Jayachandra N. Sakamuri

Improved frequency control from wind power plants considering wind speed variation

A fast frequency controller (FFC) for wind power plants (WPPs), which produces a temporary overloading power reference based on frequency deviation and rate of change of frequency, is proposed in this paper. Contrary to standard controllers proposed in the literature, the gains of the FFC are optimized for different wind speeds ensuring an improved frequency control from WPPs over the whole wind speed range. Two options for temporary frequency control implementations from WPPs are analyzed and compared. Moreover, the impact of mechanical, electrical and control limitations at different wind speeds and its effect on frequency control is discussed in the paper. Results show that by optimizing the gains, an improved frequency control can be obtained compared to standard controllers which apply a fixed gain over whole the wind speed range.

Coordinated Voltage Control in Offshore HVDC Connected Cluster of Wind Power Plants

This paper presents a coordinated voltage control scheme (CVCS) for a cluster of offshore wind power plants connected to a voltage-source converter-based high-voltage direct current system. The primary control point of the proposed voltage control scheme is the introduced Pilot bus, which is having the highest short-circuit capacity in the offshore AC grid. The developed CVCS comprehends an optimization algorithm, aiming for minimum active power losses in the offshore grid, to generate voltage reference to the Pilot bus. During the steady-state operation, the Pilot bus voltage is controlled by dispatching reactive power references to each wind turbine (WT) in the wind power plant cluster based on their available reactive power margin and network sensitivity-based participation factors, which are derived from the dV/dQ sensitivity of a WT bus w.r.t. the Pilot bus. This method leads to the minimization of the risk of undesired effects, particularly overvoltage at the terminals of the WT located far away from the AC collector substation, by dispatching lower reactive power references compared with the ones nearer to the substation. In addition, this paper proposes a control strategy for improved voltage ride through capability of WTs for faults in the offshore grid, thus leading to improved dynamic voltage profile in the offshore AC grid.

Dynamics of voltage source converter in a grid connected solar photovoltaic system

This paper emphasises the modelling and control of a voltage source converter (VSC) for three phase grid connected PV system. The transfer functions for inner current control and outer DC link voltage control for VSC are derived. The controllers for VSC are designed based on PI and K factor control methods and the performance of VSC using both the methods are presented with the simulations performed using PSCAD/EMTDC. The design of LC filter to meet the specified THD requirement for grid connected VSC is presented and the corresponding harmonic analysis is performed for different solar radiation conditions. The effectiveness of the proposed controller is illustrated by evaluating the response of PV system for power system voltage sag and swell conditions as well as grid faults.

Coordinated control scheme for ancillary services from offshore wind power plants to AC and DC grids

This paper proposes a new approach of providing ancillary services to AC and DC grids from offshore wind power plants (OWPPs), connected through multi-terminal HVDC network. A coordinated control scheme where OWPPs AC grid frequency modulated according to DC grid voltage variations is used to detect and provide the ancillary service requirements of both AC and DC grids, is proposed in this paper. In particular, control strategies for onshore frequency control, fault ride-through support in the onshore grid, and DC grid voltage control are considered. The proposed control scheme involves only local measurements and therefore avoids the need of communication infrastructure otherwise required for communication based control, and thus increases the reliability of the control system. The effectiveness of the proposed control scheme is demonstrated on a MTDC connected wind power system developed in DIgSILIENT PowerFactory.

Suitable method of overloading for fast primary frequency control from offshore wind power plants in multi-terminal DC grid

Increased penetration of offshore wind power plants (OWPPs) demands frequency control services from them. Overloading the wind turbine, for few seconds after the under frequency event, to utilize its kinetic energy seems promising option for fast primary frequency control. Two methods of overloading the wind turbine (WT), with and without considering the impact of WT dynamics and variation of WT output power during the overload, are proposed in the literature. In this paper, these two methods are applied for fast primary frequency control from OWPPs connected through multi-terminal DC grid considering the operation of the WT at below rated wind speed. Moreover, the impact of release of overload on the dynamics of the wind turbine, therefore on the associated AC and DC grids are studied in this paper. Finally, the suitable overloading method is proposed based on the simulation and experimental results. The time domain simulations for fast primary frequency control are performed on an OWPP connected through a 3-terminal DC grid using DIgSILENT PowerFactory. The experiments are performed on OWPP model integrated to a laboratory scale 3-terminal DC grid test set up. Based on the simulations and experimental results, overloading method which considers the variation of WT output power during the overload provides better performance during and after release of the overload.

Coordinated control of wind power plants in offshore HVDC grids

This paper presents a coordinated reactive power control for a HVDC connected cluster of offshore wind power plants (WPPs). The reactive power reference for the WPP cluster is estimated by an optimization algorithm aiming at minimum active power losses in the offshore AC Grid. For each optimal reactive power set point, the OWPP cluster controller generates reactive power references for each WPP which further sends the AC voltage/ reactive power references to the associated WTs based on their available reactive power margin. The impact of faults at different locations in the offshore grid, such as wind turbine (WT) terminal, collector cable, and export cable, on the dynamic voltage profile of the offshore grid is investigated. Furthermore, the dynamic reactive power contribution from WTs from different WPPs of the cluster for such faults has also been studied. Keywordsoffshore wind power plant; reactive power control; HVDC; offshroe grid faults

Coordinated fast primary frequency control from offshore wind power plants in MTDC system

In this paper, coordinated fast primary frequency control (FPFC) from offshore wind power plants (OWPPs) integrated to surrounding onshore AC power system through a three terminal VSC HVDC system is presented. The onshore AC grid frequency variations are emulated at offshore AC grid through appropriate control blocks, based on modulation of the DC grid voltage. The proposed FPFC produces a power reference to the OWPP based on the frequency deviation and its rate of change measured in the offshore AC grid. Moreover, the impact of wind speed variations on the OWPP active power output and the dynamics of wind turbine are also discussed. The corresponding impact of OWPPs active power output variation at different wind speeds on the power system frequency control and DC grid voltage is also presented. The results show that the proposed coordinated fast primary frequency control from OWPPs improves the power system frequency while relieving the stress on the other AC grid participating in frequency control.

Simulation and testing of novel Ferro-Transient Suppression Circuit for CVT

In this paper a typical 400 kV CVT is simulated for Ferroresonance and Transient Response conditions using Active and Passive Ferroresonance Suppression Circuits (FSCs) available in the literature. The effect of the FSC and other parameters on the transient response is analyzed. A novel thyristor based Electronic FSC is proposed in this paper which is suitable to suppress both the Ferroresonance and the transient response oscillations in a CVT. The hardware of the Electronic FSC is designed and validated for both the Ferroresonance and the transient response conditions using the simulations as well as the laboratory tests. The simulations and laboratory test results show that, with the new Electronic FSC, the Ferroresonance is suppressed within 5 cycles and the CVT is meeting all transient response classes (3PT1, 3PT2, and 3PT3) of IEC 60044-5. The simulation of the CVT is performed using PSCAD/EMTDC. Some practical concerns of this solution are also addressed.