Power System Computer Aided Design Research Articles

LS-Solar-PV System Impact on Line Protection

Large-scale photovoltaic power station access to the grid will profoundly change the fault current characteristics of the power station’s outgoing lines. This change results in adaptive problems in traditional protection phase selection components, which may cause incorrect actions in reclosing, protection ranging, and distance protection. Based on the fault current characteristics of the large-scale photovoltaic power station transmission line, this paper analyzes the adaptability of the phase current difference mutation and the sequence component phase selection component in protecting the Photovoltaic (PV) power plant side of the transmission line. Based on the fault current analytical formula, the phase relationship between the phase current difference and the current sequence component under different control targets, such as suppressing negative sequence current, suppressing the active power fluctuation, and suppressing the reactive power fluctuation, is derived. The operational performances of the phase–phase current difference of the abrupt phase selection component and the sequence component phase selection component of the power station side are degraded, which may cause incorrect operation of the phase selection component. Based on the actual engineering parameters of a PV power plant, a simulation model was built in Power System Computer Aided Design (PSCAD) to verify the correctness of the theoretical analysis.

Unified High-Speed EMT Equivalent and Implementation Method of MMCs With Single-Port Submodules

This paper proposes a unified high-speed electromagnetic transient (EMT) equivalent and implementation method for modular multilevel converter (MMC) with arbitrary single-port submodule (SM) structures. The proposed method, interacts with the users by the incidence matrix, branch admittance matrix and the column vector of the current sources as input, then a nodal admittance matrix is automatically generated for each individual SM and the internal node information of the SMs are transferred to the interfacing nodes. Hence, the internal nodes are eliminated without obtaining the complicated analytical solutions, the reduced-order equivalent circuit of each SM is obtained and all the SMs within each arm are cascaded into one ultimate Thévenin equivalent. At the end of each time step, the EMT solver updates all the previously eliminated internal node information so that nothing is lost under steady and transient states. The proposed approach is validated by EMT models of an enhanced double half-bridge sub-module based MMC-HVdc system on power systems computer aided design (PSCAD)/EMTdc.

Comparison and analysis of the fault arc characteristics of flexible DC transmission lines

VSC-HVDC grid is one of the important development directions of the smart grid in the future. The arc characteristics of transient fault on the transmission line are the theoretical basis for determining reclosing time and reclosing strategy of DC circuit breaker. In this study, the process of the transient single-pole ground fault on the true bipolar VSC-HVDC transmission line is analysed and the arc model of the fault is determined according to the fault characteristics. Then a two-terminal voltage source converter-high voltage direct current (VSC-HVDC) system including this arc model is built in power systems computer aided design (PSCAD). The primary and secondary arc characteristics of the transient single-pole ground fault on the transmission lines are simulated and compared with the arc in AC system. It is shown that the arc extinction time of the flexible DC transmission system is lesser than that of the AC system at the same voltage level.

Numerical calculation of the lightning transient voltage distribution on converter transformer winding based a dual winding model

The insulation co-ordination of converter transformer winding has a great influence on power system security. An equivalent circuit of a 500 kV converter transformer winding structure, including a LC equivalent network circuit and an ideal transformer in parallel, was firstly established. A combined 12-pulse rectifier equivalent circuit model, mainly consisting of six single-phase converter transformer and 12 converter valves, was built in Power Systems Computer Aided Design (PSCAD) software. In certain operating states of the converter value, the transient voltage distribution was analysed when lightning strokes the DC bus of a typical converter station. The results show that the dual winding model can present the transient voltage characteristics of the converter transformer. The maximum lightning transient voltage is located at the end of the star-connected winding near the neutral point and reaches 1270 kV, and also the maximum gradient voltage occurs at the winding near the terminal point. In different operating states for the converter valve, the transient voltages of a three-phase winding behave as per the similar distributing rules. For the star-connected phase winding, the lightning transient voltage of the winding disc reaches the maximum magnitude when lightning strike occurs at the peak of the phase voltage on the valve side.

Analysis of influencing factors on sub‐synchronous oscillation of wind‐thermal‐bundled system sent out via AC/DC transmission system

Sub-synchronous oscillation may occur when the wind-thermal-bundled system sent out via AC/DC transmission (WTBH) is integrated in power grid. Causing large-scale wind-turbine tripping, even lead the system to collapse. First, in this study the mechanism of sub-synchronous oscillation of this system is analysed. Second, the influencing factors such as the ratio of the wind-thermal-bundled installed capacity, the series compensation parameters of AC transmission line, the different operating conditions of wind power etc., are analysed, which can cause different degree impact on the sub-synchronous oscillation. Finally, the system is simulated under the Power Systems Computer Aided Design (PSCAD) environment. The results show that under the same conditions, the higher series compensation, the greater transmission capacity of the AC line, the greater risk of the sub-synchronous oscillation. Under the condition of full load, the greater proportion of installed capacity of Doubly fed Induction Generator (DFIG) is, the less risk of sub-synchronous oscillation; Under the lower wind speed condition, reducing the number of wind turbine can eliminate the risk of sub-synchronous oscillation, and the oscillation frequency vary with the number of wind turbine.

Wavelet‐based EMTR method for fault location of VSC‐HVDC transmission lines

A wavelet-based electromagnetic time reversal method for locating faults in voltage source converter-high-voltage direct current (VSC-HVDC) transmission lines is proposed in this study. This principle, firstly, is to use wavelet decomposition to filter the 1-mode fault currents, which are recorded at ends of the transmission line. Secondly, the filtered currents are mirrored on time axes, which called time reversal. Thirdly, the time-reversed currents are set as current sources at ends of the lossless mirror lines. Through assuming a fault at each point of line, the assumed fault currents are calculated, and the RMS of fault current at actual fault location is maximal according to a strict scientific proof. Theoretically, the Wavelet-based Electromagnetic Time Reversal (WEMTR) method is robust against fault types and resistance. A multilevel converter-HVDC system is established in Power Systems Computer-Aided Design (PSCAD)/Electromagnetic Transients including DC (EMTDC) to verify the feasibility of the proposed method, and the transmission line is simulated by frequency independent phase model. The results show that, the fault location can be calculated exactly without high sampling rates.

Cost-Based Droop Scheme for Converters in Interconnected Hybrid Microgrids

Hybrid microgrids are the future of the power system as they bring the advantages of ac and dc networks under a common roof. As the power system continues to expand, hybrid microgrids will be connected together for increased reliability. One of the main concerns of such an arrangement is economical power sharing among multiple grids. This paper aims to address this issue by proposing a cost-based droop scheme for interlinking converters (ILCs). At the basic level, a novel voltage-frequency droop is presented which manages bidirectional power flow among the grids. Any change in ac grid frequency is translated into a dc bus voltage variation which allows active power flow from an underloaded to the overloaded grid. The conventional secondary control within the individual ac and dc grids is used to eliminate the inherent frequency and dc voltage deviations, restoring them to the nominal values. Economical power sharing among multiple grids is realized by embedding the incremental costs into the voltage-frequency droop and minimizing the total active power generation cost based on equal incremental cost principle. Different operational modes are discussed for interconnected grids, and simulations performed in power system computer-aided design (PSCAD) are used to confirm the validity of the scheme for each mode.

Novel control strategy of grid‐connected photovoltaic power supply for frequency regulation

The maximum power point tracking (MPPT) mode is widely applied in photovoltaic (PV) power generation system. In this article, instead of MPPT, a novel control strategy of grid-connected PV power supply for frequency regulation is proposed. First, the static frequency characteristic curve of PV plant is proposed, and a method of synthesising the PV power output power–voltage characteristic curve and static frequency characteristic curve into frequency regulation curve of PV power supply is introduced. So the PV plant can participate in frequency regulation of the system. Through simulation analysis in power systems computer aided design (PSCAD)/electromagnetic transients including DC (EMTDC), PV plant realises the function of frequency regulation when load changes, so the variation of system frequency can be reduced and the frequency stability of power system can be improved, and the effectiveness of the strategy is verified.

High Step-Up Quasi-Z Source DC–DC Converter

In this paper, a high step-up quasi- Z Source (QZS) dc–dc converter is proposed. This converter uses a hybrid switched-capacitors switched-inductor method in order to achieve high voltage gains. The proposed converter have resolved the voltage gain limitation of the basic QZS dc–dc converter while keeping its main advantages, such as continuous input current and low voltage stress on capacitors. Compared to the basic converter, the duty cycle is not limited, and the voltage stress on the diodes and switch is not increased. In addition to these features, the proposed converter has a flexible structure, and extra stages could be added to it in order to achieve even higher voltage gains without increasing the voltage stress on devices or limiting the duty cycle. The operation principle of the converter and related relationships and waveforms are presented in the paper. Also, a comprehensive comparison between the proposed and other QZS based dc–dc converters is provided which confirms the superiority of the proposed converter. Simulations are done in power systems computer aided design (PSCAD) in order to investigate the maximum power point tracking (MPPT) capability of the converter. In addition, the valid performance and practicality of the converter are studied through the results obtained from the laboratory built prototype.

Internet of Things Platform for Energy Management in Multi-Microgrid System to Improve Neutral Current Compensation

In this paper, an Internet of Things (IoT) platform is proposed for Multi-Microgrid (MMG) system to improve unbalance compensation functionality employing three-phase four-leg (3P-4L) voltage source inverters (VSIs). The two level communication system connects the MMG system, implemented in Power System Computer Aided Design (PSCAD), to the cloud server. The local communication level utilizes Modbus Transmission Control Protocol/Internet Protocol (TCP/IP) and Message Queuing Telemetry Transport (MQTT) is used as the protocol for global communication level. A communication operation algorithm is developed to manage the communication operation under various communication failure scenarios. To test the communication system, it is implemented on an experimental testbed to investigate its functionality for MMG neutral current compensation (NCC). To compensate the neutral current in MMG, a dynamic NCC algorithm is proposed, which enables the MGs to further improve the NCC by sharing their data using the IoT platform. The performance of the control and communication system using dynamic NCC is compared with the fixed capacity NCC for unbalance compensation under different communication failure conditions. The impact of the communication system performance on the NCC sharing is the focus of this research. The results show that the proposed system provides better neutral current compensation and phase balancing in case of MMG operation by sharing the data effectively even if the communication system is failing partially.

A Developed Structure for DC–DC Quasi-Z-Source Converter with High Voltage Gain and High Reliability

In this paper, a developed structure for DC–DC quasi-Z-source (QZS) converters is proposed. First, the proposed two-stage structure is presented and analyzed. Then, the proposed structure is extended to [Formula: see text] stages and its relations are calculated. Compared with other conventional structures, the proposed structure has higher voltage gain and higher reliability. The proposed topology is suitable for high power applications. To have the correct performance of conventional QZS converter, all impedance network elements must be intact. In the case of small failure in one of the elements, the operation of the whole system is disrupted. The proposed structure has high reliability because when one stage fails, the fault management system separates that stage from the other stages and the remaining stages continue to transmit power. In this paper, in addition to analyzing the operation of the proposed converter in different operating modes, calculations of voltage gain, voltage stresses across capacitors and reliability analysis are also presented. Reliability is calculated according to well-known Markov model. Moreover, a comprehensive comparison in terms of voltage gain and reliability is made between the proposed converter and the other conventional structures. Also, the rating values of inductors and capacitors are designed. Finally, experimental and simulation results are presented by using power system computer-aided design (PSCAD) software to verify the theories.

Analysis of Propagation Delay for Multi-Terminal High Voltage Direct Current Networks Interconnecting the Large-Scale Off-Shore Renewable Energy

Voltage-source-converter-based multi-terminal high voltage direct current (MTDC) networks are extensively recognized as a viable solution for meeting the increasing demand of electrical energy and escalating penetration of renewable energy sources. DC faults are major limitations to the development of MTDC networks. The analysis of variable constraints has become mandatory in order to develop a reliable protection scheme. This paper contributes in assessing the propagation delay with the analytical approximation in MTDC networks. The propagation delay is analyzed in the time domain by taking only the forward traveling wave into account and considering the initial voltage step of magnitude at the fault position. Numerous simulations were carried out for different parameters and arrangements in Power System Computer Aided Design (PSCAD) to explore the proposed expressions. The results accurately depicted the time development of fault current. The results obtained from the real-time digital simulator (RTDS) confirmed that the proposed approach is capable of evaluating propagation delay in MTDC networks. Moreover, the influence of fault resistance is also taken into account for investigating its effect on the system parameters.

Transient inrush current detection and classification in 230 kV shunt capacitor bank switching under various transient‐mitigation methods based on discrete wavelet transform

To avoid the failure of instantaneous overcurrent relays (50) owing to fails triggered by transient inrush currents during capacitor-bank switching, this study describes a new approach to detect and classify high-transient inrush current. These currents are due to the energisation of a shunt capacitor bank, rated 4 × 72 Mvar/230 kV, in a substation system in Thailand. The simulation tool power systems computer-aided design (PSCAD) is used to simulate the transient inrush current using six transient-mitigation methods, i.e. (i) a base case, (ii) a pre-insertion resistor, (iii) a pre-insertion inductor, (iv) a current-limiting reactor, (v) a series 6% reactor and (vi) synchronous closing control. Inrush current signals from PSCAD were used as inputs for discrete wavelet transforms. On a scale from 1 to 30, the maximum value of the wavelet coefficient is used to detect the inrush current. The high value of the standard deviation of the wavelet-scale analysis is used to discriminate between the high transient inrush current and the normal capacitor rate current. The results obtained show that the newly proposed method effectively detects and discriminates the capacitor switching inrush current, both isolated and back-to-back switching, with high accuracy.

Hierarchical control strategy for MVDC distribution network under large disturbance

The DC distribution technology brings challenges and opportunities to the penetration of large-scale distributed renewable energy sources. In a medium-voltage DC (MVDC) distribution network, operation mode switch occurs owing to some large disturbances. To manage these disturbances and achieve seamless switch of the operation mode, a hierarchical control strategy with two layers, namely local and global layers, is proposed in this work. The novel local layer controller called P–U–I controller is designed to enhance the voltage stability, improve system controllability and suppress overcurrent under large disturbances. This controller only needs local information and does not rely on fast communication. Furthermore, as the scale of the DC distribution network expands continuously, the number of system operation mode increases in geometric progression. Therefore, a global layer controller based on breadth-first search algorithm is proposed. This controller can automatically identify the system topology and adjust the control mode of converters to optimise the system operating characteristics. Thus, this hierarchical control strategy can accurately control the system power in steady state, suppress overcurrent under large disturbances and suit large-scale DC distribution networks. Finally, a three-terminal MVDC distribution network simulation model established on power systems computer-aided design and RT-LAB validates the proposed strategy.

A new topology for nonisolated multiport zero voltage switching dc‐dc converter

SummaryIn this paper, a new multiport zero voltage switching dc‐dc converter is proposed. Multiport dc‐dc converters are widely applicable in hybrid energy generating systems to provide substantial power to sensitive loads. The proposed topology can operate in 3 operational modes of boost, buck, and buck‐boost. Moreover, it has zero voltage switching operation for all switches and has the ability to eliminate the input current ripple; also, at low voltage side, the input sources can be extended. In addition, it has the ability of interfacing 3 different voltages only by using 3 switches. In this paper, the proposed topology is analyzed theoretically for all operating modes; besides, the voltage and current equations of all components are calculated. Furthermore, the required soft switching and zero input currents ripple conditions are analyzed. Finally, to demonstrate the accurate performance of the proposed converter, the Power System Computer Aided Design(PSCAD)/Electro Magnetic Transient Design and Control(EMTDC) simulation and experimental results are extracted and presented.

High voltage gain dc–dc converters based on coupled inductors

In this study, two new high voltage gain dc-dc converters are proposed. In the proposed converters, two coupled inductors are used to increase and decrease the output voltage more than the traditional dc-dc converters in boost and buck operations, respectively. In these converters, by increasing the turns ratio of the coupled inductors, the voltage conversion ratio is increased for the whole range of duty cycles. The proposed topologies are analysed in all operating modes and the values of current stresses of all switches, voltage stresses on all switches, input current ripples and voltage gains are calculated for boost and buck operations. Finally, the accurate performance of the proposed converters is verified through experimental and Power System Computer Aided Design (PSCAD)/Electro Magnetic Transient Design and Control (EMTDC) simulation results.

Method for high-impedance fault detection

Correct detection of high-impedance faults (HIFs) is crucial because they produce a serious threat for humans, livestock and property. HIF currents, typically with fault resistances between 10 and 100 kΩ, cannot be detected with traditional protection functions and they behave randomly consisting of unstable and unpredictable dynamic fluctuations in the amplitude, harmonic levels etc. One main challenge is to reliably separate HIFs from normal network load and other switching events. In this study, new HIF detection method is proposed based on extensive power systems computer aided design (PSCAD) simulation studies and tests with real-life HIF measurements. The proposed method is applicable to different MV networks with different grounding practices. Depending on the grounding type, HIF detection is based on the use of zero sequence current I o or negative sequence current I 2. Method works in both 50 and 60 Hz networks and does not need voltage measurement. It also works as tripping protection function with sufficiently short operation time, is simple/flexible and has good usability from end user point of view.

Grid connected photovoltaic power plants: new aspects in switching procedures with vacuum circuit breakers involved

This study presents vacuum circuit breaker switching investigation on a grid connected photovoltaic power plants. The focal point of this research is to discuss potential overvoltages during various switching operations in terms of the medium-voltage insulation coordination procedure. Influence of the LC filters of solar converters on high-frequency transients was highlighted, with main focus on generation of arc re-strikes and pre-strikes during opening or closing of the vacuum circuit breaker. Scheduled and emergency switching operations were analysed by means of laboratory experiments and Power Systems Computer Aided Design (PSCAD) simulations. Alternative switching sequences were proposed for mitigation of transient overvoltages.

Multivalued Coefficient Prestorage and Block Parallel Method for Real-Time Simulation of Microgrid on FRTDS

The microgrid containing a large amount of high frequency power switches and nonlinear components has put forward high requirements for power system real-time simulation technology. Multivalued coefficient prestorage can reduce the calculation steps in real-time simulation. In order to reduce the storage pressure of the multivalued coefficients, the whole network is divided into multiple subnetworks that can be simulated in parallel, and only the parameters for computing input variables and internal variables are prestored. The multiport hybrid equivalent is performed to reduce the number of simultaneous network equations. The input variables are tied to state variables of the circuit so that the iterative calculation is limited to the local network. The devised methodology is validated through simulation of a low-voltage microgrid on a field programmable gate array (FPGA)-based real-time digital simulation (FRTDS) platform at a 5 μs time step. Comparison with a power systems computer aided design (PSCAD)/electromagnetic transients including DC (EMTDC) model shows that the proposed method is effective.

Study on the Gas-Insulated Line Equivalent Model and Simplified Model

The gas-insulated line (GIL) is one technical solution to allow the transmission of electricity underground at a high voltage level, yet its equivalent model is quite complicated. Based on an examination of the geometrical structure of the GIL and the way the metallic enclosure is grounded, this paper analyzed the electromagnetic and electrostatic coupling among the inner conductors and the metallic enclosures of the three phases. Then, the paper proposes a modeling method for the widely-used short-distance GIL based on the PI-model (the model consisting of two lumped admittance at each terminal and a lumped impedance in between). The GIL parameters were later simplified with the coupling effect of the metallic enclosure considered, and a simplified PI-model was produced. Finally, the proposed PI-model and its simplified version were built on the Power Systems Computer Aided Design (PSCAD) platform, and their effectiveness verified by simulation results.