With the large-scale integration of distributed generators (DGs), the coupling relations between transmission network (TN) and distribution networks (DNs) are getting closer. Coordinating the black-start processes of TN and DNs is beneficial to improving restoration efficiency. Therefore, a distributed black-start optimization method for global transmission and distribution (T&D) network is proposed in this paper. Firstly, a black-start optimization model of global T&D network is established to minimize the outage costs. Then, it is decomposed into the TN black-start optimization sub-problem and several DN black-start optimization sub-problems based on the analytical target cascading (ATC). The mixed integer quadratic programming (MIQP) is used for sub-problem modeling. The models take into account the restoration operations of different grid components at each black-start time step. The transmission control center (TCC) and distribution control centers (DCCs) only need to exchange partial information including the boundary power at each time step for distributed coordination, to obtain the coordinated black-start scheme for T&D network. Finally, the effectiveness of the proposed model and method is validated using a global T&D network case.

This paper presents the interaction analysis of external disturbances and internal dynamics of MMC-HVDC (Modular Multilevel Converter High Voltage Direct Current) systems considering sub-module (SM) capacitor voltage ripples, and proposes an enhanced control strategy for disturbance suppression. The disturbance equation indicating the interaction of the external MMC output currents and internal SM capacitor voltage ripples are firstly established. Then based on the disturbance equation, the dynamic analytical expressions of the differential-mode capacitor ripple voltages and the converter currents are derived in the d-q reference frame, which explains that the resultant current term i0 can be regarded as an equivalent disturbance that will lead to transients in the capacitor voltages during external perturbations. Next, the disturbance observer is introduced to track the resultant disturbance current term and an enhanced control for the MMC based on disturbance suppression strategy is proposed to improve the system dynamic response during external transients. Furthermore, system stability characteristics under the enhanced control with different parameters are explored through theoretical analysis. Finally, a benchmark single-terminal MMC-HVDC simulation model is established in PSCAD/EMTDC to verify the proposed control.

The Modular Multilevel Converter based High Voltage Direct Current Transmission (MMC-HVDC) systems can provide active inertia support for the connected ac grid for fast frequency recovery. However, the submodule (SM) capacitance voltages will be affected by the transient power flow and there exist dc voltage fluctuation issues during the active inertia support process. This paper proposes a multi-mode active inertia support strategy for MMC-HVDC systems considering the constraint of dc voltage fluctuations. The analytical relationship of dc voltage fluctuations and ac frequency variations when MMC adopts the virtual inertia control is firstly established, and then different inertia support operation modes are divided according to the allowable dc voltage fluctuation ranges. Next, the multi-mode active inertia support strategy for MMC-HVDC is proposed, which can provide flexible active inertia support on the premise of reasonable dc voltage fluctuations according to different frequency variations and recovery requirements. Besides, the inertia support process of MMC is equivalent to the zero-input response of first-order circuit to calculate the maximum sustainable supporting time, which ensures that the dc voltage fluctuation constraint can be well satisfied in different operation modes. Finally, several simulation results verify the effectiveness of the proposed control strategy.

The communication-dependent inertia supporting of MMC-HVDC systems transmits the ac frequency information to the MMC station via phase measured units (PMUs), and provides frequency support for the connected ac grid. However, the random disturbances including the communication time delays and denial of service (DoS) attacks in the communication networks introduce new threats on the stable operation of the MMC-HVDC and the connected ac system. This article proposes a Markov-based stochastic stabilization control for MMC-HVDC systems with inertia supporting under random disturbances. Firstly, the Markov time-delay jump model of MMC-HVDC systems with inertia supporting is investigated under random disturbances, while the communication delay and DoS attacks are modeled as the stochastic Markov jumps with bounded time-varying delay and state discontinuities. Then the time delay term is regarded as a kind of equivalent disturbance acting on system input and compensated through the second-order communication disturbance observer (CDO). Besides, a Markov-based finite-time stabilization controller is designed for MMC-HVDC to mitigate the influence of random DoS attacks without need of information about the detailed DoS attack states. Simulations of several cases on PSCAD/EMTDC platform show that the inertia supporting control of MMC-HVDC can remain stable and unaffected in realizing fast frequency recovery for the ac grid under the random disturbances in the communication networks.

Large-scale renewable energy grid-connection will lead to a serious weakening of the inertia level of the power grid, which will have a non-negligible impact on the frequency stability of the system. This paper proposes an emergency control strategy for high proportion renewable power system that can make full use of the adjustment ability of multi-type power generations to maximize the frequency recovery effect after disturbances. Firstly, the frequency response characteristics of multi-type power generations in high proportion renewable energy power system are analyzed, and the frequency aggregation response models including thermal power units, wind turbine units and photovoltaic energy storage units are derived. On this basis, a system frequency emergency control model considering the adjustment characteristics of multi-type power generations is established. The objective function of the model is to minimize the frequency modulation cost after disturbance accident, while the constraint conditions include the output power of multi-type power generations and the transient index of frequency stability. The constraint of frequency change rate is transformed into the minimum inertia demand of the system, which can reduce the influence of imprecise measurement of disturbance occurrence time on the accuracy of scheme formulation. Finally, combined with the established example system model, it is verified that the theoretical calculation errors of the maximum frequency deviation and quasi-steady-state deviation against the actual values can be limited within 1% and the effectiveness of proposed emergency frequency control scheme is verified under different disturbances.

The Modular Multilevel Converter High Voltage Direct Current Transmission (MMC-HVDC) technology is considered to be the most feasible choice for high-voltage and high-power transmission systems, and its flexibility and high controllability provide a new solution for renewable energy grid integration. The MMC topology contains a large number of capacitors, which enables it to provide a certain active inertia support for the connected AC system. Different from a synchronous machine, the active inertia control of an MMC can flexibly adjust a system’s inertia-supporting power by changing the control parameters. By introducing a variation of the Sigmoid function with amplitude-limiting capability, this paper proposes an adaptive active inertia control strategy for the MMC-HVDC system. The proposed scheme adjusts the inertia constant adaptively according to the frequency change rate of the AC system, which can better respond to the frequency recovery performance. Finally, the MMC-HVDC simulation model is established in PSCAD/EMTDC to verify the effectiveness of the proposed control strategy.