Phase-shifting Transformer Research Articles

Defining Transmission System Operators’ Investment Shares for Phase-Shifting Transformers Used for Coordinated Redispatch

The implementation of network codes within the framework of European Transmission System Operators (TSOs), involves redesigning the process of executing remedial actions aimed at maintaining the power system on a daily basis. One of the key elements of this redesign is the co-optimisation of all accessible measures, bringing a cost-optimal result and providing network security for the entire Capacity Calculation Region (CCR). This specifically means that the currently installed Phase Shifting Transformers (PSTs) are expected to be utilised for the benefit of the whole CCR, with no special priority to any issues incurred by the owner. Therefore, this paper addresses any questions regarding the rules of financing (investment shares per TSO) to be applied for future PST installations. The investment shares are calculated based on the exemplary implementation of a new European procedure – cost-sharing of remedial actions. Consequently, another long-term application of this process is postulated. In order to support the claims with numerical evidence, two scenarios with new PST investments are analysed. The conclusions drawn show that the largest investment burden can be imposed upon zones different from the area of which the new PST installation has taken place. As a result, joint TSOs’ investments may be a potential solution to financing new devices used for future coordination of remedial actions.

Pre-Selection of the Optimal Sitting of Phase-Shifting Transformers Based on an Optimization Problem Solved within a Coordinated Cross-Border Congestion Management Process

The current European policy roadmap aims at forcing the TSOs to coordinate remedial actions used for relieving the congestions in the synchronous power system. In this paper, an optimization problem for coordinated congestion management is described and its results obtained for a real European use cases created in the H2020 EU-SysFlex project are presented. First of all, these results prove the feasibility of a central optimization problem for the coordination of the cross-border congestion management process. Next, the formulated optimization problem is used to tackle the issue of planning the investments in phase-shifting transformers (PSTs), for the purpose of increasing the efficiency/decreasing the cost of congestion management. Finally, this paper introduces two optimization-based indicators for pre-selecting the investment sites, which may be used to support the decision makers aiming at decreasing the costs of coordinated congestion management.

Transmission expansion planning via power flow controlling technologies

The transition from conventional towards renewable energy systems requires additional transmission capacities and higher operational flexibility to overcome contingency situations. This study presents a novel transmission expansion planning approach for the simultaneous placement of AC systems and power flow controlling technologies including high voltage direct current, phase shifting transformers (PSTs) and thyristor controlled series compensators (TCSCs). The approach is formulated as a mixed-integer linear programming optimisation problem and the operating point of power flow controlling devices is endogenously determined by minimising expansion costs. Both PSTs and TCSCs are integrated into the optimisation problem by means of new mathematical formulations. PSTs are presented in more detail considering the PST impedance, which is modelled as an equivalent phase shift. The controllable series impedance of TCSCs is also represented as an equivalent phase shift. Exemplary results based on a synthetic 120 bus system show a significant potential of PSTs and TCSCs to reduce expansion costs as well as to lower the requirement for new transmission capacities. The simultaneous employment of these technologies reduces the expansion costs within the test system between 18.7 and 32.4% compared to those costs generated by the sole reinforcement by AC systems.

Research on output frequency of linear phase-shifting transformer

The article briefly introduced the basic structure of linear phase-shifting transformer and the working principle of inverter system. After the previous work of the research group, based on the maxwell simulation software, the high-frequency output of the linear phase-shifting transformer is subjected to finite element simulation analysis, and an improved design is proposed for the high-frequency output. Finally, the feasibility of simulation analysis is verified through experiments, which provides strongly support for subsequent prototype optimization.

Improving Power Flow Control in AC Transmission System Using Phase Shifting Transformer (PST)

Obtaining optimal power flow has been a challenge over the years for power system engineers. Several ways and technologies ranging from conventional to the use of modern Flexible Alternating Current Transmission Systems (FACTS) devices have been incorporated into the power system to ensure optimal flow of power. FACTS devices used are capable of mitigating several power system problems ranging from line overloading, voltage instability, transient instability, and Power system congestion and so on. Phase shifting transformer (PST) is a FACTS device that is capable of controlling power flow problems in a network by varying its phase angle. PST, when installed at appropriate locations on the power system is capable of redirecting and redistributing power flow from overloaded lines to the less loaded ones, hence ensuring an optimal performance and improved performance of the power system.This paper evaluates the effect of incorporating the PST on the IEEE 5 bus system. Newton-Raphson load flow technique is employed in modeling the IEEE 5 bus system and the PST. Simulation was carried out using MATLAB software package for both the steady state condition and with the incorporation of PST. Results showed that overloading on some transmission lines was checked and mitigated by redistribution of active power across the system. There was also reduction of losses on the lines. Keywords: Phase Shifting Transformer, Power System, Power flow, MATLAB, Newton-Raphson. DOI: 10.7176/CEIS/11-4-03 Publication date: June 30 th 2020

Design and Application of Linear Phase-shifting Transforme

In power conversion equipment, the Phase-shifting Transforme plays an important role in reducing harmonic pollution and improving the quality of output waveform. Based on the principle and structure of linear motor, a linear phase-shifting transformer is designed in this paper to overcome the disadvantages of the traditional phase-shifting transformer with complicated winding structure and difficult expansion. This paper introduces the basic principle of the linear phase-shifting transformer, deduces the relevant formulae of the electromagnetic design of the linear phase-shifting transformer, carries out the finite element simulation, applies the linear phase-shifting transformer to the multiple stacked inverter system, and proves the effectiveness of the designed linear phase-shifting transformer.

Performance evaluation of phase-shifting transformer for integration of renewable energy sources

In recent years, the installed power capacity of renewable power plants has been increasing because of the technological developments, industrialization and rapid population growth. This case has led to the extension of electric transmission networks. Extending transmission networks, on the other hand, means the long transmission lines and line losses. The efficient control of electricity transmission systems can be achieved through an efficient selection of devices used in transmission lines as well as the proper management of networks. These devices, called Flexible Alternating Current Transmission System (FACTS) devices, are integrated into transmission lines and provide energy efficiency in the transmission system. In this study, a novel Two-Core Asymmetric Phase-Shifting Transformer (APST) model, which is a serial FACTS device developed for the efficient use of transmission lines, will be realized. The most important factor in the selection of the Two-Core APST model is to obtain a general model that is valid for all quadrature and non-quadrature dual-core APSTs instead of Quadrature Booster type APSTs, which are frequently used in the literature. The designed Two-Core APST model will be tested in an electrical transmission system consisting of a 154 kV source voltage level and loads. Moreover, this model will be applied in IEEE 9-bus system in MATLAB program and changes in transferred power will be examined. Changes in active power transfer and voltage will be observed by the step change of the APST.

Use of Hybrid Phase Shifter for Congestion Management in A Grid with RESs and EVs: Transmission Efficiency Benefit in Comparison with Phase Shifting Transformer

Integration of renewable energy sources (RESs) and electric vehicles (EVs) to electric power grids is increasing. These RESs and EVs may introduce major problems to grid such as transmission lines congestion. Owing to the causative factors nature, congestion may regularly happen and continue forlong commulative time. Thus, transmission efficiency (TE) is a major factorwhen relieving congestion. Congestion can be relieved by using phase shifting transformers (PSTs), hybrid phase shifters (HPSs), or flexible AC transmission system (FACTS) devices. However, PSTs have technical drawbacks such as their large steps, which may result in increased losses, and FACTS devices cost is high. This paper investigates benefits of using an HPS rather than a PST in terms of TE. As HPS operates continuously, it provides more precise control of active power flow than PST. A modified IEEE-14 bus test system is used and a security margin is kept in each simulated case with HPS/PST. Results revealed higher TE when an HPS is used. Thus, RESs and EVs can be more optimally hosted with HPSs.

Novel Control Approach for a Hybrid Grid-Forming HVDC Offshore Transmission System

This article describes a hybrid topology of high-voltage direct current (HVDC) for offshore wind farms using a series connection of a voltage source converter (VSC) and six-pulse diode rectifier (6P-DR). In this topology, the offshore side VSC (OF-VSC) acts as a grid-forming converter to maintain the PCC (point of common coupling) voltage of offshore wind farms (WF) and frequency. In addition, the OF-VSC functions as an active power filter to suppress the 5th, 7th, 11th, and 13th order harmonic current components produced by the 6P-DR, making it almost sinusoidal. Due to the 6P-DR being used in the hybrid converter, this new configuration reduces the total cost of the converters and losses, while preserving the power flow to the onshore gird. Compared to the fully-rated converter and hybrid converter based on a 12-pulse diode rectifier, the power loss and cost are reduced, and in addition, the proposed hybrid converter does not require a phase shift transformer nor a high number of diodes. A 200 MW in an HVDC transmission system using the hybrid configuration was simulated in PSCAD. The results show that the system operated correctly and the harmonic components were filtered.

Robust and Cost-Effective Synchronization Scheme for a Multicell Grid Emulator

Steadily increasing requirements for grid-connected converters, their growing unit power, and output voltage call for flexible high-power medium-voltage grid emulators. Four-quadrant cascaded H-bridge topology features high output voltage resolution and high effective switching frequency, which enable high-dynamic high-fidelity grid emulation. This article shows that despite nonidealities (turn ratios, phase-shift angles, and different stray inductances) in a typical phase-shifting multiwinding transformer, active front-ends of the cells can be effectively synchronized to their respective supply voltages using transformer primary-side voltage measurements and nameplate values. Consequently, cell input filters can be omitted, making the grid emulator more compact, cheaper, faster, and exhibiting more robust grid synchronization. Theoretical developments and designs are verified by means of high-fidelity time-domain simulations.

Conceptual Design of the Power Supply for Magnetic Island Divertor Configuration on J-TEXT

The ac power supply system of the magnetic island divertor coil is widely applied to reduce the peak thermal load of the boundary target plate. It was found that this system should have the current frequency in the range of 0-100 Hz and the amplitude of 5 kA at 100 Hz. In this article, a new main circuit is proposed for the variable-frequency power supply system. The proposed circuit consists of a six-pulse, thyristor rectifier and a parallel, H-bridge inverter. The control strategy applies the carrier phase-shifted, sinusoidal pulsewidth modulation (CPS-SPWM) to form an almost sinusoidal current and calculate the LC filter parameters of the bus and inverter. Based on the requirements and load parameters of the power supply, it is found that the voltage and current double-closed loop PI control accelerates the system dynamic response and improves the nonlinear, load adaptability of the system. Moreover, the phase-shifting transformer with high-overload capability is applied in the proposed power supply to reduce the volume of the transformer and in advance charging. On the other hand, the water-cooling mode is adopted and the structure of water-cooled plate is designed. Moreover, optimal device and layout selection is performed to achieve reasonable modularization. Finally, the proposed method is applied to complete the main components of the space layout of the power supply. In the studied cases, the power density of 0.42 W/L is obtained.

Transcutaneous Energy Transmission for Ventricular Assist Device with a Couple of Air-core Transcutaneous Transformers

Transcutaneous energy transmission systems that supply power to the ventricular assist device implanted in the human body have been studied. The magnetic field radiated from a transcutaneous transformer exceeds the limit set by the international standard CISPR 11. Therefore, we proposed a magnetic-field-reduction method using both 0° and 180° phase-shift transformers. In this study, we measured the magnetic field both in the case of using a single transcutaneous transformer (Type A) and in the case of using a couple of transcutaneous transformers (Type B). The receiving power of the load resistance was fixed at 14.4 W. As a result, the magnetic field became 59.0 dBµA/m in Type A, and 49.3 dBµA/m in Type B. Furthermore, the transmission efficiency was 57.2% in Type A and it was 45.2% in Type B when the distance between the couple of transcutaneous transformers was 30 cm. These results indicated the effectiveness of our method in magnetic-field reduction.

Application of voltage phase angle control in power system with unevenly loaded supply network

In real power system situations of uneven loading of supply lines included in the section, forming connection between the source and the power system or separate dedicated load node, often occur. In such situations one of power transmission lines (PTL) will be loaded below the nominal installed capacity, while the other parallel communication line will be overloaded. Elimination of uneven load in PTL can be performed by the phase angle control between the voltages at the beginning and end of the connection. One of the effective means for the phase angle control is a phase-shifting transformer (PST), the control of which is provided by the redistribution of power flows in individual PTLs of the section with the elimination of their uneven loading. On the example of simulation and computational experiments carried out in the power system of Azerbaijan, the effectiveness of use of voltage phase angle control in the power system with a significant uneven loading in the controlled sections of the supply network of the system is shown. Analysis of the effect of elimination of the unevenness on reducing losses in the network and increasing the efficiency of using power generation at stations is given.

A novel particle movement bee colony algorithm with Dynaflow controller for line loadability enhancement

The extended utilization of sustainable power sources leads to significantly impulsive and erratic power streams in the transmission networks. By executing power stream controlling devices, it is possible to extend transmission limit. This paper displays the development of steady state power flow model of Dynaflow controller – a recently introduced member of flexible AC transmission systems (FACTS) family that includes a thyristor controlled phase-shifting transformer, a multi module thyristor-switched series capacitor with coordinated control. This paper incorporates Dynaflow controller in optima power flow solutions to upgrade the network line loadability in power systems. The Particle Movement Bee Colony Algorithm based hybrid optimization method is used for dealing with the optima power flow issue for enduring steady-state studies. The attainability of the recommended methodology was attempted on IEEE 30-bus system for several objectives. The outcome of the suggested algorithm with Dynaflow controller gives better solutions to improve the line loadability under the selected set of most severe network contingency conditions.

Three‐phase thyristor controlled reactor using two sets of delta connected switches with low current harmonics

This work presents a three-phase thyristor-controlled reactor (TCR), which makes use of two constituent banks of controlled reactors, with coordinated control of the triggering angle of both the banks. The first reactor bank has a set of `delta' connected back-to-back thyristor switches with a balanced set of reactors in series with each switch, while the second reactor bank also has back-to-back thyristor switches in `delta' connection but with a balanced set of reactors in series with each line. It is demonstrated that this arrangement prevents some major current harmonics in the supply line by proper choice of the maximum kVAr ratio between the two reactor banks. The use of delta connected switches in both banks reduces the current rating of the switches. Hence, without using any phase-shifting isolation transformer or filters, the proposed technique permits control of reactive power over a reasonable range, with a low current total harmonic distortion meeting existing harmonics standards. Simple construction and operation of the proposed TCR make it an attractive and cheaper solution. The modelling, analysis and performance assessment of the new TCR are presented.

Phase-Shifting Transformer Combined with the Novel SHEPWM for Harmonics Reduction in Medium-Voltage Inductive Loads

In this paper, a novel approach is proposed to reduce the impact of harmonics in the high-power medium-voltage squirrel-cage induction motor. An inverter with a minimum switching frequency is required for this load type to reduce the snubber and switching losses. Phase-shifting transformer is selected as an interface between the three-phase supply and the inverter. This is to reduce the impact of low order harmonics and consequently reducing total harmonic distortion (THD). Selective harmonic-elimination pulse width modulation (SHEPWM) is employed as a control technique in the inverter to eliminate specific low-order harmonics. Thereby, many low order harmonics are reduced further. By this way the THD of the system is reduced significantly and consequently the better and cleaner energy is obtained. In this research, PSpice simulation verified by MATLAB is used to estimate the harmonics content of the currents in high-power medium voltage induction motor.

A Series-Connected Multilevel Converter: Topology, Modeling, and Control

Cascaded H-bridge (CHB) is a promising topology for medium-voltage high-power applications. CHB consists of low-power H-bridge modules in each phase, and these modules are connected in a cascade manner to achieve multilevel operation. However, CHB requires a phase-shifting transformer with multiple secondary windings to generate an isolated dc source for each module, and this requirement increases the overall system cost. In this paper, a series connection of low-power modules is employed to realize a new power converter called a series-connected multilevel converter. Each phase of the proposed topology requires a single isolated dc source. Hence, the required number of secondary windings is considerably reduced. This approach also minimizes the complexity and cost of the overall system. The operation of the proposed topology is presented by using three-level neutral point clamped modules in each phase. Model predictive control (MPC) is employed to control the dc-bus capacitor voltage and output currents of the proposed topology. A discrete-time mathematical model of the proposed topology is also developed to predict the future behavior of control variables. The performance of the proposed topology with MPC is verified through MATLAB simulations and a scaled-down laboratory prototype.

Control strategy optimization for energy efficiency and harmonic mitigation in multi-series converters

To provide electrical power for the Research System of Superconducting Magnets (RSSMs) including the background field superconducting magnet and the tested superconducting objects, the high power phase-controlled converter will be used to develop the power supply system. However, because of its inner nonlinear feature, the current harmonics and the reactive power are injected into the AC power supply system. To improve the quality of the power supply system for RSSMs, an improved synchronous control strategy is suggested for the superconducting magnet power supply system, which comprises four series-connected six-pulse converters fed by a phase-shifting transformer, respectively. According to the proposed control strategy, the basic unit is two 12-pulse converters and the control method will be changed in terms of load fluctuations which are represented by the per unit value of converter output voltage. As a result, harmonic is greatly reduced but the power factor is also high.

Modeling Phase Shifters in Power System Simulations Based on Reduced Networks

Phase shifters are becoming widespread assets operated by transmission system operators to deal with congestions and contingencies using non-costly remedial actions. The setting of these controllable devices, which impacts power flows over large areas, may vary significantly according to the operational conditions. It is thus a key challenge to model phase shifters appropriately in power system simulation. In particular, accounting for the flexibility of phase shifters in reduced network models is a vibrant issue, as system stakeholders rely more and more on reduced models to perform studies supporting operational and investment decisions. Different approaches in the literature are proposed to model phase shifters in reduced network. Nevertheless, these approaches are based on the electrical parameters of the system which are not suitable for reduced network models. To address this problem, our paper proposes a methodology and assesses the impact of this contribution in terms of accuracy of the modelling on reduced network models. The approach was applied to a realistic case-study of the European transmission network that was clustered into a reduced network consisting of 54 buses and 82 branches. The reduction was performed using classical clustering methods and represented using a static power transfer distribution factor matrix. The simulations highlight that including an explicit phase shifter transformers representation in reduced models is of interest, when comparing with the representation using only a static power transfer distribution factor matrix.

Investigation of control of power flow by using phase shifting transformers:Turkey case study

Transmission systems are needed to be upgraded based on expected/unexpected load growth factor by years. However, it is not so easy to install and upgrade the transmission system, which requires transmission planning calculation ahead of time. Traditionally, transmission companies built extra transmission lines to meet that load growth, but it is not easy and cost-effective to upgrade the system every time loads increase. Some unexpected load growth may occur for some load points that is not in the part of planning calculation. For those situations, the transmission system may face serious congestion problems. Transmission companies have been looking for a way to control power flow rather than building extra electricity capacity. With the development of technology, more complex and integrated flexible alternating current transmission system technologies that can control power flows by changing the voltage amplitude, the angle, and the impedance of the transmission line are now widely used. Phase-shifting transformer (PST) is one of the most widely used devices that can be used for controlling power flow. PST can vary the amount or direction of the active power flow by injecting voltage at different phases into the transmission line on which PSTs are installed. In this study, possible location case studies of PST in the Turkish transmission network will be investigated. Static analyses will be performed using the PSS/E and Python programs. The maximum and minimum production status of distributed wind power plants will be analyzed comparatively in the energy corridor with PST. Capacity improvement and power flow dispatch from each case study with and without PST will be compared. Finally, we propose a mathematical model to suitably determine the angle of PST in order to minimize real power losses that are caused by PST and provide N-1 security.