Welding inverter

Demonstration of normal and low voltage ride through (LVRT) operation of a constant current electric arc welding inverter.

Introduction

Welding is an integral and energy intensive part of the construction, manufacturing, and maintenance industries. In electric arc welding, the melting energy is provided by establishing an arc between two electrodes, where one of them is the metallic workpiece being welded. There are several technologies for electric arc welding, each with their own unique power quality requirements. For instance, welding technologies such as metal inert gas (MIG) and submerged arc welding (SAW) requires constant voltage power supply. The constant voltage is shown on the left of Figure 1, also known as the "straight" or "flat" characteristic.

Figure 1: Constant voltage “straight" characteristic (left) and Constant current “droop" characteristic (right) [1]

Meanwhile, shielded metal arc welding (SMAW) and gas tungsten arc welding (GTAW) requires constant current power supply. The profile for constant current power sources are shown on the right of Figure 1 , also called the droop volt/ampere curve. In either case, a change in the voltage causes a small change in the welding current. The welder controls the length of the arc by regulating the voltage, which essentually controls the heat input to the arc to produce the required weld.

The droop volt/ampere curve (Figure 1, right) shows cases of both open circuit voltage and short circuit current. A higher value of open circuit or no-load voltage makes establishing the welding arc easier and extinguishing the arc more difficult. It is desired to have a short circuit current with a value higher than the nominal load in order to prevent the electrode from sticking to the workpiece when striking the arc [2].

This model implements the constant current power supply topology used in arc welding inverter applications, together with a closed loop control. Performance in a variable grid voltage condition is also tested.

Model description

The electrical part of the model is shown in Figure 2. The grid is modeled using voltage sources, resistance, and inductance for each phase. Grid parameters for inductance and resistance are set in Model Initialization panel.

Figure 2: Welding inverter power stage topology



A three-phase diode rectifier is used to convert AC voltage into DC, which is further smoothed by a DC link capacitor. A single-phase inverter connects to the other side of the DC link, which inverts DC voltage to high frequency AC voltage. Switching frequency (fs) for a single-phase inverter is set in the Model initialization panel. High frequency AC voltage is fed to the high frequency transformer, which steps down the voltage to the desired level. Then, voltage is rectified by the diode full-wave rectifier and the current to the load is fed via the output inductor. The load is modeled using the resistance component R_load. Measurement of the output current for the control loop is done using ampere meter.

The control part of the model is represented by a blue color and is shown in Figure 3. The single-phase inverter can be controlled in open-loop and closed-loop mode. Operation mode selection is done through the SCADA Input component.

Figure 3: Control part of the model



The closed-loop control is implemented in the Controller subsystem, which is shown in Figure 4. The reference current value is set using the SCADA input component. Parameters for the PI regulators kp and ki are set in the Model initialization panel. The controller calculates the duty cycle value based on the reference current value and the measured output current.

Figure 4: Controller subsystem



Table 1. HIL device resource utilization
No. of processing cores 2
Max. matrix memory utilization 14.72%
Max. time slot utilization 80%
Simulation step, electrical 0.5 µs
Execution rate, signal processing 100 µs

In this model, the core coupling element is used to overcome hardware limitations on the number of power converters per core, since the overall weight of power converters in this model is 5 whereas a maximum weight of 3 is allowed per core. The core coupler element divides the electrical circuit in two parts, allowing two cores to run, each with a 0.5µs simulation time step.

More information about Circuit Partitioning can be found here.

Simulation

This application comes with a pre-built SCADA panel (Figure 5). The panel offers most essential user interface elements (widgets) to monitor and interact with the simulation in runtime. You can customize it freely to fit your needs.

Figure 5: Welding inverter SCADA panel



The following cases describe normal operational mode and behavior during a low-voltage ride-through (LVRT) fault.

  1. Normal operation mode: Grid voltage is set to 230V using the Grid Voltage slider. Reference value for the output current is set to 300A using the Output Current Reference slider widget. For this operation point, the calculated duty cycle has a value of 0.32. According to the duty cycle, the PWM signals are generated as shown in the top graph of Figure 6. The inverter output voltage (Vprim) is generated based on PWM signals, as shown in the bottom graph of Figure 6.

    Figure 6: PWM signals and inverter output voltage



    In Figure 7, the output current waveform is shown. This demonstrates that the rms value is close to the reference current of 300A.

    Figure 7: Output current



    Figure 8 shows that the controller keeps the reference value of the current while voltage changes, as long as it is within operational range. This is because the welding inverter modeled here is controlled to behave as a constant current power supply. The controller passes its duty cycle limit of 0.5 at around 1.9s, which makes it unable to provide the reference current value for the set voltage level. This effect is visible in the bottom graph of Figure 8.

    Figure 8: Constant current power supply



  2. Operation of the inverter during three phase LVRT: LVRT is implemented using the Macro Button widget. The amplitude of the ideal voltage source is set from 230V to 115V for a period of 500ms, after which the voltage is restored to its starting value. The three-phase LVRT can be observed with the phase voltage measurements shown in the bottom graph of Figure 9.

    Figure 9: Inverter operation during three phase LVRT



    The middle graph of Figure 9 shows a drop in the output current. This is caused by reaching the duty cycle limit, which renders the system unable to keep current at the dsired level. At 0.9s, the recovery of the grid voltage caused the total welding current to overshoot. This shows that voltage disturbances at the grid side can impact the quality of the welding process.

Table 2. Minimum requirements
Files
Typhoon HIL files

examples\models\power supplies\welding inverter

welding inverter.tse,

welding inverter.cus

Minimum hardware requirements
No. of HIL devices 1
HIL device model HIL402
Device configuration 1

References

  1. https://weldknowledge.com/2016/02/06/characteristics-of-arc-welding-power-sources/
  2. https://www.researchgate.net/publication/224350020_A_two-stage_power_converter_for_welding_applications_with_increased_efficiency_and_reduced_filtering