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This chapter describes a new method for operating the offshore wind farm (OWF) with diode rectifier unit (DRU)-high-voltage direct current (HVDC) (offshore side), where a medium voltage (MV) submarine cable is in parallel operation with DRU-HVDC link. In order to avoid uncontrolled current flow through the MV submarine cable, a phase shifting transformer (PST) is applied on the onshore side of the MV submarine cable. The application of PST is to ensure the smooth blackstart and stable operation of the OWF and DRU-HVDC link. Both static and dynamic behaviors of the proposed method are presented in this chapter and the simulation results validate the proposed method.
Institute of Electrical Power Engineering, University of Rostock, Germany
*Address all correspondence to: lijun.cai@uni-rostock.de
1. Introduction
Since the wind energy is renewable and environmental natural resource, the utilization of wind power plant increased quickly. In the future, the development of wind power utilization will focus on large offshore wind farms (OWFs) [1, 2, 3, 4].
Especially, many planned OWFs become larger and more distant from the onshore grid. Conventional HVAC transmission is not flexible and limited due to large charging currents of the submarine cables. With the development of power electronics, OWFs with voltage source converter (VSC)-high-voltage direct current (HVDC) grid connection become more popular. The VSC-HVDC technology is based on insulated gate bipolar transistors (IGBTs), and it offers significant advantages over the thyristor-based line-commutated converter-HVDC (LCC-HVDC) technology. VSC-HVDC converters can be used to supply weak grids, offer blackstart capability, and can provide decoupled active and reactive power controls [1, 2, 3, 4].
Due to the continuous increase of the OWF capacity, the capacity of offshore converter stations should also be increased. This could result in a larger dimension and heavier offshore converter stations. In order to reduce the costs of offshore VSC-HVDC converter station and offshore platform, the diode rectifier unit (DRU) concept for offshore converter stations is proposed in [5, 6, 7], as shown in Figure 1.
Figure 1.
OWF with DRU-HVDC.
1.1 Advantages of DRU
Compared to the VSC-HVDC and LCC-HVDC converter stations, the main advantages of DRU unit are summarized as follows [5, 6, 7]:
Simpler structure.
Works robust since diodes do not need protection against the steep rise of current when fired and are not susceptible to failure events within the recovery time [5].
Compared to IGBTs, diodes have lower switching and conducting losses due to a lower on-stage voltage and switching frequency [5, 6, 7].
Easy transport and installation: according to [7, 8], replacing the VSC offshore converter station by a DRU, the total top side volume could be reduced by 80% and weight could be reduced by 65%. The installation time could be reduced by 20%.
According to [7, 8], by applying DRU, the transmission loss could be reduced by 20% and transmission capacity could be increased by 30%. The total costs could be reduced by 30%.
Higher reliability, modular design, and reduced operation and maintenance costs [5, 6, 7, 8].
1.2 Technical challenges and possible solutions for DRU-HVDC link
Compared to the well-controlled VSC-HVDC, the DRU is a passive device without any controllability, and therefore it cannot provide the AC reference voltage for the OWF [8, 9]. Furthermore, in order to meet the power quality and reactive power requirements on the point of common coupling (PCC), filters and reactive compensations should be equipped on the DRU [10].
In order to deal with the above-mentioned challenges, different solutions were proposed:
First solution: since the DRU has not the blackstart capability, an additional 33 kV or 66 kV medium voltage (MV) AC cable is suggested in [5] for the startup of OWFs, as shown in Figure 1 [5]. However, during the startup of OWFs, there could be a period where the DRU-HVDC and medium voltage AC (MVAC) cables are in parallel operation. Large current on the MVAC cable could occur because of the uncontrolled DRU operation. After the startup of the OWFs, the MVAC cable will be disconnected and part of the offshore wind turbine will take over the control of the OWF [5, 9], e.g. grid forming controls (because the DRU is a passive rectifier without any control capability). Therefore, modifications on the wind turbine controllers (from grid following to grid forming) are necessary. Furthermore, the coordinated control of the wind turbines in OWF is necessary [9].
Second solution: in [6], a hybrid topology with parallel operation of DRU and VSC is suggested for controlling the voltage and frequency of the offshore grid, as shown in Figure 2. However, considering the dimension and costs of the offshore VSC converter station, the cost-effectiveness could be significantly reduced [11]. Therefore, the first solution is concentrated in this chapter.
Figure 2.
Hybrid topology on the offshore side.
1.3 Main objective of this chapter
The main objective of this chapter is to ensure the stable operation and improvement of the dynamic behavior of OWF with DRU-HVDC grid connection. Therefore, a new method is applied, where the phase shifting transformer (PST) is used on the onshore side of the MVAC cable [12].
This chapter is organized as follows: following the introduction, the proposed system structure, startup procedure, wind turbine and HVDC models are introduced in Section 5. In Section 6, both static and dynamic behaviors are analyzed. Finally, brief conclusions are deduced.
The proposed method is shown in Figure 3, where the PST is applied on the onshore side of MVAC cable. The main objective of PST is for the smooth startup and stable operation of the OWF [12, 13, 14].
Figure 3.
Proposed system structure.
Similar to the method proposed in [5], the MVAC cable is used for the startup of OWF. Due to the application of PST, the MVAC cable and DRU-HVDC link could be operated in parallel (no additional switching procedure necessary).
2.2 System startup
The startup procedure of the OWF with DRU grid connection is summarized as follows (the numbers are illustrated in Figure 3):
Start the onshore VSC-HVDC converter (⑥) (which is connected to the onshore power system (⑦) directly).
Start the PST (①).
Charging the MVAC cable (②).
Charging one of the MVAC cable strings in the OWF (③).
Start further wind turbines on the charged OWF string (depend on the capacity of the MVAC cable (②)).
Use the OWF park controller (③) [15] to regulate the reactive power flow on the MVAC cable (②) (equals to the natural charging reactive power of the MVAC cable (②)). The total apparent power (active power generated by the OWF and the MVAC cable (②) charging reactive power) should be less than the capacity of the MVAC cable (②).
Start the DRU (④).
Charging the DC cable (⑤).
Start the rest wind turbines of the OWF (③).
Use the PST (①) to regulate the active power transfer on the MVAC cable (②) to a minimum value (≈0MW). This enables the parallel operation of MVAC cable and the DRU-HVDC grid connection.
Use the OWF (③) park controller to regulate the reactive power flow on the MVAC cable (②) (equals to the natural charging reactive power of the MVAC cable (②)).
2.3 Advantages of the proposed method
The proposed method could have following advantages:
The PST will regulate the active power flow and the OWF park controller (③) [15] will regulate the reactive power flow on the MVAC cable (②). Overloading is avoided for the parallel operation of MVAC cable (②) and DRU-HVDC link (④,⑤,⑥).
MVAC cable (②) will not be switched off after the startup of OWF (③) and therefore no additional switching is necessary.
No modification of the wind turbine controller and onshore VSC-HVDC converter controller is necessary.
The wind turbine operated in the OWF could be the doubly fed induction generator (DFIG) or full converter (FC). Both wind turbine models are briefly summarized in this section.
The onshore VSC-HVDC converter applies the conventional DC voltage and AC voltage (or reactive power compensation) controller. The DRU model is also introduced in this section.
3.1 DFIG wind turbine model
The basic configuration of a DFIG wind turbine is shown in Figure 4 [1, 2, 3].
Figure 4.
Structure of DFIG.
The stator of the induction machine is connected directly to the grid, and the rotor is connected to the grid through two converters: line-side converter (LSC) and rotor-side converter (RSC). The LSC and RSC consist of two three-phase pulse-width modulated (PWM) converters, and they have a common DC bus. Conventionally, both converters are of three-phase two-level type having three legs, each of which consists of two IGBTs and two anti-parallel diodes as illustrated in Figure 5. Voltage control in these converters is done by the PWM using a carrier frequency in the order of kHz [1, 2, 3]. In this chapter, the general DFIG controllers introduced in [1, 2, 3] are applied.
Figure 5.
Two-level VSC circuit.
3.2 FC wind turbine model
The typical configuration of the FC wind turbine is given in Figure 6. Usually, the generator could be a multi-pole synchronous generator designed for low speed, and this allows for gearless design. The generator can either be electrically excited or permanent magnet synchronous generator. For allowing variable speed operation, the synchronous generator is connected to the grid through two full power converters (LSC and RSC), where they convert the variable frequency output power of the generator to AC power with grid frequency [1, 2, 3].
Figure 6.
Structure of FC.
3.3 DFIG and FC controller
The controls of both DFIG and FC wind turbines are achieved by controlling LSC and the RSC utilizing vector control techniques [1, 2, 3].
Vector control allows decoupled control of both active and reactive power. RSC is used to control the active and reactive powers delivered to the grid. LSC is used to maintain the DC bus voltage regardless of the magnitude and direction of the rotor power. The reactive power controllability of the LSC is always applied to reinforce fault-ride-through (FRT) capability and provides grid voltage/reactive power control [1, 2, 3].
3.4 Onshore VSC-HVDC converter model
In this chapter, the onshore VSC-HVDC converter applies the modular multilevel converter (MMC) topology, and it is illustrated in Figure 7 [15]. Each converter phase consists of upper and low multi-valve units. Each multi-valve unit has a modular structure with series-connected sub-modules (SMs). Each SM contains a capacitor and two IGBTs/diodes as illustrated in Figure 8 [15]. This chapter is concentrated on the operation of OWF and DRU, and the half-bridge SMs are applied [15].
Figure 7.
Detailed MMC topology.
Figure 8.
MMC sub-module.
3.5 Onshore VSC-HVDC controller
The onshore MMC injects the active power transmitted by the offshore DRU to the onshore AC grid while maintaining the DC voltage at desirable level. In addition, it supports the onshore AC grid voltage in steady state operation and during faults. It uses a vector control [1, 2, 3, 15].
The frame of the onshore VSC-HVDC is shown in Figure 9 [15].
Figure 9.
Control frame of the onshore VSC-HVDC converter.
3.6 DRU model
As described in [5], the DRU combines a transformer with a diode rectifier and DC smoothing reactors in a common tank filled with synthetic ester. In this chapter, the 6-pulse DRU is considered [16].
All the system is modeled in PowerFactory [15]. The main system components, e.g. offshore DRU, DC cable, onshore VSC-HVDC, 33-kV MVAC submarine cable and PST, are shown in Figure 10, where the OWF is operating with nominal power.
Figure 10.
DRU and VSC-HVDC with nominal power output of OWF.
The system illustrated in Figure 3 is simulated in this section. Both static and dynamic behaviors of the proposed method are considered.
4.1 System parameter
4.1.1 33-kV cable parameter
The ABB 33-kV submarine cable is applied in this simulation and the parameters are given in Table 1 [15].
33-kV submarine cable parameter
Value
Rated current (kA)
0.437
Rated voltage (kV)
33
AC resistance (ohm/km)
0.0754
Reactance (mH/km)
0.36
Capacitance (μF/km)
0.23
Cable length (km)
120
Table 1.
33-kV submarine cable parameter.
4.1.2 PST parameter
A standard transformer model is modified to enable the PST function. The parameters are given in Table 2 [15]. For simulating the energization, saturation is also considered.
System parameter
Value
Rated power (MVA)
60
Rated voltage HV (kV)
220
Rated voltage MV (kV)
33
Vector group
YN/D
Short-circuit voltage (%)
20
Copper loss (kW)
180
Knee flux (p.u.)
1.1
Linear reactance (p.u.)
200.6431
Saturated reactance (p.u.)
0.2
Saturation exponent
15
Table 2.
PST parameter.
4.1.3 Wind turbine settings
The OWF consists of 20 strings, and 10 FC wind turbines (SN=5.6MVA,PN=5MW) are equipped on each string. One string structure is given in Figure 11a. The total OWF capacity is 1 GW [15].
Figure 11.
OWF operates with 20% of nominal power. (a) One string of OWF and (b) OWF with DRU.
4.1.4 Static operation
Firstly, the static operation of the proposed method is considered. OWFs operating with 20% and 100% of nominal power are selected for the demonstration of the proposed approach.
4.1.4.1 OWF operating with 20% of the nominal power
Figure 11a shows the power flow of one string in the OWF, where the active power of each wind turbine is 20% of its nominal power (1 MW). The wind turbines on the string are connected by 33-kV submarine cable and the cable length (between two wind turbines) is 1.5 km.
Power control for the 33-kV cable between onshore and offshore:
Active power control: the active power is controlled by the PST and it is 0 MW on the offshore side. The total active power generated by the OWF is transferred by the DRU-HVDC link, as shown in Figure 11b.
Reactive power control: the reactive power is controlled by the OWF park controller (offshore grid station) [15] to ensure the cable is operating on its natural charging reactive power (9.4 MVar), as illustrated in Figure 11b.
As illustrated in the power flow result, all node voltages are in nominal range.
4.1.4.2 OWF operating on nominal power output
Figure 12a shows the power flow of one string in the OWF, where each wind turbine generates nominal power (5 MW).
Figure 12.
OWF operates with nominal power. (a) One string of OWF and (b) OWF with DRU.
As illustrated in Figure 12b, the offshore grid station can control the reactive power on the 33-kV onshore-offshore cable with its charging reactive power (9.4 MVar, shown in Figure 12b). Similar to the results in Section 6.1.4.1, the total active power generated by the OWF is transferred by the DRU-HVDC link.
It is clear that all the node voltages are in the nominal range.
4.1.4.3 Startup of OWF and HVDC system
Since the dynamic behaviors during the startup of the PST and 33-kV onshore-offshore cable were discussed in [13], this chapter is concentrating on the startup of OWF wind turbines and DRU.
4.1.4.4 Startup of OWF
The wind turbines of the OWF are started with 20% of the rated power (1 MW). At 0.5 s, the first string of the OWF is switched on. From 1 s to 5.5 s, the wind turbines on this string are started sequentially.
At 6 s, the second string of the OWF is switched on and from 6 s to 10 s, and the wind turbines on the second string are started.
In order to demonstrate the capability of OWF park controller (offshore grid station) [15], the reactive power of the 33-kV cable between onshore and offshore is set to 14 MVar, as shown in Figure 13.
Figure 13.
Reactive power on the 33-kV cable (offshore side).
After starting the first string (1–5.5 s), since the wind turbines on the first string are working on 20% of their nominal power, the total reactive power capability of the first string (10 wind turbines) is not enough to maintain the regulated reactive power on the 33-kV cable (14 MVar).
After the startup of the second string, the reactive power of the 33-kV cable can be controlled effectively.
During the startup of the OWF, it is obvious that there are oscillations on the reactive power. This is mainly due to the parameter of the onshore-offshore 33-kV cable and the saturation parameters of the PST.
4.1.4.5 Startup of OWF transformer and 150-kV submarine cable
After starting the OWF wind turbine, the OWF transformers (illustrated in Figure 11b) are started at 12 s. Then two 150-kV submarine cables (illustrated in Figure 11b) are switched on at 15.5 s.
Figures 14 and 15 show the current and the voltage of the OWF transformer. The transformer inrush current is limited effectively by means of the controllable switch of the circuit breaker (switching on at voltage maximum) [15] and the voltage is also in the nominal range.
Figure 14.
Current of the OWF transformer.
Figure 15.
Voltage of the OWF transformer.
4.1.4.6 Startup of DRU
After starting the two 150-kV submarine cables, the DRU is switched on at 18.5 s.
Figure 16 shows the current and the voltage of on the DRU. Both the voltage and current are in the nominal range. Since the AC filters on the DRU are deactivated, the DRU current contains 5th, 7th, 11th, and 13th harmonics as the fast fourier transformation (FFT) analysis shown in Figure 17.
Figure 16.
Current and voltage on the DRU.
Figure 17.
FFT analysis of the DRU current.
After starting DRU, the whole OWF and DRU-HVDC link are in normal operation.
The stable and economical planning of OWF with HVDC grid connection is quite important for the future power systems. This chapter proposed a new method, where the application of PST ensures the blackstart and stable operation of the OWF with DRU-HVDC link. Applying the proposed method, no modification of the wind turbine controllers and onshore VSC-HVDC controllers are necessary. Moreover, since the PST is located onshore, it reduces the maintenance, operation, and installation costs of the PST.
1.Karaagac U, Mahseredjian J, Saad H, Jensen S, Cai LJ. Examination of fault ride-through methods for off-shore wind farms connected to the grid through VSC-based HVDC transmission. In: Paper Published in 11th International Workshop on Large-Scale Integration of Wind Power into Power Systems; November 13-15, 2012; Lisbon Portugal
2.Cai LJ, Yin H, Lan YL, Lan T, Wu X, Eckel HG, et al. Sub-synchronous harmonic impedance analysis of doubly-fed induction generator wind turbine. IFAC-PapersOnLine. 2019;52(4):165-169
3.Cai LJ, Fan Z. Doubly-fed induction generator offshore wind farm with VSC-HVDC grid connection: start-up procedure. In: International Conference on Renewable Power Generation RPG. London, UK: IET; 2015
4.Bozhko SV et al. Control of offshore DFIG-based wind farm grid with line-commutated HVDC connection. IEEE Transactions on Energy Conversion. 2007;22(1):71-78
5.Hammer T, Seman S, Menke P, Hacker F, Szangolies B, Meth J, et al. Diode-Rectifier HVDC link to Onshore Power Systems: Dynamic Performance of Wind Turbine Generators and Reliability of Liquid Immersed HVDC Diode Rectifier Units, CIGRE 2016, 21, rue d’Artois, F-75008 PARIS. Paris: CIGRE;
6.Hoffmann M, Hemdan N, Kurrat M. AC Fault Analysis of DRU-VSC Hybrid HVDC Topology for Offshore Wind Farm Integration. VDE/IEEE Power and Energy Student Summit. London: IEEE; 2018
7.2nd Generation DC Grid Access for Large Scale Offshore Wind Farms. Siemens AG; 2015. Available from: https://rave-offshore.de/files/downloads/konferenz/konferenz-2015/Session7_2015/7.2_Menke
8.Yu L, Li R, Xu L. Operation of offshore wind farms connected with DRU-HVDC transmission systems with special consideration of faults. Global Energy Interconnection. 2018:608-617
9.Neumann C, Eckel HG, Achenbach S. A fault handling current control strategy for offshore wind power plants with diode rectifier HVDC transmission. In: Paper Published in 17th International Workshop on Large-scale Integration of Wind Power into Power Systems; October 17-19, 2018. Stockholm, Sweden: IEEE;
10.Blasco-Gimenez R, Aparicio N, Ano-Villalba S, Bernal-Perez S. LCC-HVDC connection of offshore wind farms with reduced filter banks. IEEE Transactions on Industrial Electronics. 2013;60(6):2372-2380. DOI: 10.1109/TIE.2012.2227906
11.Ramachandran R, Poullain S, Benchaib A, Bacha S, Francois B. On the black start of offshore wind power plants with diode rectifier based HVDC transmission. In: 2019 21st European Conference on Power Electronics and Applications (EPE '19 ECCE Europe). Europe: ECCE; 2019. pp. 1-10. DOI: 10.23919/EPE.2019.8914779
12.Patent P26336DE: Verbindungzwischen Offshore-Energiesystemen und Landstromnetzensowie Schwarzstartverfahrenfür Offshore-Energiesysteme
13.Cai LJ, Meng X, Latif Q, Eckel HG, Weber H. Application of phase shifting transformer (pst) for blackstart and stable operation of offshore wind farm with diode-rectifier unit HVDC link. In: Paper published in 19th Wind Integration Workshop; November 11-13, 2020. Berlin, Germany: IEEE;
14.Cai LJ, Meng X, Zhang H, Eckel HG, Weber H. Dynamic behavior of phase shifting transformer (PST) for blackstart and stable operation of offshore wind farm with diode-rectifier unit HVDC link. In: Paper Published in 20th Wind Integration Workshop, September 29-30, 2021. Berlin, Germany: IEEE;
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