DC power supply device and railway substation incorporating the same

Abstract

The DC power supply device according to the invention comprises a transformer (1) having a primary side (5) and a secondary side (6), a diode rectifier (2) whose input side is connected to the secondary side (6) of the transformer (1), an inverter (3) whose output side is connected to the secondary side (6) of the transformer (1), and a controller (4). The inverter (3) is controlled by the controller (4) and generates reactive power and / or harmonics on the secondary side (6) of the transformer (1) so as to regulate a DC voltage on the output side of the diode rectifier (2) to a target value. The controller (4) receives at an input side at least one DC signal output by the diode rectifier (2) and controls the inverter (3) using the at least one DC signal.

Description

Technical Field

[0001] The present invention relates to a DC power supply device that supplies direct current for traction of electric trains, tramways, etc. Depending on the specific application, the DC power supply device is incorporated into a DC railway substation.

Background Art

[0002] DC power supply devices generally include a rectifier connected to an AC power distribution network via a transformer, and the rectifier is formed from one or more diode bridges. The drawback of such a DC power supply device is that the DC line voltage decreases as the load (e.g., vehicle power) increases due to the presence of series leakage reactance and series resistance. This voltage drop leads to the following problems. · In order to correct the voltage drop, the distance between substations must be restricted · Under high load, the DC catenary voltage may drop excessively, and as a result, the current may increase excessively, which causes operational problems such as heat dissipation, heat generation, or circuit interruption in the power converter installed in the vehicle.

[0003] Thyristor rectifiers are gradually being adopted in new substation systems to address these problems. The DC voltage can be controlled. However, thyristor rectifiers do not have no drawbacks. · Existing traction rectifier units need to be completely replaced · They are more expensive than diode rectifiers · They exhibit a lower power factor

[0004] In addition to the voltage drop problem, both diode and thyristor rectifiers exhibit unidirectional power flow; that is, during braking, energy flowing through the DC line that cannot be captured by other trains must be wasted by large braking resistors. To address this limitation, regenerative inverters based on either insulated-gate bipolar transistors (IGBTs) or thyristors are being incorporated into railway substations. Substations equipped with inverters are called reversible substations. Currently, several power electronics architectures for reversible substations have been found, and these can be broadly classified into three groups. Diode rectifiers related to inverters, as disclosed in the patent documents: Australian Patent Application Publication No. 523146, Chinese Patent No. 10277429, Chinese Patent No. 105226969, Chinese Utility Model Publication No. 204333980, European Patent Application Publication No. 3091631, European Patent No. 2343213, Chinese Patent Application Publication No. 102267405, and Chinese Utility Model Publication No. 202906763. • Thyristor rectifiers related to inverters, as disclosed in the paper "Efficient recovery of braking energy by reversible DC substations" by D. Cornic, presented at the Conference on Electrical Systems for Aircraft, Rail and Marine Propulsion (2010) • IGBT-based bidirectional pulse-width modulation (PWM) converter (see U.S. Patent No. 10554117).

[0005] Damping power is typically 25% to 30% of the driving power. The large power level differences in each power flow direction make the combination of diode rectifiers and pulse-width modulation (PWM) inverters, particularly IGBT-based PWM inverters, a cost-effective choice. Furthermore, these can be retrofitted (i.e., the transformer-rectifier section does not need to be replaced). However, they still have limitations in terms of DC link voltage controllability.

[0006] Several solutions, such as those disclosed in Chinese Patent Application Publication No. 102774294, Chinese Patent Application Publication No. 102267405, and Chinese Utility Model Publication No. 202906763, propose adjusting the DC voltage by active power injection, i.e., by sharing the active power supplied to the DC line between the PWM inverter and the diode rectifier. However, a high inverter power rating is required to mitigate the DC voltage drop over the entire operating range of the rectifier.

[0007] Another solution is proposed in the Chinese Utility Model Publication No. 212323740, which involves compensating for the reactive and harmonic currents drawn by the diode rectifier by supplying reactive and harmonic currents of the same magnitude but in opposite phase. Using this method, the DC voltage on the output side of the rectifier is inevitably stabilized at or near the no-load voltage of the rectifier, i.e., at a voltage considerably higher than the nominal voltage of the rectifier. This significantly increases the required rated power of the inverter and, consequently, the power consumption of the inverter, reducing the overall energy efficiency of the system. Furthermore, this method requires monitoring of reactive currents and harmonics circulating at the AC terminals of the diode rectifier, typically necessitating the addition of current and voltage sensors, thus affecting the complexity and cost of the system. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Australian Patent Application Publication No. 523146 [Patent Document 2] Chinese Patent No. 10277429 Specification [Patent Document 3] Specification of Chinese Patent No. 105226969 [Patent Document 4] Chinese Utility Model Publication No. 204333980 Specification [Patent Document 5] European Patent Application Publication No. 3091631 [Patent Document 6] European Patent No. 2343213 [Patent Document 7] Chinese Patent Application Publication No. 102267405 Specification [Patent Document 8] Chinese Utility Model Publication No. 202906763 Specification [Patent Document 9] U.S. Patent No. 10554117 [Non-patent literature]

[0009] [Non-Patent Document 1] D. Kohnick, "Efficient Recovery of Braking Energy by Reversible DC Substations," Conference on Electrical Systems for Aircraft, Rail and Marine Propulsion, 2010. [Non-Patent Document 2] I. Quesada et al., "Evaluation of the Boundary of the Solution Space of Harmonic Cancellation Techniques," Journal of Electrical Engineering, Vol. 88, No. 1a, pp. 21-25, 2012. [Non-Patent Document 3] L. Limongi et al., "Digital Current Control Systems," IEEE Industrial Electronics Magazine, Vol. 3, No. 1, 2009. [Overview of the Initiative]

[0010] The present invention aims to improve upon the above-mentioned drawbacks, and to achieve this objective, provides a DC power supply device comprising: a transformer having a primary and a secondary side; a diode rectifier with its input side connected to the secondary side of the transformer; an inverter with its output side connected to the secondary side of the transformer; and a controller arranged to control the inverter so that the inverter generates reactive power and / or harmonics on the secondary side of the transformer to adjust the DC voltage on the output side of the diode rectifier to a target value, wherein the controller receives at least one DC signal output by the diode rectifier on its input side and controls the inverter using at least one DC signal.

[0011] Particular embodiments of the present invention are defined in the appended dependent claims.

[0012] The present invention also provides a railway substation including the DC power device defined above.

[0013] Other features and advantages of the present invention will become apparent from the following detailed description which is made with reference to the accompanying drawings.

Brief Description of the Drawings

[0014] [Figure 1] It is a diagram showing a DC power device according to the present invention. [Figure 2] It is a diagram showing different possible embodiments of a transformer, a diode rectifier and an inverter of a DC power device according to the present invention. [Figure 3] It is a diagram showing different possible embodiments of a transformer, a diode rectifier and an inverter of a DC power device according to the present invention. [Figure 4] It is a diagram showing different possible embodiments of a transformer, a diode rectifier and an inverter of a DC power device according to the present invention. [Figure 5] It is a diagram showing different possible embodiments of a transformer, a diode rectifier and an inverter of a DC power device according to the present invention. [Figure 6] It is a diagram showing different possible embodiments of a transformer, a diode rectifier and an inverter of a DC power device according to the present invention. [Figure 7] It is a diagram showing a preferred embodiment of a transformer, a diode rectifier and an inverter of a DC power device according to the present invention. [Figure 8] It is a diagram showing a closed-loop controller of a DC power device according to the present invention, which is arranged to control the inverter so that the inverter generates reactive power to adjust the DC output voltage of the diode rectifier. [Figure 9]This figure shows the DC output voltage versus load current of a DC power supply device in two cases: without DC voltage adjustment according to the present invention, and with DC voltage adjustment according to the first operating example of the present invention. [Figure 10] This figure shows the response of the DC output voltage of a DC power supply device when a train is running, with and without DC voltage regulation according to the present invention. [Figure 11] This figure shows another preferred embodiment of the transformer, diode rectifier, and inverter of the DC power supply device according to the present invention. [Figure 12] This figure shows a closed-loop controller configured to control the inverter shown in Figure 11 so that the inverter generates both reactive power and harmonics to adjust the DC output voltage of the diode rectifier. [Figure 13] This figure shows the simulation results (curve C1) of the DC output voltage of a DC power supply device versus the DC load power of a diode rectifier using the control method disclosed in Chinese Utility Model Publication No. 212323740, and the simulation results of the DC output voltage of a DC power supply device versus the DC load power of a diode rectifier in two cases: with DC voltage adjustment (curve C2) and without DC voltage adjustment (curve C3) according to the second operating example of the present invention. [Figure 14] This figure shows the simulation results (curve C4) of the apparent power of the inverter of a DC power supply device using the control method disclosed in Chinese Utility Model Publication No. 212323740 versus the DC load power of its diode rectifier, as well as the simulation results of the apparent power of the inverter of the DC power supply device according to the present invention versus the DC load power of its diode rectifier in two cases: with DC voltage adjustment (curve C5) and without DC voltage adjustment (curve C6) according to the second operating example of the present invention. [Figure 15] This figure shows the simulation results of the DC output voltage of the DC power supply device according to the present invention versus the DC load power of its diode rectifier in two cases: with DC voltage adjustment (curve C7) and without DC voltage adjustment (curve C8) according to the third operating example of the present invention. [Figure 16]This figure shows the simulation results of the apparent power of the inverter of the DC power supply device according to the present invention versus the DC load power of its diode rectifier in two cases: with DC voltage adjustment (curve C9) and without DC voltage adjustment (curve C10) according to the third operating example of the present invention. [Figure 17] This figure shows some alternative embodiments of a controller for controlling an inverter of a DC power supply device according to the present invention, which uses the DC rectifier output current as the input signal instead of the DC rectifier output voltage. [Figure 18] This figure shows a predetermined relationship between the DC rectifier output current and the reactive power setting point that can be used in the embodiment shown in Figure 17. [Figure 19] This figure shows the predetermined relationship between the DC rectifier output current, AC voltage, and reactive power setpoint that can be used in the embodiment shown in Figure 17. [Figure 20] This figure shows another alternative embodiment of a controller for controlling the inverter of a DC power supply device according to the present invention, which uses both the DC rectifier output current and the DC rectifier output voltage as input signals. [Modes for carrying out the invention]

[0015] Referring to Figure 1, the DC power supply device according to the present invention for a DC railway substation comprises a transformer 1, a diode rectifier 2, an inverter (DC / AC converter) 3, and a controller 4. The primary side 5 of the transformer 1 receives power from an AC distribution network, typically a three-phase AC distribution network, more specifically a three-phase medium-voltage AC distribution network. The secondary side 6 of the transformer 1 supplies AC power to the input side of the diode rectifier 2. The diode rectifier 2 outputs a DC voltage Vrect, which is supplied to the railway tracks for train traction. The inverter 3 receives the DC voltage Vrect at its input side, and its output side is connected to the secondary side 6 of the transformer 1. One or more diodes 7 on the input side of the inverter 3 make the inverter 3 unidirectional. The inverter 3 is preferably a pulse-width modulation (PWM) inverter, such as a PWM inverter based on an IGBT, MOSFET (metal-oxide-semiconductor field-effect transistor), or IGCT (integrated gate-conjugate thyristor). Various inverter topologies can be used, including 2-level inverters and multi-level inverters.

[0016] During the traction phase, the railway tracks are supplied with a DC voltage Vrect obtained from the AC power grid via transformer 1 and diode rectifier 2, and controller 4 controls inverter 3 so that the DC voltage Vrect is adjusted to a target value Vrect*, as will be explained later. During the braking phase, the DC power generated by the braking of the train is converted to AC power by inverter 3 and injected into the AC power grid via transformer 1. This prevents the DC braking power from being dissipated into the DC power grid and allows a significant portion of the traction power to be recovered.

[0017] Various embodiments are possible for the transformer 1, diode rectifier 2, and inverter 3. As shown in Figures 2 to 6, the transformer 1 can be a delta-Y transformer, and the diode rectifier 2 can comprise two diode bridges 8, such as two 6-pulse diode bridges, each connected to each of the secondary windings of the delta-Y transformer. The inverter 3 can include two 3-phase power semiconductor bridges 9, each receiving a DC voltage Vrect through its respective DC filter 10 and connected at its respective output to the two secondary windings of the transformer 1 via its respective AC filter 11 (see Figure 2). Taps can be used on the secondary windings of the transformer 1 to adjust the AC voltage level on the output side of the inverter 3. Instead of using taps, a transformer 12 (see Figure 3) or an automatic transformer 13 (see Figure 4) may be provided between each AC filter 11 and the corresponding secondary winding of the transformer 1. Instead of having two power semiconductor bridges 9, the inverter 3 may include a single three-phase power semiconductor bridge 9 that receives a DC voltage Vrect via a DC filter 10 and, at its power output, is connected to the secondary winding of transformer 1 via a delta-Y transformer 14 and AC filter 11 (Figure 5) or via an auto-transformer 15 and AC filter 11 (Figure 6).

[0018] When one or more transformers 12, 14 are used on the output side of the power semiconductor bridge 9 (Figures 3 and 5), a single diode 7 is sufficient on the input side of each DC filter 10 due to the galvanic isolation provided by the transformers 12, 14. When no transformers are used or one or more automatic transformers 13, 15 are used on the output side of the power semiconductor bridge 9 (Figures 2, 4, and 6), two diodes 7 (one for each polarity + and -) are provided to avoid recirculation.

[0019] DC and AC filters 10 and 11 typically include inductors, and possibly capacitors and resistors. To prevent overmodulation of inverter 3, voltage level adaptation may be required on the AC side. Such adaptation can be performed by a tap incorporated into transformer 12 (Figure 3), automatic transformer 13 (Figure 4), or transformer 1 (Figure 2).

[0020] The architectures shown in Figures 2 to 4 can be adapted to a series connection of rectifier diode bridges commonly found in 3000Vdc railway networks. In this case, the inverter power semiconductor bridge 9 would be connected in series rather than in parallel.

[0021] Figure 7 shows in more detail an inverter 3 having a two-level voltage source converter topology, based on the architecture of Figure 5. In Figure 7, for simplification, protection circuits, contactors, precharge circuits, etc., are omitted. However, the winding leakage inductance at the terminals of transformers 1 and 14 is shown and used to perform the AC filtering duty cycle. To further reduce the switching harmonics of the AC voltage on the output side of inverter 3, additional AC filtering components (inductors, capacitors) can be added in series or parallel with the leakage inductance of transformer 14. Key voltage and current measurements for a control embodiment are shown, including the DC voltage Vrect, the DC link voltage Vdc_link at the input of the three-phase power semiconductor bridge 9 and the output of the DC filter 10, the AC currents ia, ib, and ic at the output of the three-phase power semiconductor bridge 9, the AC voltages va, vb, and vc in one of the secondary windings of transformer 14, and the power semiconductor (here IGBT) switching signals Q1 to Q6.

[0022] Figure 8 shows a typical embodiment of controller 4, which is a closed-loop controller based on a multi-loop control scheme. Controller 4 includes a phase-locked loop 20, abc to dq transformer units 21, a DC link inverter control line 22, and a DC rectifier control line 23.

[0023] The phase-locked loop 20 is adjusted to set the "q" coordinate of the AC voltage vq to 0, and converts the three-phase voltages va, vb, and vc into voltage amplitude Vd, phase angle θ, and angular frequency ω.

[0024] The abc to dq transformer unit 21 uses the phase angle θ to convert the three-phase currents ia, ib, and ic into two components id and iq of the synchronous reference frame (dq frame).

[0025] The function of the DC link inverter control line 22 is known in itself. This function consists of adjusting the DC link voltage Vdc_link so that the power semiconductor bridge 9 receives sufficient voltage for its operation. The DC link inverter control line 22 includes a DC voltage closed-loop controller 24 which has in series a subtractor 25 that subtracts a target value Vdc_link* from the inverter DC link voltage Vdc_link and a proportional-integral (PI) compensator 26 that outputs an active power setpoint P*. The DC link inverter control line 22 further includes an AC current closed-loop controller 27, which comprises in series a scaling unit 28 that converts an active power setpoint P* into a reference current id* using an AC voltage amplitude Vd, a subtractor 29 that subtracts the actual current id output by the abc to dq transformers 21 from the reference current id*, a proportional-integral compensator 30, another subtractor 31 that subtracts voltages ω, L, and iq from the output voltage of the proportional-integral compensator 30 (where L is the equivalent AC inductance as seen from the output side of the inverter 3), and an adder 32 that adds the output signal of the subtractor 31 to the voltage amplitude Vd sent via the scaling unit 42 to output a voltage command Vid for the inverter 3.

[0026] The DC rectifier control line 23 includes a DC voltage closed-loop controller 33, which has in series a subtractor 34 that subtracts the rectifier DC voltage Vrect from a target value Vrect* and a proportional-integral (PI) compensator 35 that outputs a reactive power setpoint Q*. The DC rectifier control line 23 further includes an AC current closed-loop controller 36, which has in series a scaling unit 37 that converts the reactive power setpoint Q* to a reference current iq* using the AC voltage amplitude Vd, a subtractor 38 that subtracts the actual current iq output by the abc to dq conversion unit 21 from the reference current iq*, a proportional-integral compensator 39, and an adder 40 that adds the voltage output by the proportional-integral compensator 39 to voltages ω, L, and id (where L is the equivalent AC inductance as seen from the output side of the inverter 3) and outputs a voltage command Viq for the inverter 3.

[0027] Voltage commands Vid and Viq, along with the phase angle θ, are input to the pulse width modulator 41. The pulse width modulator 41 outputs switching signals Q1 to Q6 for controlling the power semiconductor bridge 9 of the inverter 3. The pulse width modulator 41 can implement various known modulation schemes such as spatial vector modulation, sinusoidal modulation, a type of discontinuous pulse width modulation, or others.

[0028] When the train is running, inverter 3 cannot supply power to the DC network due to the presence of blocking diode 7. Since the inverter DC link voltage Vdc_link is definitely higher than the diode rectifier DC voltage Vrect (i.e., Vdc_link*>Vrect*), diode 7 is in the blocked state. Compensator 26 drives the active power setpoint P* to a small value that can compensate for the power loss of inverter 3.

[0029] When the train is braking, the active power setpoint P* is set to the total regenerative power generated from the DC network to the AC network. In this scenario, diode 7 begins to conduct, and Vrect ≈ Vdc_link.

[0030] Regarding DC rectifier control, when the train is running, the compensator 35 drives the reactive power setpoint Q* so that Vrect is adjusted to the target value Vrect*. When the train is braking, Vrect increases beyond a certain limit of voltage control, and Q* is set to zero.

[0031] The AC current closed-loop controllers 27 and 36 allow adjustment of the power factor of the AC current at the output of the inverter 3. While the AC current closed-loop controllers 27 and 36 are useful in both operating modes (train running and train braking), a control embodiment without such an internal AC current closed-loop is also possible.

[0032] Figure 9 shows the rectifier DC voltage Vrect as a function of the load current (i.e., the DC current consumed on the output side). Figure 9 shows the rectifier DC voltage Vrect as a function of the load current (DC current consumed on the output side of the diode rectifier 2 by the DC network, 100% load current corresponding to the nominal DC current) for the case without DC voltage adjustment according to the present invention (inverter 3 is disabled; continuous line in Figure 9) and with DC voltage adjustment according to the present invention (inverter 3 and DC rectifier control line 23 are enabled; dashed line in Figure 9). As shown in the figure, without DC voltage adjustment, the voltage decreases as the load current increases, causing the current to increase relative to a given power value, resulting in heat dissipation, heat generation, and the risk of circuit damage. When inverter 3 is enabled, the DC rectifier control line 23 causes inverter 3 to generate reactive power on the secondary side of transformer 1 and adjust the DC voltage Vrect to a target value, which may be constant or vary according to the load current. In the diagram, the DC voltage Vrect begins to adjust at 10% load current and gradually decreases to 750V, the nominal voltage of diode rectifier 2 and the DC network, at 50% load current. Above 50%, the DC voltage remains adjusted to 750V. The inverter's DC link voltage Vdc_link is then adjusted via the active power setpoint P*.

[0033] Therefore, the present invention has the advantage of allowing the use of an inexpensive, reliable, and long-life rectifier, i.e., a diode rectifier 2, without causing the DC voltage drop that diode rectifiers typically exhibit when combined with a transformer.

[0034] Compared to existing solutions for DC voltage regulation based on active power generation, the advantage of the present invention is that the rated power required for inverter 3 to achieve rectifier voltage regulation is reduced. In fact, reactive power is usually not useful in DC power supply devices. The present invention uses the reactive power circulating in the AC section of the device for the purpose of regulating the rectified DC voltage.

[0035] Furthermore, the unidirectional nature of inverter 3 allows it to be sized solely for regenerative power rather than total power, thus enabling it to have smaller dimensions. Inverters typically require a rated power in the range of 25% to 30% of the rectifier power for the purpose of recovering braking energy. In this invention, inverter 3 can be sized solely for regenerative power, and there is no need to increase the rated power of the inverter for voltage regulation purposes.

[0036] The simulation results are shown in Figure 10. The nominal voltage of diode rectifier 2 and the DC network is 750V, and the nominal power of diode rectifier 2 is 3MW. The railway power is gradually stepped from 50% to 300% (100% load corresponding to nominal power). When inverter 3 is inactive, the rectifier voltage drops from 765V to 665V. When the reactive power controller is activated and inverter 3 is enabled, the rectifier voltage is adjusted to 750V under all power levels. Up to approximately 100% load power, inverter 3 generates some induced power (negative reactive power). When the load power increases beyond 100%, inverter 3 generates capacitive power (positive reactive power). The higher the level of rectifier voltage compensation, the higher the required reactive power.

[0037] It will be apparent to those skilled in the art that many modifications are possible in the embodiment shown in Figure 8. For example, in the DC voltage closed-loop controllers 24, 33 of the DC link inverter control line 22 and / or DC rectifier control line 23, components based on PID (proportional-integral-derivative) compensators, model predictive control, etc., can be used instead of proportional-integral compensators. The AC current closed-loop controllers 27, 36 can use control loop solutions other than those disclosed above, such as steady-state frame control or hysteresis control. Furthermore, the reactive power setpoint range can be restricted to positive values ​​only, in which case the dashed line in Figure 9 will follow a continuous line for load currents between 0 and 100%, and will only perform a boost of the rectifier DC voltage Vrect.

[0038] To further reduce the amount of current required from inverter 3 to adjust Vrect, the adjustment function provided by the present invention can be complemented by a harmonic compensation function. In this case, the rectifier DC voltage Vrect can be increased by compensating (partially or entirely) for the current harmonics drawn out by diode rectifier 2, which, like reactive power, generate a voltage drop to transformer 1. The required compensation level can be commanded by a DC voltage closed-loop controller, similar to controllers 24 and 33, or it can be set to maximum compensation. Harmonic compensation can be implemented using existing technologies applicable to active power filtering, such as a selective harmonic cancellation method disclosed in the paper "Evaluation of the Solution Space Boundary of Harmonic Cancellation Techniques" by I. Quesada et al., published on pages 21-25 of Vol. 88, No. 1a of the Journal of Electrical Engineering (2012), or a modulation scheme based on a current control scheme using a harmonic compensation network disclosed in the paper "Digital Current-Control Schemes" by L. Limongi et al., published in Vol. 3, No. 1 of the IEEE Journal of Industrial Electronics (2009).

[0039] An example of an embodiment combining harmonic compensation and rectifier output voltage control is described below. Figure 11 shows a detailed embodiment of the architecture using the AC interconnection concept shown in Figure 3. This includes a transformer 1, a diode rectifier 2 with two 6-pulse diode bridges 81 and 82, and an inverter 3 with two IGBT inverter bridges 91 and 92. The IGBT inverter bridges 91 and 92 are interconnected with the diode bridges 81 and 82 via transformers 121 and 122, respectively. In this example, current measurement signals ia, b, and c and the IGBT switching command signal Q for each inverter bridge 91 and 92 are used. 1-6 These are controlled separately. Signals with sub-index "1" correspond to inverter bridge 91, and signals with sub-index "2" correspond to inverter bridge 92. The input currents to diode rectifier bridges 81 and 82 are denoted as ia1_rect, ib1_rect, ic1_rect, ia2_rect, ib2_rect, and ic2_rect, and the DC output current of diode rectifier 2 is denoted as idc_rect.

[0040] Figure 12 shows an embodiment of controller 4 based on current control with harmonic compensation combined with a rectifier output voltage control method. Controller 4 is a closed-loop controller and includes units 20, 25, 26, 28, 34, 35, and 37 similar to the corresponding units in Figure 8. The input rectifier currents ia1_rect, ib1_rect, and ic1_rect at the first diode bridge 81 are measured and input to the harmonic compensation reference calculation unit 431. This unit 431 incorporates abc-to-dq frame transformation and filtering functions. Unit 431 outputs two reference signals id1_rect_h* and iq1_rect_h*, which include the harmonics of the diode bridge 81 to be compensated by controller 4, in a sign-matched state (the fundamental wave component is filtered). The three-phase currents ia1, ib1, and ic1 of the first inverter bridge 91 are converted into two components id1 and iq1 in a synchronous reference frame by the abc-to-dq transformer 211.

[0041] In the adder / subtractor 291, signal id1 is subtracted from the sum of the reference signal id1_rect_h* and the reference signal id* from the scaling unit 28. A dedicated harmonic compensator 441 is typically added in parallel to a proportional-integral compensator 301 similar to compensator 30 in Figure 8 to enhance the compensation capability of the current controller 4 at the target harmonic frequencies. Signal ω.L.iq1 is subtracted in the adder / subtractor 311 from the sum of the outputs of units 441 and 301, and the result is fed to adder 321, which adds it to the output of the scaling unit 421, which receives the voltage Vd output by the phase-locked loop 20. Adder 321 outputs a voltage command Vid1.

[0042] In the adder / subtractor 381, signal iq1 is subtracted from the sum of the reference signal iq1_rect_h* and the reference signal iq* from the scaling unit 37. A dedicated harmonic compensator 451 is typically added in parallel to a proportional-integral compensator 391 similar to the compensator 39 in Figure 8 to enhance the compensation capability of the current controller 4 at the target harmonic frequencies. Signal ω.L.id1 and the outputs of units 451 and 391 are added by the adder 401 to generate another voltage command Viq1. Voltage commands Vid1 and Viq1 are fed to the pulse width modulator 411 along with the phase angle θ output by the phase-locked loop 20. The pulse width modulator 411 outputs switching signals Q11, Q21, Q31, Q41, Q51, and Q61 to control the first inverter bridge 91.

[0043] The same units as 211, 431, 291, 301, 441, 311, 321, 421, 381, 391, 401, and 451 are provided for the three-phase currents ia2, ib2, and ic2 of the second inverter bridge 92 and the input rectifier currents ia2_rect, ib2_rect, and ic2_rect of the second diode bridge 82, and for simplicity, they are shown as a single block 462. In this way, voltage commands Vid2 and Viq2 are generated and supplied to the pulse width modulator 412 along with the phase angle θ output by the phase-locked loop 20. The pulse width modulator 412 outputs switching signals Q12, Q22, Q32, Q42, Q52, and Q62 to control the second inverter bridge 92.

[0044] The harmonic compensators 441, 451 (and those in block 462) may be based on resonant compensators resonating at 6·ffund (which compensates for the negative sequence of the fifth harmonic and the positive sequence of the seventh harmonic), and possibly other frequencies such as 12·ffund, where ffund is the fundamental frequency. As a result, the inverter voltage commands Vid and Viq are modulated so that the inverter AC current partially compensates for the rectifier AC current harmonics. Other embodiments for harmonic compensation based on multiple synchronous reference frames and other schemes are also possible (see the paper "Digital Current Control Schemes" by L. Limongi et al., published in IEEE Journal of Industrial Electronics (2009), Vol. 3, No. 1).

[0045] Instead of adding to control lines 22 and 23 as shown in the embodiment of Figure 12, harmonic compensation may be replaced with DC rectifier control line 23, i.e., a control line similar to control line 23 in Figure 8 may be used, but to generate harmonics instead of reactive power, and the harmonics are generated for the purpose of adjusting the rectifier DC voltage Vrect.

[0046] As is evident, in the embodiments of the present invention described above, the controller 4 receives a DC voltage Vrect as input and uses this voltage as a feedback signal to control the inverter 3 to adjust the DC voltage Vrect to a target value Vrect*. The target value Vrect* may be constant or may vary as a function of the load current. In the latter case, a conventional DC current sensor located on the output side of the diode rectifier 2 can provide DC load current data to the controller 4, enabling the controller 4 to change the target value Vrect* according to a predetermined rule.

[0047] By using a DC voltage as the feedback signal input to controller 4, the present invention has the advantage of not requiring an expensive detection scheme, including voltage and current sensors for measuring the reactive power and harmonics at the input (AC side) of the diode rectifier in order to compensate for the reactive power and harmonics, with respect to the device disclosed in Chinese Utility Model Publication No. 212323740.

[0048] Furthermore, as shown by the simulation curve C1 in Figure 13, the DC voltage of the device disclosed in Chinese Utility Model Publication No. 212323740 is maintained near the no-load voltage (approximately 790V in Figure 13) and therefore well above the nominal voltage (750V) because it is regulated over the entire operating range of the diode rectifier by fully compensating for the reactive power and harmonics drawn out by the diode rectifier. As a result, the power consumption by the inverter is high (see Figure 14, curve C4). In the simulation of the device disclosed in Chinese Utility Model Publication No. 212323740, a transformer is incorporated between the DC positive terminals of the rectifier bridge because otherwise the amount of the 5th and 7th harmonics of the AC current circulating between the inverter and the diode rectifier would be excessive.

[0049] On the other hand, in the present invention (see Figure 13, curve C2), the controller 4 can control the inverter 3 as follows: -When the DC voltage Vrect drops from the no-load voltage of the diode rectifier 2 to the nominal voltage of the diode rectifier 2 (750V in this embodiment), the inverter 3 becomes inoperable. -When the DC voltage Vrect reaches the nominal voltage of the diode rectifier 2, the inverter 3 becomes active and adjusts the DC voltage to the nominal voltage over the operating range of the diode rectifier 2, from nominal power (3000kW in the example) to maximum overload (300% of nominal power in the example).

[0050] In this way, the power consumed by the inverter is significantly reduced (see Figure 14, curve C5), improving global energy efficiency and equipment lifespan. In particular, the rated power of inverter 3 does not need to be higher than the power required for energy recovery, and inverter 3 only needs to operate when the DC voltage drops below the nominal voltage level.

[0051] Figures 13 and 14 also show, for comparison, the changes in DC voltage Vrect and apparent power of inverter 3 when inverter 3 is disabled (curves C3 and C6).

[0052] In the embodiments shown in Figures 13 and 14, similar to the embodiments shown in Figures 9 and 10, only reactive power is generated by the inverter 3 of the DC power supply device according to the present invention to adjust the DC voltage Vrect. However, unlike the examples shown in Figures 9 and 10, where induced power is generated before 100% load to initiate adjustment, in the examples of Figures 13 and 14, the reactive power is capacitive only and is generated as if from 100% load.

[0053] Figures 15 and 16 show another example of operation of the DC power supply device according to the present invention. In this example (see curve C7), it is assumed that the maximum overload capacity of the diode rectifier is 450%. The DC voltage Vrect is adjusted to the nominal voltage over an operating range from the nominal load power to, for example, 300% load power, and then adjusted to a target value that is below the nominal voltage but remains higher than the unadjusted DC voltage (see curve C8), decreasing as a function of load power. In particular, as shown in the figure, the changing target value can be selected such that the voltage drop compensation remains constant above 300% load power and up to the maximum overload, i.e., 450%, thereby making it possible to clamp the power consumption of inverter 3 (see curve C9), and inverter 3 does not need to be oversized solely for the purpose of adjusting the DC voltage.

[0054] Adjusting the DC voltage Vrect to the nominal voltage of the diode rectifier 2 and starting the adjustment at the nominal voltage is preferable for reasons of simplification and energy efficiency. However, in alternative embodiments, the adjustment may be started at a predetermined value of DC voltage Vrect that is lower than the no-load voltage of the diode rectifier 2 but different from (higher or lower than) the nominal voltage.

[0055] In a general embodiment, the present invention allows adjustment to begin when the DC voltage Vrect, which drops from the no-load voltage of the diode rectifier 2, reaches a predetermined voltage (equal to or different from the nominal voltage), and the DC voltage Vrect is adjusted to a constant or variable target value that is equal to or below the predetermined voltage. The difference between the no-load voltage and the predetermined voltage is typically at least 25%, preferably at least 50%, and more preferably at least 75%, of the difference between the no-load voltage and the nominal voltage of the diode rectifier 2. The difference between the no-load voltage and the predetermined voltage is typically at most 125%, preferably at most 110%, of the difference between the no-load voltage and the nominal voltage of the diode rectifier 2.

[0056] In all embodiments described above, the controller 4 controls the inverter 3 using a DC voltage Vrect as a feedback signal. However, in other embodiments, the controller 4 can control the inverter 3 using the DC current output by the diode rectifier 2 instead of, or in addition to, the DC voltage Vrect. Figure 17 shows such an embodiment, in which the DC voltage closed-loop controller 33 in Figure 8 or the corresponding DC voltage closed-loop controller in Figure 12 is replaced by a lookup table or analysis function 47 that modulates the reactive power setpoint Q* as a function of the DC rectifier output current idc_rect, using a predetermined relationship between the DC rectifier output current and the reactive power setpoint. This input-output relationship is calculated to compensate for the DC-side voltage drop caused by an increase in the DC rectifier output current, and the AC voltage Vd can also be an input. Figures 18 and 19 show two embodiments of the input-output relationship based on a simple linear method. The example in Figure 18 uses only the DC rectifier output current as input. The example in Figure 19 uses both the DC rectifier output current and the AC voltage as inputs. Adjusting the DC voltage using the DC rectifier output current instead of the DC voltage itself results in lower accuracy because the relationship between reactive power injection, AC voltage, DC voltage, and DC current is model-based; that is, there is no closed-loop control operation to compensate for the difference between the target DC voltage level and the measured DC voltage level. However, such control also has the following significant advantages: • Response time is basically defined by the inverter's internal current loop and has a large control bandwidth. • Since there is no need to adjust the external control loop, potential stability issues can be avoided. • DC rectifier does not require a voltage sensor.

[0057] Both techniques can also be used in combination, with DC voltage closed-loop control performing fine-tuning of the reactive power setpoint and precisely controlling the voltage to a target level, while DC current measurement can speed up the system's response time. Figure 20 shows an embodiment that integrates both methods.

Claims

1. A transformer (1) having a primary side (5) and a secondary side (6), A diode rectifier (2) whose input side is connected to the secondary side (6) of the transformer (1), An inverter (3) whose output side is connected to the secondary side (6) of the transformer (1), The system includes a controller (4) configured to control the inverter (3) so that it generates reactive power on the secondary side (6) of the transformer (1) in order to adjust the DC voltage on the output side of the diode rectifier (2) to a target value, The controller (4) receives at least one DC signal output by the diode rectifier (2) on its input side, and uses the at least one DC signal to control the inverter (3). The controller (4) is configured to control the inverter (3) such that the inverter (3) becomes inactive when the DC voltage drops from the no-load voltage of the diode rectifier (2) to a predetermined voltage, and becomes active when the DC voltage reaches the predetermined voltage and adjusts the DC voltage to the target value. A DC power supply device characterized by the following features.

2. A transformer (1) having a primary side (5) and a secondary side (6), A diode rectifier (2) whose input side is connected to the secondary side (6) of the transformer (1), An inverter (3) whose output side is connected to the secondary side (6) of the transformer (1), The system includes a controller (4) configured to control the inverter (3) so that it generates harmonics on the secondary side (6) of the transformer (1) in order to adjust the DC voltage on the output side of the diode rectifier (2) to a target value, The controller (4) receives at least one DC signal output by the diode rectifier (2) on its input side, and uses the at least one DC signal to control the inverter (3). The controller (4) is configured to control the inverter (3) such that the inverter (3) becomes inactive when the DC voltage drops from the no-load voltage of the diode rectifier (2) to a predetermined voltage, and becomes active when the DC voltage reaches the predetermined voltage and adjusts the DC voltage to the target value. A DC power supply device characterized by the following features.

3. The target value is equal to or lower than the predetermined voltage. The DC power supply device according to claim 1 or 2.

4. The target value changes as a function of the DC current output by the diode rectifier (2). The DC power supply device according to claim 3.

5. The difference between the no-load voltage and the predetermined voltage is at least 25%, preferably at least 50%, and more preferably at least 75%, of the difference between the no-load voltage and the nominal voltage of the diode rectifier (2). A DC power supply device according to any one of claims 3 to 4.

6. The difference between the no-load voltage and the predetermined voltage is a maximum of 125%, preferably a maximum of 110%, of the difference between the no-load voltage and the nominal voltage of the diode rectifier (2). A DC power supply device according to any one of claims 3 to 5.

7. The predetermined voltage is substantially equal to the nominal voltage of the diode rectifier (2). A DC power supply device according to any one of claims 3 to 6.

8. The secondary side (6) of the transformer (1) has at least two secondary windings connected to the diode rectifier (2) and the inverter (3), respectively. A DC power supply device according to any one of claims 1 to 7.

9. The transformer (1) is a delta-Y transformer. A DC power supply device according to any one of claims 1 to 8.

10. The diode rectifier (2) includes at least one six-pulse diode bridge, A DC power supply device according to any one of claims 1 to 9.

11. The diode rectifier (2) includes at least two diode bridges (8), A DC power supply device according to any one of claims 1 to 10.

12. The inverter (3) includes, for example, at least one power semiconductor bridge (9) based on an IGBT, MOSFET, or IGCT. A DC power supply device according to any one of claims 1 to 11.

13. The controller (4) is configured to pulse-width modulate the inverter (3). A DC power supply device according to any one of claims 1 to 12.

14. The inverter (3) is unidirectional. A DC power supply device according to any one of claims 1 to 13.

15. The controller (4) includes an AC current closed-loop controller (36) driven by a DC voltage closed-loop controller (33). A DC power supply device according to any one of claims 1 to 14.

16. The controller (4) is configured to control the inverter (3) so that it generates reactive power and harmonics on the secondary side (6) of the transformer (1) so that it adjusts the DC voltage on the output side of the diode rectifier (2) to the target value. A DC power supply device according to any one of claims 1 and 3 to 15.

17. The at least one DC signal includes the DC voltage, A DC power supply device according to any one of claims 1 to 16.

18. The controller (4) includes a DC voltage closed-loop controller (33) that controls the inverter (3) based on the DC voltage. The DC power supply device according to claim 17.

19. The at least one DC signal includes a DC current output by the diode rectifier (2). A DC power supply device according to any one of claims 1 to 18.

20. The controller (4) controls the inverter (3) based on the DC current using a loop-up table or an analysis function (47). The DC power supply device according to claim 19.

21. A railway substation comprising the DC power supply device according to any one of claims 1 to 20.

22. The inverter (3) is configured to adjust the DC voltage at the output side of the diode rectifier (2) under the control of the closed-loop controller (4) when the vehicle is in traction, and to recover DC power when the vehicle is braking in order to inject it into the AC power distribution network. A railway substation according to claim 21.

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