Elsevier

Renewable Energy

Volume 101, February 2017, Pages 945-963
Renewable Energy

Assessment of a non linear current control technique applied to MMC-HVDC during grid disturbances

https://doi.org/10.1016/j.renene.2016.09.050Get rights and content

Highlights

  • Assessment of a grid side MMC.

  • Non linear current control of MMC-HVDC.

  • Dynamic performance is improved.

  • Tests with one, two and three phase to ground faults.

  • A solution to improve electric protections operation.

Abstract

Nowadays, the most advanced solution to connect offshore wind farms to the mainland grid is the use of High Voltage DC converters. Among the variety of High Voltage DC technologies available, the Modular Multi-Level Converter stands out. It is totally different to the traditional two or three level Voltage Source Converters, and it uses a voltage source control scheme presenting advantages such as a better harmonic content, but also some disadvantages such as a more complex control system.

However, its dynamic performance can be improved by using a non linear current control technique, taking advantage of its faster response during electrical transients when compared to voltage source or linear current techniques. In this assessment, it has been compared with the voltage source control technique by using a model of an offshore wind farm connected to the mainland grid. In order to make a more comprehensive study, tests with one, two and three phase to ground faults have been carried out. Finally, a solution to improve electric protections operation by increasing the current injected by the Modular Multi-Level Converter in the Point of Common Coupling above the converter rated current value during voltage sags has been assessed.

Introduction

High Voltage DC (HVDC) systems are being used with great success in offshore wind farms with mainland grid connections because of the advantages such as the decrease in the cable price for long distances, or the absence of reactive power absorption in the transmission line [1], [2].

A less widespread technology, because of its novelty and complexity, is the Modular Multi-Level Converter (MMC), Fig. 1, proposed for HVDC applications in 2003 by Marquardt [3] and first used commercially in the Trans Bay Cable project in San Francisco [4]. This topology presents some advantages such as: energy storage is distributed, it is a modular topology so it is easily scalable due to the high number of levels, filters and transformer may not be necessary and the resulting switching frequency is high [5], [6], [7]. Some disadvantages are the cost of semiconductors and drivers. MMC technology has been used in one of the world's largest wind farms (DolWin2), to build a 135 km long HVDC line.

As it is known, electrical grids with a high penetration of renewable energy are subject to compliance with strict grid codes developed by the System Operators to ensure the proper functioning of these systems and their reliability from the point of view of the grid. One of the most relevant grid code requirements is the Low Voltage Ride Through (LVRT) capability of wind generators [8], [9], [10]. In the case of a voltage sag in the connection point, typically due to a short-circuit in some point of the grid, generators must remain connected instead of tripping, and do it according to the characteristics of the voltage sag to be withstood, duration, depth, and time profile imposed by the corresponding grid code.

Once the LVRT problem in wind farms has been overcome, the next step for renewable energies is to participate in other ancillary services [11], [12] such as voltage control in the Point of Common Coupling (PCC) [13], [14] or primary and secondary frequency support [15].

Another desirable characteristic would be to reproduce the inertial response of the spinning reserve provided by synchronous generators in the electrical grid. This would contribute significantly to improve system stability when facing power oscillations in the grid during electrical disturbances, and it is particularly useful in power systems with high penetration of renewable energies with a high degree of variability in power generation, such as wind power or photovoltaic. Finally, this would also contribute to make the injected current more similar to the one provided by synchronous generators, therefore improving the operation of electrical grid protections.

All of these services depend on the control system design of the grid side converter, in this case a MMC converter. Depending on the circumstances, it may be more advantageous to use a voltage or current control, a PLL based synchronization or a virtual inertia emulator, etc.

There are a number of interesting issues to work on, related to the control of MMC, such as converter operation under unbalanced grid faults [16], the calculation [17], [18], suppression [19] or reduction [20] of circulating currents, as well as voltage modulators [5], [16], [19] or current source control systems [21]. In Ref. [21], the non-linear current control (NLCC) for connecting an MMC to a balanced grid was presented; in this paper, control strategies to operate during grid faults have been incorporated to NLCC, as well as a system to increase the current injected by the MMC into the grid during grid faults.

In this paper, several possibilities and combinations of control strategies for single-phase to ground, phase to phase or three-phase faults were studied. The structure of the paper is as follows, after the introduction, the MMC topology (Section 2), non linear current control (NLCC) and voltage control (Section 3) are briefly explained. Section 4 explains the overall control strategy for wind farm side and grid side converter of the HVDC, and the NLCC and voltage control schemes to control the MMC when the grid is balanced. Section 5 is devoted to the control schemes when the grid is unbalanced, including the algorithm for calculating the positive and negative sequences, and the schemes for NLCC and voltage control. Section 6 presents a system using a transformer with taps to increase the current injected into the grid during faults. In Section 7 the simulation results are presented. Finally, the conclusions are presented in Section 8.

Section snippets

Structure and basic equations of MMC

The structure of an MMC (Fig. 1) contains three phases; each is formed by an upper arm, a lower arm and two small coupling inductances L. Each of the six arms consists of several switching modules (SM) connected in series; in the example of Fig. 1 each arm includes five SMs.

Although SMs can be implemented in a variety of topologies, the most common are the half-bridge (HBSM) (Fig. 2a) and full bridge (FBSM) (Fig. 2b) [22]. The simplest topology (HBSM) features lower losses and is cheaper,

Non linear current control algorithm

NLCC is a simple way to get the MMC follow the current reference of the three phases iabc. It is simpler than voltage control because it does not need PI regulators or dq axes decoupling equations. It is based on keeping the current of each phase ig(g=a,b,c) within a band value ε around the current reference ig[igε;ig+ε] (Fig. 4a), by choosing the appropriate level of the MMC output voltage vo (Fig. 4b). The MMC output voltage vo can take n + 1 possible values; in the example of Fig. 4c

Control of the HVDC converter in the case of a balanced grid

When the AC voltage connected to the MMC is balanced and there is no a fault, the objective of the control system should be to regulate the active P and reactive Q power of the AC side. MMC converters are usually voltage source controlled, but a NLCC can be used too. In the paper, when the AC side is balanced and it is not under fault, the voltage control was selected. When the AC voltages are under balanced or unbalanced faults, the systems changes to the NLCC. The reason for the change

Control of the HVDC converter in the case of unbalanced faults

Every so often line faults are unbalanced, either single phase to ground or phase to phase faults. In these cases, grid codes are usually less strict than in three phase faults but also impose some limitations such as the ban of active and reactive power absorption from the grid during the fault, as for example in the Spanish P.O.12.3.

Thus, during such faults, preventing power absorption from the grid is the most important goal to be met by the grid side converter. However, it can also

Transient overcurrent injection into the electrical grid

In electrical grids with a high penetration of renewable energies, problems with grid code compliance may arise as a consequence of their particular behavior during electrical transients, mainly voltage sags but also in the near future when the grid faces frequency variations.

While traditional power plants are able to transiently generate power values above their rating, and their transient behavior is conditioned by the mechanical inertia of the electrical generator, these effects don't exist

Simulation results

In this section, the on-shore MMC station of a wind farm was simulated under grid faults. The on-shore MMC was connected to the wind farm through HVDC cables and it was connected to the main AC grid using HVAC cables with an inductance 2Ll (Fig. 14). The fault was generated in the center of the AC line, and it was modelled using switches Sabc and inductances Lf; the fault can be three-phase, phase to phase grounded or single phase, depending on the number of switches Sabc that are closed. When

Conclusions

Currently, MMC topology is mostly used in HVDC systems, and always used with voltage control, although studies have been carried out to explore its possibilities operating as a current source. In this paper, the operation of a MMC-HVDC converter using a non linear current source algorithm, NLCC, and its application to wind energy has been addressed.

Specifically, a MMC-HVDC system with NLCC connected to the mainland grid has been simulated, and three-phase-to-ground, phase to phase grounded, and

References (33)

  • R. Marquardt, Modular multilevel converter: an universal concept for HVDC-networks and extended DC-bus-applications,...
  • G. Bergna et al.

    An energy-based controller for HVDC modular multilevel converter in decoupled double synchronous reference frame for voltage oscillation reduction

    IEEE Trans. Ind. Electron.

    (2013)
  • Industry, Tourism and Commerce Spanish Ministry

    Operation Procedure O.P. 12.: response requirements in front of voltage dip at wind farms utilities

    BOE

    (2006)
  • E.O.N. Netz

    Grid Code High and Extra High Voltage, E.ON Netz GmbH, Bayreuth

    (2006)
  • EirGrid

    Grid Code – Version 3.4, EIRGRID, Ireland, 16th. October

    (2009)
  • P. Faria, T. Soares, Z. Vale, H. Morais, Distributed generation and demand response dispatch for a virtual power player...
  • Cited by (24)

    • Numerical methods for power flow analysis in DC networks: State of the art, methods and challenges

      2020, International Journal of Electrical Power and Energy Systems
      Citation Excerpt :

      Several control strategies have been proposed, such as the model predictive approach [24], passivity-based control design [25,26], classical proportional--integral (PI) controls [27,28], artificial neural networks [29,30], sliding planes [31–34], fuzzy logic [35–38], adaptive control [39,40], feedback control [35,32,41], and backstepping control [19,42]. It is also important to mention that there are other alternatives to building HVDC systems based on various converter topologies, such as modular multilevel converters (MMCs) [43–45] or line-commutated converters [46] that can be controlled using the aforementioned linear or nonlinear control strategies [47,48]. Finally, the dynamic analysis of HVDC systems also comprises a stability analysis of the entire network for determining the existence of equilibrium points and for studying the transient stability performance of the grid under large disturbances, as discussed in [4] for VSC--HVDC systems and in [7] for MMC--HVDC configurations.

    • A review on current injection techniques for low-voltage ride-through and grid fault conditions in grid-connected photovoltaic system

      2020, Solar Energy
      Citation Excerpt :

      Here, the associated types of grid faults remain SLG, LL and DLG (Byung-Ik et al., 2012; Hyo-Sang et al., 2010; Sadeghkhani et al., 2018). Conventionally, a sudden stoppage of the GCPV system, due to severe grid faults, would trigger the islanding protection and shut down the inverter causing power outages, voltage flickers and energy losses (Yang et al., 2014b; Papanikolaou, 2013; Ramirez et al., 2017). Therefore, the LVRT technique had been proposed by researchers to control and regulate the issue of grid faults and to comply with the GCs.

    • MMC as nonlinear vector current source for grid connection of wave energy generation

      2019, International Journal of Electrical Power and Energy Systems
      Citation Excerpt :

      MMCs mainly use voltage modulators, either high frequency modulators such as Phase Disposition–Sinusoidal PWM [7], Multilevel PWM [8] and Multilevel SVM [9], or low frequency such as Near Level Control [10–12]. In the same way as other electronic converters, a control loop is responsible for the active power control through the DC link voltage control and an inner control for the reactive power or the electrical grid voltage and in order to fulfill the grid low voltage ride through code [13]. Although this technology was initially designed for a voltage source converter, it is also possible to use it as a nonlinear current source [14–17], by generating a voltage waveform that keeps the electric current within a sinusoidal hysteresis band.

    • Enhanced controller for grid-connected modular multilevel converters in distorted utility grids

      2018, Electric Power Systems Research
      Citation Excerpt :

      Their equations are calculated in Ref. [18], their suppression in Ref. [17], and their reduction in unbalanced systems in Ref. [19]. Other aspects of MMC studied are: models and control systems in unbalanced grids [20,21], influence of the commutation frequency and the number of SMs in the harmonic content [22], DC bus protection using thyristors included in the SMs [23] and current source control [24]. Regarding the 3-phase grid-connected renewable systems, several vector control strategies are studied in the scientific literature.

    View all citing articles on Scopus
    View full text