James L. Kirtley

An improved transformer top oil temperature model for use in an on-line monitoring and diagnostic system

In this paper, the authors examine dynamic models of power transformer top oil temperature for use in an online monitoring and diagnostic system. Data taken from large transformers in the field indicate that the IEEE model of top oil temperature rise over ambient temperature does not adequately account for daily variations in ambient temperature. The authors propose a modification that accurately predicts top oil temperature and can be implemented in an online system. This model is verified using data from a large power transformer in service.

Microgrid Stability Characterization Subsequent to Fault-Triggered Islanding Incidents

With the growing deployment of microgrids, it has become urgent to investigate the microgrid behavior during transient faults and subsequent islanding conditions. The load type and the manner in which distributed generations (DGs) are controlled can have substantial impacts on the dynamic performance of microgrids. In this paper, impacts of different control schemes of the inverter-based DG and microgrid load types on the microgrid stability subsequent to fault-forced islanding are investigated. A microgrid model, simulated on Matlab/Simulink software, is analyzed including a mix of synchronous and inverter-based DG and a combination of passive RLC and induction motor (IM) loads. Simulation results show that in the presence of IM loads, the microgrid may lose its stable operation even if the fault is isolated within a typical clearing time. The critical clearing time of a microgrid is highly dependent on the microgrid control strategy, DG interface control, and load type. Induction motor loads can prove problematical to microgrid transient stability, particularly in situations in which the voltage dip can cause the induction motor to “pull out”.

A Simple Technique for Islanding Detection With Negligible Nondetection Zone

Although active islanding detection techniques have smaller nondetection zones than passive techniques, active methods could degrade the system power quality and are not as simple and easy to implement as passive methods. The islanding detection strategy, proposed in this paper, combines the advantages of both active and passive islanding detection methods. The distributed-generation (DG) interface was designed so that the DG maintains stable operation while being grid connected and loses its stability once islanded. Thus, the over/undervoltage and over/underfrequency protection method would be sufficient to detect islanding. The main advantage of the proposed technique is that it relies on a simple approach for islanding detection and has negligible nondetection zone. The system was simulated on PSCAD/EMTDC and simulation results are presented to highlight the effectiveness of the proposed technique.

Superconducting rotating machines

The opportunities and limitations of the applications of superconductors in rotating electric machines are given. The relevant properties of superconductors and the fundamental requirements for rotating electric machines are discussed. The current state-of-the-art of superconducting machines is reviewed. Key problems, future developments and the long range potential of superconducting machines are assessed.

Two Degrees of Freedom Active Damping Technique for

In grid connected photovoltaic (PV) systems, lowpass filters are utilized to reduce injected current harmonics. LCL filters have recently drawn attention for PV system grid interfaces due to their small size and they have shown better attenuation to switching harmonics than simple L filters. However, the LCL filter causes resonance resulting in oscillation and instability issues. This paper proposes an effective active damping technique by introducing a two-degree-of-freedom (2DOF) PID control structure. The 2DOF control structure allows the independent action of PI and D terms giving two degrees of freedom. The design is based on a typical three-phase grid-tied PV system. The active damping control loop is formed by using the existing grid side inductor currents and thus eliminating the need of additional sensors. The relative stability is illustrated in frequency domain by using bode plots. A real-time hardware-in-loop study is performed to validate the performance of the proposed 2DOF technique to damp out the LCL filter resonance.

LCL

Massachusetts Institute of Technology (MIT) has developed an adaptive, intelligent, monitoring system for large power transformers. Four large transformers on the Boston Edison system are under continuous surveillance by this system, which can summon attention to anomalous operation through paging devices. The monitoring system offers two advantages over more traditional (not adaptive) methods of tracking transformer operation: while temperature measurements have been made for some time and can even be incorporated into SCADA systems, comparison of actual measurements with predictions based on operating conditions and ambient temperature can provide a much more sensitive measure of actual transformer condition; and while oil samples are taken from transformers and analyzed for chemical species that might indicate abnormal operation, continuous monitoring of oil chemistry can give immediate indication of something going wrong, perhaps in time to remove the transformer from service and prevent a catastrophic failure. The authors describe the monitoring system and field experience.

Filter-Based Grid Connected PV Systems

In previous literature, constant RLC loads were assumed to impose, on the islanding detection method, the hardest detectable case. For this reason, distributed-generation (DG) islanding studies are usually analyzed and performed by using constant RLC loads. In this paper, different types of loads are taken into account by modeling the loads voltage and frequency dependence. The performance of the over/undervoltage and over/underfrequency protection (OVP/UVP and OFP/UFP) method is examined for the different load models and the results are compared with the constant RLC load case. The analysis is conducted on a constant power-controlled DG designed to operate at unity power factor. A generalized formula, for calculating the nondetection zone of the OVP/UVP and OFP/UFP method, is derived in terms of the loads voltage and frequency dependence parameters. The analysis is further extended on a constant current-controlled DG interface.

Monitoring the health of power transformers

This paper introduces a novel coordinated voltage-control (CVC) scheme for distributed generations (DGs) that relies on adaptively changing the roles (master or slave) of the devices [inverter-based DG, diesel generator, and online tap changer (OLTC)] within the smart grid, depending on system conditions. In addition, the proposed scheme imposes different control response and bandwidth on the devices to coordinate the reactive power among distributed generations (DGs) and OLTC steps. The main objective of the proposed method is twofold: 1) to maximize the reactive power reserve of DGs and, hence, facilitate reaction during contingency situations and 2) to provide voltage regulation during normal operating conditions. The simulated distribution system includes inverter-based DGs (photovoltaic and wind turbine), diesel generator, and OLTC and the potential of the CVC scheme is evaluated and analyzed in view of improving voltage profile, maximizing the reactive power reserve, enhancing fault ride through and improving the transient stability margin. The control algorithm is examined under steady state, load excursion, and three-phase-to-ground fault conditions. The results demonstrate the ability of the proposed CVC scheme to satisfy the targeted objectives with significant improvement in the maximum critical clearing time. The proposed scheme is independent of real-time measurements and is widely adaptive to the dynamics of power systems, thus making it quite suitable for utility implementation.

Performance of the OVP/UVP and OFP/UFP Method With Voltage and Frequency Dependent Loads

A simulation study is performed for the purpose of determining the transient stability characteristics of superconducting alternators. This simulation is compared with an equal area criterion method. It is found that superconducting machines have good transient stability characteristics, that field forcing is not particularly helpful nor necessary. It is also found that the equal area criterion is useful for computing critical clearing times, if voltage behind subtransient reactance is held constant.

A Dynamic Master/Slave Reactive Power-Management Scheme for Smart Grids With Distributed Generation

In this paper, past experience and present knowledge are used to speculate on the applicability of superconductors in the field windings of large synchronous machines. The prospects look quite attractive for 3600 r/min and 1800 r/min turbine generators in the range of ratings from 1000-10,000 MVA. The prospects are also bright for synchronous condensers and peaking generators provided refrigeration can be supplied in an economical manner.