The Scheme of Movable DC Ice Melting Device Based on the Engine-Generator Topology

In this article, an entire scheme of DC ice-melting devices based on the engine-generator topology is proposed to solve the problem of poor portability caused by the current devices. It is found that the engine-generator topology suffers from a power mismatch when starting with load, which may eventually lead to starting failure. A soft-start strategy is proposed to control the power required by the load by controlling the duty cycle of the switching tube to achieve real-time power matching. Compared with the most popular phase-controlled rectifier DC ice melting device, the entire scheme of the device is proposed, which not only achieves continuous voltage and current adjustment but also effectively reduces the ripple of the output and control complexity. The feasibility of the proposed soft-start strategy and the entire scheme of the device is verified by simulation results.


INTRODUCTION
Line ice cover disaster is one of the most serious disasters suffered by the power grid, often causing serious losses to the power system [1].The situation of China's power grid in the face of ice disasters is severe, and many transmission lines and power facilities will be covered with ice and snow in the freezing weather environment.With the increasing demand for electrical energy, it has become urgent to solve the transmission line ice coverage faults.
Thermal ice melting can achieve short-time ice melting easy to implement and simple to operate, which is mainly divided into two categories: AC ice melting and DC ice melting.AC ice-melting technology has many years of application history and experience.However, AC ice melting has difficulties such as large reactive power required for ice melting power supply and difficulty in matching line impedance [2], [3].Compared to AC ice melting, the power capacity required for DC ice melting is much smaller than AC ice melting under the same ice melt power, and there is no need for impedance matching and load transfer, which reduces ice melt operation [4].Therefore, DC ice melting is an effective means to perform line deicing.
After decades of research and development, the DC ice melting scheme based on power electronic rectification technology stands out among a host of schemes and has been unanimously recognized by experts and scholars at home and abroad.In [5], a diode uncontrolled rectifier DC ice melting device has a series of advantages such as small size, lightweight, low cost, simple control, low loss, and high power factor.However, the device cannot achieve voltage and current regulation with poor adaptability.In [6], a thyristor-based controlled rectifier DC ice melting device was proposed with continuously adjustable output voltage and current, and can also be used as a static reactive power compensator [7], [8], which is the most widely used DC ice melting device.However, due to the large harmonic and reactive power of this device, filters need to be added for harmonic suppression and reactive power compensation, which will increase the size and cost of the entire device.In [9], a fully controlled rectifier DC ice melting device based on switching tubes is proposed, which has a larger regulation range and smaller harmonics for output voltage and current and is capable of achieving unit power factor.However, the switching tubes used in this device lead to a significant increase in the cost and control complexity of the whole device.
Most of the previously described DC ice-melting power supplies are fixed devices with poor mobility.Although a movable DC ice melting power supply is proposed in [10]- [12], the electrical energy is still derived from the grid.In other words, they only achieve the movability of the rectifier, but not the mobility of the whole ice-melting device.One solution for portable and removable power is the engine-generator topology, however, there is less research on the application of this topology to DC ice melting in China and abroad.
In this article, a movable DC ice-melting device based on engine-generator topology is proposed.The problems of the engine-generator topology and the soft-start strategy are analyzed in Section 2. The entire scheme is detailed in Section 3. Simulation results are presented in Section 4. Finally, Section 5 concludes the article.

Problems with the engine-generator topology
Normally, the DC ice melting power supply needs to be connected to the transmission line before starting the operation.However, for the engine-generator topology, there is an input-output power mismatch during the starting process, which may lead to starting failure [13].The input power curve of the engine and the output power curve of the generator is shown in Figure 1, where Pmec is the mechanical power input to the engine and Pload is the load power output from the generator.When the engine speed is in the low-speed interval I, the engine has no load-carrying capacity and the output torque is small, so the slope of the power characteristic is small; when the high-speed interval II is reached, the engine has the load-carrying capacity and the output torque increases significantly, and the slope of the curve increases.For the generator, the output voltage is approximately proportional to the rotational speed, then the output power of the generator can be considered as a square linear relationship with the rotational speed.During the starting process, the engine can only start properly if the engine output power curve is always above the generator output power curve.However, when the engine is just started, the speed is in the low-speed range, and the mechanical power curve of the engine is located below the output power curve of the generator at this time.In other words, whenever the engine-generator topology is started directly with a load, the engine will stall due to the mismatch between the input and output power.To solve the problem of starting with load in the engine-generator topology, the industry has commonly adopted the solution of adding a clutch between the engine and the generator.However, the addition of the clutch increases the maintenance cost and control complexity of the system.In this paper, a softstart strategy that does not require additional mechanical devices is proposed with a simpler control method and more reliable operation.The strategy is based on the Buck circuit in power electronics technology, and its principle is shown in Figure 2. By adjusting the duty cycle of the PWM wave to achieve the regulation of the load voltage, we achieve the regulation of the load power, so that the input and output power is always balanced.Finally, the engine can be started normally.
Equated with ice-covered lines to a resistance-sensitive model, the output voltage and current can be obtained as: where D is the duty cycle, and VS is the DC bus voltage.
The output power of the generator needs to be controlled to increase linearly to ensure that the power change of the engine can always track the change of the upper load power.Therefore, the modulating wave needs to be selected as an open-square function, and the expression for the duty cycle is: Substituting Equation (2) into Equation ( 1), the output voltage and current can be further written as: According to Equation (3), the expression of output power can be obtained as: The output voltage, current, and power (torque) waveforms of the soft start strategy throughout the starting process are shown in Figure 3.

ENTIRE SCHEME OF THE MOVABLE DC ICE-MELTING DEVICE
The entire scheme of the movable DC ice-melting device based on the engine-generator topology is shown in Figure 4, which mainly consists of an engine with a starter, a multi-branch generator, an uncontrolled rectifier module with a switching matrix, a controller, and a soft-start device.The whole device adopts a modular design approach.When a problem occurs with an individual module, it is only necessary to replace the corresponding module, which improves the maintainability of the device.For ice-covered lines, due to the line impedance of the conductor and ground differing greatly, corresponding to the ice melting conditions are also very different.The ice melting condition of the conductor is a low-voltage high current, while the ground is a high-voltage small current.To make full use of the power as much as possible to achieve substantial voltage and current regulation, and to meet the ice melting needs of the conductor and ground respectively, a multi-branch motor scheme is adopted.Switching of ice melting conditions is achieved by the series-parallel connection between multiple branches, which greatly improves the adaptability of the device.
When we design the rated voltage of each branch of the generator, power matching needs to be considered.The rotational speed is considered to be proportional to the voltage and the torque is proportional to the current.At a certain speed, the maximum output power of the generator depends on the electromagnetic torque of the generator.When the current corresponding to the electromagnetic torque can provide the current required by the load, the power of the generator and the load can be successfully matched and ice melting is possible.The power curves of the generator and the load are shown in Figure 5.To ensure that the generator can always provide the power required by the load, the load power curve needs to be always below the generator output power curve, so the rated voltage design value of the generator must be lower than the voltage Umax in the figure.Considering various factors such as system cost and power quality, the uncontrolled rectifier is finally selected as the rectification strategy, and multiple uncontrolled modules are cascaded through the switching matrix.Since the transmission line has distributed inductance and the output voltage of the uncontrolled rectifier circuit has low harmonic content, the values of the filter inductor and filter capacitor can be designed to be smaller, which can reduce the size of the device.
The soft-start strategy proposed in Section 2 can not only be used for the starting process but also achieve continuous regulation of output voltage and current by controlling the PWM duty cycle, which not only overcomes the problem of poor adaptability of the traditional uncontrolled rectifier ice melting device but also retains a series of advantages such as small losses and high power factor in the uncontrolled rectifier device.Although the introduction of switching tubes increases the control complexity of the device to a certain extent, it is also much lower than the phase-controlled and fullycontrolled rectifier devices.

Simulation Results
Simulation is done by using MATLAB/Simulink.The parameters of the PMSM are shown in Table 1.The series and parallel output waveforms of the two ice-melting devices with different duty cycles or phase-shifted trigger angles are obtained under the same filter conditions, as shown in Figure 6.The parameters of the load and filter are shown in Table 2. Compared with the phase-controlled rectifier, the voltage ripple of the uncontrolled rectifier with a soft-start strategy is not much different, but the suppression effect of the current ripple is very significant.In addition, the output ripple of the phasecontrolled rectifier increases significantly with the increase of the phase-shift trigger angle, while the output ripple of the soft-starting device is almost unchanged.In the case of load variation, it can be seen that a large value change in voltage and current is achieved with approximately constant output power.

Soft-start strategy
The load voltage, current, and power waveforms for a direct and soft start in series operation are shown in Figure 7, which is approximately the same as the theoretical waveform in Figure 3.However, there are deviations between the simulated waveform and the theoretical waveform, mainly in two aspects: one is that the output is a stepped waveform, and the other is that there is an overshoot in the initial stage.The reason for the output as a step wave is that the frequency of the carrier wave is high and the slope of the modulating wave is low, resulting in the same modulating wave value and the same PWM duty cycle over several consecutive carrier cycles, so the output amplitude will remain constant over several carrier cycles.The overshoot in the output during the initial phase is caused by the generator charging the DC bus capacitor.However, thanks to the soft-start strategy, this overshoot is suppressed to some extent and the shock to the device is within acceptable limits.The results indicate that the soft-start strategy works well.As a result, the performance of the softstart strategy and the entire scheme of the device based on the engine-generator topology can be improved greatly.

Conclusions
In this article, an entire scheme of DC ice-melting devices based on the engine-generator topology is proposed to solve the problem of poor portability caused by the current devices.However, the enginegenerator topology cannot be started directly with the load due to the mismatch between the engine output power and the power required by the load, otherwise, it will lead to a failed start.To solve the power matching problem, a soft-start strategy is proposed to control the power required by the load by controlling the duty cycle of the switching tube to achieve real-time power matching.Compared with the most popular phase-controlled rectifier DC ice melting device, the entire scheme of the device is proposed, which not only achieves continuous voltage and current adjustment but also effectively reduces the ripple of the output and control complexity.The feasibility and reliability of the proposed soft-start strategy and the entire scheme of the device are verified by simulation.

Figure 1 .
Figure 1.Engine and load power characteristic curves

Figure 3 .
Figure 3. Output voltage, current, and power (torque) waveforms with a soft-start strategy

Figure 4 .
Figure 4.The entire scheme of the movable DC ice-melting device based on the engine-generator topology

Figure 5 .
Figure 5.The power curves of the generator and the load

Figure 7 .
Comparison of direct start and soft start output waveforms (a) Load voltage; (b) Load current; (c) Load power.

Table 2 .
Parameters of the load and filter