Modeling and Control of Multi-Terminal HVDC with Offshore Wind Farm Integration and DC Chopper Based Protection Strategies

 

ABSTRACT:

Multi-Terminal HVDC based on three-level neutral-clamped voltage source converters (VSC) is an ideal approach for the integration of DFIG wind farms to the power grid. However, dc-link faults and ac faults are major concerns for the safety and consistency of VSC-HVDC system. This paper demonstrates methods employing both full bridge and half bridge DC-DC converters for the fast clearance and protection of dc and ac ground faults respectively. In addition, control strategies incorporating decoupling control and feed-forward compensation on both grid side and wind farm side VSCs are also presented. Normal operations are observed to examine the performance of the MT-HVDC system, and also dc-link fault and three-phase ground fault at inverter side are simulated to verify the effectiveness of the approach employing DC-DC converters to suppress dc current overshoot in case of dc-link fault and mitigate dc voltage overshoot during three-phase ac ground fault. This proposed MT-HVDC transmission system and the fault-ride through capabilities provided by the dc choppers is validated by the simulation studies using detailed Matlab/Simulink model for normal operation, dc and ac ground faults.

KEYWORDS:

  1. VSC-HVDC
  2. DFIG
  3. DC chopper
  4. Faults

 SOFTWARE: MATLAB/SIMULINK

 BLOCK DIAGRAM:

Fig. 1 Topology of the proposed multi-terminal VSC-HVDC system.

EXPECTED SIMULATION RESULTS:

 

Fig. 2 Simulation results of MT-HVDC during normal operation: (a) active power of wind farm, (b) dc voltage, and (c) ac rms current.

Fig. 3 Simulation results of 6 DFIG units during normal operation: (a) active power, (b) reactive power, (c) ac rms voltage, and (d) back-to-back dc-link voltage of DFIG unit.

Fig. 4 Simulation results of MT-HVDC during dc pole-to-pole fault with and without full bridge dc chopper protection: (a) dc voltage, and (b) dc current.

Fig. 5 Simulation results of MT-HVDC during three-phase ac ground fault at inverter side with and without half bridge dc chopper protection: (a) ac rms voltage at inverter side, (b) dc voltage overshoot without protection measures, and (c) dc voltage with protection measures.

CONCLUSION:

This paper investigates a multi-terminal VSC-HVDC system, which integrates two DFIG wind farms to the ac grid. The control strategies of both WFVSC and GSVSC stations are discussed in detail, and two approaches employing both full bridge and half bridge dc choppers are extended and displayed. Simulation studies are carried out in normal, dc pole-to-pole and ac ground fault operations, and the result verifies the effectiveness of the proposed MT-HVDC system in both the performance of wind power delivery and the protection measures for various fault conditions. Specifically, the dc voltage drop and dc current overshoot are eliminated during dc fault with full bridge dc choppers, while only a 8% voltage overshoot is observed with the implementation of half bridge dc choppers in case of three-phase ac ground fault.

REFERENCES:

[1] S. G. Hernandez, E. M. Goytia and O. A. Lara, “Analysis of wide area integration of dispersed wind farms using multiple VSC-HVDC links,” in Proc. of EPE, Sevilla, pp. 17-26, 2008.

[2] S. Towito, M. Berman, G. Yehuda and R. Rabinvici, “Distribution generation case study: electric wind farm doubly fed induction generators”, in Proc. Convention of Electrical and Electronics Engineering(CEEE), Israel, pp. 393-397, Nov. 2006.

[3] N. Flourentzou, V. G. Agelidis, and G. D. Demetriades, “VSC-based HVDC power transmission systems: an overview,” IEEE Trans. Power Electron., vol. 24, no. 3, pp. 592-602, Mar. 2009.

[4] L. Xu, L. Yao, and C. Sasse, “Grid integration of large DFIG-based wind farms ssing VSC transmission,” IEEE Trans. Power Syst., vol. 22, no. 3, pp.976-984, Aug. 2007.

[5] L. Weimers, “HVDC Light: A new technology for a better environment”, IEEE Power Eng. Review, vol. 18, no. 8, pp.19-20, Aug. 1998.

Dynamic Behavior of DFIG Wind Turbine Under Grid Fault Conditions

 

ABSTRACT:

The use of doubly fed induction generators (DFIGs) in wind turbines has become quite common over the last few years. These machines provide variable speed and are driven with a power converter which is sized for a small percentage of the turbine-rated power. This paper presents a detailed model of induction generator coupled to wind turbine system. Modeling and simulation of induction machine using vector control computing technique is done. DFIG wind turbine is an integrated part of distributed generation system. Therefore, any abnormalities associates with grid are going to affect the system performance considerably. Taking this into account, the performance of DFIG variable speed wind turbine under network fault is studied using simulation developed in MATLAB/SIMULINK.

KEYWORDS

  1. DFIG
  2. DQ Model
  3. Vector Control

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig. 1 Simulink model of DFIG system

EXPECTED SIMULATION RESULTS:

 Time (sec)

 Fig. 2 Stator currents during balance condition

Time (sec)

Fig. 3 Rotor currents during balance condition

   Time (sec)

Fig. 4 Speed and torque during balance condition.

Time (sec)

Fig. 5 Acive and reactive power during balance condition

CONCLUSION:

This paper presents a study of the dynamic performance of variable speed DFIG coupled with wind turbine. The dynamic behavior of DFIG under power system disturbance was simulated using MATLAB/SIMULINK.Accurate transient simulations are required to investigate the influence of the wind power on the power system stability. The DFIG considered in this analysis is a wound rotor induction generator with slip rings. The stator is directly connected to the grid and the rotor is interface via a back to back power converter. Power converter are usually controlled utilizing vector control techniques which allow the decoupled control of both active and reactive power flow to the grid. In the present investigation, the dynamic DFIG performance is presented for both normal and abnormal grid conditions. The control performance of DFIG is satisfactory in normal grid conditions and it is found that, both active and reactive power maintains a study pattern in spite of fluctuating wind speed and net electrical power supplied to grid is maintained constant.

REFERENCES:

[1] T. Brekken, and N. Mohan, “A novel doubly-fed induction wind generator control scheme for reactive power control and torque pulsation compensation under unbalanced grid voltage conditions”, IEEE PESC Conf Proc., Vol 2, pp. 760-764, 2003.

[2] L. Xu and Y. Wang, “Dynamic modeling and control of DFIG-based wind turbines under unbalanced network conditions”, IEEE Trans. On Power System, Vol 22, Issues 1, pp. 314-323, 2007.

[3] F.M. Hughes, O. Anaya-Lara, N. Jenkins, and G. Strbac, “Control of DFIG based wind generation for power network support”, IEEE Trans. On Power Systems, Vol 20, pp. 1958-1966, 2005.

[4] S. Seman, J. Niiranen, S. Kanerva, A. Arkkio, and J. Saitz, “Performance study of a doubly fed wind-power induction generator Under Network Disturbances”, IEEE Trans. on Energy Conversion, Vol 21, pp. 883-890, 2006.

[5] T. Thiringer, A. Petersson, and T. Petru, “Grid disturbance response of wind turbines equipped with induction generator and doubly-fed induction generator”, in Proc. IEEE Power Engineering Society General Meeting, Vol 3, pp. 13-17, 2003.