Optimization study of overvoltage problem in PV distribution network based on electrical energy router

Photovoltaic power generation has the advantages of being clean and efficient, but it poses a threat to the voltage quality of distribution networks due to unstable power output. In order to solve the overvoltage and power utilization problems of a high-penetration PV distribution network, this paper proposes a tidal current analysis model based on a four-port power router. First, the steady-state modeling of different ports of the electrical energy router is carried out, and the corresponding voltage control strategy is proposed. Then, by establishing a unified tidal current calculation model, a solution for the optimal operation of the distribution network is proposed. Finally, based on the overvoltage data of a regional power grid, the impact on the voltage level of the grid-connected PV system before and after the addition of the power router is compared. From the analyzed data, it is concluded that the introduction of a four-port power router for corresponding port control strategies can effectively improve overvoltage problems in high-permeability PV distribution networks.


Introduction
Photovoltaic power generation has been developed on a large scale throughout the country, especially in the northwest region, due to its environmental friendliness and economic viability [1].With the high penetration of PV energy into the grid, it poses a major threat to the safety of the distribution network, potentially causing tidal reversal, resulting in voltage overruns at the common connection point of the PV grid-connected system.Voltage crossing limit not only affects the quality of power to the local load but also increases the losses in the distribution equipment of the lines and transformers, causing system overload and affecting the penetration rate of the PV power system [2].
Distribution networks have a variety of means for reactive voltage regulation, which can be divided into local control, central control, and distributed control [3], including on-load regulation transformers [4], energy storage devices, and PV inverters [5].The diversification of power forms has placed higher demands on the power management of distribution networks [6].The emergence of the electrical energy router has brought a new breakthrough in solving the problem of PV grid-connected overvoltage and distribution network management.The electrical energy router can achieve flexible control of network currents like a router in the information Internet and realize functions such as tidal current distribution, voltage improvement, network monitoring, and fault isolation through flexible control means [7].At present, many researchers are mainly oriented towards the topology of the electrical energy router [8], and there is less research on the mathematical model of the power router and the tide calculation model and voltage optimization method of the distribution network after joining the power router [9].
This paper proposes a solution for PV distribution networks with an energy router by modeling the ports of the electrical energy router.The feasibility of the proposed distribution network architecture and its tidal optimization scheme is verified by constructing a five-node distribution network simulation model with node voltages as the optimization targets, solving the problems of voltage overrun and low electrical energy utilization.

Equivalent model of an electrical energy router
To solve the problem of optimizing the operation of the distribution network, a four-port power router topology is proposed, including four ports for the AC load, the PV array, the grid, and the energy storage system, as shown in Figure 1 ) ( cos sin ) The active power injected into the DC bus is equal to the magnitude of the active power flowing into the AC side of the rectifier, Cl P can be expressed as: ,

DC-DC port steady-state equivalent model
The power electronics equivalent of the DC/DC port is shown in Figure 3 and consists of a loss equivalent resistor and an ideal transformer connected in series [10].
The port voltage and the power on the primary and secondary sides can be expressed as:

DC-DC switching bus model
For a DC-DC switching bus with a total of M ports of AC ports (marked by x ) and DC ports (marked by y ) of the electrical energy router, the power is injected into the bus in the positive direction, and the equation of balance at the DC bus end can be expressed as: where dc P  includes dynamic losses related to the actual exchange power and static losses independent of the operating state of the electrical energy router.Equations ( 1)-( 7) form the steady-state model of the power router.

Model for high penetration PV access to the distribution network
Multiple PVs are connected to the distribution network, as shown in Figure 4.The voltage magnitude of one of its users m is:

Electricity router voltage control strategy
The system operates under grid-connected grid conditions, with the distribution grid connected to the power router, the PV and the distribution grid coordinating the processing for the energy supply of the AC and DC loads.Port 2 is equivalent to a PQ node, whose control equation can be expressed as follows.
Table 1 shows the specific control modes for each port.

Integrated tidal analysis model for distribution networks with electrical energy router
(1) Virtual nodes and association matrices to describe the coupling between the ports of the power router and the network nodes.For the AC/DC ports, the model can be simplified, as shown in Figure 5.Each AC/DC port can be seen as a form of inclusion of the rectifier input ports beyond the overall network.This model port bus association matrix il M is: (2) A normalized tidal model is as follows: 1 1 ( cos sin ) 0 ( sin cos ) 0 ( cos sin ) 0 Equations ( 8)-( 14) form a tidal equation for a distribution network system containing a power router.

Optimization objectives
The voltage data is measured here using the provisions of the national standard "GB12325-2008" for voltage deviation at nodes.With the minimum voltage deviation as the control target, we have the following equation.

Constraints
(1) Tidal equation constraints for AC networks: 0 0 (2) AC network line tide upper and lower boundary constraints: (3) DC port voltage balance equation: The control variables all need to satisfy their own upper and lower-bound constraints: Equations ( 15)-( 19) are the optimization model for the distribution network with minimized voltage deviation, which is combined with the above models for simulation.

Example analysis of a four-port electrical energy router for five node distribution networks
The improved system model is shown in Figure 6, where a power router module containing four ports is added to the five-node distribution system.The relevant parameters of the distribution network and the PV plant are shown in Table 2.  7(a).00:00 02:00 04:00 06:00 08:00 10:00 12:00 14:00 16:00 18:00 20:00 0.96 0.98 From Figure 7(a), we can see that the overvoltage phenomenon is more serious around 10:00-17:00.Therefore, we control the voltage deviation according to the optimization objective to achieve the optimal tide to obtain the results shown in Figure 7(b).We can see that after the distribution network line improvement, the overvoltage phenomenon in both places has been significantly improved.
The highest point of overvoltage 13:00 data is selected for observation.Figure 8 is the blue bar graph for the improvement of the port power before.At this time, PV reached the maximum output power, and the line overvoltage phenomenon is very prominent.Through the coordination of power to the storage port, the average active power of the storage end increased, and the load end power was reduced to avoid the occurrence of the overvoltage phenomenon.The improved port power is shown in the orange bar graph.

Discussion
By proposing a more scenario-rich four-port power router steady-state model, the power router and the distribution network are analyzed in terms of the whole tide based on various control schemes for the voltage of the distribution network, with a more global idea to solve the voltage overrun problem.In this paper, although it is only combined with a simple five-node distribution network model to solve basic engineering problems, it is hoped that by solving the voltage overrun problem in highpenetration PV distribution networks, this small entry point may bring a new perspective to other researchers.Also, if there are some needs to expand to more ports, we can just pick the right bus connection for them and match the tidal relationship for the different ports.

Figure 2 .Figure 3 .
Figure 2. Power electronic architecture of the AC/DC port of the energy router

Figure 4 .
Figure 4. Diagram of multiple PV connections to the distribution network

Figure 5 .
Figure 5. AC/DC port virtual node diagram

Figure 6 .
Figure 6.Structure of five node distributed PV distribution network in a grid

Figure 7 .
Figure 7. Time-voltage curve of two transformer monitoring points on a distribution line in a region

Figure 8 .
Figure 8. Optimisation results for individual currents after energy router improvements .

Table 1 .
Control modes for each port of the four-port electrical energy router

Table 2 .
Distributed PV distribution network structural parameters , two distribution transformer test points at loads 3 and 4 are used as examples, and the voltage data from 0:00 to 19:45 on 19 May 2022 at a region randomly selected as shown in Figure