Connected AC System Research Articles

Incentive Mechanism Design for Emergency Frequency Control in Multi-Infeed Hybrid AC-DC System

In multi-infeed hybrid AC-DC (MIDC) systems, the emergency frequency control (EFC) with LCC-HVDC systems participating is of vital importance for system frequency security. Nevertheless, when regional power systems are operated by different decision-makers, the LCC-HVDC systems and their connected AC systems might be unwilling to participate in the EFC due to the costs and losses. In this paper, to incentivize the LCC-HVDC systems and their connected adjacent AC systems to participate in the droop-based EFC, a novel control-parameter-based incentive mechanism is proposed, which can flexibly deal with various possible power imbalances. Then, a non-cooperative-based incentive game model is formulated to quantitatively analyze the incentive mechanism in the MIDC system. An algorithm for seeking the Nash equilibrium is designed, and the uniqueness of Nash equilibrium is proven. Moreover, the individual rationality, incentive compatibility and social optimality of the proposed mechanism are analyzed and proven. The effectiveness of the proposed incentive mechanism is verified through a case study.

Adaptive Active Inertia Control Strategy of MMC-HVDC Systems for Flexible Frequency Support

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.

Power Equalization Control Strategy for MMCs in Hybrid-Cascaded UHVDC System

Based on the hybrid-cascaded topology of ultra-high-voltage direct current (UHVDC) engineering, this study clarified the mechanism of unbalanced power generation among modular multilevel converters (MMCs) at the inverter side following the fault of the AC system at the rectifying side, and then proposed the power equalization strategy for MMCs. By performing closed-loop control on the active power deviation between constant-voltage and constant-power MMCs, it was possible to achieve automatic power equalization among MMCs after the occurrence of a fault so as to avoid the detrimental effect of a single MMC’s power fluctuation on the connected AC system. Meanwhile, the control enabling logic was designed to ensure the reliable input and stable exit of the control strategy throughout the disturbance period. Finally, a PSCAD/EMTDC platform was used to simulate various types of faults in the AC system at the rectifier side in order to validate the effectiveness of the proposed power equalization strategy.

Cyber-Attack Detection: Modeling and Roof-PV Generation System Defending

The continuous increase of renewable energy installation in the power system such as roof-PV systems decreases the system inertia. It is a non-negligible challenge to the system's operation stability. In addition, with the increasing number of cyber attack events reported globally, the operation of the roof-PV systems may threaten their connected AC system operation during the contingency. Utilizing the grid-connected converters (GCCs), its fast regulating characteristic could rapidly increase or decrease the solar generation output in seconds, which will bring significant influences to the system operation once the cyber attack happens, especially if the system has integrated high proportional renewable energies. This paper proposed a cyber-attack detection model to solve this problem to eliminate its effect on the roof-PV generation system. In this model, the synchrosqueezed wavelet transforms (SWT) are first applied to extract the time-frequency information of frequency measurement. Then a recurrent layer aggregation-based convolutional neural network is introduced to identify the features of cyber attacks using the results from SWT. The comparison experiments indicate that the proposed model has profound performance on the detection accuracy of the roof-PV generation system for cyber attack detection.

Novel control approach for enhancing multi-performance of two-stage converter during power transfer from solar photo-voltaic array to AC grid

This paper presents a nonconventional control approach for augmenting various performances of two-stage power converter (DC-side boost converter and three-phase voltage source inverter (VSI)) during power transfer from solar photo-voltaic array to AC grid. In the application, intermediate DC link voltage is traditionally regulated to settle at a constant and high value. This fixed value is not optimum in respect of various performances of the converters under variable power transfer scenarios. In this respect, variable values of DC link voltage under different power transfer conditions are necessary, which is absent in conventional scheme. For achieving the control flexibility, two modifications of traditional control scheme are proposed as, omission of intermediate DC link voltage regulating loop and controlling of modulation index of current loop between 0.98 and 1 for VSI. In effect, significant multi-benefits are obtained as, (i) reduction of voltage stress on power switches to enhance their life span, (ii) reduction of size, cost and enhanced lifetime of DC link capacitor, (iii) improved efficiency of conversion system and (iv) reduction of total harmonic distortion (THD) during maximum power injection from solar photo-voltaic (PV) array to connected AC system. First three benefits are produced due to reduction of DC link voltage for achieving required power transfer and fourth benefit is achieved due to high valued modulation index (between 0.98 and 1). Both simulation and experimental studies verify superior performances of two-stage converter with the help of proposed logic under variable solar power conditions. Derived mathematical condition on interplay between DC link voltage value and power transfer amount at different power factors (unity and leading) provides insight of proposed control approach.

Inverter‐side robust damping controller design of hybrid HVDC with cascaded multi‐infeed MMC converters in asynchronous situation

Objective it is known that the rectifier side control usually suffers from the time delay and long distance communication problems, which could make the damping controller to be invalid and decrease the system stability. Specifically, when the system is in asynchronous operation, the rectifier controllers cannot damp the oscillations in inverter side AC system. Thus, to design the HVDC damping controller at inverter side becomes a feasible solution to avoid these problems. Methods Based on the MMC control strategy, to design the damping controllers at inverter side, the mixed H2/H∞ robust method with regional pole placement is investigated for the two MMC inverters in constant active power control based on the master-slave control mode, where the robustness, control effort and the damping ratios can all be guaranteed simultaneously. During the design process, the simplified system models, namely the TLS-ESPRIT identification algorithm, is also used for controller study. Finally, two robust controllers are obtained based on the proposed method and the inverter side damping controllers are added at the MMCs of hybrid HVDC. Results The damping controllers for hybrid HVDC with cascaded multi-infeed MMCs can be designed at inverter side, which endows the hybrid HVDC with more control functions compared with the classical HVDC systems. The damping controllers can be added at the cascaded multi-infeed MMC inverters in constant active power control, which can enhance the connected AC systems' small stabilities. Conclusions Based on the multi-objective robust theory, the control effectiveness and robustness of the designed controllers can be guaranteed simultaneously, which can damp the low frequency oscillations significantly in different operating situations.

Novel MTDC droop scheme with decoupled power control for enhancing frequency stabilities of weak AC systems

Traditional way to enable frequency support provided by MTDC system for the connected weak AC systems is to modify the power reference of each converter under standard power-sharing droop control based on the weak grid's frequency deviations. However, this scheme artificially couples all asynchronous AC systems to tackle with the frequency disturbances of any connected AC systems, which might result in broader frequency disturbance propagations. To overcome this, this study proposes a decoupled frequency control (DFC) scheme to improve weak AC systems frequency stabilities. First, the relationship between the active power and power set points of each converter is derived by the linearization of MTDC power flow equations. Then, the frequency coupling properties of each AC system with traditional frequency control are derived, and it shows that the frequency disturbance will be propagated to all the connected AC systems with traditional control. Furthermore, the control law of each converter in the proposed DFC is developed through the defined decoupled matrix, and the independent frequency regulation characteristic of each converter can be obtained. Finally, numerical simulations have demonstrated the effectiveness of the proposed DFC scheme in events of load changes and converter loss in the main and weak AC grid.

Impact of dc grid protection strategies on frequency stability of the connected ac system: a quantitative analysis

Various options for dc grid protection strategies are currently investigated which differ in terms of speed and selectivity. In order to compare these options fairly, the impact on the connected ac grid needs to be studied. This paper analyzes the ac grid impact in terms of frequency stability and calculates the reserve capacity necessary to retain the stability. Assuming fast frequency reserves as a possible mitigation, a probabilistic method is provided to determine a trade-off between under frequency load shedding and additional reserve requirement. In general, non-selective strategies require higher and faster reserves compared to their fully selective counterparts for both cases analyzed in this paper. Both the selectivity and the speed of protection have an influence on the reserve requirement. The dc grid topology also plays an important role in the reserve sizing by determining the impact of dc contingencies on the ac grid. The concept of reserve sizing based on temporary loss of supply shows value in terms of lower capacity requirement. A grid specific CBA study shows that it is possible to compensate the higher investment cost associated with dc breakers by the benefit of lower reserve procurement cost.

Start‐up and recovery method with LCC–HVDC systems participation during AC/DC system black‐starts

With the increase in a number of online high-voltage direct current (HVDC) systems in China, a big question for their use as black-start and restoration of power system outages arises. Line-commutated converters (LCC)-HVDC systems can significantly reduce recovery time. The black-start-up and recovery conditions for LCC-HVDC systems and the connected AC systems are discussed in this study. The appropriate start-up and recovery modes are identified and the results of the control characteristics of LCC-HVDC systems are analysed in detail. The stability requirements of AC systems in terms of LCC-HVDC system effects are evaluated. Furthermore, the restoration using LCC-HVDC systems, including the priority sequence index of the multi-HVDC links and the optimisation model of the mentioned problem, is discussed in detail. Compared to the conventional black-start strategy, the applicability and effectiveness of the proposed method are illustrated using a part of the actual Guangzhou power system as a test system.

Impact of DC protection strategy of large HVDC network on frequency response of the connected AC system

Integration of renewable energy generations requires the transmission of bulky power over long distance, and high-voltage direct current (HVDC) transmission systems become a more preferable choice compared to conventional HVAC systems. For HVDC systems, one of the important concerns is the DC protection strategy which can significantly impact on the connected AC system performance, e.g. system frequency. The maximum loss-of-infeed for a AC network is highly dependent on the duration of the power outage, and the impacts of DC fault protection arrangements which result in different speed of power restoration on the connected AC system, on the system frequency, have not been properly understood. Different DC protection arrangements using DC disconnectors, fast and slow DC circuit breakers on frequency response of the connected AC networks are investigated. A three-terminal meshed HVDC system is studied to demonstrate system behaviour during DC faults.

Commutation Failure Elimination of LCC HVDC Systems Using Thyristor-Based Controllable Capacitors

The adverse impacts of commutation failure (CF) of a line-commutated converter (LCC)-based high-voltage direct current (HVdc) system on the connected ac system are becoming more serious for high-power ratings, for example, the development of ultra-HVdc systems. Aiming to solve the problem of CF particularly for higher power/current LCC HVdc systems, this paper proposes a new method, which utilizes a thyristor-based controllable capacitor (TBCC), to eliminate CFs. The topology of the proposed TBCC LCC HVdc and its operating principles are presented. Then, mathematical analysis is carried out for the selection of component parameters. To validate the performance of the proposed method, modified LCC-HVdc and capacitor-commutated converter (CCC)-based HVdc systems based on the modified CIGRE HVdc system are modeled in a real-time digital simulator. Simulation studies for zero impedance single-phase and three-phase faults are carried out, and comparisons are made with both LCC-HVdc and CCC-HVdc systems. Furthermore, voltage and current stress of the TBCC are investigated and power-loss calculations are presented. The results show that the proposed method is able to achieve CF elimination under the most serious faults while the increase of power losses due to the TBCC is small.

Selection methods of main circuit parameters for modular multilevel converters

The modular multilevel converter (MMC) will be used in power systems more and more widely. The main circuit parameter design of the MMC impacts the initial investment and the operating performance of the system. In this study the parameter design problem of several important elements in the main circuit of the MMC, such as the link transformer, the arm reactor, the sub-module (SM) capacitor and the SM power electronic devices are analysed. Based on the fundamental equivalent circuit of the MMC, the design method for the rated valve side no-load voltage of the link transformer is proposed; and the variation range of the rated valve side no-load voltage is determined as (1.00-1.05)/2 times of the DC side voltage. By introducing the concept of the equivalent capacity discharging time constant H , the nearly constant relationship between the fluctuation ratio of the SM capacitor voltage and the H -value for different projects is discovered, which means the H -value is independent of particular projects. In this study, the corresponding parameters of several practical projects are analysed and the recommended H -value is given as 40 ms. The voltage and current stress of the SM power electronic devices is investigated for the four extreme operating conditions by the analytical formulae of the MMC; and it is discovered that the current stress of the SM power electronic devices changes greatly with the operating condition. By introducing the concepts of the phase unit series resonance frequency and the circulating current resonance frequency, the principle for designing the parameter of the arm inductor is established. Through analysing the corresponding parameters of several real projects, the recommended value of the phase unit series resonance frequency is given as the rated frequency of the connected AC system.

Start-Up and Shut-Down Control Strategies for MMC-Based Multi-Terminal HVDC Systems

AbstractThis paper presents start-up and a shut-down control strategies for modular multilevel converter (MMC) based multiterminal high-voltage direct-current (HVDC) systems (MMC-MTDC). An AC system charging method with voltage slop control and a DC line charging method with sequentially unblock control are firstly discussed. Based on the above two charging methods and considering the strength of the connected AC systems, a start-up control scheme suitable for MMC-MTDC systems is described. The scheduled shut-down control strategy is divided into three stages: in energy feedback stage, four methods including increasing modulation index, adjusting transformer tap, introducing third harmonic voltage injection and inserting redundant submodules (SMs) are presented; and the second stage is energy dissipation stage, in which the SM capacitor voltage is discharged to its lower limit; The SM capacitors are fully discharged by SM internal resistor in the third stage. Finally, a simulation model of five-terminal M...

Generic inertia emulation controller for multi‐terminal voltage‐source‐converter high voltage direct current systems

A generic inertia emulation controller (INEC) scheme for multi-terminal voltage-source-converter (VSC)-based high voltage direct current (HVDC) systems is proposed in this study. The INEC can be incorporated in any grid-side VSC station, allowing the multi-terminal HVDC (MTDC) terminal to contribute an inertial response to connected AC systems during system disturbances, in a fashion similar to synchronous generators. The DC-link capacitors within the MTDC are utilised by the INEC scheme to exchange stored energy with the AC system by varying the overall DC voltage level of the MTDC network within a safe and pre-defined range. A theoretical treatment of the INEC algorithm and its implementation and integration within a conventional VSC control system are presented, and the impact on the total DC capacitance required within the MTDC network to ensure that DC voltages vary within an acceptable range is discussed. The proposed INEC scheme is validated using a MATLAB/SIMULINK model under various changes in demand and AC network faults. The model incorporates a multi-machine AC power system connected to a MTDC transmission system with multiple converter-interfaced nodes. The effectiveness of the INEC in damping post-fault oscillations and in enhancing AC grid frequency stability is also investigated.

HVDC System 적용 Double-tuned 필터의 설계 방법 연구

The ac side current of an high voltage direct current (HVDC) converter is characterized by highly non-sinusoidal waveform. If the harmonic current is allowed to flow in the connected ac system, it may cause unacceptable levels of distortion. Therefore, ac side filters are required as part of the total HVDC converter station, in order to reduce the harmonic distortion of the ac side current and voltage to acceptably low levels. The ac side filters are also employed to compensate network requested reactive power because HVDC converters also consume substantial reactive power. Among different types of filters, double-tuned filters have been widely utilized for HVDC system. This paper presents two design methods of double-tuned filter; equivalent method and parametric method. Using a parametric method, in particular the paper proposes a new design algorithm for a realistic system. Finally, the performance of the design algorithm is evaluated for a 80㎸ HVDC system in Jeju island with PSCAD/EMTDC program. The results cleary demonstrate the effectiveness of proposed design method in harmonics elimination and steady-state stability.

AC/DC System Dynamic Performance - Transient Stability Augmentation with Dynamic Reactive Power Compensation

The dynamic performance of an ac/dc system is evaluated for a system configuration which consists of a large generating plant synchronized to a relatively sparsely connected ac system with a back-to-back HVdc tie to a relatively strong asynchronous ac system. The emphasis of dynamic performance is on transient stability, where stability augmentation is achieved primarily by improving the transient reactive power capability of the ac/dc system. Additionally the effects of machine fault torques are evaluated. The particular study system configuration is suitable for operation of large generating plants near the borders of asynchronous power systems with power sharing by utilities in each asynchronous system.