Energy router for friendly access of electrified railway to microgrid and control method

Abstract

The invention discloses an energy router for friendly access of an electrified railway to a micro-grid, the energy router is mounted at the joint of an alternating-current power grid and a direct-current micro-grid and comprises a midpoint transformer, a tuned filter and a half-bridge converter, the half-bridge converter is composed of a plurality of half-bridge sub-modules (HB-SMs), one end of the half-bridge converter is connected with a DC power grid (MVDC) through the tuned filter, and the other end of the half-bridge converter is connected with the DC micro-grid through the tuned filter. The other end of the middle-point transformer is connected with an AC power grid (MVAC) containing a railway load through the middle-point transformer; one end of the tuned filter is connected with a DC power grid, and the other end is connected with the half-bridge converter to filter DC ripples; one side of the midpoint transformer is connected with the half-bridge converter, meanwhile, the midpoint of the midpoint transformer is connected with the DC power grid, the other side of the midpoint transformer is connected with the AC power grid containing the railway load, and the electric isolation effect is achieved. And the number of converters is reduced through the topological structure and the corresponding control method, so that the capacity demand of the converter is reduced, and the investment cost is reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of electrified railway traction power supply, and in particular to a control method for an energy router for friendly access of an electrified railway to a microgrid. Background Art

[0002] This invention is applicable to the commonly used AC / DC dual-current traction power supply system for rail transit. The electrified railway traction system is vital to a country's economic growth. At present, single-phase MVAC traction power supplies dominate, in which electricity is converted from a three-phase MVAC / HVAC grid to a single-phase MVAC power supply of 25kV, 50Hz. In recent years, the number of locomotives has continued to increase, and the frequency of operation has gradually increased. With this, there are challenges in the power quality of the traction power supply system, such as voltage imbalance on the AC side, demand for reactive power, and the presence of negative sequence components. Especially with the increasing popularity of capacity sharing functions in electric railways, these challenges pose significant risks to the stability and reliability of the power grid system.

[0003] With the development of renewable energy generation and the high penetration rate of energy storage units, MVDC has been booming due to its advantages of easy integration, low transmission loss, and convenient access to renewable energy and energy storage. Connecting MVDC with MVAC can, on the one hand, realize the energy exchange between the two power supply systems and facilitate unified power flow management. On the other hand, it can solve the related problems of rail traction power supply in MVAC, such as power quality and neutral section problems, through the regulation of MVDC side devices.

[0004] In 2003, R. Mraquardt et al. proposed the concept of modular multilevel converter (MMC), which made a breakthrough in the interconnection of AC and DC systems. For the interconnection of MVDC and MVAC systems containing electric railways, the traditional MMC based on single-phase half-bridge submodules (HB-MMC) can be used. However, the traditional HB-MMC has four arms, which not only increases the number of devices, but is also susceptible to DC and AC faults. In the event of a fault, due to the large fault current, excessive stress may be caused on the equipment, thereby damaging the equipment. Therefore, the proposal of an energy router with low capacity demand is of great significance to the improvement of railway traction power supply and power quality of the power grid. Summary of the invention

[0005] To solve the above problems, the present invention proposes an energy router for electrified railway friendly access to microgrid, whose topology reduces the number of converters, reduces the converter capacity requirement, reduces the investment cost, and has fault interception capability.

[0006] An energy router for friendly access to a microgrid for electrified railways, comprising: a midpoint transformer (1), a tuned filter (2), and a half-bridge submodule (HB-SMs) (3); an MVDC voltage (V DC )(4) connected to the MVAC single-phase network (V ac )(5), the midpoint transformer (1) has two primary windings and one secondary winding. The primary windings consist of the same number of turns and are wound in a uniform direction around the transformer core. The primary windings are connected to the MVDC grid (4) through two chain links (C-Ls) (6) and a tuned filter (parallel LC filter) (2). Each CL (6) contains a number of cascade-connected HB-SMs (3). Each HB-SMs contains two IGBTs (7) with anti-parallel diodes and an SM capacitor (8). The cascade connection of HB-SMs (3) is responsible for the waveform formation in each CL (6), thereby generating the required voltage waveform. A sorting algorithm is used to regulate and balance the voltages of each capacitor.

[0007] The midpoint of the primary side of the transformer (1) is connected to the negative pole of the MVDC (4), and the positive pole of the MVDC (4) is connected to the C-Ls (6) through the tuned filter (2). The tuned filter (2) is connected between the MVDC (4) grid and the C-Ls (6), so that no basic AC elements can flow into the MVDC grid.

[0008] The energy router for electrified railway friendly access to microgrid is characterized by reducing the connection of arms or chain links.

[0009] The energy router for electrified railway friendly access to microgrid is characterized by adopting a midpoint transformer (1) for electrical isolation, and the midpoint transformer adopts appropriate modulation technology to eliminate DC magnetic flux in the transformer.

[0010] The energy router for electrified railway friendly access to microgrid is characterized in that the inductance of the arm-type inductor can be combined with the leakage inductance of the midpoint transformer (1), thereby eliminating the need for an additional inductor.

[0011] The energy router for the electrified railway friendly access to the microgrid is characterized in that, in order to achieve the required voltage handling capability and facilitate multi-level operation, the CL (6) of the converter is composed of N HB-SMs connected in series. CL (6) acts as a controllable voltage source and includes DC and AC components. The AC component of the CL is used to transfer the power of the converter to the single-phase MVAC traction power supply, while the DC component in the CL is used to establish energy balance and recover energy from the MVDC. Each CL current must be set to the full peak value of the AC current plus half of the DC current, expressed as:

[0012]

[0013] The control method of the energy router for the friendly access of an electrified railway to a microgrid is characterized in that it has a fault blocking capability for different short-circuit faults.

[0014] A: DC pole-to-pole fault on the MVDC side;

[0015] When a pole-to-pole fault occurs on the DC side of the MVDC (4), all IGBTs (7) are blocked. The parallel LC filter (2) blocks the AC current entering the fault location. After the IGBT (7) is blocked, the possible current paths are the anti-parallel diode D2 of the left chain ring and the anti-parallel diode D1 of the right chain ring and the SM capacitor Cc (8). At this time, the equivalent SMs capacitor voltage is 2V DC , which is higher than the primary winding voltage. Therefore, the right CL diode D2 and the left CL diode D1 become reverse biased, no current flows in the CL (6) of the converter, and the converter is protected and the fault is blocked.

[0016] B: AC line-to-line fault on the MVAC side;

[0017] In case of AC line-to-line fault, the converter is blocked by removing the gate pulses of the IGBT (7). The parallel LC filter (2) acts as a short circuit for the DC component. The possible current path is from the MVDC (4) side through the upper anti-parallel diodes D1 and Cc (8) of the two SMs of the CL to the fault location. But the equivalent SM capacitor voltage of the CL is 2V DC , higher than V DC Therefore, D1 gets reverse biased and no current flows through CL(6) of the converter, and the converter is protected against AC line-to-line short-circuit fault.

[0018] The energy router for the electrified railway friendly access to the microgrid is characterized in that a pre-charging scheme for the SM capacitor is proposed, comprising the following steps

[0019] Step 1: Divide the total number of SMs in C-Ls (6) equally into two groups (Group A and Group B). Assuming there are N SMs in CL, then N / 2 SMs form Group A, and the other N / 2 SMs form Group B.

[0020] Step 2: In the initial stage, the AC circuit breaker remains open. Therefore, the current can flow from the MVDC (4) side through the pre-charging resistor and CLs and return through the negative terminal of the MVDC grid. When both IGBTs are blocked (i.e., S1 and S2 are both turned off), the DC current flowing through CL is positive, and the SM capacitor can be charged. This is called the charging stage. In the bypass, S1 is turned off, S2 is turned on, and the bypass SM capacitor is charged.

[0021] Step 3: The S2 switches of the SMs of group A and group B are given high-frequency square wave PWM technology (duty cycle is 50%) in a complementary manner (ie, if S2 of group B is in the on state, then S2 of group A is in the off state).

[0022] Step 4: During half a cycle of the PWM switching frequency, N / 2 SMs are charged to V DC Similarly, in the next half cycle, the other N / 2 SMs are charged to V DC .

[0023] Step 5: The equivalent SM capacitor voltage of CL is 2V DC At this point, the pre-charging of the SMs is completed and the converter can switch to normal operation by bypassing the pre-charging resistors using a circuit breaker on the MVDC side.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] 1) The present invention proposes an energy router for electrified railways to be friendly connected to a microgrid, whose topology has twice the arm voltage amplitude and higher AC output voltage than the traditional HB-MMC, which meets the system power supply requirements and reduces power supply losses.

[0026] 2) The energy router for the electrified railway friendly access to the microgrid proposed in the present invention has only two arms / CL arms, and the total number of SMs and the number of switches are 4n and n respectively. The level formed on the AC side of the converter is 4n+1, which means that the primary voltage of the transformer is highly sinusoidal, which reduces the harmonic content of the AC side voltage and improves the power quality of the system.

[0027] 3) The topological structure of the energy router for the electrified railway friendly access to the microgrid proposed in the present invention has a fault isolation function, which can protect the equipment from the influence of AC and DC short circuit faults, which is impossible for the traditional HB-MMC.

[0028] 4) The energy router for the electric railway friendly access to the microgrid proposed in the present invention has a CL current of Same as the traditional MMC, the peak value of the AC voltage is V DCTherefore, the turns ratio of the midpoint transformer is 0.5:0.5:1, while the turns ratio of the conventional MMC is 1:2. However, in both CL-based MMCs, since both AC and DC currents flow through the transformer, there is a possibility that the transformer will be saturated. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 A schematic diagram of the topological structure of an energy router for friendly access to a microgrid for an electrified railway in the present invention; in the figure, 1 is a midpoint transformer; 2 is a tuned filter (parallel LC filter); 3 is a half-bridge submodule (HB-SMs); 4 is a MVDC grid; 5 is a MVAC single-phase grid; 6 is a chain link (C-Ls); 7 is an IGBT with an anti-parallel diode; and 8 is a SM capacitor.

[0030] Figure 2 It is a flow chart of a method for precharging SM capacitors in an energy router for friendly access to a microgrid for an electrified railway in the present invention;

[0031] Figure 3 A block diagram of the overall structure control strategy of a controller of an energy router for friendly access to a microgrid for an electrified railway in the present invention; DETAILED DESCRIPTION

[0032] The present invention proposes an energy router for friendly access to a microgrid for electrified railways, comprising: a midpoint transformer (1), a tuned filter (2), and a half-bridge submodule (HB-SMs) (3); an MVDC voltage (V DC )(4) connected to the MVAC single-phase network (V ac )(5), the midpoint transformer (1) has two primary windings and one secondary winding. The primary windings consist of the same number of turns and are wound in a uniform direction around the transformer core. The primary windings are connected to the MVDC grid (4) through two chain links (C-Ls) (6) and a tuned filter (parallel LC filter) (2). Each CL (6) contains a number of cascade-connected HB-SMs (3). Each HB-SMs contains two IGBTs (7) with anti-parallel diodes and an SM capacitor (8). The cascade connection of HB-SMs (3) is responsible for the waveform formation in each CL (6), thereby generating the required voltage waveform. A sorting algorithm is used to regulate and balance the voltages of each capacitor.

[0033] The midpoint of the primary side of the transformer (1) is connected to the negative pole of the MVDC (4), and the positive pole of the MVDC (4) is connected to the C-Ls (6) through the tuned filter (2). The tuned filter (2) is connected between the MVDC (4) grid and the C-Ls (6), so that no basic AC elements can flow into the MVDC grid.

[0034] Specifically, the energy router for electrified railway friendly access to microgrid is characterized by reducing the connection of arms or chain links.

[0035] Specifically, the energy router for electrified railway friendly access to microgrid is characterized by adopting a midpoint transformer (1) for electrical isolation, and the midpoint transformer adopts appropriate modulation technology to eliminate DC magnetic flux in the transformer.

[0036] Specifically, the energy router for electrified railway friendly access to microgrid is characterized in that the inductance of the arm-type inductor can be combined with the leakage inductance of the midpoint transformer (1), thereby eliminating the need for an additional inductor.

[0037] Specifically, the energy router for the electrified railway friendly access to the microgrid is characterized in that, in order to achieve the required voltage handling capability and facilitate multi-level operation, the CL (6) of the converter is composed of N HB-SMs connected in series. CL (6) acts as a controllable voltage source and includes DC and AC components. The AC component of the CL is used to transfer the power of the converter to the single-phase MVAC traction power supply, while the DC component in the CL is used to establish energy balance and recover energy from the MVDC. Each CL current must be set to the full peak value of the AC current plus half of the DC current, expressed as:

[0038]

[0039] Specifically, the control method of the energy router for the electrified railway friendly access to the microgrid is characterized by having a fault blocking capability for different short-circuit faults.

[0040] A: DC pole-to-pole fault on the MVDC side;

[0041] When a pole-to-pole fault occurs on the DC side of the MVDC (4), all IGBTs (7) are blocked. The parallel LC filter (2) blocks the AC current entering the fault location. After the IGBT (7) is blocked, the possible current paths are the anti-parallel diode D2 of the left chain ring and the anti-parallel diode D1 of the right chain ring and the SM capacitor Cc (8). At this time, the equivalent SMs capacitor voltage is 2V DC , which is higher than the primary winding voltage. Therefore, the right CL diode D2 and the left CL diode D1 become reverse biased, no current flows in the CL (6) of the converter, and the converter is protected and the fault is blocked.

[0042] B: AC line-to-line fault on the MVAC side;

[0043] In case of AC line-to-line fault, the converter is blocked by removing the gate pulses of the IGBT (7). The parallel LC filter (2) acts as a short circuit for the DC component. The possible current path is from the MVDC (4) side through the upper anti-parallel diodes D1 and Cc (8) of the two SMs of the CL to the fault location. But the equivalent SM capacitor voltage of the CL is 2V DC , higher than V DC Therefore, D1 gets reverse biased and no current flows through CL(6) of the converter, and the converter is protected against AC line-to-line short-circuit fault.

[0044] Specifically, the energy router for the electrified railway to be friendly connected to the microgrid is characterized in that a pre-charging scheme for the SM capacitor is proposed, comprising the following steps:

[0045] Step 1: Divide the total number of SMs in C-Ls (6) equally into two groups (Group A and Group B). Assuming there are N SMs in CL, then N / 2 SMs form Group A, and the other N / 2 SMs form Group B.

[0046] Step 2: In the initial stage, the AC circuit breaker remains open. Therefore, the current can flow from the MVDC (4) side through the pre-charging resistor and CLs and return through the negative terminal of the MVDC grid. When both IGBTs are blocked (i.e., S1 and S2 are both turned off), the DC current flowing through CL is positive, and the SM capacitor can be charged. This is called the charging stage. In the bypass, S1 is turned off, S2 is turned on, and the bypass SM capacitor is charged.

[0047] Step 3: The S2 switches of the SMs of group A and group B are given high-frequency square wave PWM technology (duty cycle is 50%) in a complementary manner (ie, if S2 of group B is in the on state, then S2 of group A is in the off state).

[0048] Step 4: During half a cycle of the PWM switching frequency, N / 2 SMs are charged to V DC Similarly, in the next half cycle, the other N / 2 SMs are charged to V DC .

[0049] Step 5: The equivalent SM capacitor voltage of CL is 2V DC At this point, the pre-charging of the SMs is completed and the converter can switch to normal operation by bypassing the pre-charging resistors using a circuit breaker on the MVDC side.

Claims

1. An energy router for friendly access of electrified railway to microgrid, comprising: A half-bridge converter consisting of a midpoint transformer (1), a tuned filter (2) and a plurality of half-bridge sub-modules (HB-SMs) (3); MVDC voltage (V DC )(4) connected to the MVAC single-phase network (V ac )(5), the midpoint transformer (1) has two primary windings and one secondary winding. The primary windings consist of the same number of turns and are wound in a uniform direction around the transformer core. The primary windings are connected to the MVDC grid (4) through two chain links (C-Ls) (6) and a tuned filter (parallel LC filter) (2). Each CL (6) contains multiple cascaded HB-SMs (3). Each HB-SMs contains two IGBTs (7) with anti-parallel diodes and an SM capacitor (8). The cascade connection of HB-SMs (3) is responsible for the waveform formation in each CL (6), thereby generating the required voltage waveform. A sorting algorithm is used to regulate and balance the voltage of each capacitor. The midpoint of the primary side of the midpoint line frequency transformer (1) is connected to the negative pole of the MVDC (4), and the positive pole of the MVDC (4) is connected to the C-Ls (6) through the tuned filter (2). The tuned filter (2) is connected between the MVDC (4) grid and the C-Ls (6), so that no basic AC components can flow into the MVDC grid.

2. The energy router for electrified railway friendly access to microgrid according to claim 1 is characterized in that: Reduces the number of arms or links to connect.

3. The energy router for electrified railway friendly access to microgrid according to claim 1 is characterized in that: A midpoint transformer (1) is used for electrical isolation, and the midpoint transformer adopts appropriate modulation technology to eliminate the DC magnetic flux in the transformer.

4. The energy router for electrified railway friendly access to microgrid according to claim 1 is characterized in that: The inductance of the arm inductor can be combined with the leakage inductance of the midpoint transformer (1), thereby eliminating the need for an additional inductor.

5. The energy router for electrified railway friendly access to microgrid according to claim 1 is characterized in that: To achieve the required voltage handling capability and facilitate multi-level operation, the converter's CL (6) consists of N HB-SMs connected in series. CL (6) acts as a controllable voltage source and contains both DC and AC components. The AC component of the CL is used to transfer the converter power to the single-phase MVAC traction supply, while the DC component in the CL is used to establish energy balance and recover energy from the MVDC. Each CL current must be set to the full peak value of the AC current plus half of the DC current, expressed as:

6. The control method of an energy router for friendly access of an electrified railway to a microgrid according to any one of claims 1 to 5, characterized in that: It has fault blocking capability for different short-circuit faults. A: DC pole-to-pole fault on the MVDC side; When a pole-to-pole fault occurs on the DC side of the MVDC (4), all IGBTs (7) are blocked. The parallel LC filter (2) blocks the AC current entering the fault location. After the IGBT (7) is blocked, the possible current paths are the anti-parallel diode D2 of the left chain ring and the anti-parallel diode D1 of the right chain ring and the SM capacitor Cc (8). At this time, the equivalent SMs capacitor voltage is 2V DC , which is higher than the primary winding voltage. Therefore, the right CL diode D2 and the left CL diode D1 become reverse biased, no current flows in the CL (6) of the converter, and the converter is protected and the fault is blocked. B: AC line-to-line fault on the MVAC side; In case of AC line-to-line fault, the converter is blocked by removing the gate pulses of the IGBT (7). The parallel LC filter (2) acts as a short circuit for the DC component. The possible current path is from the MVDC (4) side through the upper anti-parallel diodes D1 and Cc (8) of the two SMs of the CL to the fault location. But the equivalent SM capacitor voltage of the CL is 2V DC , higher than V DC Therefore, D1 gets reverse biased and no current flows through CL(6) of the converter, and the converter is protected against AC line-to-line short-circuit fault.

7. The energy router for electrified railway friendly access to microgrid according to claim 1, characterized in that: A pre-charging scheme for SM capacitors is proposed, comprising the following steps: Step 1: Divide the total number of SMs in C-Ls (6) equally into two groups (Group A and Group B). Assuming there are N SMs in CL, then N / 2 SMs form Group A, and the other N / 2 SMs form Group B. Step 2: In the initial stage, the AC circuit breaker remains open. Therefore, the current can flow from the MVDC (4) side through the pre-charging resistor and CLs and return through the negative terminal of the MVDC grid. When both IGBTs are blocked (i.e., S1 and S2 are both turned off), the DC current flowing through CL is positive, and the SM capacitor can be charged. This is called the charging stage. In the bypass, S1 is turned off, S2 is turned on, and the bypass SM capacitor is charged. Step 3: The S2 switches of the SMs of group A and group B are given high-frequency square wave PWM technology (duty cycle is 50%) in a complementary manner (ie, if S2 of group B is in the on state, then S2 of group A is in the off state). Step 4: During half a cycle of the PWM switching frequency, N / 2 SMs are charged to V DC . Similarly, in the next half cycle, the other N / 2 SMs are charged to V DC . Step 5: The equivalent SM capacitor voltage of CL is 2V DC At this point, the pre-charging of the SMs is completed and the converter can switch to normal operation by bypassing the pre-charging resistors using a circuit breaker on the MVDC side.

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