Alex Y. Wu

Fault-current limiter applications in medium- and high-voltage power distribution systems

Electronically triggered fault current limiters have been used in medium and high voltage power distribution systems, and they are used to limit the available short-circuit current to a lower level, so that under-rated switchgear can be operated safely. However, to insure the safe and proper usage of these devices, there are a few major operating parameters which need to be considered: (1) selection of trigger level; (2) selection of di/dt; (3) the effect of the capacitor bank and stray capacitances; and (4) coordination between these devices and phase overcurrent relays at the faulted breaker. This paper examines these major operating parameters and evaluate the performance of the current limiters under various simulated fault conditions, and develop application guidelines for current limiters.

A low harmonic electric drive system based on current-source-inverter

This paper presents a new electric motor drive system topology, which is based on a pulse-width-modulation (PWM) current source inverter (CSI). It is aimed at reducing the current harmonics drawn from paper mill distribution systems and the electric utility. Meanwhile, the voltage harmonics feeding into the electric motors are significantly reduced. Furthermore, the voltage rate of rise of the PWM-CSI drives is low compared to the currently dominated PWM voltage source inverter (VSI) based motor drive systems. The paper also presents a proposal to modify the current paper mill machine motor drive systems in order to achieve an overall low harmonic level. The new topology is verified by computer simulations. The problems of the new motor drive system are also described.

The Analysis of Current Transformer Transient Response and Its Effect on Current Relay Performance

The current transformer transient response will affect the performance of protective relaying. Various factors are examined which will influence current transformer transient response. Formulas are developed for estimating the time to saturation and the time to desaturation of a current transformer during a fully saturated condition. Three current transformer and relaying arrangements are also presented for minimizing the impact of current transformer transient response.

Risks From Utility Supply Disruption

This article elaborates on the properly required protection and how its operation will prevent the undesired consequences to the ICG owner, the utility, and the general public. This article also discusses actions that take place when the utility supply is disrupted, creating an islanding condition and states reasons why protection required by regulatory agencies, local utilities, and documents such as IEEE Standard 1547 IEEE Standard for Interconnecting Distributed Resources with Electric Power Systems are required of an individual ICG. Consequences of not having the protection in place can damage the generator and/or its prime mover plus be a hazard to public safety. Examples of these consequences are given. This article will provide a clearer understanding to ICG owners of why they are required to have specified protective equipment in place.

Switching transient analysis and specifications for practical hybrid high resistance grounded generator applications An IEEE/IAS working group report #2

This paper reports on the continuing efforts of an IAS Working Group to investigate industry concerns with excessive stator fault-point burning damage in conjunction with various industrial generator grounding and ground fault protection practices. Previous working group efforts were reported in a series of papers discussing typical voltage bus connected industrial generator applications. These papers proposed a new method of grounding, called hybrid grounding, that offered the ability to limit damage while still providing the required level of ground fault current under all operating conditions. This new Working Group paper reports on the detailed design requirements for hybrid grounding. The paper reports the results of switching transient studies that formed the bases for recommended overvoltage protection. It also provides guidance in selection of equipment and fault protection required for hybrid grounding. The experience gained with several hybrid grounding applications is also reported.

The Application of Computer-Aided Grounding Design Techniques to a Pulp and Paper Mill Grounding System

The combination of computer analysis and measurements used to determine the optimum configuration for the grounding system for a large pulp and paper mill which includes three substation grounding grids, each located in a significantly different nonuniform soil, is presented. A thorough analysis and measurement program prior to construction of a new facility which can identify major problems before they occur is shown. Minimum cost solutions to the problems are proposed.

Modified medium voltage arc flash incident energy calculation method

The IEEE Standard 1584-2002 provides a guide for conducting arc-flash hazard analyses for low- and medium-voltage systems. Users, however, need to be aware that the models in the guide were based on the measurements of arc incident energy under a specific set of test conditions combined with theoretical work. In the medium-voltage arc-flash analysis, the incident energy calculation, according to the guide, is a function of system voltage, gap between conductors, the distance from the possible arcing point to the worker, the bolted three-phase fault, and the arc clearing time from the source-side protecting device. However, the effect of fault current decaying is not considered in the current guide. According to an IEEE-recommended short-circuit calculation method, the fault contributions from generators and induction and synchronous motors would usually decay over the short-circuit period based on their short-circuit time constants and the excitation system. Therefore, the standard calculated incident energy from short-circuit point of view would be higher than the value if the effect of decaying current were considered. On the other hand, the arc clearing time according to the current guide is derived from the initial bolted three-fault current, which would result in a shorter arc clearing time. As a result, the standard calculated incident energy value from arc clearing point of view would be lower than the value if the effect of decaying current were considered. Therefore, there are two conflicting elements contained in the current guide, which may result in an inconclusive personal protective equipment requirement. This paper will present a modified incident energy calculation method based on the decaying three-phase fault current and also based on the arc clearing time derived from decaying three-phase fault current at the time of fault interruption. A sample calculation and a comparison of the results of the current method and the modified method are presented in this paper.

MV Generator Ground fault arcing power damage assessment

An industrial Medium Voltage (MV) system usually consists of multiple power sources such as utility tie transformers and generators. These are normally low-resistance grounding as shown in Fig. 1. When a ground fault occurs at the generator stator, ground currents from its own neutral circuit and external power sources will flow into the fault and cause damages to the stator winding. The IEEE Generator Grounding Working Group issued a guideline for generator grounding practices, which recommends using a hybrid grounding system to minimize the ground fault damage induced by its own neutral grounding source. This paper will evaluate the total ground fault damages based on the arcing power energy to derive a maximum MV system ground current that would limit the ground damage to an acceptable level.

Switching transient analysis and specifications for practical hybrid high resistance grounded generator applications An IEEE/IAS Working Group report #2

This paper reports on the continuing efforts of an IAS Working Group to investigate industry concerns with excessive stator fault-point burning damage in conjunction with various industrial generator grounding and ground fault protection practices. Previous working group efforts were reported in a series of papers discussing typical voltage bus connected industrial generator applications. These papers proposed a new method of grounding, called hybrid grounding, that offered the ability to limit damage while still providing the required level of ground fault current under all operating conditions. This new Working Group paper reports on the detailed design requirements for hybrid grounding. The paper reports the results of switching transient studies that formed the bases for recommended overvoltage protection. It also provides guidance in selection of equipment and fault protection required for hybrid grounding. The experience gained with several hybrid grounding applications is also reported.

Investigation of electric drive system responses to lightning related voltage sags

Lightning strokes on utility transmission lines may cause flashovers between line to line or line to ground, which would consequently result in voltage sags at the lower AC voltage buses. A voltage sag at the AC bus of electric drives would cause a voltage drop at the drive DC bus voltage which may have an adverse impact on electric drive systems. Both AC and DC drives are sensitive to the DC bus voltage drop. Electric drive tripping, which would cause paper machine shutdown, is a frequent occurrence during lightning seasons. This paper presents the result of an investigation of the drive responses to the utility lightning strokes for a paper mill which has both AC and DC drives. Specifically, the following topics are discussed: (1) voltage sag phenomena related to the fault caused by lightning strokes on the utility transmission systems; (2) electric drive system responses to the voltage sags; a computer model on a three-phase system was developed to investigate the problems; and (3) possible system and drive modifications for riding through voltage sags. The impacts of the modifications have also been evaluated to assure proper paper machine operations.