2. INTRODUCTION
Page 1
Introduction
Switchgear and protection methods play a vital role in every high voltage system.
Electrical switches, fuses and other devices aim to rid the system of faults in minimal
time, minimizing the damage caused to equipment and the cost of repairs.
Dependability and security of protection must be reliable in order to safeguard the
entire system, ensuring continuity and security of supply as well as liability and the
safety of personnel in electricity transmission and distribution systems.
This project will focus on four types of switchgear and protection, primarily the
operation, application and constructional features of a high voltage fuse and
subsequently, the same aspects of a high voltage switch.
It will then proceed to look at two circuit breakers and describe the operation of a
Distance Protection Scheme, giving an example of a possible application. To conclude,
the operational features and applications of various types of Transformer protection
including restricted earth fault, Buchholz and winding temperature will be detailed.
3. INTRODUCTION
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Table of Contents
Introduction ________________________________________________________________ 1
High Voltage Fuse ___________________________________________________________ 3
Applications___________________________________________________________________________ 3
Constructional Features __________________________________________________________________ 3
Operation ____________________________________________________________________________ 4
High Voltage Switch__________________________________________________________ 6
Applications___________________________________________________________________________ 6
Constructional Features __________________________________________________________________ 6
Operation ____________________________________________________________________________ 7
Sulphur Hexafluoride (SF6) Circuit Breaker _______________________________________ 8
Vacuum Circuit Breaker_______________________________________________________ 9
Distance Protection Schemes _________________________________________________ 10
Translay Feeder Protection System__________________________________________________________ 10
Transformer Protection ______________________________________________________ 11
Restricted Earth Fault ___________________________________________________________________ 11
Buchholz ____________________________________________________________________________ 12
Winding Temperature ___________________________________________________________________ 12
References________________________________________________________________ 13
4. HIGH VOLTAGE FUSE
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High Voltage Fuse
APPLICATIONS
A high voltage fuse is a protective device utilized in high voltage power systems from
1.5kV to 138 kV. Typical applications of a high voltage fuse are:
Protection of an instrument transformer used for metering electricity, or
Protection of a small power transformer where the expense of a circuit breaker
is unjustified.
This project will focus on the following type of high voltage fuse:
HRC (High Rupture Capacity)
CONSTRUCTIONAL FEATURES
As demonstrated in figure 1.0, a typical HV HRC fuse consists of a metallic strip or
wire fuse element with a series of constrictions and a small cross-sectional area
compared to that of the fuse. The element is encased in a non-combustible, insulating
porcelain tube and enclosed at each end by an electrical terminal cap.
The terminal caps are conductive and the fuse is arranged in series. The flow of
current causes heat to be produced from the element due its resistance. Heat in turn
creates increased resistance and vice versa.
The eutectic fuse element is designed to carry a specific load current without
corrosion. In the event of a fault current which exceeds the normal load current, the
fuse element will vaporize, creating an open circuit.
A paramount part of any HRC fuse is the high grade refined silica sand. As current
flows through the fuse element, the sand conducts the heat away from the fuse
element, towards the durable outer fuse body. Thus, preventing any unwanted thermal
damage to the element. In addition and most importantly of all, in the event of a fault
current, the sand acts as a current limiting, arc-quenching medium by extinguishing
the arc internally.
Another feature found in many high voltage current limiting fuses is the striker
mechanism. This device is centered in the fuse and attached to one terminal cap, with
the other end of the striker pin connected electrically to the opposite terminal end by
5. HIGH VOLTAGE FUSE
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means of a highly resistive wire. Under fault condition, the fuse elements will melt and
the current will then jolt into the resistive striker wire, forcibly releasing the striker pin
either by method of a robust spring or the ignition of a pyrotechnic charge inside the
striker. The striker pin pierces the terminal cap. This can be a visual indicator that the
fuse needs replaced but is also frequently used to activate a tripping mechanism which
de-energizes all three phases connected to a switch. This process lasts only 0.1
seconds.
Figure 1.0
1. Terminal cap 5. Coil resistance
2. Porcelain tube 6. Striker device
3. ARC-perished filler 7. Star-shaped framework
4. Melting body
OPERATION
Under normal operating conditions, a high voltage HRC fuse is designed to carry the
rated current of the fuse void of any deterioration, for a period of time equal to or
greater than the lifespan of the other equipment in the system.
In the event of a low level fault current, the process of temperature rise in the fuse
element is considerably slow. This prevents the alloy-applied element constrictions
from melting simultaneously as they should. Instead the temperature increases
disproportionately, causing only a few breaks in the element and failing to operate the
striker mechanism. These breakages result in a slightly increased value of impedance,
incapable of quenching an arc. To overcome this problem, a minimal value of fault
6. HIGH VOLTAGE FUSE
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current which can be safely disrupted by a fuse has to be pre-determined by each fuse
manufacturer.
Should a high level short circuit occur the fuse will trip in a specific time. This involves
both the formation of many small electric arcs at the constricted points of the fuse
element and the operation of the striker mechanism. The arcs are quenched rapidly
through absorption by the silica sand filler which is packed tightly into the fuse and
solidifies into a powerful, insulating amalgam of glass and partly fused sand, called
fulgurite. This causes a dramatic rise in circuit impedance, causing the current to drop
to zero and in turn, creates a voltage spike.
7. HIGH VOLTAGE SWITCH
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High Voltage Switch
Essentially mechanical devices that conduct electrical current and provide an open
point in a circuit for isolation, all high voltage switches are incapable of disrupting fault
currents and must coordinate with system requirements and BIL (Basic insulation
level).
APPLICATIONS
Two applications of such high voltage switches are:
Disconnect switch – a switch capable of disrupting overload currents, utilized in
substations either on a line or on both sides of equipment for complete isolation
purposes.
Air break switch – also utilized as isolators but only in external substations,
these switches are fitted with an arc-perishing device such as an arcing horn
which quenches arcs formed during current interruption by extending the arc
beyond a point at which it can sustain itself. This feature enables this type of
switch to be operated whilst in an energized state.
CONSTRUCTIONAL FEATURES
Disconnect switches such as the one shown in figure 2.0 are simple in construction.
They consist of one conducting blade which is hinged at one end and stationary at the
other. The stationary part of the conducting blade can sometimes be fitted with a
mechanical hoop and a lock applied when in the closed position to prevent accidental
opening by extreme fault currents or bad weather conditions. Each terminal is
mounted on an appropriate insulator which must be capable of enduring voltage
surges and have good compatibility with the other system components in order to
operate effectively and efficiently. These insulators are selected by reference to BIL.
8. HIGH VOLTAGE SWITCH
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Figure 2.0
OPERATION
Under normal operating conditions, the disconnect switch will remain in a closed
position, conducting electrical current continuously without overheating.
In order for maintenance and repairs to be completed, the circuit requiring service
must first be de-energized. It is only after this step that the switches can be operated
safely. The switch can be operated manually through use of an insulated ‘hot stick’
which can be used safely from a distance to disengage the conducting blade from the
terminal, ascertaining complete isolation of that particular part of the system.
9. SULPHUR HEXAFLUORIDE (SF6) CIRCUIT
BREAKER
Page 8
Sulphur Hexafluoride (SF6) Circuit Breaker
These circuit breakers operate in SF6 gas and are widely utilized in external electricity
transmission and distribution substations today. The high insulating properties of the
gas and exceptional arc-quenching ability deems it an effective method of protection.
The circuit breaker consists of one large chamber that houses two fixed contacts
separated by a specific gap which acts as a vacuum. There are two separate
moveable cylinders which contain the SF6 gas, one containing a fixed piston which is
connected to other stationary points in the breaker. Due to the positioning and vertical
movement of the cylinders, the internal volume of them will vary according to the state
of the circuit breaker. As demonstrated by figure 3.0, the breaker opens by moving the
cylinders down towards the pistons; this reduces the internal volume of the cylinder
and compresses the gas in preparation to quench an arc. Each cylinder has vents,
which in a closed position would be blocked by the stationary contacts. As the
cylinders continue to move down, more vents become unblocked and SF6 is forcibly
released through the vents and into the vacuum of the circuit breaker where the arc is
extinguished. In a closed position, the cylinder moves upwards so the internal volume
increases which in turn reduces the pressure of the gas. A disadvantage of SF6 circuit
breakers is SF6 gas as it is a potent greenhouse gas that is 20,000 times more harmful
to the environment than C𝑂2.
Figure 3.0
10. VACUUM CIRCUIT BREAKER
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Vacuum Circuit Breaker
Mainly utilized in medium voltage applications, a vacuum circuit breaker consists of a
heavy glass or ceramic vacuum chamber which houses a pair of electrical contacts;
one stationary, one moveable. There is a metal vapour condensing shield mounted
inside the chamber, providing dielectric protection from metallic vapour to the
equipment. The moveable contact is connected to the main control device by flexible
metallic bellows. This feature provides a seal for the vacuum, eradicating the
possibility of a leak. The vacuum acts as the arc-quenching medium and is deemed
more efficient than any other form of interrupter due to its extremely high insulating
properties. When the contacts are open, the ionization of metal vapour on the contacts
cause an arc to form. The arc is extinguished promptly due to rapid condensation on
the surface of the contacts of the electrons, ions and metallic vapour emitted by the
arc. Advantages of vacuum breakers include the minimal amount of damage caused to
contacts during operation and a maintenance free lifespan for approximately three
decades. See figure 3.1 for construction.
Figure 3.1
11. DISTANCE PROTECTION SCHEMES
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Distance Protection Schemes
Distance protection schemes utilize impedance relays to detect voltage and current
and are most suitable for applications with high speed auto-reclosing for the protection
of vital transmission lines over a distance of approximately 10 miles. There are several
advantages of distance protection including; high economical and technical factors,
and provision of both primary and remote back-up functions in one scheme.
TRANSLAY FEEDER PROTECTION SYSTEM
Under normal operating conditions, there are two current transformers in this system,
one at each end of the feeder as demonstrated in figure 4.0 Both CT’s carry equal
current so have the same polarity and are connected by a closed loop through two
relays via a loop pilot wire. The current flows through the loop so the potential
difference will be equal to zero (C𝑇2- C𝑇1= 0).
In the event of a fault, the current measurements will differ causing a resultant to pass
through the relay coils and trip associate circuit breakers in order to isolate the fault
line.
Figure 4.0
12. TRANSFORMERPROTECTION
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Transformer Protection
RESTRICTED EARTH FAULT
Restricted earth fault relays only operate in the event of an internal fault at the
star/neutral point of the system.
As demonstrated in figure 4.1, there are four current transformers in this protection
scheme; one for each phase and the neutral line. The secondary CT’s of each phase
of a power transformer are connected together in series then connected to the
common terminals of a secondary neutral CT to form a relay. This arrangement causes
the unbalanced secondary current from each phase to oppose the unbalanced
secondary current of the neutral. If the potential difference between the two currents is
zero and there is no resultant, then the currents are of equal amplitudes. However, if
the two current values differ, the REF relay will be activated due to the flow of the
resultant current.
Figure 4.1
13. TRANSFORMERPROTECTION
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BUCHHOLZ
A Buchholz protection relay is gas-operated and located, encased and oil-filled on the
pipe connecting the oil-filled conservator to the main oil-filled transformer tank. In the
upper part of the Buchholz container there is a hinged float connected to a mercury
switch which changes position according to the level of oil. In the lower area there is a
hinged baffle plate connected to a mercury switch. In the event of an internal fault such
as deterioration of insulation between windings, the oil surrounding the transformer will
decompose into hydrocarbon gases. These gases will rise up to the top of the
Buchholz container gradually reducing the oil level. This reduction in oil level causes
the float to move, which changes the position of the mercury switch, closing its
contacts and triggering an alarm.
WINDING TEMPERATURE
The lifespan of transformer windings is completely dependent on winding temperature.
If excessive, heat causes rapid deterioration of winding insulation and eventually
windings, dramatically reducing their lifespan. In avoidance of this, the temperature of
the windings has to be monitored consistently. This is achieved by implementing
current transformers to take measurements of current in order to calculate the
temperature. Relays can be connected to probes and placed in oil tanks to monitor
temperature, activating an alarm if a temperature difference occurs.
14. REFERENCES
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References
Youtube.com (May 2013), Industrial Fuses, 2nd November 2014,
http://www.youtube.com/watch?v=sqcaRamjr4M
tucsontransformer.com (1995-2012), Southern States Hook Stick Disconnect Switch,
7th
November 2014,
http://www.tucsontransformer.com/img/southern-states-hook-stick-disconnect-switch-
05-1002-1.htm
electrical4u.com (No date specified), S𝐹6 Circuit Breaker, 7th
November 2014,
http://www.electrical4u.com/types-and-operation-of-sf6-circuit-breaker/
electroniclevel3.wikispaces.com (No date specified), HRC Fuses, 5th
November 2014,
http://electroniclevel3.wikispaces.com/HRC+Fuses
openelectrical.org (No date specified), Circuit Breakers, 11th November 2014,
http://www.openelectrical.org/wiki/index.php?title=Circuit_breakers
electrical4u.com (No date specified), Protection of Lines or Feeder, 11th November
2014,
http://www.electrical4u.com/protection-of-lines-or-feeder/
electrical4u.com (No date specified), Restricted Earth Fault Protection of Transformer |
REF Protection, 11th November 2014,
http://www.electrical4u.com/restricted-earth-fault-protection-of-transformer-ref-
protection/
15. REFERENCES
Page 14
WEEDY, B.M, CORY, B.J, JENKINS, N, EKANAYAKE, J.B & STRBAC, G. (2012)
Electric Power Systems. John Wiley & Sons: The Atrium.
