WO2022031157A1 - Method for controlling voltage source inverters to provide flexibility to ac electrical microgrids - Google Patents

Abstract

The invention relates to a method for controlling voltage source inverters used in AC electrical microgrids which provides for flexible operation. The method controls three modes of operation of the electrical microgrid. These are the mode in which the electrical microgrid is connected to a stable electrical grid, in which case the method regulates the AC current injected into the electrical grid by the electrical microgrid, the mode in which the electrical microgrid is connected to a weak electrical grid characterized by a short circuit ratio lower than 10, in which case the method regulates the AC voltage at the connection point between the electrical microgrid and the electrical grid, and the isolated mode in which the electrical microgrid is disconnected from the electrical grid, in which case the control method regulates the AC voltage of the electrical microgrid. The transition between the three modes takes place on the basis of the value of the short circuit ratio and the state of the switch between the electrical microgrid and the electrical grid. The control method estimates, in real time, the parameters of the electrical grid: the resistance, the inductance and the voltage, and also the current required by the local load on the electrical microgrid.

Description

A Voltage Source Inverter Control Method for Flexibility of AC Power Microgrids

DESCRIPTION:

Technical area :

The present invention relates to a method for controlling electrical microgrids in order to ensure their flexible operation by developing a control of the voltage source inverters.

Previous state:

An electrical microgrid (microgrid) is an electrical system composed of distributed energy resources, electrical loads, energy storage systems and control units. These elements are interconnected and operate with well-defined electrical limits as a single controllable entity. Electrical microgrids can be designed to provide alternating current (AC) or direct current (DC) depending on the nature of the electrical distribution lines. There are hybrid electrical microgrids that combine both types of lines (MA Hossain, HR Pota, MJ Hossain, and F. Blaabjerg, “Evolution of microgrids with converter-interfaced generations: Challenges and opportunities,” Int. J. Electr. Power Energy Syst., vol. 109, issue October 2018, pp. 160-186, 2019).

Electrical microgrids can be connected to the electrical grid, at the common coupling point, operating in grid-connected mode as they can be disconnected operating independently in isolated mode. In some cases, the electrical networks to which the electrical microgrids are connected are weak characterized by a very high line impedance leading to voltage disturbances at the common coupling point and a short-circuit ratio of less than 10 (SL Lorenzen, AB Nielsen , and L. Bede, “Control of a grid connected converter during weak grid conditions,” 2016 IEEE 7th Int. Symp. Power Electron. Distrib. Gener. Syst. PEDG 2016).

Distributed energy resources, used in electrical microgrids, produce electrical energy at the local level close to its end user. They bring together several technologies, in particular renewable energies (solar, wind, micro hydroelectric systems, biomass, etc.), fossil energies (microturbines, gas turbines, etc.), fuel cells, etc.

Thus, electrical microgrids make it possible to reduce greenhouse gas emissions, reduce electrical losses due to large transmission lines, improve the reliability of the electrical supply, ensure a strong integration of renewable energies into existing electrical networks. and reduce the cost of electric kWh. However, electrical microgrids have to face a set of constraints such as the transition between operating modes, flexibility, voltage regulation in isolated mode and overall system stability despite fluctuations and intermittency of renewable resources (A. Hirsch, Y. Parag, and J. Guerrero, “Microgrids: A review of technologies, key drivers, and outstanding issues,” Renew. Sustain. Energy Rev., vol. 90, no. September 2017, pp. 402-411, 2018).

The majority of distributed energy resources are not suitable for direct connection to the electrical microgrid. Therefore, DC/AC, AC/DC and AC/DC/AC power converters are needed as an interface between the electrical microgrid and the distributed energy resources. Voltage source inverters are the most widely used DC/AC power converters in electrical microgrids. The control of these inverters is crucial to ensure effective control of the overall operation of electrical microgrids.

Several techniques have been used for the control of voltage source inverters, for example: proportional integral derivative, proportional resonant, deadbeat corrector, predictive controller, hysteresis controller, slip mode control and LQR (AM) control. Bouzid, JM Guerrero, A. Cheriti, M. Bouhamida, P. Sicard, and M. Benghanem, “A survey on control of electric power distributed generation systems for microgrid applications,” Renew. Sustain. Energy Rev., vol. 44, pp. 751-766, 2015).

The patent CN105429297A proposes a multi-operational control strategy for electrical microgrids by dividing their operation into mode connected to a normal electrical network, mode connected to a risky electrical network, weak connection mode (when the main electrical network does not supply the power microgrid which is isolated and exhibits weak operation), isolated mode and recovery mode (reconnection to the power grid). A control strategy for each mode as well as the transition conditions between the different modes is determined.

In patent EP3134950A1, a hierarchical architecture for controlling electrical microgrids, consisting of distributed resources and loads, is proposed against meteorological threats in particular, in order to increase the resilience of the electrical microgrid. The hierarchical architecture contains three levels of control; a component control level comprising control equipment related to distributed resources and loads, an intermediate control level related to the component control level and a power microgrid control level that calculates the degree of threat related to the control level intermediate. If the threat level is low, all levels are controlled centrally. If the degree of threat is high, the control is done in a decentralized way by integrating just the level of control of the components. The electrical microgrid can operate in connected mode or in isolated mode.

In patent WO2018122726A1, a control system for electrical microgrids is proposed. The control system is based on a hierarchical structure containing two levels: a first control level for the power converters and a second control level comprising a controller adapted to the first control level. With this system, the control of the active power is linked to the control of the frequency and the control of the reactive power is linked to the control of the voltage and vice versa according to the droop method (Droop Control Method). The electric microgrid has the following operating modes: grid-connected mode, isolated mode and / or connected to smart storage systems. The electrical microgrid can supply direct current loads as well as alternating current loads.

In the present invention, the control relates to the voltage source inverters used in alternating current electrical microgrids containing at least one single distributed energy resource whatever its nature, connected to a voltage source inverter and containing, also, at least one local alternating current electrical load. Whose method calculates the control of the inverters in three modes of operation of the electrical microgrid:

-The mode connected to a stable electrical network

- The mode connected to a weak electrical network

- Isolated mode

Description of the invention:

The present invention consists in controlling voltage source inverters to allow flexible operation of alternating current electrical microgrids containing at least one single distributed energy resource, whatever its nature, connected to a voltage source inverter and also containing at least one local alternating current electrical load. The control method controls the voltage source inverters used in said electric microgrids by considering three operating modes: mode connected to a stable electric network, mode connected to a weak electric network and isolated mode.

The control strategy is described as follows:

1- If the electrical microgrid is connected to a stable electrical grid, the electrical grid imposes the voltage at the common coupling point with the electrical microgrid. The voltage remains constant despite fluctuations in distributed energy resources and local electrical loads. In this case, the control system regulates the output alternating current injected by the electric microgrid into the electric network in order to control the exchange of active and reactive powers between the electric network and the electric microgrid. This is the mode connected to a stable electrical network.

2- If the electrical microgrid is connected to a weak and unstable electrical network characterized by a short-circuit ratio of less than 10, the voltage at the common coupling point undergoes fluctuations that may exceed the limits required by international standards. Regulating the alternating voltage therefore becomes a priority. In this case, the control system regulates the voltage at the common coupling point between the electrical microgrid and the electrical grid. This is the mode connected to a weak electrical network.

3- If the electrical microgrid is disconnected from the electrical network operating in isolated mode, the system regulates the alternating voltage of the electrical microgrid. This is isolated mode. The transition between the different modes is made according to the state of the switch at the common coupling point and the short-circuit ratio. If the switch at the common coupling point is closed, the electrical microgrid operates in the mode connected to a stable electrical network. If in addition the short-circuit ratio is less than 10, the electrical microgrid operates in mode connected to a weak electrical grid. If the switch is open, the electrical microgrid operates in isolated mode. FIG. 1 presents the flowchart of the control strategy proposed by the present invention.

To increase the reliability of the electric microgrid and decrease the number of sensors used, the control system, when the electric microgrid is connected to the electric network, estimates the inductance, the resistance and the voltage of the electric network to which the electric microgrid is connected. The system also estimates the local AC load current in all operating modes.

The regulation of alternating current and voltage is done with the Backstepping technique which makes it possible to force the alternating current and voltage to follow reference values while ensuring overall asymptotic stability of the system.

The estimation of the resistance and inductance of the electrical network is ensured by the adaptive version of the Backstepping technique. The estimate of the mains voltage and local AC load current is guaranteed by the Grand Gain observer.

After the development of the control method, the stability of the system is studied through the Lyapunov theory.

The control system, in mode connected to a stable electrical network, allows the electrical microgrid to inject the appropriate amounts of active and reactive power by forcing the alternating current to follow a given reference. In mode connected to a weak network and in isolated mode, the control method forces the alternating voltage at the point of common coupling to follow a well determined reference. Thus, the control method ensures the flexibility of the electric microgrid and guarantees an uninterrupted power supply to the local loads of the electric microgrid. The control method estimates the resistance, inductance and voltage of the electrical network as well as the current of the local AC load; in this way, the number of sensors required for the control of the voltage source inverter is reduced and therefore the cost is also reduced. The control method guarantees global asymptotic stability of the whole system.

The system to be controlled is given in figure 2. It consists of a distributed energy resource, whatever its nature, connected to a voltage source inverter, an LC filter, a coupling inductor with the power grid and a local AC load. The characteristic quantities of the system are: i ₁ : the output current of the voltage source inverter.

V ₁ : the output voltage of the voltage source inverter. i ₂ : the output current of the LC filter. V ₂ : the output voltage of the LC filter and it is the voltage at the common coupling point. i _L : the load current.

V _-grid ; the voltage of the electrical network.

L' and R' are respectively the inductance and the resistance of the electrical network.

R , L and C are the parameters of the LC filter.

To achieve the objectives of the control algorithm, the following mathematical model is adopted then it is represented in the frame dq.

To perform a simulation of the results, we considered the following case study: Initially ( t = Os ), the electrical microgrid is connected to a stable electrical network. In this case, the objective is to regulate the alternating current injected (i _2d and i _2q ) in the electrical network in order to control the active and reactive powers whose desired values are 660 W and 0 VAR (in this way the algorithm allows to compensate the reactive power of the local loads of the electrical microgrid).

The voltage of the electrical network: V _g (t) = 220sin(2πft) (V) . R' = 0.12 Ω, L' = 0.16 mH and f = 50Hz.

At t = 1s, the inductance of the electrical network increases from 0.16 mH to 1.6 mH. With increasing inductance, the power grid becomes weak. The control algorithm switches from current regulation to AC voltage regulation. The AC voltage reference at the common coupling point V _ref =220sin(2πf _ref t)(V) and f _ref = 50 Hz .

At t = 2s , the switch at the common coupling point is open, the electrical microgrid operates in isolated mode and the control algorithm regulates the alternating voltage of the electrical microgrid whose reference is V _ref =220sin(2πf _ref t)(V ) and f _ref = 50 Hz .

The LC filter data: L=22mH, R=0.03Ω and C=15μF.

The simulations are performed with a resistive load. The simulation results show that:

The current injected into the electrical network follows its reference as well as the i _2q component of the current tends towards 0 A thus the reactive power injected into the electrical network is zero (figure 3).

The AC voltage at the common coupling point follows its reference in weak grid connected mode (Figure 4) and in isolated mode (Figure 5).

The estimated values of the resistance and the inductance of the electrical network converge towards their real values in mode connected to a stable network (figure 6) and in mode connected to a weak network (figure 7).

Estimates of the mains voltage as well as the local AC load current tend towards their actual values (Figure 8).

Claims

CLAIMS: 1- A method for controlling voltage source inverters in an alternating current electrical microgrid characterized by: -The regulation of the alternating current injected into the electrical network when the electrical microgrid is connected to a stable electrical network. -The regulation of the alternating voltage, when the electric microgrid is connected to a weak electric network in which the short-circuit ratio is less than 10, at the common coupling point between the electric microgrid and said weak electric network. - The regulation of the alternating current voltage, when the electrical microgrid is in isolated mode, at the point of connection of the local alternating current load with the electrical microgrid. 2- The control method according to claim 1, characterized in that the transition to mode connected to a stable electrical network takes place when the switch at the common coupling point is closed and the short-circuit ratio is greater than or equal to 10 ; 3- The control method according to claim 1, characterized in that the transition in mode connected to a weak electrical network takes place when the switch at the common coupling point is closed and the short-circuit ratio is less than 10; 4- The control method according to claim 1, characterized in that the transition to isolated mode takes place when the switch at the common coupling point is open. 5- The control method according to claim 1, characterized in that it estimates the inductance, resistance and voltage of the electrical network to which the electrical microgrid is connected in all operating modes; 6- The control method according to claim 1, characterized in that it allows the electrical microgrid to inject the appropriate quantities of active and reactive power by forcing the alternating current to follow a given reference, when the system is in connected mode to a stable electrical network; 7- The control method according to claim 1, characterized in that it forces the alternating voltage at the common coupling point to follow a well-defined reference, when the system is in mode connected to a weak electrical network and in isolated mode 8- The control method according to the preceding claims, characterized in that the regulation of the alternating current and the alternating voltage is ensured by the Backstepping method. 9- The control method according to the preceding claims, characterized in that the control system estimates the resistance and inductance of the electrical network by the adaptive backstepping method. 10- The control method according to the preceding claims, characterized in that the control system estimates the voltage of the electrical network and the current of the local alternating current load by the Grand Gain observer.