Learn the basic operating principles of circuit breakers as well as the different types. Review the range of operating mechanisms with a focus on pneumatics and compressors. MORE INFORMATION: http://www.idc-online.com/content/hv-circuit-breaker-operating-mechanisms-pneumatic-compressor-systems-24

1. Hydraulic operating mechanism for HV Circuit breakers
Technology Training that works

2. Topics
• What is hydraulics?
• Hydraulic circuits, symbols and units
• Mathematical relationships governing hydraulic machinery
• Main types of hydraulic systems and components
• Different types of valves used in hydraulic systems
• Hydraulic fluids
• Cavitation and aeration
• Safe handling of hydraulic fluids
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3. Power transmission
• Power transmission in machinery can be done through
various means such as:
– Electrical
– Mechanical
– Fluids
• Gas such as air (Pneumatic)
• Liquids such as water (Hydraulic)
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4. Hydraulics:
• Hydraulics is the transmission and control of
power, using a liquid as the flowing medium
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5. Hydraulic systems: Advantages
1) Infinite control of speed and pressure
2) Instant reaction to change of direction, including
stopping and starting
3) Self lubrication
4) Large forces can be easily transmitted
5) The pipelines can be relatively easily installed and
can be run at any angle
6) Leaks are easily detectable
7) The system is relatively compact and flexible
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6. General principle of a hydraulic system
Note: The numbers in the figure correspond to those in the previous slide
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7. Units used
• SI system of units is used as shown below
Quantity
Unit of
measurement
Symbol
Length Metre m
Mass Kilogram kg
Time Second s
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8. Derived unit-Force
• Force = Mass x Acceleration
• Units:
– Mass in kg
– Acceleration in meters/second/second
– Force in Newtons
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9. Derived unit-Pressure
• Pressure = Force/Area
• Units
– Force in Newtons
– Area in square meters
– Pressure in Pascals
• Pressure can be Atmospheric, Gauge or Absolute
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10. Pressure-Force-Area relationship
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11. Relation between Pressure and flow
• Pressure and flow in a hydraulic circuit are interrelated
• Any hydraulic circuit has a definite resistance to the flow of
hydraulic medium caused by
– Fluid path (Pipes), sudden changes of flow path such as
sectional area or direction, changes in level (elevation)
• A pump is needed to build up pressure to overcome the resistance
and cause fluid flow
• More the flow, greater the pressure developed
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12. Relation between Pressure and flow
A pump moves a liquid. Observe the development of pressure from the point of
constrained flow path (Red).
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13. Ways of expressing pressure
• Pressure can be expressed as:
– Absolute pressure
– Gauge pressure
• A pressure gauge reads the pressure with respect to the
atmosphere
• This value is called gauge pressure
• Absolute pressure=Gauge pressure + Atmospheric pressure
• When Absolute pressure < Atmospheric pressure the gauge
pressure becomes negative
• In this case, the pressure is termed as vacuum pressure
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14. Relationship explained
• Pa is the atmospheric pressure
• Pgauge is the gauge pressure
• Pab is the absolute pressure
• Pvacuum is the vacuum pressure
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15. Differential pressure
• Pressure difference between two specific points
• Does not depend on atmospheric pressure
• Useful to detect excessive pressures at a given section
– Example: A filter with clogged filter element will have
a high differential pressure (between input and exit)
compared to a healthy filter
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16. Differential pressure
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17. Pascal's Law
• Pressure applied to a confined static fluid is
transmitted equally and undiminished in all
directions throughout that fluid and acts with equal
force on equal areas
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18. Force multiplication
• Force = Pressure x Area
• Force applied at one point in a hydraulic system
can be multiplied by several orders of magnitude
• Force is applied to a cylinder with a small cross
section
• A cylinder of larger section in the same system
will multiply this force
• The factor of multiplication = The ratio of cross-sectional
areas
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19. Force multiplication principle
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20. Energy
• Energy is the ability to do work
• A body can store energy and release it in the form of work
• Stored energy can be of the following types
– Potential energy
– Kinetic energy
• A body can store potential energy by work done on it
– Example: A mass at a height, a compressed spring
• Kinetic energy is stored by a body which has been set in
motion
– Example: A mass moving at a constant velocity
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21. Hydraulic Fluid-Purpose
• To transmit force and motion between hydraulic system
components
• To lubricate the moving parts in the system like valve
spools, cylinder rods, etc.
• To achieve sealing where required
• To help flush the contaminants back to the reservoir
• To help disperse heat from the system
• To minimize corrosion by forming a protective coating
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22. Example of energy transmission
• Energy and motion transmission uses the property of
incompressibility of hydraulic fluids
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23. Lubrication by hydraulic fluids
• A necessary property to prevent wear of hydraulic
system components
• Formation of a lubricating film of hydraulic fluid
prevents direct contact
• This prevents wear since metal-to-metal contact is
avoided
• Susceptible components are: pump vanes, valve spools,
rings and rod bearings
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24. Lubrication by hydraulic fluids
• Figure shows formation of a thin film of fluid on
the metal surface
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25. Principle of lubrication
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26. Sealing action
Arrow marks indicate locations where the hydraulic fluid
forms a seal between high pressure (RED) and low pressure
zones
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27. Heat dissipation
• Hydraulic fluid transfers the heat from one part of the system
to another
• Conduction and convection mechanisms come into play for
heat transfer
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28. Properties of hydraulic fluid
• Good lubricity
• Ideal viscosity
• Chemical and environmental stability
• Large bulk modulus
• Fire resistance
• Good heat transfer capability
• Low density
• Foam resistance and demulsibility
• Non toxicity
• Low volatility
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29. Viscosity
• A measure of internal friction in a fluid
• Measured by the rate at which the fluid resists
deformation
• Thicker the fluid, higher the viscosity
• High viscosity fluid flows with greater difficulty
• Hydraulic fluids should have optimum viscosity
• Viscosity decreases as temperature increases
• Viscosity classification:
1. Absolute
2. Kinematic
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30. Absolute viscosity
• Definition:
The tangential force on a unit area of either one or two
parallel planes at a unit distance apart when the space is
filled with liquid and one of the planes moves relative to
the other at unit velocity
• Also called coefficient of dynamic viscosity
• Unit is centipoise
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31. Kinematic viscosity
• Definition:
A measure of the time required for a fixed amount of fluid to
flow through a capillary tube under the force of gravity.
• Used in most hydraulic calculations
• Unit of measurement: Centistokes
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32. Effect of high viscosity
• Higher resistance to flow causing sluggish operation
• Increase in power consumption due to frictional losses
• Increased pressure drop through valves and lines
• High fluid temperature due to friction
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33. Effect of low viscosity
• Increased losses in the form of seal leakage
• Excessive wear and tear of the moving parts
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34. Viscosity index
• A measure of the rate at which the viscosity changes
with temperature
• Usually viscosity reduces as the temperature increases
• A fluid with a high rate of change has a low viscosity
index
• A fluid with a low rate of change has a high viscosity
index
• Expressed as a number (without unit)
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35. Oxidation resistance
• Represents the resistance of the oil for chemical
breakdown
• Oxidation occurs when the fluid comes into contact
with air
• Deterioration resulting from oxidation can lead to:
– Formation of acids, varnishes and sludge which can block the
system
– Increase in viscosity of the fluid
– Corrosion as a result of the contaminants
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36. Foam resistance and Demulsibility
• Foaming occurs when air mixes with
hydraulic fluid
• Ability of the fluid to resist the
formation of foam is called foam
resistance
• Demulsibililty is the ability of the fluid
to separate from or resist mixing with
water
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37. Cavitation
• Cavitation is the formation of vapor in a working
fluid
• Symptoms of Cavitation are:
– Overheating of the fluid
– Excessive pump wear
– Excessive pump noise
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38. Countermeasures for Cavitation
Cause Countermeasures
• Fluid viscosity too high
• Fluid head too high
• Pump speed too high
• Inlet pipe line bore too
small
• Blocked inlet strainer
• Tank breather is blocked
• Collapsed inlet line
• Too many bends,
restrictions in the inlet line
Use the right viscosity grade
fluid
Correct the height of Fluid head
Use the recommended speed
Change the bore of the pipe line
Clean the strainer
Clean the breather
Rectify the inlet line
Ease the bends, clear the
restrictions
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39. Aeration
• Aeration is the phenomena of mixing of air with
fluid
• Symptoms are similar to those of Cavitation
• Causes are:
– Low oil level in the reservoir
– Leaky pump shaft seals
– Loose oil intake fittings
– Broken oil intake fittings
– Using fluid with low foam resistance
– Return line above the fluid level
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40. Aeration countermeasures
Causes Countermeasures
• Low oil level in the reservoir
• Leaky pump shaft seals
• Loose oil intake fittings
• Broken oil intake fittings
• Using fluid with low foam
resistance
• Return line above the fluid
level
Top up oil level in the reservoir
Replace the defective pump seals
Tighten the loose intake fittings
Replace the broken intake fittings
Use a high foam resistance fluid
or add anti foaming agent to the
fluid
Ensure that return line is
terminated below the fluid level
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41. Representation of hydraulic schemes
• By symbols and diagrams similar to electrical circuit
diagrams
• Schematic diagrams to show the operation of the system
• Standard symbols show:
– Components of the circuit
– Flow lines
– Circuit functioning
• Symbols follow national/ international standards
– Australia uses AS 1101
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42. Circuit Symbols for lines
Pressure Line
Control Line
Enclosing Line
Mechanical Connection
Drain and Bleed lines
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43. Component symbols-1
Mechanical Connection
Two Ports Closed
Hydraulic Pump
Gauge
Pneumatic Pump
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44. Component symbols-2
Linear Actuator
Pressure Switch
Pressure Relief
Accumulator
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45. Component symbols-3
Bleed
Check Valve
Restrictor
Shuttle Valve
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46. Recommendations for handling fluids
• Wear protective clothing
• If clothing becomes contaminated, remove and
clean it immediately
• Use clean tools and containers to avoid
contamination of fluid
• Use a pre-filter when filling up
• Use only the recommended fluid
• Do not mix different fluids when topping up a
system
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47. Avoid Mixing
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48. Safety issues
• Fluid and components may burn
• Spillage of hydraulic fluid can make floor slippery
• Some hydraulic fluids are inflammable
• Some hydraulic fluids can cause skin problems
• The pressures in hydraulic systems can be high enough to
penetrate the skin
• Heavy loads can be suspended in the air due to breakdown
of hydraulic system
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49. DO YOU WANT TO KNOW MORE?
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