Modeling And Control Of Hybrid Multi-terminal HVDC System Supplying Passive Loads

The Voltage Source Converter (VSC)-HVDC system became feasible in cases such as power transmission to islands and remote load centers due to the breakthrough in passive inverting technology. AC voltage waveforms of inverter side are low in harmonic content. However, power transmission use tradition two-level and three-level inverters is limited. The advantages of the modular multilevel voltage source inverter are somewhat offset by some drawbacks, such as the control and protection strategy complexity. The line commuted converter (LCC)-HVDC system has advantages of large transmission capacity and low cost, but the inverter needs the support of commutating voltage and unable to supply to passive network. In this paper, a hybrid multi-terminal HVDC System supplying more than one passive load is investigated. The system consists of LCC at the sending end, and a number of VSC at the receiving end. The system has large transmission capacity and excellent power quality, which saves the city transmission corridor and sends the power to islands.In order to reduce the undesired impacts and damages on the HB-HVDC system and the connected AC system during the startup procedures, appropriate startup control strategy should be dedicatedly designed. This paper designs the slope control strategy and undisturbed switching control strategy to let a hybrid multi-terminal HVDC System smooth start. Two conditions are considered:the parallel VSC converter station input at the same time or input respectively. The effectiveness of the proposed steady-state control strategy and start-up strategy of the hybrid pseudo bipolar HVDC simulation system are simulated in electromagnetic simulation software. The entire startup process is smooth and stable, after startup, the system tracking target reference accurately and stably.Hybrid Multi-terminal HVDC system is a typical multiple input multiple output high order system. The converter station as a subsystem, all has its own independent control loop. The control loops coupled together and interfere with each other due to the interactions of dc system. Any disturbance, such as the change of power flow, transient failure, etc., occurs in one station, can flowing through to the other interconnection control loop by the way of DC system, and affect the operation of the other converter stations. Due to periodic changes of the passive network, the interference between the converter stations will be frequent, and go against the voltage stability of the passive network. To reduce the cross coupling between the converter stations, the principle of the interaction between convertor stations of HMT-HVDC system supplying passive loads should be researched firstly, then reasonable control should be designed. The small-signal model of system is the precondition of coupling and decoupling control research.This paper deduces more precise model of LCC and VSC converter stations based on commutation principle of LCC inverter, modulation principle of the VSC converter and the conservation of AC/DC system active power ignoring switching loss.The global small signal model of HMT-HVDC system which includes LCC converter station and the ac system model, VSC converter station and the passive network model and DC network is deduced. The waveforms of small signal model simulation and electromagnetic transient simulation under the same small disturbance are compared. The global small signal model is proved correctly and can be used for subsequent research of coupling mechanism and decoupling control strategy.The interaction between convertor stations is investigated by utilizing small signal analysis method based on the small-signal model above. The interaction between convertor stations mainly come from the cross coupling between DC voltages of each converter station and the cross coupling of AC/DC side of the station. The cross coupling of DC voltage mainly relates to the line impedance. The cross coupling of AC/DC side of LCC converter station is determined by the DC voltage controller, the cross coupling of AC/DC side of VSC converter station is associated with inverter modulation. Then, a wide-area DC voltage feed-back decoupling control and a local DC current and DC voltage feed-back decoupling control are designed based on the small signal analysis method. The results show that the controller can reduce the interference between the converter stations and improved the stability of supply voltage.After decoupling, the original high order system is decomposed into several independent low-order subsystems, each converter station and AC system connected can be treated as a subsystem. In order to ensure the closed-loop dynamic performance and stability of the system, the controller should have good robust performance. The PI controller is dependent on the operating experience and artificial debugging; it is very hard to achieve precise control. An internal model controller and a mixed sensitivity H∞ controller are adopted in LCC and VSC station to enhance the robust performance of the hybrid HVDC system.As an example, a three-terminal hybrid HVDC system is simulated in the electromagnetic simulation software, the robust performance of the robust controller and PI controller are compared. The results show that the robust controller can be applied in real system with its simple, robust and high performance, which is superior to the PI controller.