Design and control of all-DC offshore wind power plant with MMC-based DC/DC high-power converters

This paper presents the modelling of an all-direct-current (all-DC) offshore wind power plant (OWPP) which employs DC/DC high-power converters based on modular multilevel converter (MMC) technology. A coordinated control strategy for such OWPP, designed to maximise the power output and maintain the DC voltages close to the nominal values, is also proposed. With the proposed strategy, wind turbines (WTs) are controlled to provide maximum power output according to a local maximum power point tracking (MPPT) reference signal together with a centralised curtailment signal. The response of the designed 600 MW all-DC OWPP is tested by means of dynamic simulations in MATLAB/Simulink.


P dc
Active power produced by one string of wind turbines P * Active power reference signal for one string of wind turbines V dc,M V Collection system voltage Reference value of collection system voltage V dc,HV Transmission link voltage V * HV Reference value of transmission link voltage Q Onshore reactive power Q * Reference value of onshore reactive power P M P P T Maximum power produced by one string of wind turbines P limit Power limitation signal P out Power produced by one string of wind turbines P ower out Total power delivered by the wind turbines to the collection system 1.Introduction Offshore wind is expected to play a central role to fulfil the increasing demand of electrical power from renewable energy sources in the coming decades, due to favourable wind speed conditions offshore [1,2,3].In pursuit of lowering costs, the size of wind turbines (WTs) and offshore wind power plants (OWPPs) is growing [4,5].
Moreover, new OWPPs are being developed farther and farther from shore.The preferred transmission system for remotely located OWPPs is based on high-voltage direct-current (HVDC) technology [6,7].All operating OWPPs to date utilise alternating-current (AC)-based collection systems.However, for large and remotely located OWPPs, a DC-based collection system may offer important advantages over the AC-based collection system, e.g.reduction of collection system cable cross-section due to the absence of reactive power [8,9].
Modular multilevel converters (MMCs) have become popular for HVDC transmission applications, due to their modular design, scalability and a multilevel architecture that enables low harmonic distortion and low voltage and current variations across submodules, thus reducing switching losses [10,11].
References [12,13] studied the use of MMC technology to connect DC WTs to a DC collection system.In particular, reference [13] presented the control of a 10 MW permanent magnet synchronous machine (PMSM) using a MMC-based high-power converter.The study focused on the PMSM of a single DC WT and not on a coordinated control for the entire OWPP.Current literature on design and control of all-DC OWPPs focuses on OWPPs with WTs up to few MW-size (eg. 5 MW) [14,15,16,17].However, the size of commercially available WTs has already reached 15 MW.
To fill those research gaps, this study proposes the design and control of a 600 MW all-DC OWPP with 15 MW-size DC WTs, connected to a DC collection system via DC/DC highpower converters based on MMC technology.This topology exploits the inherent advantages of MMC technology at both high and medium voltage levels.The developed coordinated control strategy allows the OWPP to deliver active power to the onshore grid with minimum losses, while maintaining the collection and transmission voltages close to nominal values.The power output is also influenced by a centralised power limitation signal which can be used by the system operator to curtail the power output in case of low power demand.
The proposed coordinated control strategy was tested with dynamic simulations in MATLAB/Simulink to confirm the suitability of MMC-based DC/DC high-power converters for wind power application.This paper is organised as follows.Section 2 describes the modelling of all-DC OWPP, Section 3 presents the coordinated control strategy for the proposed all-DC OWPP and Section 4 shows the results of the dynamic simulations.Section 5 concludes the article.

System description
This study considers a 600 MW all-DC OWPP connected to an onshore AC grid via a 200 km long, ±320 kV monopolar HVDC transmission link and corresponding onshore (DC/AC converter) substation, as shown in Figure 1.
The HVDC cables are based on the DC-HYJQ41+OFC model, with cross sectional area of 2500 mm 2 and resistance of 8.7 mΩ/km at 70 • C [18].A DC/DC high-power converter connects the HVDC link to the Medium-voltage directcurrent (MVDC) collection system offshore.The collection system connects 5 strings of WTs in parallel to the same MVDC bus.Each string connects 8 WTs of 15 MW size in parallel, for a total of 40 WTs.This layout is suitable for large OWPPs, as it makes possible to reduce the conduction losses with a considerable voltage ratio between generation and transmission (e.g.40) [19].The collection system's nominal voltage is ±50 kV, and the corresponding cables are DC-HYJQ41-F model, with cross sectional area of 800 mm 2 and resistance of 26.4 mΩ/km at 70 • C [18].
The WTs are based on the 15 MW reference WT developed by the National Renewable Energy Laboratory (NREL) [20].The distance between WTs is considered to be more than 7 times the rotor diameter, and power losses due to aerodynamic interactions (e.g.wakes) are neglected.In order to reduce simulation time while providing an adequate representation of the relevant dynamics, a current source is used to represent the subsystems behind the WT rectifier.Moreover, WTs in each string are aggregated into an equivalent WT of 120 MW size (8 WTs of 15 MW) and each string is connected to the collection system with a DC/DC high-power converter, as in Figure 1.

DC/DC High-power converters
The DC/DC high-power converter topology proposed in this study is a dual active bridge (DAB) MMC-based configuration, comprising one inverter and one rectifier connected front-to-front via a medium-frequency transformer.Both MMCs reach the desired voltage levels by series connection of half-bridge (HB) submodules (SMs).The medium-frequency transformer operates at a frequency of 150 Hz, which reference [21] describes as a realistic trade-off between achieving reduction in weight and size of the components and ensuring good performance of electrical insulation and core materials.The medium-frequency transformers are designed with voltage ratio of 1:1 to reduce the requirement for insulation.The windings are in a ∆-Y g configuration to block the propagation of the 3 rd harmonic in the OWPP.
The internal passive elements of the MMC, namely the cell capacitors and the arm inductance, are designed according to reference [22].
To calculate the arm capacitors, this study assumes an energy-to-power ratio E p = 30 J/kVA, which is a good trade-off between cost and performance of capacitors.
A nominal SM voltage of 1.6 kV was selected to calculate the number of SM.This value considers a large safety margin whether 3.3 or 4.5 kV insulated gate bipolar transistors (IGBTs) are chosen.
The arm inductance is designed to suppress the 3 rd harmonic component.However, to improve the stability of the system, the arm inductance of the MMCs at the two ends of the collection system was chosen with a higher value.
The main OWPP parameters and the MMCs passive elements calculated are reported in table A1, table A2, and table A3 in appendix.The parameters of the onshore MMC are the same as the HVDC side of the offshore MMC.

Coordinated control strategy
The proposed coordinated control strategy, as presented in figure 2, minimises the conduction losses to allow maximum power transfer, while maintaining the DC voltages close to nominal values.Starting from the aggregated DC WT, this strategy proposes to control the DC WT active power, P dc , according to its reference, P * .The DC/DC converter in the offshore substation is responsible for maintaining the collection system voltage, V dc,M V , at its nominal value, V * M V .Similarly, the onshore inverter maintains the transmission link voltage, V dc,HV , at its nominal value, V * HV .On the AC side, the onshore inverter controls the reactive power, Q, to follow its reference, Q * , equal to zero.

Wind turbine DC/DC high-power converter
The objective of the outer control loop of the WT DC/DC high-power converter is the manipulation of the power P dc , produced by a string of WTs, to follow the power reference set point, P * .Figure 3 depicts the power reference, P * , which consists of the maximum power point tracking (MPPT) signal, P M P P T , set to achieve the maximum power output from the string of WTs, and the power limitation signal, P limit .The power limitation signal is a variable signal imposed on the MPPT signal, which can be used by system operators to curtail the power production during periods of congestion or low consumption.

Offshore DC/DC high-power converter
The offshore DC/DC high-power converter control adjusts the collection system voltage V dc,M V , in response to the variations in the output power of the WTs.An effective control ensures the power balance in the DC collection system and avoids fluctuations of WTs power output, caused by interactions among WTs connected to the same bus.

Onshore AC/DC high-power converter
Similarly to the offshore DC/DC converter, the d-axis current component of the outer control loop of the onshore inverter regulates the HVDC link voltage to its nominal value to maintain the power balance at the HVDC transmission system.The q-axis current component of the outer control loop instead regulates the reactive power injected to / absorbed from the onshore AC grid to zero, as depicted in Figure 2.

Simulation results
The performance of the proposed coordinated control strategy was tested by means of dynamic simulations in MATLAB/Simulink.Figure 4 shows the power injected by each string of WTs, P outi , i ∈ {1, 2, 3, 4, 5} into the collection system.The power produced by each string is different so as to represent the wind speed variations across the OWPP.To test the control performance, rapid and large variations in wind speed were simulated.Figure 4 shows the controls' ability to follow the wind speed variations with accuracy, while remaining stable under large variations.The controls ensure no interactions between WTs, thus avoiding power fluctuations.
It is worth noting in Figure 4 the effect of P limit (red trace) on the produced power.As can be seen, the power output is curtailed during the activation period, and it tracks the MPPT signal after the curtailment signal is removed (i.e.P limit is greater than or equal to the string nominal power).Figure 5 depicts the aggregated DC power measured in four points of the OWPP, meaning at the sending and receiving ends of the collection system, being Power out WTs and MV side offshore substation (OSS), and at the sending and receiving ends of the HVDC link (HV side OSS and Delivered to shore, respectively).The control strategy ensures maximum power output.As can be noticed, the power production is curtailed between t=20 s and t=25 s, due to the action of P limit , previously discussed.The first action of P limit between t=11 s and t=16 s is not so evident due to the lower total power produced by the WTs.
Figure 6 and Figure 7 depict the collection and transmission system voltages, which are controlled to remain close to their nominal values, even during large power variations.The three larger DC voltage deviations correspond, in fact, to the three largest power variations.The DC voltage deviations were also restricted between ±10% of the nominal values to avoid the interactions with protection systems.

Conclusion
This paper presents the modelling and coordinated control strategy of an all-DC OWPP.It proposes employing MMCs at both high-and medium-voltage levels to benefit the OWPP from the advantages of MMC technology, such as the capability of handling large power at highvoltages with reduced switching losses and reduced harmonic distortion, due to low voltage variations across individual SMs.The modular structure of the MMC offers also enhanced reliability, with redundant SMs that can be installed and connected in case of failure of one or multiple SMs, reducing the risk of complete converter shutdown in case of SM(s) failure.However, such modular structure can be complex to install and it requires sophisticated control mechanisms.The modelling of the proposed all-DC OWPP is firstly discussed, then its coordinated control strategy is presented.Results of the dynamic simulations show that the all-DC OWPP controlled with the proposed control strategy is capable of providing power to the onshore grid in different operating conditions and under different power limitation levels, while minimising the losses.The collection and transmission system voltages are also maintained close to their nominal values, even during large power variations.

Figure 1 .
Figure 1.OWPP modeled.(a) Layout of all-DC OWPP.(b) Zoom on String 1 showing the modeling of the string and the number of WTs represented in each string.

Figure 3 .
Figure 3. Power reference for DC WT.

Figure 4 .
Figure 4. DC power output of each WT string and of power limitation signal.

Figure 5 .
Figure 5. DC power at different points of the OWPP.

Table A2 .
Collection system DC/DC converter arm parameters

Table A3 .
WT DC/DC converter arm parameters