Motor-generator Pair Research Articles

TSR-Based optimization using auxiliary motor in Tidal Energy Conversion System: An experimental study

The fluctuating nature of renewable energy is one of the factors that reduces its ability to compete with traditional forms of power generation. In the context of ocean current energy, tidal oscillations are the primary reason for the average power generation being significantly lower than the designed value. Increasing energy production through optimization is necessary for ocean current energy to compete successfully in the market. This experimental study is intended to increase electrical power production through electromechanical engineering with tip speed ratio (TSR) control using auxiliary motors, and adapts the Motor Generator Pair (MGP) approach, where the generator and motor are connected in series. A physical model of a Tidal Energy Conversion System (TECS) was developed. The system consists of a turbine simulator in the form of an AC motor with separate torque and speed controls, torque sensors, a hydraulic transmission system, and auxiliary motors. With this system, a typical tidal turbine profile was built and simulated TSR-based optimization control. The results of this optimization control show that power production can be increased through optimal tip-speed ratio control.

Modelling and small signal stability for islanded microgrids with hybrid grid-forming sources based on converters and synchronous machines

Grid-forming (GFM) technologies based on converters or synchronous machines (SMs) were proposed as emerging approaches for improving inertia, damping, and strength of power girds with increasing renewable energy penetration. Although the stability of GFM converters has been widely studied, the dynamics of a grid-connected generator can be dictated by the bulk power systems. Therefore, the stability of islanded microgrids with hybrid GFM sources should be investigated. To fulfill this gap, this paper adopts a general component connection method for modelling islanded microgrids integrated with multiple generators, and models for SMs, grid-following (GFL) and GFM converters are developed. Additionally, a new GFM technology known as the motor-generator pair (MGP) is investigated, and the model for small signal analysis is derived. Then, full-order state-space models for nine scenarios with hybrid GFM sources as well as simulation models which are used to verify their effectiveness are built. Finally, small-signal dynamics of different scenarios are fully examined. In the scenarios with SMs integrated, the range of oscillation frequency is wider than all converter-based scenarios, and mechanical dynamics can dominate stability; and in scenarios with only GFM converters, though the damping can be improved compared to GFL converters, low frequency oscillation may occur, and the dominant factor changes from the phase locked loop and power controller to voltage controller; then improvement in damping can be observed in MGP-based scenarios, but roots that dominate stability are sensitive to current controller and excitation. Further, critical ranges for control parameters, GFM converter penetration and electrical distance in different scenarios are investigated.

A novel grid-forming technology for transient stability enhancement of power system with high penetration of renewable energy

Owing to the limited inertia and voltage support of traditional grid-following inverters, grid-forming (GFM) technologies with voltage source characteristics, such as droop and virtual synchronous generator (VSG) control, have been proposed recently. However, it is difficult to achieve the same transient performance as that of traditional synchronous generators (SGs) because of the inherent current tolerance of power electronic devices. In this paper, a novel GFM technology with improved speed-power feedback control, defined as a motor-generator pair (MGP), is proposed. In addition, a state-space model is established to illustrate the control stability of the proposed GFM technology. The models and power angle characteristics of four GFM methods, namely droop, VSG, SG, and MGP, are first compared theoretically. Then, considering the current limitations and control delays, the transient response and interactions of the above GFM technologies are simulated in an IEEE 3-generator 9-bus system. The results show that the current saturations may limit the transient support of GFM inverters significantly and may also result in forced overcurrent and imbalanced power in the transient process, because of the slow response of SG governor in the system. Furthermore, compared with GFM inverters, MGP has more satisfactory transient stability because of its spontaneous inertia and faster response in primary frequency regulation than SGs owing to its similar response speed to inverters. Finally, an experimental platform including a 5 kW MGP prototype is established, and the transient performance of the proposed GFM is verified and compared.

Frequency Characteristics of the Synchronous Motor-Generator Pair to Elevate the Upper Limit Penetration Rate of Renewable Energy Sources

With the increase of renewable energy sources (RES) in power systems, the number of traditional synchronous generators (SGs) declines gradually, which leads to a decrease in total inertia and frequency instability in extreme situations. The synchronous motor-generator pair (MGP) system was proposed as a new grid-connection method to improve the inertial response and frequency stability of grids. To study the effects of the MGP on the frequency stability and penetration rate of RES in power grids, first the frequency response equations of a single MGP in a single-machine infinite bus system are derived; then, the system frequency response (SFR) model for a multi-machine system with the MGP integrated is built. By comparing key critical indices of the frequency response after step disturbance with this model, we discover that the MGP system can increase the upper limit penetration rate of RES by quite a percentage. And time domain simulations are used to compare the frequency characteristics for the penetration rates of RES at 25%, 50% and 75% with and without MGP. Finally, the MGP test bench is built for relevant experimental verification.

Vector control strategy for motor‐generator pair drive inverters in parallel operation

In view of the weak moment of inertia for power system with high penetration of renewable energy and limited transient voltage support of power electronics, a synchronous motor-generator pair (MGP) is proposed as a possible solution for renewable energy integration to improve grid stability. In order to realize the large-scale renewable energy integration, parallel inverters drive MGP is an ideal control scheme. Firstly, the working principle and mathematical model of MGP are introduced. Then based on a comparative analysis of control of parallel inverters and torque control of synchronous motors, the decoupling vector control method of MGP is proposed. Considering the linear relation between q-axis current and the active power in the rotor flux-oriented mode, the strategy of parallel drive for MGP with vector control is established. Different control loop for off-grid and grid-connected operation is designed, and effectiveness of this control strategy is verified by simulation. Finally, a 5 kW MGP experimental platform with dual inverters is built, the parameters are selected to test stability of the system, and power distribution and efficiency are studied. The results indicate that the parallel operation of MGP is stable under dynamic conditions, the strategy can realize the independent control of each inverter.

A control scheme based on motor-generator pair for photovoltaic power to integrate into the grid

A control scheme based on motor-generator pair for photovoltaic power to integrate into the grid

基于MGP并网的新能源厂站无功–电压特性研究

基于MGP并网的新能源厂站无功–电压特性研究

Isolation and Protection of the Motor-Generator Pair System for Fault Ride-Through of Renewable Energy Generation Systems

Renewable energy units are required to have reliable fault ride-through capability to prevent large-scale disconnection of renewable energy plants due to grid faults. Since synchronous generators have better voltage and current resistance capabilities, one of the advantages of renewable energy connected to the grid through the motor-generator Pair (MGP) system is the isolation of grid faults, which is totally different from existing fault ride-through methods. First, the structure and mathematical model of the MGP system are introduced in this paper. Then, a novel control method for the MGP system based on DC link voltage feedback is given, which is very important for fault ride-through in the control power balance on both sides of the DC bus. The isolation mechanism, damping and reactive power support functions of the MGP system on grid faults are analyzed by means of the rotor motion equation and phasor diagram of synchronous machine. A 5-kW MGP simulation and experimental platforms are built to verify the isolation and protection effects of the MGP system on a renewable energy generation system during the transient processes of grid voltage sag, overvoltage and multiple voltage sags. The results demonstrate that the MGP system can isolate grid faults on the generator side, while the oscillation amplitude on the motor side is much smaller, and the generator can provide strong reactive power support to the grid during three grid faults.

Motor‐generator pair: novel technology to transmit stable and high‐quality power for power system with high penetration of renewable energy

With the increasing penetration renewable energy, some changes are transforming the power system dynamic characteristics, resulting in new stability issues. One of the issues is the lack of inertia and damping. One is sub-synchronous oscillation (SSO) caused by inter-harmonics associated with inverter-based renewable power generators (IPRG). A novel technology is proposed to transmit stable and high-quality power for power system with high penetration of renewable energy. Thus, the object of this paper is synchronous motor-generator pair (MGP) system, a new gird-connection device, which is proposed to solve above problems. First, the two mass block shaft model of MGP is established, which verifies the small signal stability of this system, and it has high damping that can be controlled flexibly. Then, the harmonic response of MGP is analysed through the internal electromagnetic relationship. Result shows that comparing with the input power from renewable energy sources to M side, the harmonic content of output power on G side is greatly reduced, especially the even harmonic. These characteristics are verified by experiments. So transmitted by MGP, the power quality and stability are greatly improved.

Small Signal Stability of Synchronous Motor-Generator Pair for Power System With High Penetration of Renewable Energy

With the increase of renewable energy in power system, the usage of a large number of converters brings safety and stability issues to power system, including lack of reliable inertia response, insufficient abilities for supporting current, voltage and frequency and lack of reliable damping to restrain oscillation. To solve these instability issues, the synchronous motor-generator pair (MGP) is proposed as a new method of grid-connection. Firstly, the main structure and application scene of MGP are analyzed. Secondly, the classical model for the MGP small signal stability is derived. Theoretical deduction proves that the MGP can provide more damping than a single generator with the same mass block. Then, 5kW MGP experimental system is built. The calculation parameters of the small signal model for the experimental system are determined by fitting experimental data repeatedly. On the basis, the calculated and measured frequency responses are compared under the same disturbance. The agreements between calculations and measurements show that the MGP small signal model can reflect damping and inertia level of MGP system. Finally, same disturbance experiment is carried out on a single-generator experimental system with the same mass block. The frequency response curves of the two modes under the same disturbance show that the MGP is more conducive to system stability.

Control of a back‐to‐back converter as a power transfer system using synchronverter approach

This study presents an innovative approach where both sides of a back-to-back (BtB) system are controlled using the synchronverter approach, allowing to control the power transfer through the converters as it would be in a real motor-generator pair system. A novel way to transfer the DC voltage control loop in the BtB converter is proposed, letting to a softer and less noisy power transferring, as shown in simulation results. The modelling equations of the synchronverter and the traditional dq frame in a BtB converter are presented in this work. In addition, a comparison between these two control approaches using three different performance indexes is done, showing that the power transfer behaviour using the synchronverter approach is significantly improved. Finally, the ability of the BtB converter working as a power balancer system (PBS) to manage congestion between two distribution networks in a test bed case is tested. Results show that it is possible to obtain the PBS power quality benefits without the additional phase-locked loop units and the decoupling process, necessary with the dq frame, resulting in a simpler controller design.

Experiment Study on the Control Method of Motor-Generator Pair System

The environmental problem is a major practical problem that China and the world are faced with. It is a trend for the future power grid to be dominated by the increasing penetration of renewable energy. However, the high penetration power system may face many new problems, including reduced inertia of the power system, the stability of the frequency stability, and the weak grid characteristics of the output system may limit the delivery capacity of renewable energy base. The motor-generator pair (MGP) system can deal with these problems efficiently, but the current MGP system control method is still flawed. In this paper, the mathematical model of the MGP system is analyzed. The characteristics of the source-grid difference control method is analyzed by simulation and experiment. In the experiment, the shortcoming of existing control method that the control system is sensitive to the power grid frequency change is found. The optimized control method based on power feedback is proposed. The feasibility of the power feedback control method is proved by the experimental and simulation. The result shows that the source-grid phase difference method based on power feedback can make the MGP system follow the power reference value to transmit active power stably and reduce the frequency sensitivity of the MGP system.

Enhancement of Grid Stability with A Possible Configuration of Motor-Generator Pair for Renewable Energy Integration

Enhancement of Grid Stability with A Possible Configuration of Motor-Generator Pair for Renewable Energy Integration

Motor‐generator pair: a novel solution to provide inertia and damping for power system with high penetration of renewable energy

Increasing penetration rate of renewable energy sources makes it a trend for power system to be dominated by power electronics, which brings challenges for reliability and stability: first, unlike synchronous generator, converters do not have inherent inertia which is important for frequency response. Second, more complex interaction induced by renewable energies causes problems of power system dynamics, for example damping oscillation. In addition, converters cannot support high fault current, leading to limitation of some existing control schemes during transient events. However, compared with converters, synchronous generator has some advantages to solve aforementioned problems. Therefore, this study proposes a synchronous motor-generator pair (MGP) system as a possible grid-connection way for high penetration of renewable energies to improve stability. State equations for analysis of small signal stability are established based on proposed equivalent rotor angle, which is verified by experiment. Results of mode shape analysis and transient simulation show that MGP can effectively enhance frequency response, damp oscillation and isolate and weaken the negative effects brought by wind farm. Effects on damping ratio with different proportions of MGP are also investigated in different penetration rates.

A possible configuration with motor-generator pair for renewable energy integration View Document

High penetration rates of renewable energy will bring stability problems for the future power grid. One of the critical issues is lack of inertia. In this paper, a synchronous motor-generator pair (MGP) system is proposed as a possible solution for renewable energy integration to enhance inertia and improve grid stability. First, feasibility studies of MGP on inertia, damping, efficiency, and cost are presented. Second, an analytical model is established based on its rotor angle relation. An active power control scheme based on voltage phase difference between renewable energy source and grid is then proposed, and state equations of MGP are derived for small signal stability. Next, two experiments are designed and implemented to verify stable operation and active power regulation of the MGP system. A single-machine infinite bus system is tested to investigate small signal stability and frequency response of MGP. The results show that the MGP system has a solid base in physics and is a feasible solution for providing enough inertia and improving small signal performance in the power grid with high penetration of renewable energy. The paper concludes with a discussion on future research directions to gain a better understanding of MGP.

Synchronous Motor-Generator Pair to Enhance Small Signal and Transient Stability of Power System With High Penetration of Renewable Energy

A synchronous generator is a key element in maintaining stability in the traditional power system and performs well. However, the high penetration of renewable energy will challenge the stable operation of the future power system, because the characteristics of converter-based resources differ from those of a synchronous generator. As a result, a weak grid caused by a lack of short circuit capacity and inertia, damping, and fault ride-through abilities become important issues for grid stability. To solve these problems, the synchronous motor-generator pair (MGP) was proposed as a possible approach. In this paper, first, state equations considering different frequencies on both sides of the MGP are established. The model for small signal stability is further modified to include the wind generator. On this basis, two systems are tested to investigate the performance of the MGP in small signal stability, torsional modes, rotor angle stability, and voltage support. Finally, on the basis of proposed control strategy, a closed-loop active power control of the MGP is realized by the 3-kW experimental system. The results show that the active power of the system is controllable. The MGP can provide sufficient inertia for frequency stability, restrain oscillation, and maintain rotor angle stability by higher damping and support the voltage.