Hybrid through-type traction power supply device and its control method

Abstract

A hybrid through-type traction power supply device and a control method thereof, the device comprises a balanced traction transformer, two single-phase multi-tap step-down transformers, and a two-phase-single-phase converter, the two-phase-single-phase converter comprises two single-phase full-bridge input modules and a single-phase full-bridge output module; the balanced traction transformer is connected to a three-phase power grid, the two output ends of the balanced traction transformer are respectively connected to the input ends of different single-phase multi-tap step-down transformers, the output ends of different single-phase multi-tap step-down transformers are respectively connected to the input ends of different single-phase full-bridge input modules, the output ends of the two single-phase full-bridge input modules are connected to the input ends of the single-phase full-bridge output modules in a common DC side manner, and the output end of the single-phase full-bridge output module is connected to the traction contact network. The single-phase full-bridge input module of the present invention shares a common DC side, which can effectively reduce the system transformation cost, and the two-phase power sharing on the input side of the present invention and the DC side voltage stability can solve the negative sequence power quality problem of the public power grid.

Description

Technical Field

[0001] The present invention relates to the technical field of electric power control, and in particular to a hybrid through-type traction power supply device and a control method thereof. Background Art

[0002] At present, most electrified railways use out-of-phase traction power supply, that is, the power system forms mutually orthogonal out-of-phase voltages through balanced traction transformers (three-phase-two-phase) to supply power to traction loads. However, the out-of-phase power supply has serious power quality problems such as negative sequence, reactive power, and harmonics, and the existence of electrical phase separation in the out-of-phase power supply has seriously restricted the further development of electric railways.

[0003] In recent years, with the rapid development of power electronics technology, the through-type traction power supply method based on power electronic devices has provided an opportunity to solve the above problems. Domestic and foreign researchers plan to completely transform the existing out-of-phase traction power supply system and use three-phase-single-phase traction power supply devices to achieve through-type power supply. However, this method requires a large number of power electronic devices and the overall system cost is high. In order to make full use of the existing traction substation equipment, reduce the system transformation cost, and realize through-type traction power supply, some researchers have proposed introducing a two-phase-single-phase converter on the secondary side of the existing balanced traction transformer to achieve through-type power supply, such as Figure 1 , Figure 2 However, the two-phase to single-phase converter in this transformation scheme requires more active and passive devices, and the investment and construction cost of the system transformation is still high.

[0004] From an industry perspective, solving the problems of electrical phase separation and negative-sequence power quality in heterogeneous power supply systems, as well as reducing the cost of reconstruction and construction of traditional heterogeneous traction power supply systems, are of great significance for achieving full connectivity of electrified railways. Summary of the invention

[0005] The present invention provides a hybrid through-type traction power supply device and a control method thereof. The device adopts a single-phase full-bridge input module to share the DC side, which effectively reduces the number of active devices and passive devices, thereby reducing the cost of transforming the traditional traction system into a through-type power supply. The method can effectively achieve two-phase power sharing on the input side and DC side voltage stability, solving the negative sequence power quality problem of the public power grid.

[0006] In order to solve the above technical problems, the present invention adopts the following technical methods: a hybrid through-type traction power supply device includes a balanced traction transformer and a hybrid two-phase-single-phase conversion unit, wherein the hybrid two-phase-single-phase conversion unit includes two single-phase multi-tap step-down transformers and a two-phase-single-phase converter, wherein the two-phase-single-phase converter includes two single-phase full-bridge input modules and one single-phase full-bridge output module;

[0007] The input end of the balanced traction transformer is electrically connected to the three-phase power grid, and the two output ends of the balanced traction transformer are respectively electrically connected to the input ends of different single-phase multi-tap step-down transformers, and the output ends of different single-phase multi-tap step-down transformers are respectively electrically connected to the input ends of different single-phase full-bridge input modules. The output ends of the two single-phase full-bridge input modules are electrically connected to the input end of the single-phase full-bridge output module in a common DC side manner, and the output end of the single-phase full-bridge output module is electrically connected to the traction contact network.

[0008] Furthermore, an input inductor is connected in series to the input end of the single-phase full-bridge input module, and a capacitor and a single-phase full-bridge output module are electrically connected to the output end of the single-phase full-bridge input module in a DC-sharing manner.

[0009] Furthermore, the output end of the single-phase full-bridge output module is electrically connected to the traction contact network after being connected in series with an output inductor.

[0010] Preferably, the single-phase full-bridge input module and the single-phase full-bridge output module are IGBT devices or MOSFET devices or power electronic devices made of SiC or GaN materials.

[0011] As another aspect of the present invention, the control method of the hybrid through-type traction power supply device includes the control of the single-phase full-bridge input module and the control of the single-phase full-bridge output module. The control of the single-phase full-bridge input module is used to stabilize the DC side voltage, realize the power sharing of the single-phase full-bridge input module, and provide active support for the single-phase full-bridge output module. The control of the single-phase full-bridge output module adopts a constant power control method to provide the required power for the load.

[0012] Preferably, the control method of the hybrid through-type traction power supply device also includes electrical isolation control of the three-phase power grid: the balanced traction transformer converts the three-phase voltage from the power grid into a two-phase voltage, and the single-phase multi-tap step-down transformer electrically isolates the two-phase voltage after the conversion.

[0013] Preferably, the control of the single-phase full-bridge input module comprises the following steps:

[0014] S1, collect and determine the relevant parameters of the two-phase to single-phase converter: real-time detection of the AC side voltage u of the single-phase full-bridge input module αi 、u βi With the AC side current i αi 、i βi , and the DC side voltage U dci , determine the value of input inductance and DC side capacitance; set the DC side voltage reference value U of the single-phase full-bridge input module dc_ref , and the reactive power command Q of the hybrid through-type traction power supply device ref ;

[0015] S2, coordinate axis transformation: the AC side voltage u detected in step S1 αi 、u βi With current i αi 、i βi Perform dq transformation to obtain the d-axis voltage u of the single-phase full-bridge input module in the dq coordinate system di , q-axis voltage u qi and d-axis current i di , q-axis current i qi ;

[0016] S3, active reference current i di_ref Calculation: Substitute the DC side reference voltage value U set in step S1 dc_ref The detected DC side voltage U dci After squaring, the square error of the DC voltage is obtained. Then the DC side voltage square error Input to the PI regulator and multiply by the proportional coefficient C i / 2, and get the active reference current i di_ref ;

[0017] S4, reactive reference current i qi_ref Calculation: The reactive power command Q set in step S1 is ref Multiply by the proportional coefficient 1 / n to get the reactive power command Q of the single-phase full-bridge input module refi , and then Q refi Multiply by the factor -2 / u di 3 Get the reactive reference current i qi_ref ;

[0018] S5, active reference voltage u di_ref Calculation: Set the active reference current i di_ref and d-axis current i di Difference, get the active current error Δi di ; The d-axis current i di Multiply by the coefficient ωL αi Get the active current decoupling value I di ; The active current error Δi di First input the PI regulator, then take the inverse and the reactive current decoupling amount I qi , d-axis voltage u di Superposition is performed to obtain the active reference voltage u di_ref ;

[0019] S6, reactive reference voltage u qi_ref Calculation: The reactive reference current i qi_ref and q-axis current i qi The reactive current error Δi is obtained by subtractingqi ; The q-axis current i qi Multiply by the coefficient ωL αi Get the reactive current decoupling value I qi ; The reactive current error Δi di Input PI regulator, then take the inverse and decouple the active current I di The difference is finally equal to the q-axis voltage u qi Superposition is performed to obtain the reactive reference voltage u qi_ref ;

[0020] S7, PWM control signal PWM of single-phase full-bridge input module αi With PWM βi Calculation: Set the active reference voltage u di_ref With reactive reference voltage u qi_ref Perform dq inverse transformation to obtain the modulation voltage u of the single-phase full-bridge input module αi_ref with u βi_ref , the modulation voltage u αi_ref with u βi_ref Input PWM modulator, output FID each port control signal PWM αi With PWM βi .

[0021] Compared with the prior art, the advantages of the present invention are as follows:

[0022] (1) The hybrid through-type traction power supply device involved in the present invention has higher power supply reliability than the traditional three-phase-single-phase traction power supply device. Even if the two-phase-single-phase converter of the present invention fails, the system can still switch to the traditional out-of-phase power supply mode and use the balanced traction transformer to continue to supply power to the traction load, effectively improving the power supply reliability of the system.

[0023] (2) The hybrid through-type traction power supply device involved in the present invention makes full use of the balanced traction transformer of the existing traction substation and effectively reduces the number of active and passive components by means of a single-phase full-bridge input module sharing the DC side, thereby greatly reducing the cost of transforming the traditional traction system into a through-type power supply.

[0024] (3) The control method of the hybrid through-type traction power supply device involved in the present invention can effectively realize the power sharing of the two phases (α and β phases) on the input side and the stability of the DC side voltage, and solve the negative sequence power quality problem of the public power grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a topological diagram of the existing through-type traction power supply device based on single-phase PET;

[0026] Figure 2 It is a topological structure diagram of the existing through-type traction power supply device based on single-phase back-to-back modules;

[0027] Figure 3 This is a topological structure diagram of the hybrid through-type traction power supply device involved in the present invention;

[0028] Figure 4 A block diagram of a control method for a single-phase full-bridge input module of a hybrid through-type traction power supply device according to the present invention;

[0029] Figure 5 It is a load power waveform diagram of the hybrid through-type traction power supply device in the embodiment of the present invention;

[0030] Figure 6 It is an electrical waveform diagram of single-phase full-bridge input modules α1 and β1 of a hybrid through-type traction power supply device in an embodiment of the present invention;

[0031] Figure 7 is the DC side voltage U of the single-phase full-bridge input modules α1 and β1 of the hybrid through-type traction power supply device in the embodiment of the present invention dc1 Waveform graph. DETAILED DESCRIPTION

[0032] In order to facilitate the understanding of those skilled in the art, the present invention is further described below in conjunction with embodiments and drawings. The contents mentioned in the implementation modes are not intended to limit the present invention.

[0033] like Figure 3 As shown, a hybrid through-type traction power supply device includes a balanced traction transformer and a hybrid two-phase-single-phase conversion unit. The hybrid two-phase-single-phase conversion unit includes two single-phase multi-tap step-down transformers and a two-phase-single-phase converter. The two-phase-single-phase converter includes two single-phase full-bridge input modules and one single-phase full-bridge output module. The single-phase full-bridge input module and the single-phase full-bridge output module are IGBT devices or MOSFET devices or power electronic devices made of SiC or GaN materials.

[0034] like Figure 3 As shown, the input end of the above-mentioned balanced traction transformer is electrically connected to the 110kV / 220kV public three-phase power grid, and the two output ends u of the balanced traction transformer are electrically connected to the 110kV / 220kV public three-phase power grid. α with u β The phases (27.5kV) are electrically connected to the input ends of different single-phase multi-tap step-down transformers. The device electrically isolates the three-phase power grid through two single-phase multi-tap step-down transformers. It is worth mentioning that the input ends of the two single-phase multi-tap step-down transformers are respectively connected in series with input inductors L αi and L βi(i=1,2…10). In addition, the present invention determines the number of taps on the low-voltage side of the single-phase multi-tap step-down transformer n and the sum of the number of single-phase full-bridge input modules and single-phase full-bridge output modules of the device 3n according to the model of the power electronic device in the actual project. In this embodiment, n=10. The output terminals u of the two single-phase multi-tap step-down transformers αi with u βi (i=1,2…10) are electrically connected to a single-phase full-bridge input module α of a two-phase to single-phase converter i or β i (i=1,2…10) input terminal, two single-phase full-bridge input modules α i and β i The output end of the filter capacitor C is connected in a common DC side manner. i (i=1,2…10) and single-phase full-bridge output module γ i (i=1,2…10), single-phase full-bridge output module γ i (i=1,2…10) by connecting in series on the AC side to increase the withstand voltage level and connect an output inductor L γ It is then connected to the traction contact network to provide power for the traction load.

[0035] The control method of the hybrid through-type traction power supply device mentioned above mainly includes the electrical isolation control of the three-phase power grid, the control of the single-phase full-bridge input module and the control of the single-phase full-bridge output module. The balanced traction transformer converts the three-phase voltage from the power grid into a two-phase voltage, and the single-phase multi-tap step-down transformer electrically isolates the two-phase voltage after the conversion; the single-phase full-bridge input module is mainly responsible for stabilizing the DC side voltage and realizing the single-phase full-bridge input module α i or β i Power is evenly distributed to provide active support for the single-phase full-bridge output module; the single-phase full-bridge output module usually adopts a constant power control method to provide the required power for the load. Since the output module control method is relatively mature, this implementation only provides a detailed introduction to the control method of the single-phase full-bridge input module.

[0036] like Figure 4 As shown, the control of the single-phase full-bridge input module includes the following steps:

[0037] S1, collect and determine the relevant parameters of the two-phase to single-phase converter: real-time detection of the AC side voltage u of the single-phase full-bridge input module αi 、u βi With the AC side current i αi 、i βi , and the DC side voltage U dci , determine the value of input inductance and DC side capacitance; set the DC side voltage reference value U of the single-phase full-bridge input module dc_ref, and the reactive power command Q of the hybrid through-type traction power supply device ref ;

[0038] S2, coordinate axis transformation: the AC side voltage u detected in step S1 αi 、u βi With current i αi 、i βi Perform dq transformation to obtain the d-axis voltage u of the single-phase full-bridge input module in the dq coordinate system di , q-axis voltage u qi and d-axis current i di , q-axis current i qi ;

[0039] S3, active reference current i di_ref Calculation: Substitute the DC side reference voltage value U set in step S1 dc_ref The detected DC side voltage U dci After squaring, the square error of the DC voltage is obtained. Then the DC side voltage square error Input to the PI regulator and multiply by the proportional coefficient C i / 2, and get the active reference current i di_ref ;

[0040] S4, reactive reference current i qi_ref Calculation: The reactive power command Q set in step S1 is ref Multiply by the proportional coefficient 1 / n to get the reactive power command Q of the single-phase full-bridge input module refi , and then Q refi Multiply by the factor -2 / u di 3 Get the reactive reference current i qi_ref ;

[0041] S5, active reference voltage u di_ref Calculation: Set the active reference current i di_ref and d-axis current i di Difference, get the active current error Δi di ; The d-axis current i di Multiply by the coefficient ωL αi Get the active current decoupling value I di ; The active current error Δi di First input the PI regulator, then take the inverse and the reactive current decoupling amount I qi , d-axis voltage u di Superposition is performed to obtain the active reference voltage u di_ref ;

[0042] S6, reactive reference voltage u qi_refCalculation: The reactive reference current i qi_ref and q-axis current i qi The reactive current error Δi is obtained by subtracting qi ; The q-axis current i qi Multiply by the coefficient ωL αi Get the reactive current decoupling value I qi ; The reactive current error Δi di Input PI regulator, then take the inverse and decouple the active current I di The difference is finally equal to the q-axis voltage u qi Superposition is performed to obtain the reactive reference voltage u qi_ref ;

[0043] S7, PWM control signal PWM of single-phase full-bridge input module αi With PWM βi Calculation: The active reference voltage u di_ref With reactive reference voltage u qi_ref Perform dq inverse transformation to obtain the modulation voltage u of the single-phase full-bridge input module αi_ref with u βi_ref , the modulation voltage u αi_ref with u βi_ref Input PWM modulator, output FID each port control signal PWM αi With PWM βi .

[0044] In order to verify the economy and superiority of the hybrid through-type traction power supply device involved in the present invention, three schemes are compared below.

[0045] ①Scheme 1: Through-type traction power supply device based on two sets of single-phase PET, such as Figure 1 As shown;

[0046] ②Scheme 2: Through-type traction power supply device based on single-phase back-to-back modules, such as Figure 2 As shown;

[0047] ③ The present invention: a hybrid through-type traction power supply device, such as Figure 3 shown.

[0048] The comparison table between the present invention and the existing solutions is as follows.

[0049] Table 1 Comparison between the present invention and the existing through-type traction power supply device

[0050]

[0051] Note: The number of power modules of the present invention in Table 1 refers to the sum of the number of single-phase full-bridge input modules and single-phase full-bridge output modules.

[0052] From the comparison results in Table 1, it can be seen that under the same device selection, the present invention can reduce the number of power modules and IGBTs by 50% and the number of capacitors by 87.5% compared with Scheme 1, and the present invention only requires two single-phase multi-tap transformers, while Scheme 1 requires 20 single-phase high-frequency transformers; compared with Scheme 2, the present invention can reduce the number of power modules and IGBTs by 25% and the number of capacitors by 50%. Therefore, compared with the prior art, the present invention can significantly reduce the cost of traction system transformation and improve the system economy.

[0053] The present invention builds a hybrid through-type traction power supply device model in the MATLAB / Simulink simulation platform to verify the effectiveness and correctness of the control method of the hybrid through-type traction power supply device involved in the present invention. The simulation parameters are shown in Table 2.

[0054] Table 2 Simulation parameters of hybrid through-type traction power supply device

[0055] parameter Value <![CDATA[The secondary voltage u of a single-phase multi-tap step-down transformer αi and u βi > 2.75kV <![CDATA[Single-phase multi-tap step-down transformer secondary inductance L αi 、L βi > 5mH <![CDATA[DC-side voltage U dci > 4kV <![CDATA[DC-side capacitor C i > 4000μF <![CDATA[The voltage u on the catenary side γ > 27.5kV <![CDATA[Switching frequency f vsc > 5kHz

[0056] In order to simulate the actual working state of the hybrid through-type traction power supply device during operation and verify the effectiveness of the control method of the present invention, the present invention fully considers the dynamic fluctuation of the traction load power and sets the dynamic load power waveform as follows: Figure 5 As shown. There is no traction load within 0-0.1s, P load =0; traction load P at 0.4s load Sudden increase to 20MW; traction load P at 0.7s load It fell to 16MW.

[0057] Since 20 single-phase full-bridge input modules α i With β i The control method of (i=1,2…10) is the same as the output waveform. Here, we take the single-phase full-bridge input modules α1 and β1 as an example for comparative analysis. The electrical waveforms of the single-phase full-bridge input modules α1 and β1 are as follows: Figure 6 As shown. Figure 6 (a1) and Figure 6 As shown in (b1), at 0.1s, the load power suddenly increases from 0 to 10MW, and the output power of the single-phase full-bridge input modules α1 and β1 quickly increases to 0.5MW (20 single-phase full-bridge input modules evenly share the total load power); at 0.4s, the load power suddenly increases from 10MW to 20MW, and the output power of the single-phase full-bridge input modules α1 and β1 quickly increases to 1MW; at 0.7s, the load power drops from 20MW to 16MW, and the output power of the single-phase full-bridge input modules α1 and β1 drops to 0.8MW. Figure 6 (a2) and Figure 6(b2) are the AC side current waveforms of the single-phase full-bridge input modules α1 and β1 respectively. It can be seen from the figure that the output currents of the single-phase full-bridge input modules α1 and β1 can effectively track their reference power instructions. Figure 6 (a3) and Figure 6 (b3) are the AC side voltage waveforms of the single-phase full-bridge input modules α1 and β1. It can be seen from the figure that the AC side voltage u α1 with u β1 The phase difference is 90°. Therefore, in the process of constantly changing traction load, the single-phase full-bridge input modules α1 and β1 can effectively realize the load power sharing and solve the grid-side negative sequence current problem caused by unbalanced load of the traction transformer.

[0058] Figure 7 is the DC side voltage U of the single-phase full-bridge input module α1 and β1 dc1 Waveform, due to the sudden change of load during 0.1s, 0.4s, and 0.7s, the DC side voltage U dc1 There will be drops and sudden rises, but overall, the DC side voltage can return to a stable state in about 0.1s, and the drop is less than 80V (2%).

[0059] From the above, it can be seen that the hybrid through-type traction power supply device proposed in the present invention makes full use of the existing traction substation equipment, and adopts the means of sharing the DC side of the single-phase full-bridge input module to reduce the number of active and passive device modules required, effectively reducing the system transformation and construction costs. The control method of the hybrid through-type traction power supply device proposed in the present invention can suppress the transient impact caused by load power conversion and improve the dynamic response speed while realizing the power sharing of the single-phase full-bridge input module, thereby providing a reliable guarantee for the stable operation of the hybrid through-type traction power supply device. In general, the hybrid through-type traction power supply device and its control method proposed in the present invention have strong engineering practicality, provide a new way for the transformation of electrified railways to through-type power supply, and provide important reference and reference for the engineering and practical application of future through-type electrified railways.

[0060] The above embodiments are preferred implementation schemes of the present invention. In addition, the present invention may also be implemented in other ways. Any obvious replacement without departing from the concept of the present technical solution is within the protection scope of the present invention.

[0061] In order to make it easier for ordinary technicians in the field to understand the improvements of the present invention over the prior art, some drawings and descriptions of the present invention have been simplified, and for the sake of clarity, some other elements are omitted in this application document. Ordinary technicians in the field should realize that these omitted elements may also constitute the content of the present invention.

Claims

1. A hybrid through-type traction power supply device, comprising a balanced traction transformer, characterized in that: It also includes a hybrid two-phase-single-phase conversion unit, which includes two single-phase multi-tap step-down transformers and a two-phase-single-phase converter, and the two-phase-single-phase converter includes two single-phase full-bridge input modules and one single-phase full-bridge output module; The input end of the balanced traction transformer is electrically connected to the three-phase power grid, the two output ends of the balanced traction transformer are electrically connected to the input ends of different single-phase multi-tap step-down transformers, the output ends of different single-phase multi-tap step-down transformers are electrically connected to the input ends of different single-phase full-bridge input modules, the output ends of the two single-phase full-bridge input modules are electrically connected to the input end of the single-phase full-bridge output module in a common DC side manner, and the output end of the single-phase full-bridge output module is electrically connected to the traction contact network; The input end of the single-phase full-bridge input module is connected in series with an input inductor, and the output end of the single-phase full-bridge input module is electrically connected to a capacitor and a single-phase full-bridge output module in a DC-sharing manner; The control of the single-phase full-bridge input module is used to stabilize the DC side voltage, realize the power sharing of the single-phase full-bridge input module, and provide active support for the single-phase full-bridge output module. The control of the single-phase full-bridge output module adopts a constant power control method to provide the required power for the load; The control of the single-phase full-bridge input module comprises the following steps: S1, collect and determine the relevant parameters of the two-phase to single-phase converter: real-time detection of the AC side voltage of the single-phase full-bridge input module , With AC side current , , and the DC side voltage , determine the value of input inductance and DC side capacitance; Set the DC voltage reference value of the single-phase full-bridge input module , and the reactive power command of the hybrid through-type traction power supply device ; S2, coordinate axis transformation: the AC side voltage detected in step S1 , With current , conduct Transform, get Coordinate system of single-phase full-bridge input module Shaft voltage , Shaft voltage and Shaft current , Shaft current ; S3, active reference current Calculation: Substitute the DC side reference voltage value set in step S1 The DC side voltage detected After squaring, the square error of the DC voltage is obtained. , and then the DC side voltage square error Input to the PI regulator and multiply by the proportional coefficient , get the active reference current ; S4, reactive reference current Calculation: The reactive power command set in step S1 Multiply by the scale factor Get the reactive power command of the single-phase full-bridge input module , and then Multiply by coefficient Get reactive reference current ; S5, active reference voltage Calculation: The active reference current and Shaft current Difference, get the active current error ;Will Shaft current Multiply by coefficient Get the active current decoupling amount ; The active current error First input the PI regulator, then take the inverse and decouple the reactive current , Shaft voltage Superposition is performed to obtain the active reference voltage ; S6, reactive reference voltage Calculation: Reactive reference current and Shaft current The reactive current error is obtained by subtracting ;Will Shaft current Multiply by coefficient Get the reactive current decoupling amount ; The reactive current error Input PI regulator, then take the inverse and decouple the active current Make a difference, and finally Shaft voltage Superposition is performed to obtain the reactive reference voltage ; S7, PWM control signal of single-phase full-bridge input module and Calculation: Active reference voltage With reactive reference voltage conduct The inverse transformation obtains the modulation voltage of the single-phase full-bridge input module and , the modulation voltage and Input PWM modulator, output FID port control signal and .

2. The hybrid through-type traction power supply device according to claim 1, characterized in that: The output end of the single-phase full-bridge output module is electrically connected to the traction contact network after being connected in series with an output inductor.

3. The hybrid through-type traction power supply device according to claim 2, characterized in that: The single-phase full-bridge input module and the single-phase full-bridge output module are IGBT devices or MOSFET devices or power electronic devices made of SiC or GaN materials.

4. The hybrid through-type traction power supply device according to claim 3 is characterized in that: The electrical isolation control of the three-phase power grid is: The balanced traction transformer converts the three-phase voltage from the power grid into a two-phase voltage, and the single-phase multi-tap step-down transformer electrically isolates the converted two-phase voltage.

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