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Monday, 26 February 2018

Phase Angle Calculation Dynamics of Type-4 Wind Turbines in rms Simulations during Severe Voltage Dips


To conduct power system simulations with high shares of wind energy, standard wind turbine models, which are aimed to be generic rms models for a wide range of wind turbine types, have been developed. As a common practice of rms simulations, the power electronic interface of wind turbines is assumed to be ideally synchronised, i.e. grid synchronisation (e.g. phase locked loop (PLL)) is not included in simplified wind turbine models. As will be shown in this study, this practice causes simulation convergence problems during severe voltage dips and when the loss of synchronism occurs. In order to provide the simulation convergence without adding complexity to the generic models, a first-order filtering approach is proposed as a phase angle calculation algorithm in the grid synchronisation of the rms type-4 wind turbine models. The proposed approach provides robustness for the simulation of large-scale power systems with high shares of wind energy.



Fig. 1 Test network for the fault cases


 Fig.2 Simulation results with a PI-based PLL method and the proposed LPF method, during a severe fault (VPoC = 0.1%) (a) VPoC (pu), (b) Active current (Id) reference (solid grey), actual in PLL case (dashed grey) and actual in LPF case (dotted black) (pu), (c) Reactive current (Iq) reference (solid grey), actual in PLL case (dashed grey) and actual in LPF case (dotted black) (pu), (d) WT terminal voltage angle (wrap between ±180) in PLL case (dashed grey) and in LPF case (dotted black) (degrees)

Fig. 3 Simulation results of the proposed LPF method with threshold triggering function during a severe fault (VPoC = 0.1%) (a)VPoC (pu), (b) Active current (Id) reference (solid grey) and actual (dotted black) (pu), (c) Reactive current (Iq) reference (solid grey) and actual (dotted black) (pu), (d) WT terminal voltage angle (wrap between ±180) (degrees)

Owing to the large share of wind power in power systems of certain countries, especially in Europe, the need of power system analysis with WPPs arises. WT and WPP models have been developed by the academia and WT manufacturers, which are chosen as rms models in order to provide computational simplicity and speed, considering large-scale simulations. In addition, standards for the developed WT and WPP models have been developed by IEC and WECC working groups. As a common requirement of the grid codes, the developed models are utilised for short-term voltage stability and fault ride-through studies. The conventional method of instantaneous phase angle calculation, i.e. ideal grid synchronisation, performs well with moderately low voltage faults. However, it is shown that the physical fact of the LOS of WT converters during severe voltage dips is observed to cause simulation non-convergence problems with the instantaneous angle calculation method in rms simulations.
In this paper, a first-order LPF is proposed as angle calculation method in converter-based WTs in order to solve the nonconvergence errors during severe voltage faults. The proposed LPF method is shown in order to solve the non-convergence errors even for solid faults (i.e. a zero impedance three-phase fault). In addition, implementing a threshold triggering function to activate the LPF-based calculation only during severe voltage dips avoided any possible impairment during healthy steady-state operations. Moreover, the frequency deviation due to the LOS problem, which needs to be solved with additional control methods, is shown to be represented with the LPF method. Hence, the proposed LPF method gives the possibility to detect if the WPP experiences the LOS and hence to solve it.
The recently published IEC 61400-27-1 WTs electrical simulation models standard [1] has adopted the proposed idea of utilising a first-order LPF as phase angle calculation in rms simulations, which helps to obtain robust simulations, especially when a large-scale (e.g. Pan-European) power system with high share of wind power is simulated and analysed.
[1] IEC 61400-27-1 Ed. 1: ‘Wind turbines – part 27-1: electrical simulation models – wind turbines’, February 2015
[2] Asmine, M., Brochu, J., Fortmann, J., et al.: ‘Model validation for wind turbine generator models’, IEEE Trans. Power Syst., 2011, 26, (3), pp. 1769– 1782
[3] ENTSO-E: ‘Network code for requirements for grid connection applicable to all generators (NC RfG)’, European Network of Transmission System Operators for Electricity ENTSO-E, 2015
[4] Goksu, O., Teodorescu, R., Bak, C.L., et al.: ‘Instability of wind turbine converters during current injection to low voltage grid faults and PLL frequency based stability solution’, IEEE Trans. Power Syst., 2014, 29, (4), pp. 1683–1691
[5] Erlich, I., Shewarega, F., Engelhardt, S., et al.: ‘Effect of wind turbine output current during faults on grid voltage and the transient stability of wind parks’. IEEE Power and Energy Society General Meeting, July 2009, pp. 1–8