Design and development of single-phase inverter for locomotives based on SVPWM

For locomotive in high-power inverter power supply output efficiency is low, the level is not stable, push-pull circuit before the switch tube of high pressure, low utilization rate of the original winding transformer problem, designed and developed a kind of single phase inverter power supply based on SVPWM control. The inverter is designed with a modular structure with a rated power of 4KW, which is used to supply power to the cab electrical apparatus and auxiliary air compressor at both ends of the locomotive. It is composed of a filter circuit, a front-end boost control drive circuit, a full-bridge DC boost circuit, an inverter control circuit, a voltage and current sampling circuit, a back-end full-bridge inverter circuit and a protection circuit. The front stage of the inverter adopts a full bridge chopper circuit topology, while the rear stage achieves SVPWM control through the Vacon NXS control box. Finally, a power supply prototype was produced through simulation verification, and the results showed that the designed power supply met the design requirements.


Introduction
With the development of national economy and rail transit industry, people's travel demand for 220V power supply in rail vehicles is increasing, which also puts higher requirements on the power supply of rail transit vehicles [1] .The water dispenser, socket, microwave oven, refrigerator and other electrical appliances in the driver's cab on both ends of the vehicle, auxiliary air compressor and so on all use 220V/50HZ AC.
The inverter power supply of the current rail vehicles mostly uses the inverter power supply of the push-pull topology in the front stage.Although the push-pull topology has a simple drive circuit, it has high requirements for the voltage withstand of the switching tube, and the value applied to the switching tube is almost twice the voltage before the converter [2] .Secondly, the push-pull structure has a low utilization rate for the primary side winding of the transformer [3] .Aiming at these two problems, this paper designs a locomotive inverter power supply with full-bridge chopper circuit topology at the front stage [4][5] .The front-end chopper circuit topology can not only solve the above problems, but also suppress the common mode and effectively reduce the electromagnetic interference of the inverter power supply [6] .
Because the inverter power supply is applied to rail vehicles, higher requirements are put forward for the reliability of the inverter power supply.In this design, Insulated Gate Bipolar Transistor (IGBT) is applied to the inverter power supply, and the inverter part no longer uses the previously commonly used Sinusoidal Pulse Width Modulation technology (Sinusoidal Pulse Width Modulation, IGBT).SPWM [7- 8] and Space Vector Pulse Width Modulation (SVPWM) [9] .Compared with SPWM, the output harmonic content of SVPWM is lower and the voltage utilization rate is increased by 15%, which can effectively 2 improve the energy efficiency ratio of the power supply [10] .

Overall architecture
The hardware design of a single-phase inverter power supply system consists of a filtering circuit, a front-end boost control drive circuit, a full bridge DC boost circuit, an inverter control circuit, a backend full bridge inverter circuit, and a protection circuit.The design block diagram is shown in Figure 1.The main circuit is the power conversion circuit, consisting of a front full bridge boost chopper circuit and a rear inverter circuit.To output 220V single-phase AC power from this inverter power supply, relying on locomotive 74V DC power cannot achieve the voltage level required by the rear inverter circuit, so a boost process is required in the front stage.The control circuit includes two parts: the front boost control drive circuit and the inverter control circuit.The control circuit not only controls the normal operation of the power conversion circuit, but also outputs sampling signals from various parts of the main circuit, adjusts the waveform of the output switch control signal, and achieves the purpose of controlling the working state of the main circuit.At the same time, it receives the protection signal output from the protection circuit to complete circuit protection.This inverter power supply uses filtering circuits in the input/output and intermediate circuits to ensure the output quality of the power supply.

Boost control circuit
The boost control drive circuit uses a UC2825N chip to output two PWM signals and divide them into four PWM signals to drive two IGBT modules By setting the RT and CT pin values of UC2825N, we can obtain the desired PWM duty cycle and frequency.The values of RT and CT can be calculated using Equation 1 and Equation 2. DMAX is the maximum duty cycle that can generate PWM signals.The DMAX of this inverter power supply is set to 86%, and the PWM frequency is set to 50KHZ.The peripheral circuit of UC2825N is shown in Figure 2. The UC2825N chip is powered by 15V input from pins 13 and 15, and pins 11 and 14 are output pins that output two PWM signals; Pins 10 and 12 are grounded; Deadband set to 4 μ s.Fully meet the design requirements without affecting the normal operation of the reverse full bridge chopper circuit.
The control circuit board can also receive voltage and current feedback signals to control PWM signal output.The boost drive circuit board is designed to offset the negative voltage of the PWM signal, which makes the full bridge chopper circuit more stable and reliable, and will not cause IGBT to conduct and damage the circuit structure due to low-level burrs in the control signal.

Protection circuit
The input voltage and current are collected and transmitted to the DC chopper control circuit board through a series of comparison circuits, which control the UC2825N chip to output the PWM signal controlling the IGBT.This ensures that the front stage full bridge chopper circuit can stably output 350V direct current for voltage and current conversion in the rear stage inverter circuit.In the front-end boost chopper circuit, the current is collected at the input end of the transformer through a current transformer, and then entered into the boost control board for comparison.Then, current feedback regulation is achieved through the UC2825N chip; The circuit diagram of the current feedback part is shown in Figure 3.In Figure 3, ISA and ISB are the connection points of the current transformer, which pass through a rectifier circuit composed of diodes D1~D4 and enter the UC2825N chip to achieve current feedback.
After the front-end rectifier circuit, set a voltage collection point and enter the front-end boost control board to achieve voltage feedback regulation.The voltage feedback circuit is shown in Figure 4 and Figure 5.In Figure 4, VOUT represents the DC voltage output by the front stage full bridge chopper circuit.After passing through the LM258AP chip comparator, it is isolated by U11 and U12 optocouplers and enters the following circuit.

Inverter circuit
The inverter control circuit uses the VACON NXS control box, as shown in Figure 6.The Vacon NXS is a compact but high-performance control box with rich I/O interfaces, which makes it highly compatible and popular in integrated systems.Its SVPWM algorithm is set up, and parameter modification is flexible and simple.
The NXS frequency converter only needs 24V to work and output signals, and it itself contains a protection circuit, making it safe and reliable to use, with stable output.This is very suitable for the usage scenario of this inverter power supply.The use of Vacon NXS can completely achieve the goal of simplifying control circuits, improving control accuracy, and ensuring operational reliability.This control box and inverter drive board are a complete set of products, which mainly correspond to a threephase inverter power supply.(3) Based on the operating environment and power capacity of the inverter power supply, in practical engineering design, the output voltage of the inverter power supply should be AC242-248V when it is unloaded, and AC230-248V when it is loaded, in order to ensure the output quality of the inverter power supply.Considering a certain margin, the intermediate voltage of the inverter power supply should be between DC348-352V when it is unloaded and DC330-350V when it is loaded.

Front stage chopping test
The PWM waveform output by the full bridge chopper circuit is shown in Figure 7.It can be seen that two PWM waveforms are output and the dead zone is reasonable, basically meeting the design requirements.The output amplitude is close to 15V, and the output frequency is 50KHz.After passing through the driving circuit board, the PWM output voltage is pulled down as a whole, and the low level is a negative voltage.This can make the IGBT conduct and turn off more reliable, and there will be no situation where the IGBT misleads and damages the module due to the sharp wave of the waveform.From the simulation results in Figure 8, it can be seen that after Clark transformation, the synthesized three-phase cosine values are still in the form of sine waves, with the amplitude being two-thirds of the original amplitude.The construction of the inverter power supply and load is shown in Figure 9, and the load test results of the inverter power supply are shown in Table 1.The voltage of 74V DC after passing through the full bridge boost chopper circuit becomes 350V.After testing, the intermediate DC voltage meets the design requirements.The test results show that the inverter power supply can still operate normally under 4KW full load and all voltage indicators are within the allowable range.Input voltage /V Intermediate voltage /V Output voltage /V Load power /W

Conclusion
In order to solve the problems of unstable and low efficiency of locomotive 220V AC power supply, this paper designs a single-phase inverter power supply for locomotives based on SVPWM control technology and pulse width modulation technology, with a power of about 4KW and a stable output of 220V/50Hz AC power supply.This inverter power supply abandons the push-pull structure used in traditional inverters, resulting in higher voltage utilization and better output stability.After testing and analysis, the designed power supply meets the design requirements.

Figure 1 .
Figure 1.Overall Design Scheme of Inverter Power Supply for Internal combustion locomotive.

Figure. 5
Figure.5 Voltage feedback circuit diagram after isolation.

Figure. 6
Figure.6 Vacon NXS Control Box.This single-phase inverter power supply design can use the UUW line voltage in the three-phase of the drive board to output single-phase AC power.The voltage required for the inverter circuit is calculated using equations 3 and 4. In equation 3, UUWm is the amplitude of the line voltage UUW, U3 is the required 220V AC power, and U2 is the intermediate voltage between the front chopper circuit and the rear inverter circuit. 2 =   2√3

Figure 9 .
Figure 9. Load test diagram of inverter power supply.

Table 1 .
Power Load Test Results.