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Thursday, 13 March 2014
09:00 - 10:30 Electrical aspects and grid integration
Science & Research  


Room: Llevant
Session description

The grid integration of wind power has always been a challenge. In this session a range of speakers from academia and industry will address the impact of wind on network operation and the development of a large scale offshore HVDC grid, offshore systems, the state-of-the-art in HVDC technologies, as well as more specific topics including the design of offshore networks from a reliability perspective and the transient response of HVDC links.

Learning objectives

  • Understand the effect of wind power on grid operation and development via case studies
  • Get an overview of the state of the art in HVDC offshore grid research
  • Get acquainted with specific topics of offshore grids, including reliability and transient response aspects
Lead Session Chair:
Stavros Papathanassiou, National Technical University of Athens, Greece

Co-chair(s):
John Olav Tande, SINTEF, Norway
Sotirios Nanou National Technical University of Athens, Greece
Co-authors:
Sotirios Nanou (2) F P Stavros Papathanassiou (2) Stavros Papathanassiou (2)
(1) National Technical University of Athens, Kifissia, Greece (2) National Technical University of Athens, Athens, Greece

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Presenter's biography

Biographies are supplied directly by presenters at EWEA 2014 and are published here unedited

Sotirios I. Nanou received the Diploma in Electrical and Computer Engineering, in 2009, and the Postgraduate Diploma in Energy Production and Management, in 2011, both from the National Technical University of Athens (NTUA), Greece, where he is currently working towards the Ph.D. degree in the Laboratory of Electric Machines and Power Electronics. He worked for the Distribution Division of the Public Power Corporation of Greece in distribution equipment studies and the design of HV GIS substations. His research mainly deals with grid integration of offshore wind farms and island networks through VSC-HVDC links.

Abstract

Evaluation of a communication-based fault ride-through scheme for offshore wind farms connected through VSC-HVDC links.

Introduction

Present day grid codes impose technical requirements for the connection of offshore generating units [1]. Fault Ride-Through (FRT) capability is a basic technical requirement which foresees that an offshore station should remain in operation during onshore grid faults. Several FRT schemes can be found in the literature applicable to offshore wind farms connected through High Voltage Direct Current (HVDC) transmission, such as energy dissipation means (DC resistors), regulation of the offshore frequency [2], controlled voltage drop [3], etc. In this paper, a FRT scheme is investigated, which is based on communication between the offshore HVDC station and the controller of each wind generator.

Approach

In order to evaluate the FRT capability of the offshore wind farm (WF) under study, a suitable simulation model has been developed in SimPowerSystems Toolbox of Matlab/Simulink using the EMT simulation method. A 300 MW offshore wind farm with Voltage Source Converter (VSC)-based HVDC connection to the onshore grid has been considered as a study case.
It is assumed that the offshore wind farm is equipped with a central park controller which performs the task of active and reactive power regulation when needed. According to [4], such communication interfaces are usually implemented using Supervisory Control And Data Acquisition (SCADA)-protocols. The WF controller delivers active power set-points to each wind turbine (WT) to reduce the active power generated by the WTs, when onshore grid faults occur. Utilizing the existing communication infrastructure to perform the aforementioned task is a simple and effective solution. However, a detailed investigation is required to evaluate its performance in the presence of communication delays.
The FRT capability of the offshore WF is evaluated via simulation under various operating conditions. Specifically, different onshore voltage dips have been applied during operation near rated power, taking into account voltage dip magnitude and duration requirements imposed by grid codes [1]. The communication delay inevitably introduced in the overall control scheme is taken into account in order to investigate its impact on the FRT response of the system. For this purpose, a parametric analysis is conducted, simulating the WF response under various transport delay values. The main assessment criterion is the maximum dc over-voltage of the HVDC link during fault conditions.

Fig.1: Generic layout of an offshore WF connected through VSC-based HVDC transmission.

Main body of abstract

The generic layout of the WF is depicted in Fig. 1. The WTs are connected to the collector bus through MV cables. Each WT is equipped with a multi-pole permanent magnet synchronous generator (PMSG), using full power converters. To simplify converter modeling, conventional 3-phase two-level converters have been assumed and an aggregate 300-MW PMSG WT is used to represent the entire WF. PWM control is applied, with switching frequencies of 1.95 kHz for the HVDC and 2.25 kHz for the PMSG converters [3].
The dc voltage dynamics are strongly affected by the total dc capacitance (sum of VSC and cable capacitances). Based on [5], the sub-module capacitance in the state-of-the-art multi-level VSCs for HVDC applications is chosen so that the relation between the total energy storage and the converter rating is 30-40 kJ/MVA, which should lead to a voltage ripple in the range of 10%. In this paper, a value of 40 kJ/MVA has been chosen for the total DC capacitance Cdc of the equivalent two-level converter used in the simulations (Fig. 1). This selection has been made in order to reproduce the expected voltage rise during power flow imbalances.
In the following, the control concept employed for the HVDC and PMSG VSCs is briefly discussed, along with the proposed FRT scheme. The offshore HVDC VSC, shown in Fig. 1, transmits the active power produced by the WF, while regulating the AC voltage and frequency in the WF grid [2]. The main task of the onshore HVDC VSC is to control the dc voltage to its reference value, thereby exporting the incoming DC power to the grid.
The typical configuration of a Full-power Converter Wind Turbine (FCWT) based on a multi-pole PMSG is illustrated in Fig. 2 [6]. It is connected to the MV grid through an output LCL filter and a step-up transformer. The PMSG is controlled by the generator side converter which implements the Maximum Power Point Tracking (MPPT) strategy, whereas the grid side converter regulates the dc link voltage.

Fig. 2: Communication-based FRT scheme for WFs equipped with FCWTs.

During an onshore ac fault, the active power of the onshore HVDC converter will be drastically reduced. To avoid an overvoltage at the dc side and a subsequent system trip, the active power generated by the WF needs to be immediately reduced.
To perform this task, dc over-voltages detected at the offshore VSC can be used as an effective indication of fault conditions on the onshore grid. As shown in Fig. 2, when the measured dc voltage exceeds a predefined threshold (1.05 pu in this study), a power reduction strategy is activated, modulating the power reduction factor σp. The de-loading signal is then transmitted to each WT via a communication link, characterized by a transport delay represented by the time delay exp(-sTcom) in the model of Fig. 2. A parametric analysis is conducted within this paper, to investigate the effect of the transport delay on the FRT capability of the HVDC system.
Figs. 3-8 demonstrate the system response following a 90% dip of the onshore grid voltage, lasting 200 ms. To evaluate FRT capability under worst case conditions, the WF is assumed to operate close to its rated power. Four different communication time delay values are tested, 1, 20, 40 and 60 ms. For transport delay values up to 40 ms, successful FRT performance is achieved. The dc over-voltage at the offshore VSC is maintained below 1.3 pu, a typical threshold above which the DC over-voltage protection is triggered [3]. However, the above threshold was exceeded when Tcom becomes 60 ms. This indicates that an upper limit exists for the time delay introduced by the communication link in order to maintain successful FRT response. This limit depends strongly on the total dc capacitance. For the capacitance used in this study, the communication-based FRT concept remains effective even when relatively high communication delays are involved. As for the WT rotor dynamics, the rotor speed increase during the fault, assisted by the action of the pitch regulator, is entirely acceptable.

Fig. 3: WF output active power.



Fig. 4: HVDC voltage at offshore VSC.



Fig. 5: DC voltage at PMSG converter.



Fig. 6: PMSG electromagnetic torque.



Fig. 7: PMSG rotor speed.



Fig. 8: WT pitch angle.

Conclusion

In this paper, a communication-based FRT approach has been presented for offshore WFs connected through a VSC-based HVDC link. Specifically, a fast communication link between the offshore HVDC station and each WT is utilized, to achieve fast WT output power reduction in case of on-shore faults. The main objective of this study is to evaluate the FRT response assuming different communication delays. For this purpose, a parametric analysis is conducted, simulating the WF response under various transport delay values. The main assessment criterion is the maximum dc over-voltage of the HVDC link during fault conditions.
Results obtained from time domain simulations demonstrate the satisfactory operation of the WF during onshore grid faults, even in the presence of non-negligible communication lag. The dc voltage in the HVDC link is generally maintained within permissible limits, whereas the WT response is also entirely acceptable. However, simulation results indicate that an upper limit exists for the time delay introduced by the communication link in order to maintain successful FRT response. This limit depends strongly on the total dc capacitance. For the capacitance used in this study, the communication-based FRT concept remains effective even when relatively high communication delays are involved.
The proposed method can be easily implemented, as it employs communication infrastructure which already exists in the state-of-the-art offshore wind farms. Further, the default control schemes of the generator side and grid side converters of each FCWT do not require any essential modification, while the high cost of fully-rated DC choppers, installed either at the HVDC station or within each WT, can be avoided.



Learning objectives
The main objective is to investigate the FRT capability of a VSC-HVDC connected offshore wind farm, using a communication-based method and evaluate the impact of the communication delay. To this end, a typical offshore WF and representative WT and HVDC control configurations are employed. It is shown that successful FRT performance is possible even in the presence of relatively high communication delays, in the range of 30-40 ms.


References
[1] “Requirements for Offshore Grid Connections in the Grid of TenneT TSO GmbH”, TenneT TSO GmbH, Dec. 2012.

[2] L. Xu, L. Yao, C. Sasse, “Grid Integration of Large DFIG-Based Wind Farms Using VSC Transmission”, IEEE Trans. Power Systems, Vol. 22, No. 3, Aug. 2007.

[3] C. Feltes, H. Wrede, F. W. Koch, I. Erlich, “Enhanced Fault Ride-Through Method for Wind Farms Connected to the Grid Through VSC-Based HVDC Transmission”, IEEE Trans. Power Systems, Vol. 24, No. 3, Aug. 2009.

[4] R. L. Hendriks, R. Volzke, W. L. Kling, “Fault Ride-Through Strategies for VSC-Connected Wind Parks”, in Proc. EWEC 2009, Marseille, France.

[5] B. Jacobson, R. Karlsson, G. Asplund, L. Harnefors, T. Jonsson, “VSC-HVDC transmission with cascaded two-level converters”, in Proc. Int. Council Large Elect. Syst. Session, 2010, pp. B4-110.

[6] S. K. Chaudhary, R. Teodorescu, P. Rodriguez, P. C. Kjaer, “Control and Operation of Wind Turbine Converters during Faults in an Offshore Wind Power Plant Grid with VSC-HVDC Connection”, in Proc. IEEE Power and Energy Society General Meeting, 2011, San Diego, CA.