Impact of impact load on transient voltage of "double high" power grid

作者： GOZ Electric 时间：2024-10-11 09:38:53 阅读：21

With the continuous expansion of the scale of renewable energy power generation, a large number of power electronic devices will be used in the power grid, making the power grid develop towards a high proportion of renewable energy and a high proportion of power electronic equipment, and the power system will present a "double high" characteristic. Renewable energy generation is intermittent and random, and power electronic devices will inject harmonics into the power grid, which will affect the stable power supply of the power grid. With the development of industry, more and more impact loads will be connected to the power grid, such as arc furnaces, rolling mills, etc., which will cause serious pollution to electric energy. At present, research focuses on the steady-state power quality problems caused by impact loads, but there is relatively little research on the transient power quality problems caused. For the research on the "double high" power grid, most of them are also focused on the impact of large-scale access of renewable energy on the power system, and there is little analysis and research on the power quality of impact loads under the background of the "double high" power grid. According to the different states of arc burning, the relevant literature combines the hyperbolic G index model and the Cai's chaotic circuit to establish an arc furnace model that can study steady-state power quality and transient power quality. The relevant literature establishes an arc furnace time-varying resistance model, which can be used for harmonic analysis. Related literature has established a photovoltaic power generation system that can be connected to the grid, and analyzed the impact of renewable energy generation on the power quality of the distribution network. Related literature has established a wind-solar-storage microgrid grid-connected system, and analyzed a series of power quality problems caused by the operation of impact loads in the microgrid. Related literature has established three impact load models for arc furnaces, rolling mills, and electrified traction loads, analyzed the working characteristics of each impact load, and studied the impact of impact load operation on power quality and how to improve power quality. This paper takes the photovoltaic power generation system in the "double high" power grid as the main research object, establishes a photovoltaic grid-connected power generation system, connects an AC arc furnace to the system, and analyzes the voltage sag problem caused by a three-phase short circuit fault in the arc furnace.

Photovoltaic grid-connected power generation system

This paper adopts a two-stage photovoltaic grid-connected power generation system, and the system structure is shown in Figure 1. The power generation system consists of a photovoltaic array, a boost circuit, a DC/AC inverter, an LCL filter, an isolation transformer, and a power grid. Affected by light, climate and other conditions, the voltage and current output by the photovoltaic cell will change accordingly, resulting in changes in the output power. This requires that the photovoltaic cell can output the maximum power in real time. In the photovoltaic power generation system, the system always outputs the maximum power by adjusting the working point of the photovoltaic cell in real time. This process is the maximum power point tracking (MPPT). In general, the voltage output by the photovoltaic cell is low, which does not meet the voltage level requirements of the power supply system and electrical equipment, and cannot meet the grid connection conditions. Therefore, the boost converter is often used as a controller to implement MPPT. The photovoltaic cell outputs direct current, which needs to be converted into alternating current by a DC/AC inverter for grid connection. The inverter control method adopts voltage and current dual-loop control. The LCL filter can effectively suppress harmonic currents, and the post-stage inductor can also suppress the impact current of the power grid. Finally, the inverted AC power is boosted by the power frequency transformer and connected to the high-voltage power grid.

Figure 1

The Institute of Electrical and Electronics Engineers (IEEE) defines voltage sag as a temporary voltage drop in which the effective value of the power frequency voltage drops rapidly to 90%~10% of the rated value and lasts for 0.5 cycles~1min. During the operation of the arc furnace, the furnace charge often contacts the electrode and short-circuits. Each electrode short-circuit fault lasts for 0.01~0.02s. When the three-phase electrodes are short-circuited at the same time, the effective value of the phase voltage at the 35kV grid connection point drops by 29.19%, and the effective value of the phase voltage at the secondary side of the furnace transformer drops by 28.86%. According to the definition of voltage sag, the voltage drop at the 35kV grid connection point caused by the three-phase short circuit of the arc furnace is a voltage sag.

This paper establishes a "double-high" power grid model connected to an impact load. This model can be used to study both steady-state power quality and transient power quality. Then, the reactive power impact, current impact and voltage sag at the 35kV grid connection point and the secondary side of the furnace transformer when the three-phase short circuit of the arc furnace is carried out, which provides a basis for reactive power compensation and transient power quality management.