Over the recent years, worldwide demand for energy has been steadily growing. It is expected that by the year 2035, renewable sources of energy would be generating more than 25% of world’s electricity, with a quarter of it being contributed by wind power according to the International Energy Agency (IEA) [1GWEC, "Global Wind", Report, Global Wind Energy Council: Brussels, 2014.]. Approximately 318GW wind power sources were installed over the world by the end of 2013 and China has installed 81GW of these wind power sources, which is approximately 25% of the total worldwide installations. Furthermore, it is expected that this would increase to 200GW by 2020 [2L. Jiang, Y. Chi, and H. Qin, "Wind energy in China", IEEE Power & Energy Magazine, vol. 6, pp. 36-46, 2011.
[http://dx.doi.org/10.1109/MPE.2011.942350] ]. A majority of the wind power bases and thermal power bases are located in the northern and western areas of Mainland China and a huge percentage of these two kinds of bases are located more than a few thousand kilometers away from the load centers. Wind power is characterized by stochastic fluctuation in nature, which requires available peaking power resources to stabilize the system. The thermal power plants which are built around the large-scale wind power bases can be an effective means for stabilizing the fluctuation of wind power output. Wind-thermal-bundled power system by DC transmission is regarded as a preferable means for power generation in the northern and western areas of Mainland China [3D. Chen, S. Ma, and Y. Song, "Research on transient stability under HVDC block fault in wind-thermal-bundled power base transmitted by AC/DC system", In: 2014 International Conference on Power System Technology (POWERCON 2014), IEEE Publisher, USA, 2014, pp. 2884-2890.
[http://dx.doi.org/10.1109/POWERCON.2014.6993894] ]. Conventional renewable source of energy transmission and the bundled system meets the basic requirements for a large also ensures smooth DC power delivery and improved transmission. However, challenges still exist related to DC transmission technology and stability of the power grid when considering the bundled system.

Since, thermal power plants, wind farms and load centers may have large distance between them, high voltage direct current (HVDC) technology, including line-commutated converter HVDC (LCC-HVDC), voltage source convertor-HVDC (VSC-HVDC) and hybrid HVDC are preferred [4R.L. Sellick, and M. Akerberg, "Comparison of HVDC Light (VSC) and HVDC Classic (LCC) Site Aspects for a 500MW 400kV HVDC Transmission Scheme", In: 10^th IET International Conference on AC and DC Power Transmission, IEEE Publisher, USA, 2012, pp. 23-29.].

LCC-HVDC and VSC-HVDC systems are two typical HVDC systems, which are used for transmission from large wind farms to grid. Conventional LCC-HVDC systems are among the most widely used HVDC technologies around world. These systems have advantageous features such as the low power loss, and highest power capability. The cost for conventional LCC-HVDC systems among the HVDC systems is significantly lower. However, the LCC system has drawbacks such as high commutation failure and increases harmonics [5M.D. Miguel, S.M. David, and S. Arnaltes, "Optimal reactive power allocation in an offshore wind farms with LCC-HVdc link connection", Renewable Energy, vol. 40, pp. 157-166, 2011.]. Recently, VSC using pulse width modulation (PWM) is being increasingly used as an alternative technology for HVDC transmission due to its independent control of both active and reactive powers and the possibility to feed a weak AC system. But VSC-HVDC systems are expensive and have high loss but smaller ratings compared to conventional LCC converters [6Y. Jie, Y. Yang, Z. He, and H. Wang, "System design of MMC VSC-HVDC demonstration project for wind farm connection", In: 10^th IET International Conference on AC and DC Power Transmission, IEEE Publisher, Bermingham, UK, 2012, pp. 14-18.].

In order to combine advantages of both LCC and VSC, a hybrid HVDC system is proposed in this paper, which was first introduced in 1994 [7Z. Zhao, and M.R. Iravani, "Application of GTO voltage source inverter in a hybrid HVDC link", IEEE Transaction on Power Electronics., vol. 9, pp. 369-377, 1994.
[http://dx.doi.org/10.1109/61.277708] ]. Since different converters are used, hybrid HVDC systems can be more advantageous than conventional HVDC systems. For inverter side of a HVDC system, capability for flexible reactive control is necessary to integrate grids. The VSC can be placed at the receiving end of an existing grid, which would offer more flexible control over conventional LCC [8B. Qahraman, and A. Gole, "A VSC based series hybrid converter for HVDC transmission", In: Electrical and Computer Engineering Conference, IEEE Publisher, Vancouver, China, 2005, pp. 458-461.
[http://dx.doi.org/10.1109/CCECE.2005.1556970] ]. For rectifier side of a HVDC system, LCC is cheap for long distance transmission and is widely used by its technical maturity [9Mai H. Nguyen, Mehdi Eghbal, and Tapan K. Saha, "Hybrid multi-terminal LCC HVDC with a VSC Converter", In: A Case Study of Simplified South East Australian system PES General Meeting, IEEE Publisher, USA, 2012, pp. 1-8.]. Hybrid HVDC systems are aimed at compensating the above mentioned disadvantages for both two types of converters, allowing for better technical and economical outcomes.

Author in [10Li Wang, "Stability Analysis of Different Offshore Wind Farms Feeding into a Large Power Grid through a Hybrid HVDC Link", In: Industrial Electronics (ISIE) 2013 IEEE International Symposium, IEEE Publisher, Taiwan, 2013, pp. 1-6.] proposed a feasible structure for a hybrid HVDC system, which has a VSC at the receiving end and a LCC at the sending side. A hybrid HVDC system can be a solution for integrating offshore wind farms which feed into the main land. Author in [11E. Raymundo, T. Olguin, and A. Garces, "Integration of offshore wind farm using a hybrid HVDC transmission composed by the PWM current-source converter and line-commutated converter", IEEE Transaction on Energy Conversion., vol. 28, pp. 125-134, 2013.
[http://dx.doi.org/10.1109/TEC.2012.2230535] ] used a two-terminal hybrid HVDC system, which consists of a pulse width modulated current source converter (PWM-CSC) and a LCC to transfer energy from offshore wind farms to onshore grids. Dynamic performance of a VSC based series hybrid HVDC has been studied for supplying a weak AC system [12R. Billinton, M. Fotuhi-Firuzabad, and S.O. Faried, "Reliability evaluation of hybrid multi-terminal HVDC sub-transmission systems", In: IET Proceedings Generation Transmission and Distribution, vol. 149. 2002, pp. 571-577.]. Controller designing for a hybrid HVDC system with a LCC and VSC has been proposed [13R.E. Torres-Olguin, M. Molinas, and T.M. Undeland, A Model-based Ccontroller in Rotating Reference Frame for Hybrid HVDC., IEEE Energy Conversion Congress and Exposition, IEEE Publisher, USA, 2010, pp. 1578-1584.
[http://dx.doi.org/10.1109/ECCE.2010.5618087] ]. Author in [14L. Guangkai, L. Gengyin, and L. Haifeng, "Operational mechanism and characteristic analysis of novel hybrid HVDC system", In: 2006 International Conference on Power System Technology, IEEE Publisher, China, 2006, pp. 1-6.] discussed the mathematical mode and operational mechanism of a hybrid HVDC link. In addition, a discussion on a MTDC system for multi-infeed to AC networks was presented in [15R. Chunyi Guo, and C. Zhao, "Supply of an entirely passive network through a double-infeed HVDC System", IEEE Transaction on Power Electronics., vol. 24, pp. 2835-2841, 2010.
[http://dx.doi.org/10.1109/TPEL.2010.2050214] , 16Y. Liu, and Z. Chen, "Power Control Method on VSC-HVDC in a Hybrid Multi-infeed HVDC system", In: IEEE Power and Energy Society General Meeting, IEEE Publisher, Canada, 2012, pp. 1-8.]. Hybrid MTDC systems with a hybrid infeed of VSC and LCC to a passive load was discussed in [17G Shilpa, and Premila Manohar, "Hybrid HVDC System for Multi-infeed Applications", In: Emerging Trends in Communication, Control, Signal Processing & Computing Applications (C2SPCA), IEEE Publisher, Banglore, India, 2013, pp. 1-5.]. A multi-terminal HVDC with all VSC converters connected to offshore wind farms with disturbance had also been proposed as an example to reflect conditions for practical applications [18W. Lu, and B.T. Ooi, "Optimal acquisition and aggregation of offshore wind power by multi-terminal voltage-source HVDC", IEEE Transactions On Power Deliver, vol. 18, pp. 201-206, 2003.
[http://dx.doi.org/10.1109/TPWRD.2002.803826] ]. So far, limited numbers of multi-terminal HVDC projects have been implemented around the world, which include the five-terminal VSC-HVDC system established in Zhoushan, China on June 27, 2014 and the south-west link between Sweden and Norway will be built in 2016.

Due to the stochastic fluctuation of wind speed, wind farms are regarded as a weak AC system [19H. Zhou, G. Yang, and J. Wang, "Control of a hybrid high-voltage DC connection for large doubly fed induction generator-based wind farms", IET Renewable Power Generation, vol. 5, pp. 36-47, 2010.
[http://dx.doi.org/10.1049/iet-rpg.2009.0171] ]. This has considerable impact on the stability of the connected power grids. In this paper, thermal power plants were used to compensate for the fluctuations in wind power. A hybrid three-terminal HVDC system designed is proposed, which includes two LCC converters at the sending terminals and one VSC converter at AC grid terminal. Transient stability of the power system with bundled transmission of wind and thermal power has been studied taking practical requirements into account specific to the Northwest Grid in China, Control strategies and principles of the proposed system have also been presented. Successful operations during wind speed variations and faults on the grid side were verified with simulations using DIgSILENT.

The proposed system was simulated using DIgSILENT. A large-scale wind farms typically consist of a ten to a few hundred wind turbines. However, for this case, it is represented as one generator which can be a simplified configuration of a hybrid MTDC system. Main parameters of the system have reference to CIGRE benchmark HVDC model [23M. Szechtman, T. Wess, and C.V. Thio, "A benchmark model for HVDC system studies", In: International Conference on AC and DC Power Transmission, IEEE Publisher, UK, 1991, pp. 374-378.].

4.1. Response to Wind Speed Variation on the Sending End

Fig. (6) shows the noise wind speed described by eq. (1) to (5). The nominal wind speed is 11m/s, then at t=0s a wind speed gust occurs which increases amplitude 5m/s with 2s of duration. The ramp wind increases the nominal wind speed at a rate of 3m/s at t=2s in 20s. The wind is noisy throughout the simulation. Fig. (8) shows the performance of the bundled system for a noisy wind speed of 11m/s. Simulation results show that the mathematical model accurately reflects mutation of a gradual nature and randomness in wind speed.

Fig. (6) Wind speed.

Fig. (7a and b) show the output active power from wind farm and thermal power plant. The stochastic and uncertainty of the wind power can be canceled by thermal power. Wind generator speed is about 1.2pu and thermal generator speed is 1pu, as can be seen in Fig. (7c). Fig. (7d) shows DC voltage at the LCC converters and VSC converter. It can be seen from the figure that all voltages are at their nominal values.

Fig. (7) Simulation results with noise wind speed (a) Wind power. (b) Thermal power. (c) Generator speed. (d) DC voltage of each converter.

4.2. Three-phased Short Current Fault of VSC Inverter on the Receiving End

In this study, it is assumed that a three-phase short circuit fault occurs on the AC side of VSC inverter at 2s and is cleared after 0.1s. Simulation results are shown in Fig. (8).

The AC voltage at the fault terminal drops down to zero temporarily, which causes a decrease in the DC voltage for other converters and is shown in Fig. (8a and b). When fault occurs and AC voltage of VSC drops to 0.5pu, an off signal will be produced to prevent damage to the IGBT by PWM to maintain power balance and system stability. Although wind active power drops to nearly zero, the output active power of the thermal power plants adds 4.3pu to compensate the wind power. Fig. (8e) shows rotor current of DFIG, it can be seen that the fault has a small impact on the DFIG.

Fig. (8) System responses to the fault at the VSC inverter. (a) AC voltage of VSC. (b) DC voltage of each converter. (c) Thermal power. (d) Wind power. (e) DFIG stator current.

4.3. Load Change at VSC Inverter on the Receiving End

Since, the hybrid MTDC system has advantages such as low convention loss, control flexibility and terminal voltage support. This section studies the dynamic performances of the proposed hybrid based scheme and shows it to be superior to a purely conventional LCC based scheme. And this section consists two cases of a three terminals HVDC system.

Case 1. The two converters at the sending end are LCCs while the recceing end is a VSC, as in Fig. (1).

Case 2. All converters are LCCs, as in Fig. (9).

Fig. (9) Configuration of conventional LCC MTDC for proposed system.

To investigate both systems’ responses to the dynamic disturbance, a 30% decrease in load at the inverter side is applied at 2s for a duration of 0.2s. System performances for this section are presented in Fig. (10).

Fig. (10) System responses to load change at the VSC inverter. (a) DC voltage of receiving end of two cases. (b) The change of thermal power of two cases. (c) The change of wind power of two cases.

Load decrease causes a decrease in the active power requirements, which increases DC voltage at the receiving converter and is shown in Fig. (10a). The VSC on the receiving end has to decrease the power absorbed from the DC network quickly to keep the DC voltage. Therefore, terminal voltage of the hybrid system remains almost unaffected. However, the conventional LCC system is affected which increases AC voltage to about 0.1pu. The thermal active power of conventional LCC system decreases from 0.85pu to 0.55pu and it uses 2.8s to recover to its original value. However, the thermal active power of the hybrid system decreases from 0.85pu to only 0.81pu and system can recover fast after the load is back to normal, seen in Fig. (10b). The active power performances of wind farms are the same to the thermal power plants, seen in Fig. (10c). This demonstrates that the hybrid MTDC system is more capable of dealing with load change than a conventional LCC system.

This paper proposes a hybrid MTDC system, which connects remote load centers into existing resources (wind-thermal-bundled power system). Furthermore, control strategies for design of each converter are proposed. Based on the different simulation results, following conclusions can be made:

1. Fluctuation in wind power embedded with noise can be compensated by thermal power and the system can tolerate such variations in wind speed.
2. A three-phase short current fault of VSC on receiving end or load change does not severely impact the sending end and there is no communication failure. The hybrid MTDC system was able to deal with load change in a passive network.
3. Peak load regulation for thermal power cannot fully meet wind power, due to this some additional research, which takes in consideration the addition of energy storage systems will be incorporated into future work.