transmiss line faults

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ELECTROTECHNICS

TRANSPORT AND DISTRIBUTION OF ELECTRICAL ENERGY

INVESTIGATION ON FAULTS IN A TRANSMISSION NETWORK

Kougang Guy Rostand

2014

Uba

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I. Table of Contents

Introduction .................................................................................................................................................................. 2

I. Common fault’s definitions .................................................................................................................................. 2

1. Voltage dips (sags) and interruption ................................................................................................................ 3

a. Voltage dips: ................................................................................................................................................. 3

b. Voltage interruption: .................................................................................................................................... 3

2. Harmonics ......................................................................................................................................................... 4

3. Over voltage ..................................................................................................................................................... 4

a. Temporal over voltage .................................................................................................................................. 4

b. Man due overvoltage .................................................................................................................................... 4

c. Atmospheric overvoltage ............................................................................................................................. 4

4. Voltage variation .............................................................................................................................................. 5

5. Unbalanced Voltage set .................................................................................................................................... 5

II. General investigation on the transmission line faults, effects, causes and solutions .......................................... 5

Conclusion .................................................................................................................................................................... 9

References .................................................................................................................................................................... 9

Introduction

All power system components are liable to faults involving anomalous current flow and insulation breakdown

between conductors or between conductors and earth. The insulation material may vary from air, in the case of a

transmission line, to oil, SF6 or a vacuum, in the case of switchgear. In this work we present an overview of a

power transmission network faults, their origin and their solutions.

I. Common fault’s definitions Electromagnetic perturbation that can disturb the transmission line equipment’s and industrial processes are

generally ranged as conducted or radiated, the most present are the following: voltage dips and interruption,

harmonics, overvoltage, overvoltage, voltage fluctuation, unbalanced voltage,

These faults can be grouped into four categories whether they affects the amplitude, frequency, wave form and

symmetry but this is not the purpose of this work.

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1. Voltage dips (sags) and interruption

a. Voltage dips: Iit is a sudden drop of the voltage to a value between 10% and 90% of the reference (nominal) value during half of

the network fundamental period. The voltage dip is therefore detected by permanent measuring the rms voltage

during each half fundamental period.

b. Voltage interruption: Iit is a particular voltage drop greater than 90% of the reference voltage.

These voltage drops can be qualified depending on the duration of the drop:

-Instantaneous: T/2 < DT <30T

-Momentary: 30T< DT <3s

-Temporary: 3s< DT <1min;

-Sustained interruption: DT >1min,

Where T is the network fundamental period; DT is the fault duration.

The voltage drop can be attributed to the network faults or to the variation of heavy network load. For example an

overcurrent over the network causes a voltage across the line impedance.

-Voltage drop due to transmission line faults:

The protection devices (circuit breakers fuse) can isolate the faulty part of the network. Even though the main

source is isolated, the rest of the network can maintain a residual voltage provided by synchronous or asynchronous

motors decelerating (0.3-1s) or by capacitor bank connected onto the network.

-Voltage drop due to heavy load variation:

The commutation of heavy duty loads (asynchronous motors, electrical furnace …) can draw up to the line

protection device short circuit courant therefore causing their temporal functioning (interruption) or line voltage

drop due to the existence of line impedance.

Brief interruption of line voltage can be due to transformer tap changing or commutation, line commutation,

engaging protection device.

Figure 1: Example of voltage sag curve

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2. Harmonics Harmonics are the current components having frequency multiple of the network reference frequency.They are due

to the existence of nonlinear load into the network specially electronic devices (rectifiers, gradators, inverters,

speed aviators, fluorescent tubes, welding arc ...). These harmonic components disturb the proper functioning of

electronic devices and automatic systems.

3. Over voltage An over voltage is considered when a voltage applied to an equipment which nominal voltage is less than the value

applied (figure 3). An over voltage can be temporally, man due or atmospheric due.

a. Temporal over voltage Having the network common frequency, they have multiple origins.

-isolation fault: When a fault of isolation between the ground and a phase occurs on network, the healthy voltage

phases are multiplied by 31/2.

-Ferro resonance: due to the presence of inductance and saturate capacitor in the network (figure 2).

-neutral conductor breakage: the lesser loaded phase voltage increases.

-faulty regulation of a generator or transformer tap,

-over compensation of reactive power due to the relative shunt capacitor bank.

b. Man due overvoltage It is due to the quick variation of the network (engaging protective devices), the overvoltage can be due to:

-load commutation,

-apparition and clearing of induced courants,

-capacitive circuit drill,

c. Atmospheric overvoltage It is due to the lighting wave conducted by a line causing the line overvoltage and increase of the ground potential

(figure 2).

Figure 2: Earth connection of

surge arrester, transformer core,

neutral point and lightning

wave to ground transmission

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Figure 3: Example of an overvoltage curve

.

4. Voltage variation The rms voltage variation being less than 10% are due to load variation slowly and the use of particular industrial

loads as welding arc, furnace.

5. Unbalanced Voltage set It is when the voltages are not regularly displayed 120°, it can be due to unbalance loading or one phase to ground

fault.

II. General investigation on the transmission line faults, effects, causes and solutions

N° Fault Effects Possible causes solutions

1 Under

voltage

-equipment under

supplied,

-equipment damage,

-fault on the network (over

current, short circuit …),

-heavy load commutation

-power cut of second level priory

consumers,

-clearing the network fault,

-use of under voltage relay

2 Over voltage -isolator/dielectric

damage,

-equipment damage (fire),

-lightning,

-bad man working operation

(from supply station, from

transformer tap changing

-single phase current short

circuit in a 3 phase line

-Use of surge arrester, lightning

conductor or rod (figure 9)

-avoid operations that can affect the

line voltage, consign some power

equipment (transformers tap changers,

capacitor bank, line/load

connection/disconnection),

-use of power line circuit breaker,

3 Over

current/load

-over heating of cables

and transformers,

- drop of line voltage,

-increase of generator

work

-increase of the energy

consumption,

-use of circuit breaker and maximum

current/heat relays (figure 5),

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4 Short circuit . Between phases

-line overheating,

-fire in the faulty area,

-network transient state

occurs,

-isolation of the faulty

line

-insulators damage due to

pollution, overheating, age

or chemical

-insulation distance

reduction (figure 6),

-external source of damage

(shovel, pickaxe shock),

-overvoltage damaging the

insulator,

-use of maximum current relay (open

network) to limit the line overheating

and instability,

-use of differential or directional relay

(ring network)

. between phase and

ground

-earth potential increase,

-earth circuits shock,

-healthy phase

overvoltage,

-cable overheating

Same as for phase to phase

short circuits

-impedant neutral point earthling,

-use of ground current relay (figure 7),

5 Over heating -melting of power

equipment

-over current,

-high room temperature

-use of temperature control panel,

-use of heat dissipater or fan,

-use of naked cable,

-use of oiled or air cooling

transformer,

6 Over

vibration

-mechanical damage

(breakage), instability

-presence of harmonic

components,

-over current

-use of low pass filter,

-use of maximum current relay

7 Unbalanced

voltage in a 3

phase system

-presence of residual

current within the

transformers,

-use of unbalanced load -balance the load in the 3 phase set

8 instability Network supply

instability

Load and source variation -good design, use of capacitor banks

9 Network

coupling

error

-generator mechanical

damage,

-other short circuit effects

Coupling takes place with

no compatible voltage hour

index

-previous measurements should be

done and use of networks locking

proper device

10 Inversion of

the power

flow

direction

-overloading of standby

generators,

-unbalance of energy,

-selectivity perturbation,

-an alternator can drive a

-the power provider network

voltage drop, the consumers

standby generator feed the

network,

-quick reengaging of the line

-use of current direction relay or

active power direction relay (figure 4)

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turbine switches, the network motor

provide a transient voltage

to the network,

-an alternator starts working

as a motor

11 Voltage

variation

-reduction of motor

torque,

-increase of magnetic

losses

-voltage drop due to

overload, transformer miss

tap changing or regulator,

-over voltage causing a mis

tap changing or regulation

-supervision of voltage regulators,

-relief of second level priority

consumers,

12 -presence of

ground

current and

one phase

working

-synchronous machine

redemption,

-unbalance set of phases

-a phase is broken,

-presence of a one phase

heavy consumer

Use of three phase measuring current

transformer and ground current relay

13 Frequency

variation

-bad behaving of

synchronous process,

-change in magnetic

losses,

-need of recoupling

networks

-presence of abundant

standby sources on an

overloaded network,

- absence of synchronization

control of the alternator on

the network during rapid

line switching,

-power cut of heavy

asynchronous motor loads,

-bad functioning of the

speed control of an

alternator

-power cutting of second level priory

consumers and heavy asynchronous

motors

-uncoupling networks using a

frequency relay (figure 8),

Figure 4: Typical ring system use of directional relay

Note: Arrows represent current flow direction upon which relays will act. A, B, C, etc, are circuit breakers operated by associated relay. 1, 2, 3, etc bus bar identification

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Figure 5: Typical relay controlled circuit breaker system. Figure 6: isolation distance

Figure 7: Busbar relays protection using relays Figure 8: line departure protections using relays

Figure 9: Example of surge arester protected system

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Conclusion Protection devices detect, locate and initiate the removal of the faulted equipment from the power network in the

minimum desirable time.

Switchgear, cables, transformers, overhead lines and other electrical equipment require protection devices in order

to safeguard them during fault conditions. In addition, the rapid clearance of faults prevents touch and step

potentials on equipment from reaching levels which could endanger life. For better design of a network protection,

a better understanding of possible faults that can occur is necessary. We have presented every transmission network

faults with their effects, causes and solutions. We should mention that the function of protection is not to prevent

the fault itself but to take immediate action upon fault recognition. Protection schemes are designed on the basis of:

safety, reliability and selectivity.

References

1-Schneider Electric: ‘Guide de conception des réseaux électriques industriels’, 6 883 427/A, 2010

2- Pierre Roccia : ‘Protection des machines et des réseaux industriels haute tension’, cahier technique 113, Merlin

Gerin, 1985

3- Colin Bayliss- Brian Hardy :’Transmission and distribution electrical engineering’,third edition, Elsevier, pages

269-498, 2007

4- R. Calvas: ‘ les perturbations électriques en BT’, cahier technique 141, Schneider Electric, 1999,

5- D. Fulchiron : ‘surtensions et coordination de l'isolement’, cahier technique 151, Schneider Electrique, 1992

6- Guy R. Kougang : ‘Etude de la densification du réseau de distribution électrique autour de la centrale hydroélectrique de Memve’ele’, chap. 3, Projet Memve’ele, UYI, 2012

transmiss line faults

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