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Application of Inverter in Smart Grid Interface

Nov 17, 2020

Application of Inverter in Smart Grid Interface

  At present, the power system is still based on the centralized control method of high-voltage operation of conventional power plants. With the continuous access of passively controlled DERs and RESs, the ratio of controllable electrical energy will gradually decrease. The limitation of the excitation variables controlled by the power system will endanger the stability of the system. In order to achieve reliable control of the power system in the future, DERs and RESs must be authorized to actively participate in the regulation of the frequency and voltage of grid state variables.


  New DERs and RESs are gradually being added to the existing system under the guidelines of detailed basic operating principles. In order to participate in this control system, the existing conventional control strategy must be adjusted. The control strategy of the inverter as the grid connection must be based on the standard conventional power plant feed mode. According to the recommended strategy, DERs and RESs can actively participate in the physical control of the power system. Simulation research shows that when there are only inverters based on the recommended control method in the interconnected microgrids, the grid can still operate. This control strategy has been successfully verified on the inverter device in the laboratory. The application of the recommended control strategy standard is only the first step to establish equal conditions for DERS and RESs with conventional power equipment and to liberalize energy supply at the physical and technical levels.


When the components connected to the grid are DERs or RESs, the conventional basic control method can be adopted and implemented by the inverter. The power transmitted to the grid can be driven by the ECS or the grid. When the ECS drives the feed, the ECS determines the power transmitted to the grid. Nowadays, a single RESs inverter typically runs in this feed mode and injects all available power into the grid. When the grid drives the feed, it is no longer the ECS, but the grid determines the power transmission. Typically, most conventional large-scale power plants operate in this way. Also, this way is potentially suitable for DERs and RESs systems, or at least suitable for hybrid power systems. In the case of ECS drive feed, the inverter control method is called the grid parallel method. The second situation can be achieved by two different inverter control methods, namely the grid formation method and the grid support method. The role of the inverter in the grid formation method is to establish and maintain the grid state variables. In the grid support mode, inverters are used to balance power. It can transmit a preset amount of power, which can be adjusted according to the needs of the power system or the reference value obtained by advanced control operation. In addition, the potential interconnection of these basic active power regulators to the secondary grid control can be similarly explained.

 All the introduced control methods can be adapted to symmetrical and asymmetrical load conditions and inverter hardware. For these basic control methods, as introduced earlier, only grid formation methods and grid support methods are suitable for the control of the entire power system that is actively applied to the physical level. The distributed generation unit cannot be controlled from the grid side in the grid parallel mode. However, in addition to sufficient basic control methods, the power generation unit control must be able to interact with the defined ultra-conventional secondary grid regulator. This requirement can also be fulfilled by the basic control method described. Since these control structures are based on conventional power system control strategies, they provide the same secondary control interface as conventional power system control. Therefore, DERs and RESs with these functions can be connected to the grid control like conventional units. The secondary active power regulator in the grid is required to adjust the grid frequency to a normal value. It provides the active power offset value at the point of interest for the control structure of grid formation and grid support.

Simulation is the first step to verify the proposed control strategy. The control strategy adopts the conventional power system control strategy. The test conditions are: two interconnected power grids, each of which contains an inverter for grid formation and an inverter for grid support.

The rated apparent power of inverters for grid formation and grid support methods are Sr=125kVA and Sr=80kVA respectively. The active power and reactive power settings of the two inverters are 6kW and 3.3kvar respectively. The cable model used in the simulation is NAYY4×50SE, Rl=0.772Ω/km, Xl=0.083Ω/km. In order to compare the load distribution of different inverters, the initial value of the droop factor of all controllers is set the same. The secondary controller is used to control the power exchange and energy balance while maintaining the normal frequency.

The initial conditions for active and reactive loads are the same for the two power grids, which are 16kW and 7.3kvar respectively. This makes the power ratings of the whole system 32kW and 14.6kvar respectively. After 15s, a step load is added to the grid 1. Active power increased by 20.2kW, and reactive power increased to 7.37kvar.

Initially, each grid-forming inverter provided 10kW of power, and grid-supported inverters provided 6kW of power. Therefore, the two grid-forming inverters and the two grid-supporting inverters equally share the load power. At 15s, 4.2kW of load step power is added to the first grid. As the load changes, all inverters respond immediately, and power generation and consumption are redistributed.

 After a period of time, the secondary controller controls the inverter action, and the step load on the first grid is only compensated by the inverter on the first grid. The exchange power is controlled to return to the previous set value. The reactive power provided by the inverter is approximately 14.6kvar. At 15s, the reactive power of the first grid increased by 70var. As mentioned earlier, this simulation did not perform secondary control on reactive power. The grid-forming inverter compensates for the increased step load, and the grid-supported inverter provides the same amount of reactive power.

   Because both primary and secondary control affect the grid frequency, the impact is on different time scales.

Since a step load is added in 15s, the frequency drop is determined by the droop control function. After the primary control quickly stabilizes the frequency, the secondary control relatively slowly adjusts the frequency back to 50Hz. The inverter voltage is almost undisturbed, and at the same time, only the current adapts to the relevant load conditions. Take the three-phase voltage and three-phase current of the first grid load as an example to illustrate the load voltage and current quality and control performance. At 15s, the load is stepped into the grid. The load voltage remains almost unchanged, while the current increases with the step load.

The second part of verifying the standardized inverter control method introduced above is to test on a sufficient hardware platform. The three control structures have been realized under both symmetric and asymmetric conditions.

  At present, the power system is still based on the centralized control method of high-voltage operation of conventional power plants. With the continuous access of passively controlled DERs and RESs, the ratio of controllable electrical energy will gradually decrease. The limitation of the excitation variables controlled by the power system will endanger the stability of the system. In order to achieve reliable control of the power system in the future, DERs and RESs must be authorized to actively participate in the regulation of the frequency and voltage of grid state variables.


  New DERs and RESs are gradually being added to the existing system under the guidelines of detailed basic operating principles. In order to participate in this control system, the existing conventional control strategy must be adjusted. The control strategy of the inverter as the grid connection must be based on the standard conventional power plant feed mode. According to the recommended strategy, DERs and RESs can actively participate in the physical control of the power system. Simulation research shows that when there are only inverters based on the recommended control method in the interconnected microgrids, the grid can still operate. This control strategy has been successfully verified on the inverter device in the laboratory. The application of the recommended control strategy standard is only the first step to establish equal conditions for DERS and RESs with conventional power equipment and to liberalize the energy supply at the physical and technical levels.


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