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In recent years, photovoltaic (PV) power generation has developed rapidly at China and abroad. PV power generation is on the trend from auxiliary power supply to main power supply even leading power supply. Due to the fluctuation and intermittent characteristics of PV power generation, and zero inertia, the integration of large-scale grid-connected PV power generation may have a negative impact on the stability of the power system. Modeling grid-connected PV power generation systems is the key factor to investigate the influence between it and other parts of power system. Modeling of PV is also a hot issue that has been studied continuously for many years around the world. GE, WECC, CIGRE, CEPRI and so on have attached great importance to this issue and have published many research results. The existing research at China and abroad is reviewed in this paper, the existing problems are analyzed, and suggestions for future research are given.

This paper studies energy storage system participating to frequency regulation of wind farms based on polyline fuzzy neural network (PFNN). Firstly, the concept of generalized inertia of wind-storage combined system is analyzed. Secondly, the frequency requirements of wind-storage combined system are analyzed, and the method of PFNN is introduced. By taking frequency change, frequency change rate and state of energy storage system as the inputs of PFNN, a frequency control method is proposed. Finally, simulations have verified the correctness of the proposed method.

Due to the renewable generation target and the continuously increasing electrical demand, a large number of wind farms and solar photovoltaic (PV)plants are being planned in China. As the electricity fault characteristics of inverter-interfaced renewable energy sources are different from the traditional synchronous generators. The fault circuit contributions are getting more and more attention from engineers. In this paper, the short-circuit calculation method considering dynamic reactive power support of renewable energy systems is proposed. Through a large number of calculations, the factors affecting short-circuit currents and trends of short-circuit current under different voltage drop depths are discussed. The results presented by this paper provide the reference for future research on control and protection of power systems with high penetration of renewable energies.

Due to wind power forecasting error, active power control may not satisfy the control objective. By analysis, the main reason is the asymmetry of power forecasting error. For this, an active power control strategy considering the asymmetry of wind power forecasting error is proposed. Using historical data, wind power forecasting error and its confidence interval are estimated. And confidence interval of wind power forecasting is introduced to establish the optimization model for dispatching active power among wind farms. Then the asymmetry power forecasting error can be compensated by the model. Based on this, operation risk is introduced to optimize the power distribution strategy. Then a simulation is designed using actual data of one wind power base in China. The results show that the proposed strategy is reasonable and effective to improve the performance of active power control of wind power and reduce the risk of power shortage.

A centralized frequency regulation strategy of PV plant based on equal adjustable capacity proportion mode is proposed in this paper. When the system frequency changes due to some disturbances, the requirement of adjust active power value is calculated according to the frequency changing, then original active power reference is added with this adjust value to obtain the final active power reference as to change the active power output of PV units to participate in system primary frequency regulation. On this basis, in order to allow all PV units in same PV plant to coordinate with each other, also to prevent some PV units from excessive regulation, a proportional regulation of power method (PROP) is implemented to allocate active power reference to each PV unit more reasonable, namely, each PV unit's output is according to equal adjustable capacity proportion. Finally, an actual PV plant model is established on DIgSILENT PowerFactory platform to verify the effectiveness of the proposed control strategy.

In this paper, a steady-state multi-terminal voltage source converter high voltage direct current (VSC MTDC) model for power system analysis in DIgSILENT PowerFactory is introduced. Power flow is one of the most important and most widely used calculation in power system analysis. Although DIgSILENT PowerFactory provides a variety of elements for building MTDC grids, it's very inefficient when using the basic elements, such as ElmVscmono and ElmDcdc, to build multi-voltage level MTDC grids for example collection of large-scale renewable energy generation cluster. The aim of this paper is to derive custom steady-state VSC HVDC models to be incorporated into built-in AC/DC power flow computations that can handle power grids with high proportion of renewable energy generation connected. The hybrid modeling methods can effectively improve the modeling efficiency and extensibility. An AC/DC power flow is implemented using DIgSILENT PowerFactory to show the validity of the steady-state models.

In order to make full use of the active function of the photovoltaic (PV) power unit itself, a decentralized frequency regulation strategy for PV plant is proposed based on droop control idea in this paper. The active power regulation value, as an additional active power reference, is obtained according to the frequency regulation coefficient of PV unit; then the active power output of the PV unit is quickly regulated to the reference value, so that the frequency recovery of power system is restored. Since each PV unit in the power plant can achieve frequency regulation directly through the change of terminal frequency value, the response time will be faster than other control modes. Finally, a PV power plant which adopts the decentralized frequency regulation system is modelled on the DIgSILENT PowerFactory platform, and the control effect of the decentralized frequency regulation is verified by simulation results under different disturbances.

In order to make full use of the ability of photo voltaic (PV) units to regulate their own reactive power to participate in voltage stability control of power system, this paper takes full account of the interaction between power grid and PV power plant and proposes a multi-level reactive power control strategy based on sensitivity analysis for PV plant. Firstly, reference of reactive power is calculated combined with the analysis of reactive power and voltage sensitivity of PV power plant and the operation status of power grid. Secondly, reactive power command is allocated to each PV unit according to the reactive power margin. Finally, a reactive power control system model based on an actual PV plant project is established to verify the correctness of proposed strategy in this paper.

Qualitative and quantitative analysis are employed to estimate the possibility of sub-synchronous resonance (SSR) for a 500kV series compensated transmission system in this paper. Firstly, the torsional mode frequencies and mode shapers in sub-synchronous band are calculated by analysis method. Secondly, the impedances in different frequencies are computed after processing this system at the instance of frequency scanning principle, and then according to impedance-frequency curve, the risk of SSR for the system is evaluated qualitatively. Finally, the time-domain simulation is adopted to the system at different operating conditions on the basis of PSCAD/EMTDC. The results show that the shaft torsional vibration modal calculated by analysis method and time-domain simulation are consistent; the shaft torsional vibration intensity and modal vary a lot from operating conditions; under the condition that the electrical and mechanical parameters are the same, with the number of parallel generator increasing, the interaction of different units is more apparent and the shaft torsional vibration intensity is more serious; the torsional vibration will not take place at the shaft in generator segment under any conditions.

The accuracy of wind plant power control strategy is the key to ensure the adaptability of the wind plant. Effective testing and verification are essential means for the correctness of the control strategy. In this paper, the principle of active and reactive power control system for wind plant is studied firstly, the implementation architecture of power control system of wind plant is analyzed then, the modules of the power control system of the wind plant is implemented according to the modular design thirdly. Finally, a wind plant model with practical engineering background is established to validate the feasibility of the proposed wind plant power control system by injecting the measured data.