Search

Modal analysis in the continuous-time state-space is a mature and effective method for analyzing the subsynchronous oscillation (SSO) problems of power systems. But it has limitations on oscillation traceability analysis due to the loss of system topology information. To address this issue, a discrete-time domain modal analysis method is developed in this paper. It represents the target system as a networked discrete-time equivalent circuit and uses the nodal method to generate the state matrix, from which the discrete-time eigenvalues in the z-plane can be solved. Based on the oscillation frequency and damping ratio defined in the discrete-time domain, the stability of the oscillatory mode can be quantitatively analyzed. Then, participation factors analysis is carried out similarly to the continuous-time method to locate the contributive components. Most importantly, the distribution coefficients of branch currents/ node voltages are finally defined and derived for in-depth analysis of the oscillatory mode. With this index, the propagation path of the oscillation in the mesh power system is expected to be identified.The proposed method has been applied in several case systems with wind farms integrated. Theoretical analysis results are highly consistent with the electromagnetic transient simulation results in PSCAD/ EMTDC, thus verifying its effectiveness. [ABSTRACT FROM AUTHOR]

• This paper studies the new problem, EVs' provision of flexible ramping products (FRPs). • The dispatching capability of EV fleets is disclosed through the feasible operating region in variable spaces. • A cloud-edge collaborated market structure for the electricity market with FRPs is proposed to facilitate a large amount of EVAs integrated with protected privacy. • A distributed optimal model combining the EVA's bidding subproblem and the market clearing subproblem is established. • The distributed optimal model is solved based on the modified analytical target cascading (ATC), in which the convergence performance is improved through heuristic constraints. • Time-coupled multi-period (TCMP) operation mode is implemented in the market to maximize the dispatching potential of EV fleets. More flexible ramping service is required due to the increase of renewable power generation in power systems. Electric vehicles (EVs) could provide such flexible ramping products (FRPs) at low cost while participating in the electricity market through aggregation. However, EVs' dispatching capability cannot be fully utilized without the right incentives. This paper addresses a distributed optimal model developed between EV aggregators (EVAs) and the independent system operator (ISO). To make such concept, a cloud-edge collaborated market structure is adopted. At the edge level, EVAs assess the dispatching capability and solve the market bidding subproblem. At the cloud level, ISO solves the market clearing subproblem considering system economy and security. The overall problem is solved by the analytical target cascading (ATC) method. Heuristic constraints are also introduced into the model to improve convergence performance. The model is tested on a modified IEEE 30-bus system. Results demonstrate that the proposed method can incentivize EVAs with different owners to shift load and provide FRPs accurately, meanwhile reducing the cost and increasing the consumption of renewable energy effectively. [ABSTRACT FROM AUTHOR]

Construction of the linearized state-space model is the key step of eigenvalue analysis for SSO study of power systems with large-scale renewable power generation (RPG) integrated. This paper proposes a novel method to construct the state matrix of the AC/DC hybrid power system with RPG within the discrete-time domain. The continuous-time state-space model of each AC/DC component as well as RPG units in the power system, is discretized together with their corresponding control. The discretized equation of each component is then represented by an equivalent circuit consisting of parallel historical current and conductance. According to the power system configuration, the discrete-time equivalent circuit of the whole AC/DC hybrid system is established, and the nodal analysis method is applied for the construction of the discrete-time state-space model of the power system. In this way, the state matrix can be acquired much more conveniently than that of continuous-time state-space method, which needs topology analysis to find independent state variables of the network from the proper tree. The proposed method has been applied in the eigenvalue analysis for the SSO study of a case system adapted from the AC/DC hybrid transmission system in northwest China. Eigenvalue results are in high accordance with time-domain simulation results in PSCAD/EMTDC. [ABSTRACT FROM AUTHOR]

The Central China Power Grid (CCPG) and North China Power Grid (NCPG) are connected together by a single AC ultra‐high‐voltage (UHV) transmission link. When suffering power disturbances, power oscillation with high magnitude and long period would happen on the UHV tie‐line, which may affect the synchronous and stable operation of the interconnected two power grids. This study proposes a novel emergency control scheme for high‐voltage direct current (HVDC) in CCPG to suppress the high peak value of the power oscillation. Based on a two‐machine equivalent model, the maximum rising slope of power oscillat'ion is used to predict the peak value of power oscillation and power disturbance which is used as the reference of emergency power modulation. Simulation verification is carried out based on the full model of the CCPG and NCPG, using Power System Analysis Software Package. Moreover, field test has also been carried out in CCPG. Both simulation results and recorded wave curve show that the proposed HVDC emergency control scheme can effectively suppress the peak value under different power disturbances. This significantly improves the transient performance of the interconnected power system and the power transfer capacity of the UHV tie‐line. [ABSTRACT FROM AUTHOR]

The paper "Complete Provision of MPC-Based LFC By Electric Vehicles With Inertial and Droop Support from DFIG-Based Wind Farm," [1] contains two errors which are detailed as follows: [ABSTRACT FROM AUTHOR]

Calculation of the complex torque coefficient for multi-machine system subsynchronous resonance (SSR) assessment had been proven a hard task. In this paper the concept of complex frequency domain port-equivalence conductance matrix (CPCM) is introduced to model the individual system component which represents the relationship between the terminal voltage and current in frequency domain, and the model of the overall power system can be established easily. Thus the complex torque coefficients of the generator unit under investigation can be calculated for SSR assessment. The advantages of this proposed method are that it can take the shaft dynamics and the rotor angle interactions among the generator units into consideration for the multi-machine power system SSR analysis, as well as it can be used to analyze the effectiveness of SSR damping devices. The effectiveness of the proposed method has been verified by both the eigenvalue analysis and the time domain simulation. [ABSTRACT FROM PUBLISHER]

Highlights: [•] A practical method to construct network state equations in SSR study is proposed. [•] Network state-variables are obtained using the network configuration features. [•] Network proper tree is not required and complex topology analysis is avoided. [•] The network state equations construction is modular in the proposed method. [•] Analysis result of a multi-machine system in China is verified by simulation. [ABSTRACT FROM AUTHOR]

The initiative participation of wind power generators in power system frequency regulation is an inevitable demand to ensure power system safe operation with large-scale wind power integration. However, it is principally difficult for conventional wind turbines to participate in power system frequency regulation. For this reason, solution to this problem is becoming a hot topic in the current studies. This paper systematically reviews the relevant research progresses in the control strategies and the capability assessment of using wind power generators in the power system frequency regulation, including the comparing among the emulating inertia control, the droop control, the rotor speed control, the pitch angle control and the coordinated control. Also, future research in this area is pointed out in the paper. [ABSTRACT FROM AUTHOR]

The initiative participation of wind power generators in power system frequency regulation is an inevitable demand to ensure power system safe operation with large-scale wind power integration. However, it is principally difficult for conventional wind turbines to participate in power system frequency regulation. For this reason, solution to this problem is becoming a hot topic in the current studies. This paper systematically reviews the relevant research progresses in the control strategies and the capability assessment of using wind power generators in the power system frequency regulation, including the comparing among the emulating inertia control, the droop control, the rotor speed control, the pitch angle control and the coordinated control. Also, future research in this area is pointed out in the paper. [ABSTRACT FROM AUTHOR]

Since wind generation is one of the most mature renewable energy technologies, it will have the greatest share of future renewable energy portfolio. Due to the special characteristics of the wind generation, it requires extensive research to explore the best choice for wind power integration. In light of the practical project experience, this paper explores the feasibility of using HVdc transmission technology, particularly multiterminal HVdc (MTDC), as one of the preferable solutions to solve the grid interconnection issue of wind generation. This paper mainly focuses on the application of the hybrid MTDC to integrate wind farms into the electric power grid. A five-terminal hybrid MTDC model system including a large capacity wind farm is set up in PSCAD/EMTDC, in which the corresponding control strategy is designed. The operation characteristic of the hybrid system is studied, and the proposed control strategy is verified through simulation under various conditions, including wind speed variation and faults on ac side and dc side. [ABSTRACT FROM PUBLISHER]

Investigates the impact of direct foreign investments (DFI) on foreign trade and transfer pricing trends in China. Exploration of transfer pricing by multinational corporations; Presentation of theoretical views on the impact of DFI on the country's economy; Study of the role of foreign-invested enterprises in China's foreign trade.

This article presents an in-depth study of the regional dualistic growth pattern of the Chinese economy from 1984 to 1993. After opening up its economy to the rest of the world in 1979, China has been able to double its gross domestic product (GDP) twice by 1994, with an average annual real growth rate of 10.5%. However, Chinese economic growth has been uneven as is evident from disparate growth rates especially between the Eastern (coastal) and the Western (far inland) regions of the country. The imbalance in economic growth between the two regions has been exacerbated since China further opened the coastal region in 1984. For instance, the economic growth rate of the Eastern region was 11.4% on average from 1984 to 1993, while it was 8.9% for the Western region. This led to an increased economic gap between the two regions. In 1993 the GDP per capita of the Eastern region was 2.3 times as much as that of the Western region. The economic disparity between the two regions can be attributed to many factors, such as economic reforms and an open-door policy in favour of the Eastern region, differences in economic structures and resource conditions, a coast-orientated regional policy, and consequently differential impacts of foreign trade and foreign investment. The uneven growth and resulting increased interregional economic disparity in China have become a new focus for development economists and policy makers. Several studies have examined the influence of economic reforms and regional policy on regional economic development in China. Another group of scholars has studied regional impacts of foreign trade and foreign direct investment (FDI) in particular. These scholars have found that the impact of foreign trade and FDI on the economic development of China are different between the Eastern and Western regions.

Presents a comparative analysis of the characteristics of direct foreign investment (DFI) in China. Suspicion that not all foreign investment is equally beneficial; Primary sources of DFI; Investigation of the entry modes of investors; Discussion on the factors behind the differences among major investing countries and regions.

• Electric vehicle aggregator capacity estimation method is proposed for LFC. • Robust mixed H 2 /H ∞ -SOF controller, synthesized via ILMI for LFC with EVs. • A comparison between mixed H 2 /H ∞ -SOF and only H ∞ -SOF. • Case studies of single and 3 area systems with different scenarios simulated. • Time delays and parameter uncertainties and load variation are considered. Besides being new loads that would call for further capacity from fossil-fuel based Conventional Sources (CS), managing Electric Vehicles (EVs) smartly i.e. using Vehicle-to-Grid (V2G), holds the opportunity to be used for Load Frequency Control (LFC). However, there exists uncertainty in available dispatch capacity and time delay linked to charging/discharging control of individual EVs. Assessing this uncertain capacity and related delay of its response for LFC is presented first in this paper. Next, for the EVs participating in multi-objective LFC (V2G) along with CSs, a robust mixed H 2 /H ∞ controller based on static output feedback is adopted. The system stability, i.e. bounding frequency and tie-line power to their nominal values in the presence of internal and external disturbance, is ensured by providing better reference tracking to both EVs and CSs. Case studies of a single and three-area system with EVs are tested for LFC, where the efficacy of the mixed H 2 /H ∞ is also endorsed by the comparison with only H ∞ controller. The results showed that the EVs and the adopted controller can respond effectively to the scenarios i.e. load variation, parameters uncertainties, effects of dead bands and time delays of both controller and EVs aggregator responses. It caused a decrease in the capacity burden on CS while this study open ways for the future grid to be pollution-free. [ABSTRACT FROM AUTHOR]

The system equivalent inertia of the power grid is gradually decreased with the increasing penetration of renewable energy resources, which leads to a higher risk of frequency fluctuation after a major fault. For the frequency emergency control in the scenario of a high-voltage direct current (HVDC) line block fault, a novel criterion for enabling the fast response of user-end distributed loads is proposed in this study based on voltage feature identification and frequency drop detection. The reactive power to voltage sensitivity coefficient is combined with the Hausdorff algorithm to increase the accuracy of the calculation. The proposed criterion can realise the HVDC block fault identification at the end of the distribution grid, so as to achieve fast decentralised decision-making for distributed loads. This method may greatly reduce the need for high-speed communication and take actions faster than the primary and secondary frequency controls of the traditional units. The criterion makes it possible for low voltage small load in the user-end to be controlled precisely and selectively to fill the large power imbalanced caused by an HVDC block fault. The validity and superiority of the criterion are verified by simulation experiments using multiple test systems. [ABSTRACT FROM AUTHOR]

The system equivalent inertia of the power grid is gradually decreased with the increasing penetration of renewable energy resources, which leads to a higher risk of frequency fluctuation after a major fault. For the frequency emergency control in the scenario of a high‐voltage direct current (HVDC) line block fault, a novel criterion for enabling the fast response of user‐end distributed loads is proposed in this study based on voltage feature identification and frequency drop detection. The reactive power to voltage sensitivity coefficient is combined with the Hausdorff algorithm to increase the accuracy of the calculation. The proposed criterion can realise the HVDC block fault identification at the end of the distribution grid, so as to achieve fast decentralised decision‐making for distributed loads. This method may greatly reduce the need for high‐speed communication and take actions faster than the primary and secondary frequency controls of the traditional units. The criterion makes it possible for low voltage small load in the user‐end to be controlled precisely and selectively to fill the large power imbalanced caused by an HVDC block fault. The validity and superiority of the criterion are verified by simulation experiments using multiple test systems. [ABSTRACT FROM AUTHOR]

Recently, dc electric springs (dc-ESs) have been proposed to realize voltage regulation and power quality improvement in dc microgrids. This paper establishes a distributed cooperative control framework for multiple dc-ESs in a dc microgrid and presents the small-signal stability analysis of the system. The primary level implements a droop control to coordinate the operations of multiple dc-ESs. The secondary control is based on a consensus algorithm to regulate the dc-bus voltage reference, incorporating the state-of-charge (SOC) balance among dc-ESs. With the design, the cooperative control can achieve average dc-bus voltage consensus and maintain SOC balance among different dc-ESs using only neighbor-to-neighbor information. Furthermore, a small-signal model of a four dc-ESs system with the primary and secondary controllers is developed. The eigenvalue analysis is presented to show the effect of the communication weight on system stability. Finally, the effectiveness of the proposed control scheme and the small-signal model is verified in an islanded dc microgrid under different scenarios through simulation and experimental studies. [ABSTRACT FROM PUBLISHER]

Following a converter outage in a Multiterminal dc (MTDC) for offshore wind farm integration, the conventional droop control shares the power mismatch to minimize the impact on the wind power generation with a fixed droop coefficient; however, it may easily lead to the converter overloading and large dc voltage deviation. The paper proposes a fuzzy logic based adaptive droop controller, which utilizes the available power capacity of the converters to update the droop coefficient. It can make a compromise between the dc voltage deviation and the power sharing. Furthermore, the impact of the droop coefficient change on the stability of the MTDC grid is analyzed, and the effect of the dc-bus capacitor on the system dynamic is identified. The stability region is determined through the eigenvalue analysis. Time-domain simulations are conducted to compare the control strategy proposed with the conventional one. It has been proved that the adaptive control scheme can constantly track dynamic behaviors of the converters, and exhibit desirable response under different outage scenarios. [ABSTRACT FROM PUBLISHER]

This paper investigates the impact of foreign direct investment (FDI) on the export performance of China at the provincial level. First, it presents a theoretical discussion of the impact of FDI on foreign trade, and then an empirical study of the impact of FDI on the export performance of regions in Chin. It has been found that the impact of FDI on exports differs across three macro-regions in China. The effect is stronger in the coastal region than in the inland regions. Although FDI shows a positive and significant impact on exports from the central region, its impact on the western region is found to be insignificant. [ABSTRACT FROM AUTHOR]

In this paper, a multiagent system-based wide-area protection and control scheme is proposed to deal with the long-term voltage instability-induced cascading trips. Based on sensitivity analysis between the relay operation margin and power system state variables, an optimal emergency control strategy is defined to adjust the emergency states timely and prevent the unexpected relay trips. In order to supervise the control process and further minimize the load loss, an agent-based process control is adopted to monitor the states of distributed controllers and adjust the emergency control strategy. A hybrid simulation platform based on LabVIEW and real-time digital simulator is set up to simulate a blackout case in the power system of Eastern Denmark and to demonstrate the effectiveness of the proposed MAS-based protection strategy. [ABSTRACT FROM AUTHOR]