HV Circuit Breaker Operating Mechanisms - Hydraulic Systems

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Hydraulic operating mechanism for HV Circuit breakers

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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


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)


Hydraulics:

• Hydraulics is the transmission and control of power, using a liquid as the flowing medium


Hydraulic systems: Advantages

1) Infinite control of speed and pressure2) Instant reaction to change of direction, including

stopping and starting3) Self lubrication4) Large forces can be easily transmitted5) The pipelines can be relatively easily installed and

can be run at any angle6) Leaks are easily detectable7) The system is relatively compact and flexible


General principle of a hydraulic system

Note: The numbers in the figure correspond to those in the previous slide


Units used

• SI system of units is used as shown below

QuantityUnit of

measurementSymbol

Length Metre m

Mass Kilogram kg

Time Second s


Derived unit-Force

• Force = Mass x Acceleration

• Units:– Mass in kg– Acceleration in meters/second/second– Force in Newtons


Derived unit-Pressure

• Pressure = Force/Area

• Units– Force in Newtons– Area in square meters– Pressure in Pascals

• Pressure can be Atmospheric, Gauge or Absolute


Pressure-Force-Area relationship


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


Relation between Pressure and flow

A pump moves a liquid. Observe the development of pressure from the point of constrained flow path (Red).


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


Relationship explained

• Pa is the atmospheric pressure

• Pgauge is the gauge pressure

• Pab is the absolute pressure

• Pvacuum is the vacuum pressure


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


Differential pressure


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


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


Force multiplication principle


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


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


Example of energy transmission

• Energy and motion transmission uses the property of incompressibility of hydraulic fluids


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


Lubrication by hydraulic fluids

• Figure shows formation of a thin film of fluid on the metal surface


Principle of lubrication


Sealing action

Arrow marks indicate locations where the hydraulic fluid forms a seal between high pressure (RED) and low pressure zones


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


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


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


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


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


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


Effect of low viscosity

• Increased losses in the form of seal leakage

• Excessive wear and tear of the moving parts


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)


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


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


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


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


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


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


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


Circuit Symbols for lines

Drain and Bleed lines

Pressure Line

Control Line

Enclosing Line

Mechanical Connection


Component symbols-1

Mechanical Connection

Two Ports Closed

Hydraulic Pump

Gauge

Pneumatic Pump


Component symbols-2

Linear Actuator

Pressure Switch

Pressure Relief

Accumulator


Component symbols-3

Shuttle Valve

Bleed

Check Valve

Restrictor


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


Avoid Mixing


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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HV Circuit Breaker Operating Mechanisms - Hydraulic Systems