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DC circuit breakers (DCCBs) play a significant role in obtaining the reliability and stability of DC power systems. Selective and fast isolating of faults minimizes the power supply outages to end electricity users and protects the power converters implemented in the DC system. During the last decades, DCCBs have been deeply investigated in industry and academia. There has been impressive progress in terms of efficiency, response time, power-density, lifetime, and electrical operating ratings. Introducing wide bandgap (WBG) devices further has motivated researchers to propose more efficient and novel topologies. This paper provides a point of reference and a background review for DCCBs from novel aspects, fulfilling absent parts in the literature. The elaborated subjects include current commutation circuitry, DCCBs trip-curves developments, newly added functions to DCCBs, as well as marketing analysis. The advancements of DCCBs are surveyed and development needs are recognized. This paper aims to identify the future research needs of DCCBs for emerging DC power systems.

As the penetration of utility-scale solar photovoltaic (PV) power plants increases, the inertia in the system is reduced and there will be increased primary frequency response requirements. To increase inertia and improve the primary frequency response, grid-forming inverters connecting PV to grid and energy storage systems (ESSs) may play an important role. Moreover, high-voltage direct current (HVdc) links can also be an enabler to transfer remote PV power generation and to improve grid stability. That is, with increased penetration of PV, discrete development of PV and ESS connecting to transmission ac grid and HVdc links is one of the solutions for stable operation of the grid. In this paper, an integrated concept for integration of PV and ESS to transmission ac grid and HVdc links is proposed that is named as multi-port autonomous reconfigurable solar power plant (MARS). The integrated development incorporates advanced control methods to provide inertial and primary frequency response, reactive power support, and transient stability to manage PV and ESS resources. In this paper, high-fidelity switched system model of the integrated system and grids are developed and detailed simulation results are provided to showcase the stable operation of the integrated system and provision of grid support functions.

To better utilize the diversity of renewable energies in the U.S., this paper proposes a cross-seam hybrid multi-ter-minal high-voltage direct current (MTDC) system for the integration of different types of renewable energies in the U. S. Based on a developed station-hybrid converter design, the proposed hybrid MTDC system further investigates the connection methods of renewable energies and develops novel flexible power flow control strategies for realizing uninterrupted integration of renewable energies. In addition, the frequency response control of the hybrid MTDC system is proposed by utilizing the coordination between the converters in the hybrid MTDC system. The feasibility of the hybrid MTDC system and the performance of its corresponding control strategies are conducted in the PSCAD/EMTDC simulation. The simulation results indicate that the proposed hybrid MTDC system could realize the uninterrupted integration of renewable energies and flexible power transmission to both coasts of U.S.

Cross-seam high voltage DC (HVDC) transmission in the United States can provide an efficient and flexible bulk power delivery highway between the interconnections. Recent studies have shown significant economic benefits of cross-seam HVDC transmission systems by taking the advantages of load diversity, frequency response, renewable energy diversity, and energy arbitrage in the different interconnections. This paper introduces a station-hybrid system scheme for cross-seam interconnections. The proposed station-hybrid system combines the advantages of line commutated converter (LCC) and voltage source converter (VSC) technologies, thus enabling reliable and flexible bidirectional power flows across the interconnections. Control strategies of the station-hybrid system are investigated with a focus on power flow reversal control under normal operation conditions and emergency power support control under emergency conditions. The feasibility and effectiveness of the proposed station-hybrid system and corresponding control strategies are verified by simulations in PSCAD/EMTDC.

In this study, a new kind of hybrid high-voltage direct current (HVDC) system is proposed. Each terminal of the proposed system consists of one line commutated converter (LCC) and one full-bridge modular multilevel converter (FB-MMC). The LCC and FB-MMC are connected in parallel so that they can share the same transmission line. The active-reactive power capability of the hybrid HVDC system is extended compared with the conventional LCC-HVDC system, and power reversal control without power interruption can be achieved by the coordination control of LCC and FB-MMC. Besides, the proposed hybrid HVDC system is capable of handling DC fault, because both LCC and FB-MMC have DC fault blocking capability. Moreover, the power rating of FB-MMC can be designed to low value while keeping the bulk-power transmission capability of LCC. A two-terminal bipolar hybrid HVDC system is built in PSCAD/EMTDC. The simulation results verify the effectiveness and feasibility of the proposed hybrid topology and corresponding control strategies.

Voltage source converter-based multi-terminal HVDC (VSC-MTDC) system has been considered for interconnecting asynchronous power grids for improved system operation efficiency and reliability. In this study, a frequency response control strategy is proposed for the VSC-MTDC systems that can enable operating reserves sharing among the interconnected asynchronous grids. The proposed control strategy consists of three frequency control components: MTDC primary frequency control, corrective frequency control, and MTDC secondary frequency control. The MTDC primary frequency control could keep the frequency of a disturbed power grid stable and avoid the frequency dropping into an unacceptable level when a sudden power imbalance event occurs. The corrective frequency control strives to guarantee the settled frequency can be within the predefined secure frequency range. The MTDC secondary frequency control provides a novel optimal allocation strategy for considering the participation of MTDC system for restoring system frequency back to the nominal frequency with the lowest cost. The simulation results, based on a modified IEEE New England 39-bus system modelled in PSCAD/EMTDC, show the effectiveness of the proposed frequency response control strategies.

DC fault handling is a critical aspect for designing and operating dc distribution systems. DC distribution protection must be rapid to avoid the damages caused by the fast growth of dc fault currents. For medium-voltage dc (MVdc) distribution systems, low-cost high-speed dc breakers are still challenging. A hybrid modular multilevel converter (MMC)-based dc fault protection and restoration method is proposed for a cost-effective dc fault protection solution for MMC-based MVdc systems. In the proposed protection solution, the converter topology and associated fault handling can bypass and interrupt the fault currents from both ac and dc sources, and thus allowing fast dc fault isolation by low-cost no-load dc disconnectors. Furthermore, the proposed dc protection solution facilitates converter fault ride-through and system restoration after fault removal. Simulation results of the pole-to-pole fault protection show the satisfactory performance of the proposed protection solution and scheme.

Power demand in large or mega cities is increasing as the world urbanizes. Metropolitan transmission networks are continuously upgrading to meet the growing demand for power and to replace old-style generation with local renewable generation and power supplies from cleaner external sources. The cost of land and its scarcity make it difficult to secure new rights-of-way for traditional-style transmission lines. As power transmission levels increase, the risk of exceeding the short circuit capability of existing switchgear equipment as well as other network components becomes a major constraint to the expansion of power networks. Strategies to enhance urban transmission networks must address all of these issues and prioritize solutions that may be easily located within urban boundaries and have short lead times from decision to commercial operation. Advanced transmission technologies can effectively increase the capacity, efficiency, and utilization of existing infrastructure and improve grid resiliency under extreme contingencies or catastrophic events. These technologies have been considered and implemented by electric utilities for urban grid enhancement and modernization.

Power grids of large or mega cities are continuously upgrading to meet the growing demand for power and replace old-style local generation with power supplies from external cleaner sources. Advanced transmission technologies can effectively increase the delivery capacity, flexibility and utilization of existing infrastructure. The focus of this paper is on VSC-HVDC subtransmission technologies for urban power grid enhancement and modernization. The discussions include urban power grid expansion issues, VSC-HVDC application scenarios, advanced technical schemes of VSC-HVDC intertie in subtransmission network, and control features for grid support under emergency conditions.

In the future, with high levels of renewable energy sources penetration, the transmission corridors will have to be flexible to operate under faster transients and variable power flows. Fast power ramping and overloading capabilities of HVDC system can be utilized to ensure transient stability of the interconnected power grid. In this paper, HVDC auxiliary controllers have been studied that can provide the power grid with additional synchronizing torques through fast DC power changes in response to arising system conditions. The effectiveness of the auxiliary controllers in improving transient stability is demonstrated by performing dynamic simulations on the detailed model of the Western Interconnection system (WECC).

This paper has analyzed the transients of DC voltage and DC currents during auto reclosing of HVDC circuit breakers to an overhead DC line fault of a VSC HVDC grid. When the line is energized by reclosing the HVDC breaker the voltage and current transients behave differently from the transient following the initial DC fault. Furthermore, the transients observed during auto reclosing differs between permanent and temporary DC faults. A robust DC line protection system that accounts for the transients following auto reclosing to avoid unwanted trips on the DC line has been designed. Numerical tests using PSCAD shows improvement in security of protecting overhead lines in VSC HVDC grids.

Resilience is a critical aspect of urban power grid modernization. VSC-HVDC solutions offer advanced technology options for urban grid enhancement with integrated system benefits. This paper introduces a method for operating VSC-HVDC intertie under emergency conditions. The proposed emergency power controller (EPC) performs real-time network condition assessments and activates appropriate grid support functions to reduce the impacts of the disturbances and mitigate the risk of spread outages. Simulations studies are performed using a realistic urban power system.

Hybrid HVDC system, which consists of the advantages of line commutated converter (LCC) and voltage source converter (VSC), is an emerging power transmission system. This paper presents a review of four LCC-VSC hybrid HVDC topologies. The first topology is the pole-hybrid HVDC system, in which the LCC and VSC form the positive- and negative-pole respectively. The second one is the terminal-hybrid HVDC system; in this topology one terminal adopts LCC and the other terminal adopts VSC. The series converter-hybrid HVDC system is the third topology wherein each terminal is formed by LCC and VSC in series. The fourth hybrid topology under consideration is a parallel converter-hybrid HVDC system with LCC and VSC connected in parallel in each terminal. The main contribution of this paper is a comprehensive analysis and comparison of the four mentioned hybrid topologies in terms of PQ operating zone, power flow reversal method, and DC fault ride-through strategy.

Sharing frequency response reserves through HVDC connections among the three Interconnections is expected to be one high value product that provides benefits to economically justify the HVDC Network. In this paper, A HVDC frequency response control scheme considered frequency response reserve allocation is developed for the VSC-HVDC links connecting asynchronous interconnections. The simulation results show that the proposed control scheme can greatly help the frequency stability of interconnection after contingencies of tripping the major generator while reducing the financial loss of disturbed interconnection during the event.

With the increasing penetration level of renewable power, the power systems’ inertias are decreasing. To ensure that the frequency is above the threshold of the under frequency load shedding, one effective way is to share the spinning reserves among asynchronous AC grids through HVDC systems. In this paper, a coordinated frequency regulation scheme that enables spinning reserves sharing is proposed for such hybrid AC/DC grids. This scheme is a communication-free method that consists of the droop control, the inertia emulation control and the frequency safety control. The combined response of the three controls can increase not only the frequency nadir but also the settled frequency of the disturbed grid. In addition, the hysteresis comparators and the power injection limits are adopted in the frequency regulation scheme to reduce the coupling effect among AC grids. Thus, under small and harmless disturbance, the frequency regulation scheme will be inactive; the disturbance in strong AC grids would not affect the weak AC systems. The frequency regulation scheme coordinates well with the automatic generation control of the grids. The feasibility and effectiveness of the proposed frequency regulation scheme are verified through simulation studies.

With the increasing penetration of wind energy into power grids, the frequency regulation capability of the system tends to deteriorate. Wind generators (WGs) could provide fast frequency response when the system suffers from disturbances. However, due to the randomness and intermittence of WG, its emergency support is not stable. The spinning reserves of the asynchronous power grids can be shared through the high voltage direct current (HVDC) links. Under a large disturbance, the HVDC link could provide fast frequency response support to maintain the operating frequency of the disturbed power grid within a secure range. In this paper, a frequency secure control strategy is proposed for the HVDC link connected to a power system with high penetration of wind energy. The proposed frequency secure control strategy has three control functions: (1) back-up frequency response control, (2) frequency compensation control, and (3) response distribution optimization. The objective of this control strategy is to improve the synthetic frequency response performance of the system under severe frequency disturbance while avoiding possible subsequent frequency drop caused by rotor speed recovery of wind turbines. Besides, the cost of ancillary service for obtaining frequency response support from interconnected power systems could be reduced. Simulation verifications are conducted on a reduced equivalent ERCOT system which is interconnected to equivalent Western and Eastern Interconnections by HVDC links. The simulation results verify the feasibility and effectiveness of the proposed frequency secure control strategy.

The VSC-HVDC transmission system has shown its significant advantage on the independent control capability of active and reactive power in power systems. This flexible controllability can be used for primary frequency control of large interconnected systems and voltage support of local areas. Based on the system models of three large interconnections in North America, the frequency control capability of VSC-HVDC transmission systems used to connect each interconnection is investigated. Simulation results show that VSC-HVDC systems can provide very fast active power support to improve the dynamic frequency performance of interconnections after contingencies of tripping the major generator(s).

The benefits offered by the DC energy distribution in different applications raised the interests towards new power architectures and apparatus. The availability of the related LV and MV apparatus and protection schemes is in fact crucial to fully exploit the opportunities opened in the energy management for the smart grid. Experimental results of testing DC circuit breakers based upon current injection and solid-state technologies are presented and their effectiveness for DC distribution protection is discussed. The possibilities opened by local measurement and communication to support the operation of DC circuit breakers for DC protection as well as the design challenges and implementation issues are investigated.

Modular Multilevel Converter (MMC) has presents great potential in high power quality, low operation loss, scalability and high reliability, which make MMC a good choice for DC transmission, MV drive and other HV or MV applications. This paper describes the average modeling and control of MMC. Firstly, the operation principles of MMC are analyzed, based on which the average MMC model is deduced. Secondly, the control methods for DC voltage control, circulating current suppression, and capacitor voltage balancing, etc are developed. Finally, the effectiveness of the proposed average model is verified by comparing the simulation results of an MVDC distribution system using detailed MMC models. The correctness of the designed control methods also demonstrated through simulation. It is also proven to be feasible of using MMC in MVDC applications.

DC fault current is contributed by various distributed energy resources in dc distribution systems. The tightly coupled dc distribution systems have relatively low line impedance values. The fault current increases fast because of the low impedance. Some converters in dc distribution systems include fault current limiting function. The controlled fault currents at different locations are very close. Thus, it is important to design a reliable and fast fault detection and location method for dc distribution systems. This paper proposes a novel local measurement-based fault location algorithm for tightly coupled dc distribution systems. The proposed fault location algorithm can estimate the equivalent inductance between a protective device and a fault in less than 1 ms. The performance of the developed protection algorithm was validated by numerical simulation and hardware tests.

The paper presents a reliability evaluation method based on fault tree analysis with set theory and minimal cut set as core algorithm, which can be used to evaluate the reliability for industrial grids with wide application of variable frequency drives. The working principle is introduced firstly, based on which the method development considering different system topology designs, backup solutions and redundancy mechanisms are analyzed in details. In the end the proposed method is applied to two cases to show the reliability performance of system with variable frequency drives. The proposed method is also suitable for analyzing the reliability performance of industrial grids with other types of power electronic converter technology.

The fault protection is a critical element to ensure the reliable and secure operation of DC distribution systems. Most DC distribution systems are tightly coupled systems with low line impedances which may result in fast current increase during a fault. Thus, it is challenging to develop a fast and reliable DC fault protection method. This paper proposes and develops a novel fault inductance based DC protection method without communication between protection units at different locations. The performance of the developed protection algorithm was validated in a Real-Time Hardware-In-the-Loop (RTHIL) test platform. The testing results indicate that the developed inductance based fault location algorithm detects and locates faults with fast speed and high accuracy. Preliminary sensitivity analysis on measurement errors are also conducted to study impacts on accuracy of estimated fault inductance.

This paper proposes a two-level framework for the operation of a competitive market for reactive power ancillary services. It is argued that the first-level, i.e., reactive power procurement, be on a seasonal basis while the second-level, i.e., reactive power dispatch, be close to real-time operation. To this effect, a reactive power procurement market model is proposed here taking into consideration system security aspects. This procurement procedure is based on a two-step optimization model. First, the marginal benefits of reactive power supply from each provider with respect to system security are obtained by solving an optimal power flow (OPF) that maximizes system loadability subject to transmission security constraints imposed by voltage limits, thermal limits, and stability limits. Second, the selected set of generators is then determined by solving an OPF-based auction to maximize a societal advantage function comprising generators' offers and their corresponding marginal benefits with respect to system security, considering all transmission system constraints. The proposed procedure yields the selected set of generators and zonal price components, which would form the basis for seasonal contracts between the system operator and the selected reactive power service providers.

This paper introduces a new concept for offshore wind turbine electrical architectures for wind farms with MVDC collection grids. With the proposed pre-transformer wind turbine drivetrain arrangement, the AC output of the wind turbine generator is transformed to a higher voltage by a step-up transformer and then rectified to a corresponding DC voltage by an active ac-dc converter. A modular multilevel converter (MMC) is used for the rectifier to achieve the variable frequency AC to DC power conversion. The MMC has been shown to have burdensome design constraints at low frequency. This paper demonstrates that these design constraints are not applicable for the wind turbine application and that the MMC is a good candidate for the wind turbine rectifier.

As part of the recently completed National Offshore Wind Energy Grid Interconnection Study (NOWEGIS), potential sites for 54 GW of offshore wind for interconnection into the US grid were identified based on lowest levelized cost of energy. Significant offshore wind energy has the potential to create steady-state congestion for the transmission grid near the interconnection points. Based on 2030 Summer Peak power flow data provided for NOWEGIS, the potentially overloaded conditions were explored on a regional basis. Offshore wind is known to be an expensive option for renewables. Every aspect of the economic benefits accrued by its use should be identified and considered when evaluating the economic justification of any project. The economic value of offshore wind was evaluated using market-based production cost simulations.

Current transformer (CT) saturation leads to inaccurate current measurement and, therefore, may cause malfunction of protective relays and control devices that use currents as input signals. This paper introduces an efficient compensation algorithm capable of converting from a sampled current waveform that is distorted by CT saturation to a compensated current waveform. Attractive features include quick response time, no cumulative estimation errors, desired sample-by-sample output, independent of CT parameters/characteristics and secondary burdens, and simplicity for online implementation. The accuracy and robustness of the introduced compensation algorithm are demonstrated through extensive test cases reflecting a wide range of variations in fault conditions and CT parameters.

This paper presents the behavior analysis of modular multilevel converter under DC pole-to-pole short-circuit fault, which is an important issue in fault management, electrical system design and MMC based power system protection and control. F irstly, the transient behavior is analyzed and the conduction overlapping angle γ is def ined. Secondly, seven possible short-circuit current paths induced by different γ values are identif ied, and the corresponding engineering short-circuit current calculation methods for both AC and DC sides are proposed. And then, the inf luences of impedance distribution factor k and equivalent short-circuit resistance RSC on short-circuit currents are elaborated. F inally, case study is used to verify the effectiveness of the proposed analysis methods.

In connection with the DOE funded National Offshore Wind Energy Grid Interconnection Study (NOWEGIS), assessments for offshore wind delivery systems are made for different technologies (HVAC vs. HVDC) and topologies (radial vs. backbone vs. grid). Further, the performance and system impacts of the various options are assessed from a steady-state perspective using power flow analyses under normal system conditions and contingency conditions. Observations are made based on the findings and general knowledge of typical system operations.

This paper presents the concepts of platformless DC connections of offshore wind turbines. It describes the possible and preferred wind turbine drivetrain arrangements capable of producing sufficiently high DC voltage. The preferred DC drivetrain comprises of a medium speed permanent magnet synchronous generator (PMSG), a step-up transformer and a modular multilevel converter (MMC) based rectifier. The focus of this paper is on the detailed modeling of medium speed PMSG, which is a critical component of the preferred wind turbine drivetrain enabling desired operating frequency range for conventional line frequency transformer and MMC based rectifier. Due to large inductance of medium speed PMSG, conventional Zero d-axis current control (ZDAC) method for MPPT may produce a significant steady-state overvoltage at the PMSG terminals. Instead, the constant mutual flux linkage (CMFL) method is implemented for torque control. Simulation results are presented which demonstrate the desired performance of the PMSG for the full range of wind speeds.

This paper addresses issues and solutions of converter modeling for system level studies. Integrated converter models are developed to model sufficient converter dynamics with accurate converter losses for system level simulations. The electrical efficiency and dynamic response of MVDC Navy shipboard distribution systems were simulated in Matlab/Simulink using the developed models.

Standardization of a collection and transmission architecture for tidal arrays would enable component designers to work towards a known specification, and the marine energy industry would be able to move forward from full scale device tests to building commercial arrays. This paper considers the requirements of tidal power arrays that are expected to be built in the coming years, explains the constraints imposed by the availability and costs of components, lists out all possible system architectures, and then evaluates them on the basis of levelized cost of energy (LCoE). The most attractive solution is a ±7.5 kV dc collection architecture, but only if wound field synchronous generators are used in the turbines. If cage induction or permanent magnet generators are to be used, floating collection hubs with full converters and 33 kV ac outputs are preferred. Improved architectures are possible with further research by power electronics engineers.

Multi-attribute decision making (MADM) methods have been widely used in the strategic planning of electric utilities for selecting the best resource strategy with regard to the chosen attributes. In many MADM problems, however, the information available to the decision-maker (DM) is often imprecise due to inaccurate attribute measurements and inconsistent priority judgments. This paper presents an interval-based MADM approach in support of the decision making process with imprecise information. The proposed MADM methodology is based on the model of lineal additive utility function but extends the problem formulation with the measure of composite utility variance. Its main contribution is to help the DM gain insight into how the imprecise data may affect the choice toward the best solution and how a set of acceptable alternatives may be identified with certain confidence. Sample case studies are provided to illustrate the concepts introduced in this paper.

This paper presents an overview of usage-based methods of transmission cost allocation under open access. Allocation of transmission costs involves both technical and regulatory issues, and as a result, the methods available in the literature differ in their definition and measure of the "extent of use" of transmission resources. The primary objective of this paper is to provide a summary of techniques used for designing fair and equitable access fees for the recovery of fixed transmission costs. The discussion is thus organized under two major subtopics: algorithms for transmission usage evaluation and alternative pricing strategies. Numerical examples are provided to show the results using different methods.

Multiattribute decision making (MADM) methods have been widely used in electric utility decision problems. This paper presents an enhanced MADM method to support the evaluation of electric generation expansion strategies. The analytical hierarchy process (AHP) is incorporated into the construction procedure of linear additive utility models to facilitate the process of eliciting single utility functions and weighting parameters. The composite utility variances are estimated accounting for individual errors from inaccurate attribute measurements and inconsistent subjective judgments. To support the decision analysis with imprecise information, an appropriate confidence interval is defined to determine the likely range of estimated utility values. Such practice could help the decision maker gain insight into how the imprecise data may affect their choice toward the best solution and how a set of acceptable alternatives may be identified.

Reliability and maintenance are key concerns for offshore wind farms. Electrical systems are responsible for more failures than mechanical or structural systems, but they are also comparatively easier to repair. Nevertheless, it is important to lower the rate of failure and improve the ease of maintenance of electrical systems. This paper presents approaches to achieve these goals at the component level as well as at the system level. ABB's PCS6000 is used as the example to discuss component level issues. This paper also compares five possible conversion-collection system architectures considering their reliability and amenability to maintenance. The failure rates can be lowered and the serviceability can be improved by incorporating these considerations properly at the design stage.

Interaction between multimachine ac systems and a multiterminal dc (MTDC) grid and the impact on the overall stability of the combined ac-MTDC system is studied in this paper. A generic modeling framework for voltage-source converter (VSC)-based MTDC grids, which is compatible with standard multimachine ac system models, is developed to carry out modal analysis and transient simulation. A general asymmetric bipole converter configuration comprising positive and negative pole converters and dc cable network with a positive, negative, and metallic return circuit is considered to enable different types of faults and dc-side unbalance studies. Detailed dynamic representation of the dc cables with distributed pi-section models is used along with the averaged model and decoupled control for the converter stations. An averaged model in Matlab/SIMULINK is validated against the detailed switched model in EMTDC/PSCAD by comparing the responses following small and large disturbances (e.g., faults on the dc side). Modal analysis is performed to identify the nature and root cause of the dynamic responses. Interaction between a multimachine ac system and an MTDC grid is examined following faults on the ac and dc sides and outage of converters. It is shown that the cause of instability in certain cases could only be attributed to the dc-side state variables. An averaged model of the converter along with the dc cable network is shown to be essential to analyze the stability and dynamics of combined ac-MTDC grids.

Modeling of VSC-based multi-terminal DC (MTDC) grids for modal analysis and stability studies is reported considering bipolar converters and DC cable network with metallic return path. The proposed model is flexible enough to accommodate different converter grounding options and unbalance on the DC side due to cable and/or converter outage. A simplified version of the benchmark test system for the envisioned offshore MTDC grid in the North Sea is studied. Modal participation factor analysis under nominal and converter outage conditions is used to ascertain the nature and root cause of the dynamic responses. Comparison of the time-domain simulation results using the proposed averaged model in Matlab/SIMULINK against a detailed switched model in EMTDC/PSCAD confirm the accuracy of the modeling approach.

Aggregation methodologies for creating equivalent wind farm models are needed for power system transient stability studies involving large wind farms. One strong argument in the literature suggests single machine equivalent representation of wind farm with the assumption that all wind turbines receive the same incoming wind speed and thus operate at the same loading condition. In this paper, we examine how the validity of such single machine equivalent is affected under different wind speeds across a feeder in a wind farm with many WTGs. In particular, we compare the total active power output from the WTGs which indicates significant difference between single machine equivalent and full wind farm model under certain wind speed ranges and distances between the WTGs. We then conclude the need to use multi-machine equivalent representation for large wind farm in order to achieve adequate accuracy under the full range of wind conditions.

From efficiency and reliability standpoints, there are three major issues in coal-fired power plant auxiliary systems: high in-house load, low power factor, and weak disturbance-ride-through capability. This paper presents a cost-effective solution to solve these problems. The proposed solution consists of Variable Frequency Drives (VFDs) with Active Rectifier Units (ARUs), mechanically switched shunt capacitor banks, and coordinated reactive power control strategies implemented in the power plant Distributed Control System (DCS). In this paper, the design principles of the proposed solution will be introduced. Coordinated control strategies of the ARUs and capacitors for which implement the steady-state power factor control and dynamic voltage support under disturbances will be described. The overall potential benefits resulting from the proposed solution will be discussed and demonstrated with example studies corresponding to a 300MW coal-fired power plant.

Increased bulk power transactions in competitive energy markets together with large scale integration of renewable energy sources are posing challenges to high-voltage transmission systems. Environmental constraints and energy efficiency requirements also have significant effects on future transmission infrastructure development. This paper reviews the recent development in HVDC technologies and discusses the needs of the hybrid AC/DC grid structure for future power systems with focus on VSC-HVDC applications in meshed AC grid. It has also been recognized that hybrid AC/DC transmission system together with the wide area measurement system (WAMS) could effectively manage the overall power grid operation security and efficiency under uncertain supply and demand conditions.