Assessment of a non linear current control technique applied to MMC-HVDC during grid disturbances
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 . 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 within a band value ε around the current reference (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
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2020, International Journal of Electrical Power and Energy SystemsCitation 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.
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