My Mind, My Heart, My File

BY ROBERT A. DURHAM & MARCUS O. DURHAM

TYPICALLY, LARGE LOADS ASSOCIATED with petroleum distribution systems are located in a relatively confined plant area. Scattered distribution systems do not often have very large capacity. The combination of large industrial requirements coupled with the geographic requirements of a utility system calls for a challenging design. One of the primary duties of electrical engineers in the petroleum industry is the design and construction of power distribution systems. In most other cases, larger systems are located within a defined plant environment [1]. Systems spread
over a large area tend to be smaller in load. The scheme discussed here is unique: a large electrical network is spread over a very large geographic area. This creates an exceptional set of circumstances. At the beginning of the design process, several goals were identified.
■ Downtime must be minimized. Due to the inherent physical characteristics of the process being powered, even short downtime has serious financial consequences.

■ In conjunction with reducing downtime, it was desired that a protection system be designed so that a fault in one area does not affect loads in another area [2].
■ Under normal conditions, a voltage drop of less than 5% at the end of the system is desirable.
This is crucial to assure that full voltage is available on the load side of the transformers [3], [4].
■ Under contingency conditions, it was preferred that the load could be served successfully from either
one of two utility sources.
■ Necessarily, the system would be designed so that the heating of conductors would not cause any
additional sag that would damage the integrity of the load service [5].
The only way to effectively address these issues is to conduct a series of computer model analyses prior to the beginning of construction.

See detail : Industrial Power System Design in a Utility Environment 

CT SATURATION CALCULATIONS – ARE THEY APPLICABLE IN THE MODERN WORLD? – PART I, THE QUESTION

Abstract - Previously, ANSI/IEEE relay current transformer (CT) sizing criteria was based on traditional symmetrical
calculations usually discussed by technical articles and manufacturers’ guidelines. In 1996, IEEE Standard C37.110-
1996 [1] introduced (1 + X/R) offset multiplying, current asymmetry and current distortion factors; officially changing
the CT sizing guideline. A critical concern is the performance of fast protective schemes (instantaneous or differential
elements) during severe saturation of low ratio CTs. Will the instantaneous element operate before the upstream breaker
relay trips? Will the differential element mis-operate for out-of zone faults? The use of electromagnetic and analog relay
technology does not assure selectivity. Modern microprocessor relays introduce additional uncertainty into the design/verification process with different sampling techniques and proprietary sensing/recognition/trip algorithms. This paper discusses the application of standard CT accuracy classes with modern ANSI/IEEE CT calculation methodology. This
paper is the first of a two-part series; Part II, the Findings provides analytical waveform analysis discussions to illustrate
the concepts conveyed in Part I.

Index Terms – Accuracy Class, Asymmetrical Current, CT Burden, CT Saturation, DC Offset, Digital Filter, and X/R ratio.

See detail : Curent Transformer (CT) Saturation Calculation

Abstract.

Existence of a relatively high negativesequence current is in itself a proof of a disturbance on the power system, possibly a fault. The paper describes the usage of the negative-sequence currents in order to both detect and positively determine the position of the fault with respect to the protected zone, and thus avoid some typical weaknesses of the power transformer differential protection. Some examples of these are long delays for heavy internal faults, unwanted operations for
external faults, and insensitivity to low-level turnto- turn faults, which can be left to develop into high-level faults – with more severe damage to the power transformer – before they can be detected.
Keywords.

Relay protection, power transformer, sensitive differential protection, negative-sequence currents, internal-external fault discriminator.

See detail :  Transformer Differential Protection Improved by Implementation of Negative-Sequence Currents

A Differential System can be arranged to cover a complete Transformer. The underlying principle of such a protection scheme is shown in the figure below.
The General Idea behind the Differential Protection is that the CT’s on the primary and Secondary side must transform the respective Line currents to the same value. For the Bias Coils in the relay to function without any damage, this transformed current may lie between 1A and 5A. Hence the function of the CT’s is to transform the line currents to the same magnitude and phase under normal operation of the Transformer.Should some imbalance occur within the Transformer, such as Interturn Faults within the Windings of the Transformer or Faults on the incoming or outgoing feeders to the transformer, the Line Currents are no longer at the balanced value. Thus, the transformed current at the relay coils is no longer the same at both ends, but different. This causes an imbalance within the differential relay, and as a result, some protection mechanism will be operated, so as to isolate the Transformer and protect it.

See detail : Transformer Differential Protection

Source : http://www.eng.uwi.tt/depts/elec/staff/alvin/ee35t/notes/Trans-Diff-Protect.html

ANSI DEVICE NUMBERS
2 Time-delay
21 Distance
25 Synchronism-check
27 Undervoltage
30 Annunciator
32 Directional power
37 Undercurrent or underpower
38 Bearing
40 Field
46 Reverse-phase
47 Phase-sequence voltage
49 Thermal
50 Instantaneous overcurrent
Etc

ANSI Standard Device Designation Numbers

1) Master Element is the initiating device, such as a control switch, voltage relay, float switch, etc., which serves either directly or through such permissive devices as protective and time -delay relays to place an equipment in or out of operation.
2) Time Delay Starting or Closing Relay is a device that functions to give a desired amount of time delay before or after any point of operation in switching sequence or protective relay system, except as specifically provided by service function 48, 62, and 79.

Relay Selection Guide 

INTRODUCTION

PROTECTIVE ZONE PACKAGES
Page
Introduction . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Basic Concepts . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Types of Distress . . . . . . . . . . . . .  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Detection Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Protection Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Generators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Motors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Feeders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Transformers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Buses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Incoming Lines . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . 31
GE Relay Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
References . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36