Distributed power generation digital-analog hybrid simulation system based on NI-PXI

1 Introduction
Faced with the dual pressures of energy crisis and environmental protection, all countries are actively researching new energy DG technology. This technology can greatly improve the utilization rate of primary energy and reduce exhaust gas emissions, but it also brings new challenges to the operation and management of traditional power grids. The grid-connected operation control of DERs, the interaction between DERs and the power grid, and the dispatching management of DERs are the basic research topics for the application of DG technology.
At present, the main means of studying the above problems are still physical simulation and digital simulation. Physical simulation is a dynamic model experiment, which has a clear physical meaning, but is greatly limited by the simulation scale and extreme working conditions. It cannot fully simulate the actual system operation, and the diversity of DERs also makes it difficult to implement physical simulation; digital simulation is software simulation. Although it is not limited by the scale and structural complexity of the research object, the simulation model is usually simplified to varying degrees, and the accuracy is not as good as the physical model simulation, and it cannot simulate unknown or difficult to describe physical phenomena with mathematical expressions. In recent years, with the development of computer technology, the digital-analog hybrid simulation technology that combines the advantages of both has received more and more attention and has been promoted and applied to a certain extent.
Here, an implementation scheme of a DG digital-analog hybrid simulation system is proposed for the research of DERs grid-connected related technologies. The scheme uses PXI as a real-time digital simulation platform and a controllable power source based on a dual PWM converter as a digital-analog hybrid simulation interface, which is connected to the dynamic model experimental system to realize digital-analog hybrid simulation. Finally, taking the grid-connected control of a doubly fed wind turbine (DFIG) as an example, a digital-analog hybrid simulation system is constructed, and the feasibility of the designed scheme is verified through experiments.

2 Digital-analog hybrid real-time simulation system
2.1 System architecture
Figure 1 shows the general structure of a digital-analog hybrid real-time simulation system. For the control research on the distributed power supply side, the hardware in the loop (HIL) real-time hybrid simulation technology can be used. The scheme is: the digital simulation model is the DG system model, the physical model is the actual system control and protection device, and the hybrid simulation interface completes the signal matching between the two, realizing the control and protection strategy under hybrid real-time simulation. For the research on the interaction between DERs and the power grid, different hybrid simulation schemes are used according to different research contents. When focusing on the transient behavior of the power supply side, a digital model of the power grid is usually constructed, and the power supply is a physical model; conversely, when focusing on the dynamic behavior of the power grid side, the power grid is a physical model and the power supply is a digital model. Regardless of which of the above hybrid simulation solutions is adopted, since the digital model and the physical model are the signal system and the energy system respectively, it is necessary to realize the information mapping from the digital signal system to the physical energy system through the digital-analog hybrid simulation interface, which requires high-performance power amplification equipment.

In view of the research background of DERs grid-connected coordinated control and energy management, a digital-analog hybrid simulation system based on the NI-PXI platform is proposed. The digital simulation part is the DERs model, such as photovoltaic power generation system or wind power generation system, etc. The physical part is the simulated power grid, which is built in the dynamic model laboratory and includes dynamic model generator sets, lines and loads. A controllable power source is used as the hybrid simulation interface. The output power of the DERs digital simulation model is fed into the physical simulation platform in real time.

In a third-party modeling environment, such as Matlab/Simulink, a digital simulation model of DERs is established, and the model dynamic link library is compiled and generated. The generated dynamic link library is deployed to the PXI VeriStand real-time engine through the VeriStand system resource manager on the host, and the real-time simulation running status is observed through the host VeriStand workspace. The PXI VeriStand real-time engine communicates with the host VeriStand workspace and the stimulus configuration file editor in real time to obtain model external parameters, such as wind speed, light, power adjustment instructions, etc., so that the real-time simulation can simulate the changes in external conditions.
2.3 Implementation of wind power grid-connected real-time simulation experiment system
Figure 3 is a digital-analog hybrid simulation experiment system built with wind power grid-connected as an example. As shown in the figure, the PXI platform runs the real-time digital model of DFIG and grid-connected control, receives the wind speed signal given by the human-machine interface, calculates the output power of DFIG in real time, and connects the output power given signal to the analog input port of the controllable power source PWM controller through the analog output port of the multi-function I/O module NI PXI-7851R, thereby realizing the given active and reactive power instructions. In addition, the grid response is fed back to the digital model of the wind turbine by collecting the grid connection point voltage.

The rotor-side converter control uses stator voltage oriented vector control technology to adjust the amplitude and frequency of the rotor voltage to achieve decoupling control of active power and reactive power on the stator side, while the grid-side converter control uses grid voltage oriented vector control to maintain DC side voltage stability. Pitch angle control is used for speed and power limit of DFIG. When the generator speed is higher than the maximum speed or the system output power exceeds the rated power, the pitch angle control system adjusts the wind turbine pitch angle β to limit the wind turbine from capturing wind energy to achieve speed or power limit.

[page]4 Control of controllable power source based on dual PWM converters
The control methods of three-phase voltage-type PWM converters are divided into direct current control and indirect current control, among which the indirect current control has a simple structure. Indirect current control based on voltage vector amplitude-phase control is adopted here. The control strategy is: PWM control on the rectifier side maintains the DC bus voltage constant and unity power factor operation, and PWM control on the inverter side adjusts the phase and amplitude of the inverter voltage vector to track active and reactive power instructions. The control principle is shown in Figure 5.

5 Experimental results
The digital-analog hybrid simulation experimental system shown in Figure 3 was established. The rated parameters of the controllable power source are: Pn=30 kW; Uo=380 V; rectifier side inductance LR=1.68 mH; inverter side LI=2.26 mH. The main parameters of DFIG are: stator resistance Rs=0.023 pu, stator leakage inductance Ls=0.18 pu, rotor resistance Rr=0.016 pu, rotor leakage inductance Lr=0.16 pu, excitation inductance Lm=2.9 pu, unit moment of inertia H=4.32 s. Figure 6 shows the results of DFIG hybrid real-time simulation experiment.

Figure 6c is a comparison curve between the power command output by the PXI real-time simulation model and the actual output power of the controllable power source. By comparison, it can be seen that the output power of the controllable power source can quickly follow the change of the power command, accurately reflecting the power change characteristics of the DFIG under the condition of wind speed change.
Figure 6d is the motor speed change curve and rotor current change waveform during a certain period of time in the DFIG real-time simulation process. As the wind speed increases, the DFIG speed increases, and the rotor current provides slip power to the DFIG at the slip frequency under the regulation of the controller, and the motor transitions from the subsynchronous operation state to the supersynchronous operation state.

6 Conclusion Aiming
at the experimental platform for the research of distributed power grid-connected control technology, a digital-analog hybrid real-time simulation system design scheme is proposed. PXI is used as the digital real-time simulation platform, and the controllable power source based on the dual PWM converter is used as the power interface between the digital part and the physical part of the simulation system. The feasibility of the scheme is verified by constructing a grid-connected hybrid simulation experimental system for a doubly fed wind turbine.