Research on Parallel Control Strategy of Grid-forming of Power Conversion System

A high proportion of power electronic equipment, distributed energy, and other new energy access to the grid system, low inertia and low damping to the power grid system, these kinds of power grid characteristics in power electric wire aging remote mountains or island areas, therefore, need energy storage system under the microgrid can provide inertial support for load or grid, improve the system damping characteristics. On this basis, the grid-forming control is proposed, so that the power conversion system can work under the off-grid, and can provide inertia and damping to maintain the system stability. In addition, large-scale energy storage systems often require multiple machines operating in parallel. In this paper, the topology of NPC for a power conversion system is studied and analyzed in off-grid mode.


INTRODUCTION
With the rapid development of the global economy, the consumption of traditional fossil energy is increasing, and the resulting environmental problems and energy crises are also intensifying, which seriously threatens human health and the sustainable development of society.To adapt to the pace of human development and promote the sustainable development of the global economy and energy, the European Union and the United States have begun to study clean energy-related technologies.New energy power generation technologies mainly based on photovoltaic and wind power will play an increasingly important role in promoting world energy development [1] .To ensure the long-term and stable development of the national economy, China has also begun to vigorously develop clean energy, which is an important measure to enhance the national energy competitiveness.This is to solve the urgent needs of climate change, protect the ecological environment, and realize the sustainable development of China's economy and society [2] .However, in both wind power and photovoltaic power, their output power and voltage are unstable with time and climate change, and due to the increasing intensity and permeability of new energy development, its large-scale access has had a great impact on the traditional power system.At the same time, due to the increasing power load, the power dispatching pressure of the power grid at the peak of power consumption is huge, which seriously affects the power supply stability of the power system and the economy of the electric energy.The emergence of energy storage technology will be for new energy power generation of power system problems to provide a series of solutions.The energy storage technology in the traditional power system can effectively inhibit new energy power generation power fluctuation, smooth load, peak load, and improve the power quality.It can ensure the power supply system more safe and reliable operation and improve the stability and economy of the power system.With the continuous application of energy storage systems in the power generation side, grid side, user side, and other fields, its development has been paid more and more attention by the power industry of various countries.According to research by relevant international authorities, China, the United States, and the European Union are expected to increase their storage capacity by more than 310 GW of power in 2050, requiring an investment of at least $380 billion.Energy storage technology will have a significant impact on the development of the world economy.The energy storage system is mainly composed of energy storage media (such as lithium battery, hydrogen production power supply), and PCS.In the microgrid, the active and reactive power of the energy storage converter is distributed by the upper energy, dispatching a unit to complete the charge or discharge of the energy storage medium to prevent overcharge or undercharge, and can also realize the parallel in grid or off-grid function of the microgrid.At the same time, the microgrid is mostly used in factories and industrial parks, as well as remote mountainous areas and islands with power supply difficulties.The energy storage converters should independently supply power to the power load in the above scenarios.At this time, the microgrid will be running offline to ensure the stability of the output voltage and frequency of the system and prevent its rapid fluctuations.A large number of studies show that structural grid-forming control can effectively solve the problem of voltage and frequency fluctuations, but the commonly used V/F control strategy, due to lack of inertia, cannot adapt to the power electronics equipment under high permeability grid stable under rapid fluctuations, therefore, increase inertia system is now focused on a grid system, and use a single PCS capacity is difficult to expand the distributed system.The parallel capacity expansion of multiple PCS and the parallel control of multiple PCS under the off-grid has become the focus of research.

PARALLEL MODEL
The parallel system of PCS is developed from the topology of a single device, and its structure is shown in Figure 1.The AC side of the two devices is connected to the AC bus after the line impedance to supply power to the load jointly.

LC Filter
Line impedance Load AC Bus ...

Figure 1. Schematic diagram of the parallel PCS main circuit
The controllers of all the devices in the whole system are completely independent, which constitutes the parallel structure of the distributed power supply.There is no communication line between the devices.Therefore, this paper adopts the parallel mode without interconnection lines.In the off-grid mode, the three-phase output voltage of both PCS is symmetric, and the phase difference is 120, so the three-phase output circuit is symmetric.The following model of the whole parallel system is discussed.
Taking two PCS in parallel as an example, the output side of each energy storage power converter is represented by a voltage source, and the parallel system is shown in Figure 2. In the parallel equivalent model shown in the figure, U 1 andU 2 are PCS off-grid AC output voltage, r 1 , r 2 , X 1 , and X 2 are the line impedance, Z L is the load, and I 1 and I 2 is PCS output current.For the AC sources in parallel, if the amplitude, phase, and frequency of the output AC voltage of each device are the same, the circulation in the system can be eliminated.However, due to the influence of hardware differences, line impedance, and other factors, it is difficult to achieve complete consistency between the output voltage of each device in the actual process, leading to the generation of circulation.The circulation does not pass through the load but instead flows between the devices.Excessive circulation will increase power tube burden, and severe cases will lead to system collapse [5] .
. Physical model of the parallel system

Analysis of circulation characteristics
Circulating current is caused by the inconsistency of the output voltage of the parallel device, which causes a voltage difference and forms a loop between the devices that, in turn, generates the current between the parallel devices.Since circulation is very harmful to the device, in parallel control, circulation suppression has become the focus of researchers.In the following, this article analyzes the circulation between parallel systems.Analysis of Figure 2 shows that in a parallel system, the output current of each device can use Equation ( 1) The magnitude of the system circulation is proportional to the amplitude difference and phase difference of the output voltage between devices, and inversely proportional to the output impedance, so the system circulation can be reduced by adding an inductor to increase the line impedance in hardware.However, large line impedances will cause the voltage output characteristics to become softer, so this method is not a fundamental solution to the parallel connection problem.Under the premise that the line impedance is unchanged, if the amplitude and frequency of the AC voltage are taken as the control quantity, and the amplitude, frequency, and phase of the AC voltage are completely consistent through parallel control.The circulation can be completely suppressed, and the purpose of reasonable power distribution of the equipment can be achieved.

VIRTUAL SYNCHRONOUS GENERATOR CONTROL
The synchronous generator is mechanical equipment from a physical point of view, and the way to ensure the correct adjustment of the system frequency is to control the mechanical output torque balance and simulate inertia and damping for frequency regulation.However, the power electronic device does not have a mechanical shaft in the physical sense, so it cannot take the same implementation method as the synchronous generator [4] .When the load power changes, a torque difference occurs, and a virtual frequency modulator is used to adjust the output frequency, which finally makes the system achieve frequency stability, so a control block diagram can be derived in Figure 3 [3] .

Parallel control strategy in multi-VSG
The drooping characteristics of the VSG can ensure that the power of the system is evenly distributed during the parallel connection.However, in the actual parallel connection process, in addition to the circulation problem after a stable parallel connection, there are also a series of problems, such as excessive initial phase difference of the parallel instantaneous device, resistive inductivity of line impedance, and physical differences between hardware filters, etc.If these problems are not solved, it will have an impact on the effect of parallel connection and even lead to the collapse of the parallel system, resulting in parallel failure [6] .

Pre-synchronization
In VSG off-grid mode, the phase signal of the output voltage of the energy storage power converter is provided by an active-frequency loop.Unlike the grid-tied mode, in the off-grid mode, the frequency and phase output of each device is referenced to itself, so the PCS is activated at different times at the moment of paralleling.The output phase can be different, or even very different.If the device is not started at the same time, in the most severe case, the phase difference between the two PCSs may reach 180 °, and then a parallel connection will generate a huge inrush current instantaneously, causing device failure and shutdown and serious damage to the power device.And for the active power adjustment in droop control is based on the phase difference between devices.If the output phase difference between the two devices is too large, the parallel control will fail [1] .
Therefore, the PCS needs to be phase pre-synchronized before the parallel operation to reduce current surges and ensure smooth regulation of VSG.Parallel pre-synchronization, which can take the form of a digital phase-locked loop, is controlled, as shown in Figure 4.

The relationship between droop ratio and power-sharing
For a parallel system with two devices, the active power droop radio is shown in Equation ( 2) The parallel adjustment process is shown in Figure 5.  2), it can be known that the output frequency of PCS 1 is large at this time.Since the power and frequency of the two devices are not equal at this time, the droop control will adjust the output frequency and phase according to Figure 5.When the frequency and phase of the output voltage of the parallel system are the same, the system is stable to a new working point.At this time, the output power of PCS 1 and PCS 2 realized the power-sharing.

EXPERIMENT
Before 2 s, the system load is 100 kW active load and 10 kVar reactive loads.At 2 hours, the system suddenly increases 20 kW active load, and at 4 s, the system suddenly reduces 20 kW active load.The resulting simulated waveform is shown in Figure 6, and the simulation parameters are shown in Table 1.From Figure 6, we can see that before 2s, the two devices allocate active and reactive power in the ratio of 1:1, that is, 50 kW active power and 5 kVar reactive power, respectively; at 2 s, the active load is 20 kW, the total load of the system is 120 kW.The two devices still allocate power according to the given ratio, each 6 kW, reactive power unchanged.Similarly, at 4 s, the active load is reduced by 20 kW, and the power distribution is still correct.And the whole process of the simulation circulation is almost zero.

CONCLUSION
This paper adopts the parallel control strategy of the virtual synchronous machine, which has the inertia and damping characteristics of the traditional synchronous machine, ensures the stability of the load disturbance, solves the problem of circulation and power equalization, and finally performs the Matlab / Simulink simulation of several parallel devices, which proves the correctness of parallel control with VSG.

Figure 3
Figure 3 Block diagram of the VSG control

Figure 4
Figure 4 Block diagram of the VSG overall control

Figure 5 .
Figure 5. Droop control At the initial time, the active power of the first device is less than the second one.According to Equation (2), it can be known that the output frequency of PCS 1 is large at this time.Since the power and frequency of the two devices are not equal at this time, the droop control will adjust the output frequency and phase according to Figure5.When the frequency and phase of the output voltage of the parallel system are the same, the system is stable to a new working point.At this time, the output power of PCS 1 and PCS 2 realized the power-sharing.