pac world magazine _ pac line do not cross

Home . June 2010 Issue . Cover Story . PAC Line Do Not Cross

PAC Line Do Not Cross

Authors:

Mohindar S. Sachdev, University of Saskatchewan, Saskatoon, SK, Canada and Pratap G. Mysore, Xcel

Energy, Minneapolis, MN, USA

INTRODUCTION

High speed may be of more importance than the potential damage to the equipment for designing

protection systems for transmission lines. But in the case of power transformers, high cost of repair or

replacement, and the possibility of fire or violent failure makes “limiting the damage” a major objective.

These and other philosophical issues are discussed in clause 5 of the guide.

Insulation failure leading to shorting of a few turns of a transformer winding results in high circulating

current in the shorted turns but this does not result in any significant change in currents at the terminals of

the transformer. This suggests a need for a protection system that is sensitive enough to detect

low-current faults and operates at high speed to limit the damage. Types of failures experienced in

transformers and the consequences of those failures are discussed in clause 6. The issue of minimum

and maximum fault currents is addressed in clause 7. It is also shown that a CT provided at the

transformer terminals for providing current to a differential relay may saturate during a system fault while no

fault current may be flowing in the transformer windings.

There is no standard way to protect all transformers. Economic considerations may have a major impact

on the selection of the protection scheme and the need for an interrupting device.

IEEE guide, C37.91TM addresses all these issues starting with field data of statistics for types of failures

experienced in transformers to help the readers in making considered decision on selecting protection

systems in their applications. Some topics discussed in the guide are briefly presented in the following

sections.

Magnetizing Inrush

The issue of magnetizing inrush current is also discussed in clause 7. When a transformer is energized,

inrush currents that are several times the full load current flow in to the transformer even if the secondary

windings of the transformer are not connected to a load. The levels of these currents depend on the

parameters of the power system and the magnetic properties of the transformer core. If one winding of a

transformer is connected to a power system and the other windings are not, an abrupt change of voltage

connected to the transformer will cause magnetizing inrush currents to flow in to the transformer. Other

transients can also create similar inrush currents of large magnitudes in one of the windings of a

transformer as is explained in clause 7.4.2 of the guide. The magnetizing inrush current has two distinct

features that separates an inrush from load currents and fault currents. They are:

Inrush currents have substantial even-harmonic components in them.

There is always a low-current region that is greater than one-quarter period of the magnetizing-

current waveform as shown in Figure 1.

Fused protection and self-powered resettable fault interrupters

Fuses are often used for protecting power transformers rated up to 10 MVA or higher if suitable fuses are

available. They cost much less than the protection systems and they require little maintenance.

Self-powered-resettable fault interrupters can be used for transformers rated more than 10 MVA, and up to

80 MVA, and of appropriate voltage rating. These fault interrupters are similar to fuses, but have the ability

to sense neutral current as well to trip all three phases. Clause 8.1 of the guide provides a detailed

explanation of selection and operation of fuses and resettable interrupting device.

Overcurrent protection

Time overcurrent relays provide protection similar to fuses in the event of transformer faults and also

provide thermal overload protection to a certain degree. The relay characteristic selected for an application

should be below the transformer thermal and mechanical damage curves as discussed in Annex A of the

guide.

Instantaneous overcurrent relays provide fast clearing for severe internal faults. These are set not to

overreach for faults on the low side of the transformer - typically set to 175% (125-200% range) of the

calculated maximum fault current for a three phase fault on the low side of the transformer. Clause 8.3 of

the guide discusses coordination issues and the currents seen by high side relays for various faults on

Delta/Wye transformers.

Differential protection

Current differential relaying is the most commonly used technique for protecting transformers that are

rated higher than 10 MVA. Differential protection provides faster detection of faults on any winding in a

transformer protection zone that is defined by the location of the current transformers (CTs). Figure 3

PAC World magazine : PAC Line Do Not Cross http://www.pacw.org/no-cache/issue/june_2010_issue/cover_story/redun...

1 of 6 29-Aug-11 6:36 AM

shows a typical transformer differential relay connection for a single phase transformer. The difference

between the currents I1 and I2, usually called differential current, is applied to an overcurrent element that

acts as the operating unit of the relay.

The ratios of the CTs are selected to

provide equal magnitudes of the two

currents. But the polarities of CTs are

chosen such that the currents provided by

the secondary windings of the CTs to the

relay operating element are 180o out of

phase during normal power flow through the

transformer and for faults outside the

protection zone of the transformer.

The currents I1 and I2 are practically out of

phase when the transformer is supplying

current to a load or when a fault is

experienced outside the protection zone of

the transformer. The operating current

IOP of the relay is the difference between

I1 representing the current entering one

winding and I2 representing the current

leaving the other winding as expressed by

the following equation.

IOP = I1 - I2

The windings of three phase transformers

are often connected in different

configurations resulting in the currents out

of the windings connected to one part of

the power system having a phase shift with

respect to currents out of the windings

connected to the other part of the system.

When electromechanical relays are used,

the CTs provided at different terminals of a

transformer are connected differently to

ensure that the currents provided to the relay from all terminals of the transformer are 180 degrees out of

phase during normal load flow and faults outside the transformer protection zone. As previously stated the

CT ratios are selected in such a manner that the magnitudes of currents provided by the CTs to the

operating coil of the relay are zero when the transformer is supplying load or is feeding an external fault.

CT arrangements for

delta-wye, delta-

wye-delta, and

autotransformers are

included in clause 8.2

of the guide. CT

connections used for

differential protection

of a Delta-Wye

transformer are shown

in Figure 2 when an

electro-mechanical

relay is used. The

guide also includes an

example that shows

that the CTs provided

at all terminals of a transformer are wye connected when a numerical relay is used to protect the

transformer. The phase shift compensation is done in the relay in such cases.

The operating element of a differential relay is an overcurrent element shown as O in Figure 2. Because

the ratios of CTs provided at the different terminals cannot be perfectly matched for all operating

conditions, restraining elements, shown as R in Figure 2, are also included in differential relays. The

restraining elements are intended to ensure that the relay does not operate during external faults.

Restraint windings provide stability from CT

mismatch and CT output differences due to

CT saturation. Such relays, also known as

percentage differential relays, operate if the

differential current is above a set

percentage of restraint current.

The following two alternative ways of

including restraint in a differential relay are

discussed in clause 8.2.2. Only one of

these approaches is used in a differential

relay.

IR = k (| I1 | +| I2 |) where, k =0.5 or 1

IR = Max. (| I1 |, | I2 |)

The ratio of the differential current to the

operating current, defined as the slope, is

shown in the relay operating characteristics

shown in Figure 4. The use of relays with

single slope, dual slope and variable slope is discussed in the guide.

Methods to differentiate Inrush currents from fault currents

Harmonic content in the differential current is the most commonly used criteria for restraining the relay

when the transformer experiences magnetizing inrush. Clause 8.2.3 provides a detailed explanation of the

different methods used in relay designs. Examples shown below illustrate a couple of methods described

in the guide.


2 of 6 29-Aug-11 6:36 AM

A relay using the following approach is usually referred to as a Harmonic Blocking relay. One relay design

uses the following criterion:

| IOP | > k2 | I2 |

k2 is typically set between 0.15 and 0.2.

In this design, the operating current is the sum of the fundamental frequency component of the operating

current and the third and higher order harmonics and the restraining current is the second harmonic

component of the operating current.

In harmonic restraint relays, harmonic currents provide additional restraint to prevent the relay from

operating. This is illustrated in the following equation that describes the operation of one such relay.

| IOP | > s | IR | + k2 | I2 | + k4 | I4 | + k5 | I5 |

The logic of this relay is illustrated in Figure 5.

Over excitation of transformers produces

differential current that has high 5th

harmonic content. The presence of high

levels of the fifth harmonic component is,

therefore, used in these relays to prevent

the differential relay operation. Use of

Volts/Hertz protection is suggested to

protect the transformer from over

excitation.

The wave-shape recognition techniques

that are based on duration of the

low-current during magnetizing inrush have

also been used. One such approach is shown in Figure 6.

Sensitive ground fault protection – Restricted Earth fault protection

Clause 8.4 describes various methods used in the industry to detect ground faults involving the

transformer windings. The neutral current is compared with the zero sequence current derived from the

CTs provided at the terminals of a transformer. For an external fault these currents are equal in magnitude

but are out of phase. Both currents are in phase for a fault in the transformer protection zone, or current is

provided by the CT installed in the transformer neutral if the phase side circuit breaker is open. This

arrangement provides protection for ground faults in the wye connected windings only. Faults on the delta

connected winding are detected by a ground fault relay. One approach for detecting ground faults on the

wye as well as delta windings is shown in Figure 8. The relay 87G detects faults on the wye winding and

the relay 51N detects faults on the delta winding.

Clause 8.5 describes the protection of special purpose transformers, such as four winding transformers,

phase shifting transformers and grounding transformers. Other miscellaneous issues, such as negative

sequence relays, voltage controlled overcurrent relays, impedance relays controlling overcurrent relays,

overcurrent directional relays and backup protection are discussed in clause 8.6.

Mechanical devices for detecting faults

Electrical techniques for detecting faults in transformers cannot detect single turn-to-turn faults or incipient

faults. Mechanical devices are, therefore, used to detect such faults; these devices are described in

clause 9.

Decomposition of insulation and oil due to overheating, caused by high circulating current in the shorted

turn, and electrical arcs generate gases. Several types of devices take advantage of this phenomenon to

detect turn-to-turn and incipient faults. The operating principles of some of these devices, such as, gas

accumulator, gas detector, sudden oil pressure, sudden gas pressure, sudden oil and gas pressure and

static pressure relays are described in the guide.


3 of 6 29-Aug-11 6:36 AM

Thermal protection

Oil and winding temperature monitoring devices provide thermal protection preventing over heating of oil

and windings. Transformer overheating could be the result of many factors such as high ambient

temperature, loss of cooling system, abnormal loading during system stressed conditions.

The causes and undesirable results of overheating of transformers are identified in clause 10 of the

guide. The location of hot-spot in a transformer is discussed; this includes recent advancement in

detecting the hot-spot using mathematical modeling of the heating phenomenon in a transformer.

Possibilities of exploiting the modeling approach when it is necessary to overload a transformer are

discussed. The possibility of calculating the insulation-loss-of-life and using the information in protecting

the transformer are addressed as well. Other means of thermal protection, such as top-oil temperature

based devices and tank temperature based devices, and protection from over-fluxing are also discussed

in clause 10 of the guide.

Fault clearing

Different methods of isolating a transformer in which a fault has occurred are discussed in clause 11 of the

guide. It is desirable that a faulted transformer be isolated with minimum impact on the power system of

which the transformer is a part of. When a transmission or sub-transmission line is tapped for supplying

local load, several factors are considered in designing controlling devices, such as circuit breakers and

relays.

The use of circuit breakers for isolating a

transformer is a straight forward procedure

and is addressed in clause 11.2.

Sometimes it is necessary to trip a remote

circuit breaker before the faulted

transformer tap is isolated. This is the

case in the system shown in Figure 7. The

working of such a system is discussed in

clause 11.3. The types of transfer trip

schemes and automatic opening of the

motor-operated disconnect are discussed

in this clause. The use of circuit switchers

and self-powered resettable fault

interrupters for isolating a faulted

transformer are also addressed in clause

11.

Re-energizing a faulted transformer

Re-energizing a transformer after it has

been tripped by the operation of its protection system is not a simple issue. Different actions are usually

taken under different situations by different utilities. This subject is addressed in clause 12 of the guide.

Different scenarios are discussed in this clause.

Gas Analysis

As already stated, gases are produced by decomposition of oil and insulation. These gases include

acetylene, carbon mono-oxide, carbon dioxide, ethane, ethylene, hydrogen and methane. Different

combinations of gases are produced by different conditions in the transformer, such as partial discharge,

overheating of oil, arcing and overheating of the cellulosic materials.

These gases partly rise above the oil and are partly dissolved in the oil. Ratios of different gases are used

in the form of Doernenburg and Rogers ratios to determine the type of decomposition taking place in the

transformer.

Analysis of gases dissolved in the oil provides information on the material that has decomposed. This

analysis is not an exact science but gives useful information. This subject is addressed in clause 13 of the

guide. The discussion includes manual analysis and automatic analysis at regular intervals of time. The

level of total dissolved combustible gases and their rate of change observed over time are used to take

different courses of action; these are included in the guide.

Special protection schemes

Several special schemes for protecting transformers are identified and described in clause 14 of the

guide. One of the examples discussed in this clause reviews tripling of a unit generator due to sudden

loss of system load. The problem is identified and a solution is suggested in clause 14.1.

Another example

discusses the situation

when a grounding

transformer is installed

on the low side a

transformer and is

included in the

differential protection

zone of the


4 of 6 29-Aug-11 6:36 AM

transformer. The

problem in this case is

that the zero sequence

current supplied by the

grounding transformer

can cause the

differential protection to

operate during a fault

outside the protection

zone of the transformer. A solution to this problem is also described in clause 14.

Problems with differential protection schemes of single-phase transformers in three-phase banks are also

identified and solutions suggested in this clause.

Solar magnetic disturbances

Solar magnetic disturbances and other phenomenon can result in the flow of dc currents in the ground.

These currents can enter the power grids through the grounded neutrals of transformers resulting in

saturation of their cores. The saturation in turn results in the flow of excessive harmonic currents in the

transformers and the power system.

This problem is identified and described in clause 15 of the guide.

Annexes

The guide includes the following seven annexes in addition to the fifteen clauses.

Annex A: Application of the transformer through -fault-current duration guide to the protection of power

transformers

Annex B: Transformer failure statistics

Annex C: Examples of setting transformer protection relays

Annex D: Thermal overload protection

Annex E: Phase shift and zero-sequence compensation in differential relays

Annex F: Bibliography

Annex G: Additional sources of information

Annex A: Application of the transformer through-fault-current duration guide to the protection of

power transformers

The through fault current capabilities of transformers are specified in standards and also can be obtained

from transformer manufacturers. The characteristics of overcurrent protection systems should be

coordinated with the through current capability limits of transformers so that the protection systems

disconnect the transformers from service before the limits are exceeded.

The currents in the lines leading into a transformer are different from the currents in the transformer

windings when the windings are connected in delta configuration. This should be taken into consideration

when overcurrent devices are selected.

These and other issues concerning the application of through-fault-current duration guide are discussed in

Annex A. Applications to transformers of different rated capacities are included for guidance.

Annex B: Transformer failure statistics

Knowledge of the frequencies of different types of failures in transformers and equipment directly

connected to them help engineers in deciding the protection functions that should be used in an

application. North American data on transformer failure was not available. However, the statistics of failure

data for transformers rated 60 kV and above for the 1988 to 2002 years collected by the Canadian

Electrical Association and published for use by design and application engineers is included in Annex B.

The data includes component-years of exposure, subcomponents associated with it, number of outages,

frequency of outages, total outage time, mean duration, median duration and mean time for which the

operation was out of service.

Annex C: Examples of setting transformer protection relays

Three examples of setting relays are given in this Annex. The first case deals with setting differential

relays for a step-up 110 MVA, 13.8 / 230 kV unit transformer installed at a generating station and feeding

into a 230 kV network. Issues that are relevant to electromechanical relays are first discussed, and issues

that are relevant to numerical relays are then presented. The second case deals with the setting of a

numerical relay and an electromechanical relay for protecting an auto-transformer with a tertiary winding.

The ratings of the three windings are 400 MVA at 345 kV, 400 MVA at 118 kV and 80 MVA at 34.5 kV. The

400 kV and 118 kV windings form the autotransformer that is connected to networks and the 34.5 kV

tertiary winding supplies local loads. Finally, the third example demonstrates the setting procedure for a

combination of electromechanical and numerical relays for protecting a 70 MVA, 118 kV / 14.4 kV

transformer that takes energy from the 118 kV network and supplies load to a local distribution system at

14.4 kV. A combination of electromechanical and numerical relays is used to protect this transformer.

The settings of the relays selected to protect these transformers are calculated. The procedures used are

very similar to those used in electric power utilities.

Annex D: Thermal overload protection

The mechanism of temperature rise in transformers is explained and the impact of temperature rise on the

life of the insulation used in a transformer is discussed in this annex. Need for monitoring temperature of

all windings in a transformer is demonstrated. Two examples are included that demonstrate this need.

Methods of measuring hot spot temperature are also included. Thermal protection approaches using

temperature indicators and calculated hot-spot temperature are described in this annex.

Annex E: Phase shift and zero-sequence compensation in differential relays

Different methods for phase shift compensation and zero-sequence compensation used in differential

relays are described in Annex E of the guide. An example is included in which phase shift compensation

used in differential protection system applied to a delta-wye transformer is described. Reasons for the

need for internal compensation are identified. Wye, delta and double-delta connections of three-phase

CTs and their mathematical equivalents are described. A table of mathematical representations of 11

possible phase shifts is included. Internal settings for compensating for the magnitudes of CT secondary

currents and compensating for zero-sequence currents are explained, An example of these

compensations is included. The flow of zero-sequence currents during an external fault is first explained.

The use of a zero-sequence current trap is then described. The use of double-delta connections of CTs is


5 of 6 29-Aug-11 6:36 AM

then discussed.

Annex F: Bibliography

A bibliography of 42 standards, guides, books and published papers referred in the guide is given in this

annex.

Annex G: Additional sources of information

Eighty-five publications are included in Annex G. These references are divided in four groups depending

on the major thrust of each publication. The first group includes publications of general interest in

transformer protection. The second group consists of publications that describe protection of

transformers using currents and voltages experienced by them. The third category includes protection

based on pressure and gas analysis and the fourth category includes publications on protection from

overvoltages.

Biographies

Dr. Mohindar S. Sachdev has a B.Sc. from the Benares Hindu University, a M.Sc. from the Panjab

University and M.Sc. and Ph.D. from the University of Saskatchewan. He was chairman of the Computer

Relaying Subcommittee and the Relaying Practices and Consumer Interface Subcommittee of the IEEE

PSRC, as well as different working groups. In recognition of his contributions the University of

Saskatchewan bestowed on him the degree of Dr. of Science in May 1994. He is a Life Fellow of the

IEEE, the Institution of Engineers (India) and a Fellow of the Engineering Institute of Canada. He is a

Chartered Engineer in UK and a Professional Engineer in Saskatchewan, Canada.

Pratap Mysore received his BE and ME degrees in Electrical Engineering from Indian Institute of

Science, Bangalore, India. He has been with Xcel Energy since 1987 and is currently a consulting

Engineer (in-house position) in the Substation/ Transmission Engineering group. Prior to Xcel Energy, he

was with Brown Boveri Corporation (now ABB) and Tata Electric Companies in India. Pratap is actively

involved in the standards development work through IEEE PSRC. He is the chair of Substations

subcommittee, chair of the working group revising Shunt capacitor protection guide, and member of

several working groups within PSRC. He has presented many papers at regional conference and has

offered course on power system protection sponsored by University of Minnesota.

Home | Current Issue | Tutorials | White papers | Books | Tools | Events | Advertising | Classified | Forum

Terms and Conditions of Use and Privacy Policy

© PAC World - Last updated: 27 Jul 2010


6 of 6 29-Aug-11 6:36 AM

pac world magazine _ pac line do not cross

Documents