Yi Du

A Survey of Condition Monitoring and Protection Methods for Medium-Voltage Induction Motors

Medium voltage induction motors are widely used in industry and are essential to industrial processes. The breakdown of these medium voltage motors not only leads to high repair expenses, but also causes extraordinary financial losses due to unexpected downtime. To provide reliable condition monitoring and protection for medium voltage motors, this paper presents a comprehensive survey of the existing condition monitoring and protection methods in the following five areas: thermal protection and temperature estimation, stator insulation monitoring and fault detection, bearing fault detection, broken rotor bar/end-ring detection, and air-gap eccentricity detection. For each category, the related features of medium voltage motors are discussed; the effectiveness of the existing methods are discussed in terms of their robustness, accuracy and implementation complexity; recommendations for the future research in these areas are also presented.

A survey of condition monitoring and protection methods for medium voltage induction motors

Medium voltage induction motors are widely used in industry and are essential to industrial processes. The breakdown of these medium voltage motors not only leads to high repair expenses, but also causes extraordinary financial losses due to unexpected downtime. To provide reliable condition monitoring and protection for medium voltage motors, this paper presents a comprehensive survey of the existing condition monitoring and protection methods in the following five areas: thermal protection and temperature estimation, stator insulation monitoring and fault detection, bearing fault detection, broken rotor bar/end-ring detection, and air-gap eccentricity detection. For each category, the related features of medium voltage motors are discussed; the effectiveness of the existing methods are discussed in terms of their robustness, accuracy and implementation complexity; recommendations for the future research in these areas are also presented.

A review of identification and monitoring methods for electric loads in commercial and residential buildings

Electricity consumption in commercial and residential buildings account for around 70 percent of the total electricity consumption in the United States. Through advanced load identification and management technologies, electric energy consumption and carbon emissions in buildings can be reduced by providing fine management of energy usage in an efficient way. This paper gives a state-of-the-art review of monitoring and identification methods for electric loads in commercial and residential buildings, compares their applicability and accuracy on different kinds of loads, and updates possible direction for future research.

Magnetic Effects of DC Signal Injection on Induction Motors for Thermal Evaluation of Stator Windings

Active thermal protection techniques have been proposed for the thermal protection of induction motors. By injecting dc signals, the dc model of the induction motors can be used to estimate the stator winding resistance and, thus, the average stator winding temperature. The injected dc signals may lead to unbalanced magnetic saturation, which may degrade a motors performance and may increase heat dissipation in the motor. This paper presents a detailed analysis of the magnetic saturation caused by dc signal injection, with its effects discussed. It is shown that the injected dc signal induces a varying magnetic saturation in an induction motor during each cycle. The effects of the magnetic saturation on the motor currents, the motors thermal behavior, and the estimation of the stator winding resistance and temperature are analyzed. Such analysis is illustrated from both the finite-element simulation and experimental results. It is shown that the magnetic saturation does not affect the performance of the stator resistance and temperature estimation, which proves the feasibility of these active thermal protection techniques under different load conditions. In addition, it is shown that the injected dc signals have a little impact on the thermal performance of induction motors, which proves that the dc signal injection can be widely used for thermal protection without causing much thermal stress in induction motors.

A Nonintrusive Thermal Monitoring Method for Induction Motors Fed by Closed-Loop Inverter Drives

Closed-loop induction motor drives have wide applications in electric traction system and industrial processes. Accurate thermal monitoring not only protects the induction machines from overheating, but also boosts the usage and performance of the overall drive system. State-of-the-art thermal monitoring schemes for induction machines utilize thermal models, which are inaccurate for drive-fed machines and are not adaptive to varying cooling conditions. This paper proposes a nonintrusive thermal monitoring scheme for induction motors fed by closed-loop inverter drives. By applying offsets to the current controllers in the motor drive, dc currents are injected into the stator windings of the machine. The accurate estimation of the injected dc voltage is achieved by carefully analyzing and compensating for inverter nonidealities. The stator winding temperature can thus be monitored based on the estimated dc stator resistance. The method is nonintrusive because it eliminates the thermocouples, requires no hardware change to the existing motor drive, and has minimal impacts on the normal operation of the motor. The proposed method is practically implemented in a programmable inverter drive and is validated both by simulation and real-time experimental results.

A DC Signal Injection-Based Thermal Protection Scheme for Soft-Starter-Connected Induction Motors

This paper proposes a remote and sensorless thermal protection scheme for soft-starter-connected induction motors. A dc signal injection-based method is used to estimate the stator winding resistance and, thus, the stator winding temperature. Based on continuous monitoring of the stator winding temperature, a thermal condition monitoring and protection scheme is proposed using only the motors input voltages, currents, motor nameplate information, and the ambient temperature. An adaptive Kalman filter is designed to reduce the stator winding temperature estimation error. In addition, the influence of the cable resistance was investigated, and a compensation method is suggested to improve the accuracy of the estimated temperature. The requirements of the data acquisition system, such as sampling frequency and analog-to-digital conversion resolution, and their affects on the accuracy of the estimated temperature are also discussed in detail. The proposed thermal monitoring scheme has been validated from the experimental results of a 7.5-hp TEFC induction motor under various load conditions. The importance of this new thermal monitoring scheme lies in its remote and sensorless nature.

Protection of meshed microgrids with communication overlay

The higher reliability of meshed grids, whether in power or communication, when compared with radial grids is well established. However, while meshed grids provide higher reliability, protection of meshed grids is challenging. In smaller systems such as microgrids, line impedances can be small and many branches may experience approximately the same level of fault current. Locating faults quickly before damage to the equipment and system occurs becomes a non-trivial task and cannot be achieved without communication. This paper discusses a comprehensive scheme for protecting meshed microgrids with multiple levels of protection for redundancy. Power line carrier (PLC) technology is proposed as cost effective and robust means of communication, obviating the need for additional cabling.

Minimizing emissions in microgrids while meeting reliability and power quality objectives

Fossil fuel generators like diesel and natural gas gen-sets are the mainstays of microgrids for reliable power generation. This paper investigates different methods or modes to maximize fuel efficiency of the online generators in a microgrid where a substantial portion of the total generation capacity comes from fossil fuel generators. Energy storage is used to facilitate high efficiency operation of the generators and ensure conformation with the power quality standards and reliability metrics. The methods to maximize fuel efficiency, involving generator scheduling, peak shaving, and power cycling, are discussed. Various case studies are simulated to determine the impact of these three methods or modes on fuel consumption for two particular types of load profiles.

Electromagnetic design considerations for a 50,000 rpm 1kW Switched Reluctance Machine using a flux bridge

The Switched Reluctance Machine (SRM) is a robust machine and is a candidate for ultra high speed applications. Until now the area of ultra high speed machines has been dominated by permanent magnet machines (PM). The PM machine has a higher torque density and some other advantages compared to SRMs. However, the soaring prices of the rare earth materials are driving the efforts to find an alternative to PM machines without significantly impacting the performance. At the same time significant progress has been made in the design and control of the SRM. This paper reviews the progress of the SRM as a high speed machine and proposes a novel rotor structure design to resolve the challenge of high windage losses at ultra high speed. It then elaborates on the path of modifying the design to achieve optimal performance. The simulation result of the final design is verified on FEA software. Finally, a prototype machine with similar design is built and tested to verify the simulation model. The experimental waveform indicates good agreement with the simulation result. Therefore, the performance of the prototype machine is analyzed and presented at the end of this paper.

Stator Temperature Estimation of Direct-Torque-Controlled Induction Machines via Active Flux or Torque Injection

This paper proposes two thermal monitoring methods for induction machines with direct torque control (DTC). The stator resistance Rs is used as a direct indicator of average winding temperature and is estimated via a dc current offset in the stator winding. In a DTC drive system, the major challenge is how to excite the dc current offset via the only existing flux and torque control loops. The quantitative relationship between the desired dc current offset and the corresponding changes in flux linkage and torque is derived, and this shows that the dc current offset can be achieved either by superimposing flux linkage bias command [ ΔΨsα,ref ΔΨ sβ,ref ] in the flux control loop (method 1) or torque ripple command ΔTem,ref in the torque control loop (method 2), both shown in Fig. 1. Simulation results confirm the analytical analysis of the two proposed methods; while the experimental data prove that both methods achieve accurate temperature estimation under various operating conditions. The proposed simple and efficient signal-injection-based temperature estimation technique is particularly advantageous since it eliminates the need for embedded temperature sensors, requires no hardware change to conventional DTC drive systems, and has a minimal impact on the induction machines normal operation.