New Instrumentation for Electric Drives

An improved strategy for controlling DC voltage droop is proposed to solve the problems of conventional DC voltage droop control strategy, such as the received power of converter stations is distributed according to fixed proportion, the power variation amplitude is large, the power regulation ability is easy to lose when reaching the upper limit, and there is obvious voltage deviation. A three-terminal VSC-MTDC system model including wind farm was constructed by Matlab/Simulink. According to different conditions of power fluctuation caused by wind power output variation, simulation and analysis of VSC active power and DC voltage variation were carried out. Simulation results showed that the improved strategy for controlling DC voltage droop proposed in this paper made the converter station retain more power regulation margin when the converter station absorbed more active power, and could prevent the converter station from losing its power regulation ability when the power reached the upper limit, and realize reasonable power distribution among converter stations. Meanwhile, the proposed control strategy could maintain DC voltage constant, which verified the feasibility of its application in wind power grid-connected flexible DC transmission system.

A three-terminal DC transmission system based on modular multilevel converter (MMC) is developed to solve the problem of power supply for distributed islands or cities. The system, with flexible features, may generate high quality output voltage and reduce construction costs and power losses in the converter stations. However, the three-terminal DC system based on MMC faces more complex situations in operation and its control strategies. In this paper, the mathematic model of three-terminal DC system based on MMC is deduced. Then according to the structure of MMC, a method of carrier phase-shift SPWM is introduced. As the system-level control, the decoupling controller for three-terminal DC system based on MMC is designed. A multi-point DC voltage control strategy based on DC voltage error method, which doesn't need communication between converter stations, is proposed to improve the system reliability. By simulation with PSCAD/EMTDC, the feasibility and effectiveness of the proposed model and control strategies are verified.

The modular multilevel converter (MMC) station connected to the islanded renewable energy generation system needs to adopt the voltage frequency (VF) control to provide AC voltage. The single-pole converter fault will unbalance the input and output power of the DC power grid, which causes the DC voltage or the bridge arm current of the non-fault pole to exceed the protection value in the time scale of tens to hundreds of milliseconds, leading to cascading failures. To realize the fault ride-through (FRT) of single-pole converter fault, this paper analyzes the electrical characteristic of the system. Based on the analysis, the existing topology is optimized and the reasonable operation reserved margin is designed. Furthermore, the corresponding control strategy is proposed, which can not only ensure the single-pole converter block fault ride-through but can also realize economic, stable, and resilient power supply and address asymmetrical problems. Finally, the simulation model is built in PSCAD/EMTDC and the simulation results validate the effectiveness of the proposed control strategy.

For stable operation of a power system reliable control of active & reactive power is necessary. Voltage source converter based HVDC system bears the advantages of being able to change the control strategy immediately with respect to the active and reactive power changes. In this paper a function based fuzzy control system is proposed for the control of active power, DC voltage at converter station and voltage at inverter station. Basically two input one output methodology based fuzzy logic controller has been proposed. The basic features of proposed system used in VSC based HVDC systems are: 1) only two rule base system is used.2) all controllers are not self correcting. At both stations error and change in error have been taken as input to the controller. Design of high pass filter with FFT analysis has also been proposed for a better dynamic performance of the function based fuzzy controller. Computer simulation using Simulink gives clear result of the performance of the proposed controller.

The true bipolar modular multilevel converter-based multi-terminal direct current (MMC-MTDC) DC transmission line is prone to single-pole grounding fault, which may cause overload and overcurrent of the non-fault DC line with fault poles, resulting in system protection misoperation and system collapse. Therefore, the power transfer control strategy should be adopted to improve system stability. In addition, considering that the commutator stations of true bipolar MMC-MTDC system may have unipolar faults, it is necessary to adopt the control strategy of inter-pole power transfer or inter-station power transfer to improve the transmission capacity of the system under fault conditions. In this paper, a power transfer control strategy is proposed, which is widely applicable to MMC-MTDC system. In the case of line fault, the power transfer takes into account the line power margin and the power margin of converter station. The inter-pole power transfer is better than the inter-station power transfer under the converter station fault condition, and the inter-station power transfer takes into account the priority of the power margin of the converter station. At the same time, based on the Zhangbei four-terminal flexible direct current transmission project, the Zhangbei four-terminal flexible direct current transmission system is built by using PSCAD/EMTDC, and the flexibility and effectiveness of the proposed strategy are verified by simulation.

Although the control of modular multi-level converters (MMCs) in high-voltage direct-current (HVDC) networks has become a mature subject these days, the potential for adverse interactions between different converter controls remains an under-researched challenge attracting the attention from both academia and industry. Even for point-to-point HVDC links (i.e., simple HVDC systems), converter control interactions may result in the shifting of system operating voltages, increased power losses, and unintended power imbalances at converter stations. To bridge this research gap, the risk of multiple cross-over of control characteristics of MMCs is assessed in this paper through mathematical analysis, computational simulation, and experimental validation. Specifically, the following point-to-point HVDC link configurations are examined: (1) one MMC station equipped with a current versus voltage droop control and the other station equipped with a constant power control; and (2) one MMC station equipped with a power versus voltage droop control and the other station equipped with a constant current control. Design guidelines for droop coefficients are provided to prevent adverse control interactions. A 60-kW MMC test-rig is used to experimentally verify the impact of multiple crossing of control characteristics of the DC system configurations, with results verified through software simulation in MATLAB/Simulink using an open access toolbox. Results show that in operating conditions of 650 V and 50 A (DC voltage and DC current), drifts of 7.7% in the DC voltage and of 10% in the DC current occur due to adverse control interactions under the current versus voltage droop and power control scheme. Similarly, drifts of 7.7% both in the DC voltage and power occur under the power versus voltage droop and current control scheme.

As the core control unit, the pole control unit of converter station determines the performance of the whole HVDC system. Aimed the back-to-back converter station, the control strategies are analyzed and discussed in this paper; characteristic curve for converter station control is patterned. On the basis of above work, the pole control unit is designed. Through the fast control for fire angle of converter valves and the slow control for taps of converter transformer, the projected control strategies are implemented. At last, the simulation results verified that this designed pole control unit can realize expected functions.

In multi-terminal VSC-HVDC system, The most important prerequisite to ensure stable operation of the system is to maintain stability of the DC voltage, a coordinated power control strategy applies to multi-terminal VSC-HVDC system is proposed in this paper. The proposed control strategy can make the converter stations controlled by multiple fixed-point DC voltage distribute the power reasonably according to the capacity of station converters and the settings of operators, eliminating the DC voltage offset caused by change of load power, and also eliminating the adverse impact of DC line resistance on the accuracy of converters power distribution. Finally, the simulation is carried out on a three terminal physical prototype and also testified on a three terminal HVDC system in PSCAD/EMTDC, the effectiveness and validity of the proposed control strategy are verified.

The voltage-source-converter- (VSC-) based multiterminal VSC-HVDC power transmission system (VSC-MTDC) is an ideal approach to connect wind farm with power grid. Analyzing the characteristics of doubly fed induction generators as well as the basic principle and the control strategy of VSC-MTDC, a multiterminal DC voltage control strategy suitable for wind farm connected with VSC-MTDC is proposed. By use of PSCAD/EMTDC, the proposed control strategy is simulated, and simulation results show that using the proposed control strategy the conversion between constant power control mode and constant DC voltage control mode can be automatically implemented; thus the DC voltage stability control and reliable power output of wind farm can be ensured after the fault-caused outage of converter station controlled by constant DC voltage and under other faults. The simulation result shows that the model can fulfill multiterminal power transmission and fast response control.

In the high-voltage DC transmission system, the converter station is a site established to complete the conversion of AC and DC power. Whether the AC filter can operate stably will directly affect the reliability of the DC transmission system. At the same time, AC filter also have higher fault rate than other circuit breakers in the converter station. If the operating conditions of the AC filter can be predicted to a certain extent and hidden dangers can be removed in advance, serious accidents such as fault trips can be avoided to a large extent. This paper proposes a fault early warning method of AC filter circuit breaker based on the extreme learning machine (ELM). Taking a converter station in Northwest China as the background, analyze the recorded waveform of the AC filter. The opening and closing current of the circuit breaker and the corresponding operating conditions when the AC filter is switched on and off are extracted, establishing a learning training library and verifying its effectiveness. The influence of different activation functions and the number of hidden layer nodes on this method is discussed. Finally, the fault diagnosis performance of intelligent learning methods such as BP neural network, RBF neural network and genetic RBF network is compared.

With the large-scale renewable energy centralized access to the power grid and the successive operation of UHVDC transmission projects, the frequent action of AC filters and converter transformer taps in converter station caused by random disturbance of AC system has attracted wide attention. It has become a key problem of voltage and reactive power control in AC/DC hybrid power grid on quantifying the operating range of converter bus voltage according to the specific operation mode of DC system in engineering. In view of the above problems, this paper analyzes the influence of AC voltage fluctuation on discrete equipment operation. Based on the voltage and reactive power characteristic equations of converter station, the sensitivity formula of reactive power absorbed by converter station to AC voltage is derived. In addition, under the operation mode of constant power at the sending converter station and constant voltage at the receiving end, the reactive power control mode of the converter station and the tap control mode of the converter transformer are considered. Meanwhile, the minimum filtering requirements of the converter station and the safe operation requirements of the converter control angle are also considered. Using the actual operation parameters as an example, the minimum fluctuation range of AC voltage causing the action of discrete equipment is obtained through simulation by MATLAB.

The multi-terminal flexible DC distribution network can realize the power supply of distributed power supply and the flexible operation of multiple DC loads, which is an important development direction of the future DC distribution network. The coordinated control and communication coordination of each converter station and multi-converter is the key to the safety and stability of DC distribution network. Therefore, in view of the physical information coupling characteristics of multi-terminal flexible DC systems, considering the coordinated control stability characteristics and information interaction capabilities between multi-terminal flexible converter stations, a master-slave control strategy based on multi-site cooperative communication is proposed. Then, combined with the real-time power simulation system RT-LAB and the network simulation tool OPNET, a power information hybrid real-time simulation platform for multi-terminal flexible DC distribution network is constructed to analyze the power information interaction process in different communication environments and its application to DC distribution network. Finally, with the Suzhou Four-Terminal Flexible DC Network Project as an example, the effectiveness of the proposed coordinated control strategy and power information hybrid simulation platform is verified by setting network communication congestion and delay in typical scenarios such as fault-locking and DC load disturbance of the master-slave converter station.

A wide variety of commercial and industrial electrical equipment require three phase power. Most residential homes and small entrepreneur workshops are not been installed with three phase power by electric utilities because it costs significantly more than single phase installation. Due to unavailability and high cost installation of three phase power network, a power converter is very important to address this problem. The design of a single phase to three phase power converter by using static phase conversion technique will be presented in this paper. The power converter will be able to convert the single phase 220∼240 VAC supply to balanced three phase 220∼240 VAC to power the three phase AC motor. The design criterions which include the suitable selection of appropriate capacitors sizing, motor ratings and other relevant components also will be experimentally verified on laboratory prototype converter.

DC voltage control and power distribution are significant requirements for multi-terminal HVDC control system. The common voltage droop control has the drawback of failing to keep the dc voltage at a fixed value if active power is fluctuating, and voltage margin method actually controls dc voltage only by one converter at a time. A novel control strategy-Coordinated DC Voltage Control-is thus proposed. This strategy utilizes telecommunication technology to coordinate all the grid side converters to control dc voltage. A stable dc voltage as well as accurate power distribution according to the minimization of power loss or other optimal power flow algorithm can be achieved. The control effects are validated based on a five-terminal HVDC system modeling in PSCAD/EMTDC with three different scenarios, including steady state and dynamic state. The satisfactory simulation results prove the feasibility and robustness of the proposed control strategy.

The numbers of photovoltaic (PV) systems at residential premises are increasing worldwide. This thesis explores new power electronic solutions for premise energy management systems (PEMS) which incorporate a battery energy storage system (BESS) to manage the utilisation of solar and grid power as the load demand varies. A flexible, isolated three port converter (TPC) is proposed that allows unrestricted power flows between the dc connected PV systems and battery storage and the ac connected consumer loads and the external power system. The traditional PEMS solutions will seek to perform the power exchanges at a dc and/or ac bus and use multiple power converters to interface the PV and BESS components and to allow power flows between the dc and ac systems. A number of topologies exist but all require at least two separate power converters for an integrated PV-BESS that results in multistage power conversion steps for some power flows. Several three-port converter (TPC) topologies are reported in the present literature as possible improvements. However, these topologies often have important restrictions on the possible power flows. This thesis develops and demonstrates a high frequency linked three-port converter (HFL-TPC) topology for PEMS employing PV and BESS. A topological and analytical approach was taken and identified that an HFL-TPC topology could be developed by using three coupled active bridges based on dual active bridge (DAB) converters. The converter is symmetric and readily extensible to an arbitrary number of ports. All the ports are bidirectional and have the same power transfer capability. A key challenge for the HFL-TPC is developing appropriate modulation strategies. Several modulation and control strategies are reported for HFL-TPC topologies using a piecewise linear time domain approach. This approach is adequate for the dual active bridge but becomes much more complex if the number of ports increases. The practical implementation of the optimised switching variables on the digital signal processor (DSP) is very difficult to achieve. This thesis adapts the well-known power systems concepts related to real and reactive power flows between generators coupled by inductive lines to establish the key underlying principles that dictate the real and reactive flows in multiport converters. It is shown that the real and reactive flows are strongly dominated by fundamental frequency power flows. For a multiport converter, the fundamental frequency angle differences between the ports determine the real power flows while the magnitude differences largely determine reactive flows. Based on these power system concepts, the thesis develops an improved quintuple phase shift (QPS) modulation for the HFL-TPC. It achieves the required power transfers at minimum current and provides favourable switching operations to improve the efficiency. The proposed algorithm avoids the extensive computational burden associated with traditional optimisation processes which are barely tractable even for converters with a few ports. The thesis also develops the small signal models for the HFL-TPC and control decoupling networks to allow independent closed loop power control strategies. Within the literature, TPCs with a four-quadrant ac port are very rare. It is technically possible to modify one port of the proposed converter by replacing the conventional bridge converter with a matrix converter. The approach was not pursued far as this has significant practical difficulties. It is challenging to maintain soft switching. In a single phase system, the output voltage zero points make reactive power flows difficult to control. In this thesis, the ac port has been developed by adding an inverter stage. Single phase inverters have power fluctuation at the dc bus at twice the output frequency. The thesis presents methods that allow the ripple power to be directed to a nominated port. The operation of the HFL-TPC with the proposed QPS modulation and control strategies has been demonstrated by extensive simulation studies conducted in MATLAB/Simulink. A 60 V, 400 W laboratory scale prototype has been constructed and demonstrated at module level to verify the practical feasibility of the proposed optimised modulation strategies. Simulation and experimental results show that the proposed optimised QPS modulation method controls the reactive power, improves the efficiency, and maintains the zero voltage switching (ZVS) operation over a wide voltage and power range. The proposed converter is a flexible alternative that can facilitate the full range of power flows that might be expected in a PEMS system. It has general applicability across a range of industrial applications where power flows between multiple sources and loads need to be facilitated. The transfer of power systems concepts to the control of the converter modulation is a significant advance in the control of multi-port systems.