the input AC supply to an intermediate DC supply at the intermediate DC link; the second converter is arranged to convert the intermediate DC supply at the intermediate DC link to an output AC supply at first and third phases of the three-phase electrical supply; and the controller is configured to provide switching signals to the first and second converters such that the output AC supply is phase shifted by 120° relative to the input AC supply.

The power electronics autotransformer may comprise: an input transformer arranged to convert the input AC supply to a transformed AC supply provided to the first converter; and an output transformer arranged to convert an AC output from the second converter to the output AC supply.

The controller may be configured to: measure a voltage and current of each phase of the three-phase electrical supply; determine an imbalance between the currents of the three-phase electrical supply; and provide switching signals to the first and second converters to reduce the imbalance.

In accordance with a third aspect there is provided an electric railway system comprising: an overhead line and running rails for providing electrical power to an electric train; a negative feeder line; a three-phase electrical supply having a first phase connected to the overhead line, a second phase connected to the running rails and a third phase connected to the negative feeder; and a power electronics autotransformer comprising: first and second converters; an intermediate DC link; and a controller connected to the first and second converters, wherein: the first converter is arranged to receive an input AC supply from the first and second phases of the three-phase electrical supply and to convert the input AC supply to an intermediate DC supply at the intermediate DC link; the second converter is arranged to convert the intermediate DC supply at the intermediate DC link to an output AC supply at first and third phases of the three-phase electrical supply; and the controller is configured to provide switching signals to the first and second converters such that the output AC supply is phase shifted by 120° relative to the input AC supply.

Detailed Description

The invention is described in further detail below by way of example and with reference to the accompanying drawings, in which:

Figure 1 is a schematic diagram of an example 2x25 kV railway electrification system;

Figure 2 is a schematic diagram of an example 3x25 kV railway electrification system;

Figure 3 is a schematic diagram of an example arrangement of power electronics autotransformers for the system of Figure 2;

Figure 4 is a control scheme representation for generating current references for first and second converters of an example power electronics autotransformer;

Figure 5 is a control scheme representation for current control of a first converter of an example power electronics autotransformer;

Figure 6 is a control scheme representation for a DC supply and second converter of an example power electronics autotransformer;

Figure 7 is a schematic diagram of an example power electronics autotransformer;

Figure 8 is a schematic flow diagram of operation of the example power electronics autotransformer;

Figure 9 is a plot of simulated current of the 3x25 kV railway electrification as a function of time during operation of an example power electronics autotransformer; and

Figure 10 is a plot of simulated voltage sequences of the 3x25 kV railway electrification as a function of time during operation of an example power electronics autotransformer.

Figure 2 illustrates an example electric railway system 200, in which a similar conductor arrangement as that in the example in Figure 1 is used, involving an overhead feeder line 208, a return conductor 209 and running rails 207. The conductors 207, 208, 209 are supplied by transformers 202, 203 having inputs connected to a high-voltage three- phase transmission grid 201.

In place of the autotransformers 111 in the example system 100 of Figure 1, the system 200 comprises power electronics autotransformers 211. Unlike traditional autotransformers, in which input and output voltages necessarily have the same (or opposite) phases, power electronics autotransformers allow for an arbitrary phase difference between the input and the output voltages, since the intermediate DC link allows the phase of the output AC supply to be generated with an arbitrary phase relationship to the input AC supply. The phase difference between the input and output of each power electronics autotransformer 211 is selected to be 120 degrees, so that the output is in phase with the remaining phase of the three-phase supply. With the replacement of traditional autotransformers with power electronics autotransformers, the railway electrification system 200 draws a balanced current from the public grid, thereby removing the need for connection to dedicated high-voltage grids and reducing significantly the cost of the infrastructure without significantly impacting on power losses. This is because power is mainly transmitted over the three-phase transformers 202, 203 of the feeder stations, while the power electronics autotransformers 211 mainly acting as phase balancers. The power electronics autotransformers 211 therefore require a substantially reduced power rating compared with transformers 202, 203.

The system 200 is designed to balance the single-phase current drawn by the train 210, through the power electronics autotransformers 211 being configured to modify the magnitude and phase angle of the voltage output with respect to the input. Specifically, the magnitude of the voltage output is configured to be the same as the input, while the phase has a difference of 120 degrees. Therefore, the current of the transformers 202, 203 in the feeder stations is balanced, with any changes being balanced almost instantaneously. The power electronics autotransformers 211 are synchronised with the railway electrification network voltage and are current controlled.

As illustrated in Figure 2, a plurality of autotransformers 211 may be connected to the conductors 207, 208, 209, the autotransformers 211 being distributed at regular intervals. In alternative arrangements, a single autotransformer 211 with a larger capacity may be used, which may be positioned at a central point along the railway line.

Figure 3 illustrates an example power electronics autotransformer 211 in further detail. The autotransformer 211 is provided with an AC supply from two of the three phases of a high voltage three-phase distribution grid 201 via a three-phase step-down transformer 202. The transformer 202 provides a three phase AC supply at 25 kV having currents i_s,abc and voltages v_s,_abc . The AC supply is connected to the overhead line 208, running rails 207 and negative feeder 209, the train load 301 connected across the overhead line 208 and running rails 207, drawing a load current IL. The train load 301 is representative of all the trains in the section between the power electronics autotransformers 211.

Each power electronics autotransformer 211 comprises a single-phase input transformer 302 having inputs connected to the overhead line 208 and running rails 207. The input transformer 302 provides a transformed AC current zi and voltage Vi to a first converter 303. The first converter 303 converts the transformed AC supply from the input transformer 302 to an intermediate DC voltage V_dc at an intermediate DC link 304. The intermediate DC voltage v* is provided to a second converter 305, which converts the intermediate DC voltage to an AC current U and voltage V2, which is then transformed by an output transformer 306 to provide an output AC supply to the overhead line and negative feeder 209.

Although the power electronics autotransformers 211 are illustrated in Figures 2 and 3 as having input and output AC transformers 302, 306, in some cases these AC transformers may be omitted, for example where the converters 303, 305 are capable of being connected directly to the output of the transformer 202. In this case, either inductive filters or inductive capacitive filters are needed at the input and output of the power electronics autotransformers 211. In other cases, the transformers 302, 306 may be required to transform the 25 kV voltage input to and from a lower voltage suitable for the converters 303, 305. Typical AC voltage levels for these converters will be between 4 kV and 12 kV.

The first converter 303 is controlled to regulate the voltage at the intermediate DC-link 304 and compensate for the reactive power of first and second phases. The second converter 305 is controlled to reduce or eliminate the imbalance of transformer 202 by transferring active power from the third phase to the first and second phases and compensate for the reactive power of the third phase.

Using the definition of the quantities represented above in relation to Figure 3, an example control scheme for operation of each autotransformer 211 is described below, with reference to the control system block diagrams in Figures 4, 5 and 6.

Figure 4 illustrates a control system for generation of the reference currents /i_,?,re/ and ii_,ref for the converters 303, 305 of the power electronics autotransformer 211. A first control subsystem 401 comprises a phase-locked loop (PLL) 402 that determines the magnitude, V_s,d, and phase angle 0 of the space vector of the input supply voltages v_s,_abc . The phase angle 0 is used to transform the input supply current i_s,abc and voltage v_s,abc into the rotating reference frame aligned with the supply voltage, represented by current components i_s,d and i_s,_q, (of which only i_s,_q is used in the control) and voltage V_s,d using abc to dq transformation blocks 403, 406. A proportional-integral regulator (PI) 404 and amplifier 405 generate the reference current i \,_q,_ref, the amplifier having an amplification factor of NJn, where N is the turns ratio of the output transformer 302 (if present) and n is the number of power electronics autotransformers in the section fed by the three-phase transformer 202. In the example of Fig. 3, n is 2. If the output transformer 302 is not present, the value for N will of course be 1. The first control subsystem 401 has the function of compensating for any reactive power of the three- phase transformer 202. As the amplification factor is NJn, the reactive power of the three-phase transformer 202 will be compensated equally by all the power electronics autotransformers 211 connected to the section fed by the three-phase transformer 202.

The reference /i_,?,re/for each power electronics autotransformer 211 can be chosen also in a different way. For example, the power electronics autotransformer 211 closest to the three-phase transformer 202 could take up a proportion of the current i_s,_q up to its maximum current capability, while the remainder being taken by the second closest power electronics autotransformer 211 and so on until the current ii,_q,_ref adds up to N times is,_q .

In a second control subsystem 407, the average active power P_s,atc of the three-phase transformer 202 is calculated from the input supply current i_s,_abc and voltage v_s,abc using a low-pass filter (LPF) 408 to remove the effect of the negative sequence and harmonics. If the load of three-phase transformer 202 is balanced, i.e. the current is free from inverse sequence, the following equation is valid for the average active power P_s, _abc ^'.

The reference current for the phase c can be calculated from P_s,_abc taking into account that, when the current is free from inverse sequence and the reactive power is completely compensated for, the current is in phase with the correspondent voltage and proportional to it. The reference current i_s,c,ref is then calculated according to the following equation:

The reference current _,ref for converter 305 of the power electronics autotransformer 211 is calculated from i_s,c,ref using the amplification factor N ln, being N the turns ratio of the output transformer 306 (if present). If the output transformer 306 is not present, the value for N will of course be 1. With this choice, the current i_s,c,ref will be supplied in equal parts by all the power electronics autotransformers 211 connected to the section fed by the three-phase transformer 202. Alternatively, the current A_{c, re/}can be supplied by the closest power electronics autotransformer 211 closest to the three-phase transformer 202 up to their current limit and then by the second closest one up to the value of N times i_s,c,ref.

If there is any imbalance, this is detected by the PI 404, resulting in the current i ,_q,_ref and the voltage of the converter 303 being adjusted to reduce or eliminate the imbalance.

Figure 5 is a block diagram of first and second control subsystems 501, 502 for control of the first converter 303. The magnitude V , phase angle, Oi, and angular frequency, coi, of the space vector of vi, are calculated by a PLL 503. The quadrature signals of the voltage vi and current zi are calculated by respective all pass filter (APF) blocks 504, 505. The original signals and the respective quadrature signals are then used to calculate the dq components using ab to dq transformation blocks 506, 507.

The dc-link voltage is controlled with a PI voltage regulator 508 by comparing the de link voltage reference V_{d ,ref} with the actual dc-link voltage. The output of the voltage regulator 508 is the reference current h,d,ref. The dq components of the current i _d and i _q are regulated using PI controllers 509, 510. The outputs are then decoupled using the decoupling network 511 in the second control subsystem 502 to obtain the reference voltage components vi _d,mv and

The decoupling network 511 uses the angular frequency coi and the value of the leakage inductance Lf of the transformer 302. The voltage components vi _d,mv and vi are then converted into the reference voltage vi_,,„_v with a dq to ab transformation block 512. The reference voltage is used by a pulse- width modulation block (PWM) 513 to generate switching pulses for control of the first converter 303.

Figure 6 is a block diagram illustrating control subsystems 601, 602 for control of the second converter 305. The magnitude V _d, phase angle, 0₂, and angular frequency, o>2, of the space vector of V2, are calculated by a PLL 603. The quadrature signals of the voltage V2, current U, and the current U_{, re/} are calculated by respective APF blocks 604, 605, 606. The original signals and the respective quadrature signals are then used to calculate the dq components using ab to dq transformation blocks 607, 608, 609.

The dq current components i _,d and i are regulated using respective PI controllers 610, 611. The outputs are then decoupled using a decoupling network 612 in the second control subsystem 602 shown in Figure 6 to obtain the reference voltage components V _,d,inv and V _,q,inv. The decoupling network 612 uses the angular frequency C02 and the value of the leakage inductance L_f of transformer 306. The voltage components vi _d,mv and V _qjnv are then converted into the reference voltage V2_,;nv with a further dq to ab transformation block 613. The reference voltage is used by a pulse-width modulation block (PWM) 614 to generate switching pulses for control of the second converter 305.

Figure 7 is a schematic illustration of an example power electronics autotransformer 700, which comprises first and second converters 303, 305 and a controller 701. The controller 701 receives the supply voltage v_s,abc and current i_s,_abc of the output of the three phase transformer 202 and provides switching signals 705, 706 to the respective first and second converters 303, 305 to enable the three phase supply to be balanced. Upon detection of any imbalance in the supply current, for example caused by a change in current drawn by an electric train 210 on the line, the controller 703 may adjust the switching signals 705, 706 in accordance with the control scheme as described above.

Figure 8 illustrates schematically operation of the controller 701 in simple terms. In a first step 801, the controller 701 measures a voltage and current of each phase of the three-phase electrical supply. In a second step 702, the controller 701 determines an imbalance between the currents of the three-phase electrical supply. In a third step, the controller 701 provides switching signals to the first and second converters 303, 305 to reduce the imbalance. The process repeats continuously while the controller is operational, so that any imbalance is continually detected and reduced. Referring again to Figure 7, the switching signals 705 provided by the controller 701 to the first converter 303 cause the first converter 303 to regulate the voltage at the intermediate DC link 304 and compensate for the reactive power of the first and second phases input to the autotransformer 700, while the switching signals 706 provided to the second converter 305 cause the second converter 305 to reduce or eliminate any imbalance between the first, second and third phases by transferring active power from the third phase to the first and second phases and compensating for the reactive power of the third phase.

The proposed system has been simulated to demonstrate its technical feasibility. The result of this simulation is shown in Figure 9. Initially, i.e. at time zero, the power electronics autotransformers are switched off and one train is connected to the overhead line, drawing a power of 4.4 MW, corresponding to a current of 176 A rms (249 A peak). As the train is connected across 2 phases only, the current is imbalanced, confirmed by the fact that i_c(t) 903 is zero, while the other two currents 901, 902 for z_a( and z_b( respectively are at 176 A rms.

At time t = 0.1 s, the power electronics autotransformers are activated and the current become balanced after a short transient lasting around 25 ms. The new current for all phases is 103 A (146 A peak), showing that the load power is still 4.4 MW.

At time t = 0.25 s, the train power is changed as a step to 2.2 MW, corresponding to a current of 89 A rms (126 A peak). The system responds within around 25 ms to the change of load power and maintains current balancing.

To show how the system affects the 3-phase power supply, the voltage sequences of the three-phase network are shown in Figure 10, with the same time scale and switching as for Figure 9. When the power electronics autotransformers are switched off, the voltage of the supply clearly show the presence of the negative phase sequence of around 0.6% (around 0.15 kV compared to around 24.75 kV for the direct component). When the system is activated at t = 0.1 s, the negative sequence drops to less than 0.1%. This confirms that the power electronics autotransformers strongly reduce the imbalance of the supply voltage and, hence, can be effectively used to connect an AC electric railway to a three-phase power supply without affecting the balance of the power supply. Other embodiments are intentionally within the scope of the invention as defined by the appended claims.

Claims

1. A method of balancing an electrical power supply of an electric railway system, the method comprising: providing an input AC supply from first and second phases of a three-phase electrical supply to an input of a power electronics autotransformer, the power electronics transformer comprising first and second converters, an intermediate DC link and a controller connected to the first and second converters; converting the input AC supply to an intermediate DC supply with the first converter; converting the intermediate DC supply to an output AC supply with the second converter; and providing first and third phases of the three-phase electrical supply with the output AC supply from an output of the power electronics autotransformer, wherein the output AC supply is phase shifted by 120° relative to the input AC supply.

2. The method of claim 1, wherein the power electronics autotransformer comprises: an input transformer arranged to convert the input AC supply to a transformed AC supply provided to the first converter; and an output transformer arranged to convert an AC output from the second converter to the output AC supply.

3. The method of claim 1 or claim 2, wherein the first and second phases of the three-phase electrical supply are connected to an overhead line and running rails of the electric railway system, and the third phase is connected to a negative feeder of the electric railway system.

4. The method of any preceding claim, comprising: measuring a voltage and current of each phase of the three-phase electrical supply; determining an imbalance between the currents of the three-phase electrical supply; and providing switching signals to the first and second converters to reduce the imbalance.

5. The method of claim 4, comprising: providing switching signals to the first converter to cause the first converter to regulate a voltage at the intermediate DC link and compensate for reactive power of the first and second phases; and providing switching signals to the second converter to cause the second converter to reduce or eliminate any imbalance between the first, second and third phases by transferring active power from the third phase to the first and second phases and compensate for the reactive power of the third phase.

6. A power electronics autotransformer for balancing an electrical power supply of an electric railway system, the power electronics autotransformer comprising: first and second converters; an intermediate DC link; and a controller connected to the first and second converters, wherein: the first converter is arranged to receive an input AC supply from first and second phases of a three-phase electrical supply and convert the input AC supply to an intermediate DC supply at the intermediate DC link; the second converter is arranged to convert the intermediate DC supply at the intermediate DC link to an output AC supply at first and third phases of the three-phase electrical supply; and the controller is configured to provide switching signals to the first and second converters such that the output AC supply is phase shifted by 120° relative to the input AC supply.

7. The power electronics autotransformer of claim 6, comprising: an input transformer arranged to convert the input AC supply to a transformed AC supply provided to the first converter; and an output transformer arranged to convert an AC output from the second converter to the output AC supply.

8. The power electronics autotransformer of claim 6 or claim 7, wherein the controller is configured to: measure a voltage and current of each phase of the three-phase electrical supply; determine an imbalance between the currents of the three-phase electrical supply; and provide switching signals to the first and second converters to reduce the imbalance.

9. The power electronics transformer of any one of claims 6 to 8, wherein the controller is configured to: provide switching signals to the first converter to cause the first converter to regulate a voltage at the intermediate DC link and compensate for reactive power of the first and second phases.

10. The power electronics transformer of any one of claims 6 to 9, wherein the controller is configured to: provide switching signals to the second converter to cause the second converter to reduce or eliminate any imbalance between the first, second and third phases by transferring active power from the third phase to the first and second phases and compensate for the reactive power of the third phase.

11. An electric railway system comprising: an overhead line and running rails for providing electrical power to an electric train; a negative feeder line; a three-phase electrical supply having a first phase connected to the overhead line, a second phase connected to the running rails and a third phase connected to the negative feeder; and a power electronics autotransformer comprising: first and second converters; an intermediate DC link; and a controller connected to the first and second converters, wherein: the first converter is arranged to receive an input AC supply from the first and second phases of the three-phase electrical supply and to convert the input AC supply to an intermediate DC supply at the intermediate DC link; the second converter is arranged to convert the intermediate DC supply at the intermediate DC link to an output AC supply at first and third phases of the three-phase electrical supply; and the controller is configured to provide switching signals to the first and second converters such that the output AC supply is phase shifted by 120° relative to the input AC supply.

12. The electric railway system of claim 10, wherein the power electronics autotransformer comprises: an input transformer arranged to convert the input AC supply to a transformed AC supply provided to the first converter; and an output transformer arranged to convert an AC output from the second converter to the output AC supply.

13. The electric railway system of claim 11 or claim 12, wherein the controller is configured to: measure a voltage and current of each phase of the three-phase electrical supply; determine an imbalance between the currents of the three-phase electrical supply; and provide switching signals to the first and second converters to reduce the imbalance.

14. The electric railway system of any one of claims 11 to 13, wherein the controller is configured to: provide switching signals to the first converter to cause the first converter to regulate a voltage at the intermediate DC link and compensate for reactive power of the first and second phases.

15. The electric railway system of any one of claims 11 to 14, wherein the controller is configured to: provide switching signals to the second converter to cause the second converter to reduce or eliminate any imbalance between the first, second and third phases by transferring active power from the third phase to the first and second phases and compensate for the reactive power of the third phase.