With the wide application of renewable energy power generation technology, the distribution network presents the characteristics of multi-source and complex structure. There are potential risks in the stability of power system, and the problem of power quality is becoming more and more serious. This paper studies and proposes a power optimization cooperative control strategy for flexible fast interconnection device with energy storage, which combines the flexible interconnection technology with the energy storage device. The primary technology is to regulate the active and reactive power of the converter. By comparing the actual power value of the converter with the reference value, the proportional integral (PI) controller is used for correction, and the current components of d and q axes are obtained and input to the converter as the reference value of the current inner loop. The control strategy in this paper can realize power mutual aid between feeders, and at the same time, the energy storage device can provide or absorb a certain amount of power for feeders, so that the power grid can realize stable operation in a certain range.

In recent years, with the rapid development of renewable energy power generation technology based on wind and light energy and the wide application of distributed generation, the distribution network presents the characteristics of multi-source and complex structure [

Flexible interconnection technology is an important technology in active distribution network. Through flexible interconnection device, different areas in active distribution network can realize power mutual aid and realize flexible regulation of power flow between connected buses, and promote distribution network system to accept more distributed renewable energy. After adding the energy storage device, the flexible fast interconnection device with energy storage used in this paper can realize the power mutual aid between different feeders, and stabilize the power fluctuation within a certain range when the connected feeders cannot absorb or provide the corresponding power difference in time.

The idea of flexible interconnection is proposed mostly through the new power electronic equipment based on VSG, mainly including loop balance controller (LBC), loop power controller (LPC) [

Many scholars have studied the distribution network flexible interconnection switch scheme with flexible interconnection equipment as the connection port between different feeders. In 2001, Nippon Electric Power Industry Research Institute proposed to connect the feeders of distribution network in the form of ring network through LPC, and realize the mutual aid of power flow between different distribution network partitions by using the independent control based on local voltage information [

Reference connected the energy storage to the back-to-back voltage source converter through the DC-DC converter based on the double active bridge structure, and proposed a global control strategy for the energy storage flexible interconnection equipment [

Aiming at the problems of lack of flexibility and rapidity in the control strategy of flexible interconnection devices in the existing research and the optimization of the control strategy of flexible interconnection devices, this paper proposes a power optimization cooperative control strategy for flexible fast interconnection device with energy storage. The application of energy storage technology can realize the collaborative optimization of energy at all ends of the distribution network, effectively achieve demand side energy management, renewable energy consumption, improve the system operation economy, and improve the power quality. At the same time, the supercapacitor has high power density, many cycles and fast response speed, which is suitable for short-term high-power demand scenarios. The control strategy of energy storage device is added in this paper, which can effectively save transmission time in power supply reliability; In terms of personal and equipment security, since there is no need for on-site personnel to operate, the security of switching operation will be greatly improved after the flexible interconnection is realized. And it can effectively solve the problems of resource interconnection and mutual supply in different regions, and mismatch between power resources and load demand. Facing the traditional distribution network, it can realize the power mutual aid of different feeders, and the energy storage device can also provide active and reactive power support. At the same time, the working state of the whole system is controlled by adjusting the energy storage device and power electronic converter, so as to realize the comprehensive allocation and management of electric energy, and realize the optimal collaborative control of flexible fast interconnection devices with energy storage.

The structure diagram of the flexible fast interconnection device with energy storage studied in this paper is shown in

It is specified that the active power reference value of converter 1 is

The flexible interconnection device can realize the following three functions:

Funtion1: Power mutual aid

When

Funtion2: Active and reactive power support (supercapacitor discharging)

When

Funtion3: Active and reactive power support (supercapacitor charging)

When

The power control function diagram of the flexible fast interconnection device is shown in

The power mutual aid, active and reactive power support of the system depend on the accurate control of the output active and reactive power of the port converter, that is, P–Q control of the port.

The governing equation is as

In

As shown in

As shown in

In order to verify the effectiveness of the control strategy proposed in this paper, a simulation model is established in MATLAB/Simulink environment, as shown in

System parameter | Numerical value |
---|---|

Rated voltage of feeder | 380 V |

DC bus voltage | 600 V |

DC side capacitance | 50 mF |

Rated capacity of inverter | 100 kVA |

Inverter switching frequency | 3 kHz |

LCL filter inverter side inductance | 857 μH |

LCL filter network side inductance | 343 μH |

LCL filter capacitor | 90 μF |

Neutral inductance | 2.7 mH |

Among them, the supercapacitor energy storage capacity is determined by the voltage level and support time according to the project requirements. See the Funding Statement at the end of the text for specific projects.

In order to verify that the flexible interconnection device can realize the function of power mutual aid, simulation verification is designed. At 0.1 s, the active power of feeder 1 with load is 40 kW, and the reactive power is 20 kvar; feeder 2 has load active power of 40 kW and reactive power of 20 kvar. At 0.3 s, feeder 1 suddenly increases the load active power of 30 kW and reactive power of 30 kvar; the load power of feeder 2 remains unchanged, and the supercapacitor does not participate. The waveforms of VSC output power, feeder output power, feeder load power and supercapacitor state are measured, respectively, as shown in

According to the above simulation, the supercapacitor does not provide power to feeder 1 and feeder 2 at 0.1 s, and feeder 1 and feeder 2 provide active power of 40 kW and reactive power of 20 kvar, respectively; at 0.3 s, the sudden load power of feeder 1 is provided by feeder 2. When the supercapacitor is not involved, the flexible fast interconnection device can realize the power mutual aid function between the two converters, solve the problems brought by renewable energy power generation to the power system, and help maintain the stability of the power grid.

In order to verify that the flexible interconnection device can realize the function of active and reactive power support, two simulation verifications of supercapacitor charging and discharging are designed, respectively.

When at working condition 1:0.1 s, the active power of feeder 1 with load is 40 kW, and the reactive power is 20 kvar; feeder 2 has load active power of 40 kW and reactive power of 20 kvar.

At 0.3 s, feeder 1 and feeder 2 suddenly increase the load active power of 30 kW, and the supercapacitor discharges. The waveforms of VSC output power, feeder output power, feeder load power and supercapacitor state are measured, respectively, as shown in

According to the above simulation, the supercapacitor does not provide power to feeder 1 and feeder 2 at 0.1 s. Feeder 1 and feeder 2 provide active power of 40 kW; at 0.3 s, the active power of sudden load increase of feeder 1 and feeder 2 is provided by supercapacitor.

When at working condition 2:0.1 s, the active power of feeder 1 with load is 40 kW and the reactive power is 20 kvar; feeder 2 has load active power of 40 kW and reactive power of 20 kvar. At 0.3 s, feeder 1 suddenly increases the load active power of 10 kW and reactive power of 10 kvar; feeder 2 with load power unchanged, supercapacitor charging. The waveforms of VSC output power, feeder output power, feeder load power and supercapacitor state are measured, respectively, as shown in

According to the above simulation, the supercapacitor does not provide power to feeder 1 and feeder 2 at 0.1 s. Feeder 1 and feeder 2 provide active power of 40 kW and reactive power of 20 kvar respectively; At 0.3 s, the active and reactive power of the sudden increase load of feeder 1 is provided by feeder 2, and the excess energy of feeder 2 is stored in the supercapacitor to charge the supercapacitor.

From the above two different working conditions, it can be seen that adding energy storage devices, while the power mutual aid between feeders, the energy that cannot be absorbed or provided by feeders can be stored or released by the energy storage device, so as to achieve the stability of power within a certain range and greatly improve the stability of the power grid.

Aiming at the power system instability and power quality problems caused by the increase of renewable energy generation, this paper proposes a power optimization collaborative control strategy for flexible fast interconnection devices with energy storage. The primary technology is the P–Q control of VSC, which compares the actual value of active and reactive power of VSC with the reference value, inputs the compared value to the PI controller for correction, obtains the current component of d and q axis, and inputs it to VSC as the reference value of the internal current loop to realize the corresponding control requirements. First, it can realize the power mutual aid between feeders without the participation of energy storage devices, and the two power grids cooperate with each other to achieve a certain range of stable operation. At the same time, when the energy storage device participates in the power mutual aid between feeders, it can realize the function of providing or absorbing excess power for feeders, improving the utilization rate of electric energy and strengthening the stable operation ability of the power grid.

But in the future, more parts need to be improved than the efficiency of such control strategies. Efficiency plays a crucial role in the practical application of control strategies, and the research on improving control strategies will also be carried out in the future. In addition, the capacity of the supercapacitor will be further optimized in the future, so that it can be used in different scenarios, and the performance of the control strategy can be further improved.

Supported by Science and Technology Projects of State Grid Corporation of China (JF2021018).

The authors declare that they have no conflicts of interest to report regarding the present study.