IDC Technologies and The Engineering Institute of Technology (EIT)
Fundamentals of Power System Protection
by Steve Mackay
www.eit.edu.au
EIT Micro-Course Series
• Every two weeks we present a 35 to 45 minute interactive course
• Practical, useful with Q & A throughout
• PID loop Tuning / Arc Flash Protection, Functional Safety, Troubleshooting conveyors presented so far
• Upcoming: – Electrical Troubleshooting and much much more…..
• Go to http://www.eit.edu.au/free-courses
• You get the recording and slides
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The nuts and bolts of electrical power system protection
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Key Topics
• Need for protection • Characteristics and components of a protection system • Faults and protection • Earthing and its relevance to protection • Protective devices
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Protection fundamentals
• What is protection? –Avoiding the undesirable effects of abnormal electrical system behaviour by appropriate action
• What are protected? – Equipment, personnel and system (stability)
• Why are they protected? – Damage, injury (Shocks/Arc flash), collapse
• How are they protected? – Isolating the abnormal part of a system from the healthy parts with least delay
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Power System Protection Introduction • Customers always demand power on a continuous basis without interruptions. • Hence it is necessary to foresee the likely interruptions that may occur in the distribution system to detect failures and to isolate only the faulty sections. • Protective equipment or protective relay is used in a power network to detect, discriminate and isolate the faulty equipment in the network to ensure that the rest of the system is fed with continuous power and at the same time, damage to faulty section is minimized.
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Role of Power system protection
1. To safeguard the entire system to ensure continuity of supply.
2. To minimize damage and repair costs.
3. To ensure safety of personnel.
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Power System Protection: Basic Attributes 1. SelectSelectivity:SelectivityivitySelectivity::: To detect and isolate the faulty item only. 2. StabilityStabilityStability:Stability::: To leave all healthy circuits intact to ensure continuity or supply. 3. SensitivitySensitivitySensitivity:Sensitivity::: To detect even the smallest values of fault current or system abnormalities and operate correctly at its setting before the fault causes irreparable damage. 4. SpeedSpeedSpeed:Speed::: To operate speedily when it is called upon to do so, thereby minimizing damage to the surroundings and ensuring safety to personnel.
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Protection philosophy
• Emphasis on Speed for the following reasons: – To minimise damage and repair costs. – To reduce production downtime. – To prevent undue thermal and magnetic overstressing of healthy equipment on through fault. – To keep voltage depressions as short as possible in the interests of plant stability. – Above all, to enhance the safety of personnel due to arc flashes and electric shock.
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Protection system components
• Measurement of electrical parameters • Sensing abnormal behaviour • Actuating the device for isolation • Isolating • Annunciating • Powering • Enabling (ex: earthing system) www.eit.edu.au
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Possible faults
• Cable Faults - Most common due to both external like moisture, digging, etc., as well as fault currents being carried
• Transformer faults - Not always common but economics rule the decision on the capacity of standby transformers
• Busbar Faults - Catastrophic but duplication is more followed in EHV substations.
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Types of faults
• There are a number of different types of faults
• A protection system must work for all the types of faults it is meant to operate
• Protection must operate at the least possible value of the designated parameter – Note: Current is NOT the only protection parameter
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Active vs. Passive Active fault types (solid and incipient)
Solid Immediate, complete breakdown of insulation causing: - High fault currents / energy - Danger to personnel - High stressing of all network equipment due to heating and electromechanical forces and possibility of combustion - Dips on the network voltage affecting other parties - Faults spreading to other phases www.eit.edu.au
Active vs. Passive
Active fault types (solid and incipient) Incipient A fault that takes a long time to develop into a breakdown of insulation caused by: Partial discharge currents These faults normally become solid faults in time
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Active vs. Passive
Passive fault types These are not “real” faults but conditions that will cause faults due to cumulative effects, such as: Overloading (over heating insulation) Overvoltage (over stressing insulation) Under frequency Power swings (damages generators)
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Types of Three-Phase Faults
(A) Phase-to-ground (E) Three Phase-To-ground (B) Phase-to-Phase (F) Phase-to-Pilot * (C) Phase-to-Phase-to-ground (G) Pilot-to-ground * (D) Three Phase * In mines www.eit.edu.au
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Magnitudes of fault currents
• Normally impedance decides the value of fault currents - But impedance can not be reduced below a certain value
• ground currents can be limited by grounding the neutral of the source and choosing suitable grounding method
• Phase fault currents can not be controlled
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Transient and permanent faults
• Transient faults - do not damage insulation permanently (eg. Tree branches on O/H line), re-closing will be successful
• Permanent - the insulation has broken down permanently requiring repair to restore insulation levels (re-closing will fail)
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Types of faults
• Phase Faults (limited only by positive sequence impedance of system) – High Fault Currents. – Only limited by inherent impedance of Power System.
• Earth Faults – Solid grounding means high earth fault currents – Only limited by inherent zero sequence impedance of Power system.
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Consequences • Heavy currents damage equipment extensively. – Danger of fire hazard.
• This leads to long outage times. – Lost production and lost revenue.
• Heavy currents in earth bonding gives rise to high touch potentials - dangerous to human life.
• Large Fault currents are more hazardous in igniting gases. – Explosion Hazard.
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Solutions
• Phase Segregation (separating phases far apart) – Eliminates phase-to-phase faults.
• Resistance grounding – Means lower earth fault currents – Value can be chosen during design stage to limit current to desired value - say 400Amps
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Earth faults
• Most faults in systems are due to insulation failures
• The current that will flow depends on the type of system earthing adopted and the effectiveness of protection earthing
• The current flow will influence – The touch voltage (in the protective earthing) – The time of protection operation
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Types of System earthing and Earth Fault Magnitude • Unearthed: No current except through the system capacitance • Solidly earthed: High, only limited by earth circuit impedance • Impedance earthed: Mainly dependant on neutral impedance • Tuned earthed: Extremely low (< 10 amps)
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Effects of electricity on humans
Four main factors determining the seriousness of shock: • Path of current flow through body • Magnitude of current • Time that current flows for • The body’s electrical resistance
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Role of Earth Fault Protection
• Useful for Indirect Contact only
• Danger is solely decided by touch/step voltage and time for fault isolation
• Sensitivity of protection is important where fault loop resistance is likely to be high
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Effects of a current flow through the body
Perception - Let go - Spasm - 16 Constriction - tingle - 1 mA 10mA mA 70 - 100 mA - DEATH www.eit.edu.au
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Resistance of the human body
For design purposes, a resistance of 1000 Ohms is considered www.eit.edu.au
Important: Earth fault loop resistance • The impedance of the earth fault current loop starting and ending at the point of earth fault. The earth fault loop impedance comprises the following starting at the point of fault.
– The circuit protective conductor – The consumers earthing terminal and earthing conductor – The metallic or earth return path as applicable – The path through the earthed neutral point of the transformer – The transformer winding and – The phase conductor from the transformer to the point of fault
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Earth Loop Resistance and Protection • Earth Loop Resistance governs flow of earth fault current
• In LV systems, flow through earth return path can cause low fault currents due to high loop resistance and thereby protection failure
• Avoid problems by using TN type of connections (TN-S or TN- C-S/MEN systems where permissible)
• Verify by measurement of loop resistance in branch circuits
• Match fault current to protective device sensitivity www.eit.edu.au
Neutral CT (Standby Earth Fault) scheme
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Core balance (zero sequence) CT Scheme
A Residual Current device (RCD) uses this principle for obtaining sensitive protection for earth leakage currents www.eit.edu.au
Summation CT scheme (4-wire feeders)
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Important • 4 Wire systems can cause problems due to: – Unbalance (zero sequence currents) – TripleN harmonics • Ensure correct connections of CTs to avoid false trips • With multiple sources: Incorrect relay pick up-Neutral isolation for avoidance of parallel return paths www.eit.edu.au
Protective devices
• Used for sensing and isolating faulty circuits • Basic types: – Direct acting devices: Fuse – Mounted integrally with breakers: Releases – External protective devices (relays)
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Fuse-The Basic Protection Device
• A fuse is the most basic of all protective devices and performs all the protection functions normally obtained by several devices • A fuse protects against short circuits and sometimes earth faults
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Fuses - types
• ReReRe---wireablewireable Type Fusible wire – Disadvantages: Open to abuse, incorrect rating used to keep circuit in-rating drops as time goes by – Advantage: Fail safe
• CartridgCartridgee Type – Silver element enclosed in a barrel of insulating material (sometimes filled with quartz sand)
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Fuses - types
CARTRICARTRIDGECARTRIDGEDGE TYPE Advantages : – fault energy contained by insulating tube – Sealed hence does not deteriorate as fast as open type – Better grading possible – Quartz sand absorbs energy and melts across ionized metal path to quench arc – Faster and can handle very high currents up to 100 kA – Normal currents are closer to fusing currents today due to improved materials and design
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Fuses characteristics
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Fusing factor
This British Standard lays down:
• limits for Temperature rise • Fusing factor - Minimum fusing current = 1.4 Rated cont. current • Breaking capacity
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Energy “ Let Through”
Energy let through basically refers to the energy let into 2 the circuit till fusing and its value is proportional to I x t
• Fuses can limit this energey by fusing very quickly - usually under ¼ cycle
• Circuit breakers can take up to 10 cycles (10 x 20ms = 200 ms) to open i.e., 40 times more energy is released into the fault !! (compared to a fuse which breaks the current flow in ¼ cycle) www.eit.edu.au
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Energy “ Let Through”
I (rms)
I (rms)
Energy (I2 t) let through by fault 2 Energy (I t) let of one cycle duration through by H.R.C. Fuse-link Time
Peak H.R.C Fuse-link cut-off
H.R.C Fuse-link duration
Fault current one full cycle (0.02 second) www.eit.edu.au
Fuse applications
Steady loads - Normally protect against over load as well as short circuit.
Fluctuating loads eg. DOL motors with high inrush compared to normal rating, cranes, etc - Here fuses generally protect against short circuit only.
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General benefits and limitations
Benefits: • Serves two purposes: fault detector and interrupter • Main virtue – SPEED and limiting fault energy Limitations: • Can only detect overcurrent faults (not overloads) – In solidly earthed systems fuses can serve as earth fault protection also • Fixed current/time characteristic • Needs replacing after operating • Use for LV and MV applications (up to 66kV) www.eit.edu.au
Circuit breakers and protective devices • Fuses act as fault current sensors and interrupters
• Other approaches use a circuit breaker to interrupt short circuits/earth faults
• Protection can be – External to the circuit breaker (ex: relays) – Integrated within the breaker (Trip unit/release)
• External relays need auxiliary power-Expensive, maintenance-intensive www.eit.edu.au
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Circuit breaker with releases • Integrated protection-independent, self- actuating • Electro-mechanical – Themo-magnetic and electromagnetic types for IDMTL+Instantaneous current protection – Ground fault through an external relay • Current trend: Integral digital protection- versatile
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Typical device highlights
• Built-in current sensors (output 0.5 amps) • Rating plug to decide primary current settings (from p.u. values) • A/D converters for signal processing • Breakers designed to trip with minimum mechanical effort • Minimum of 20% rated current necessary for operation • PTs for voltage-dependent protection • Separate power supply
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Digital protection features
• Compact size-the same basic device used for an entire range of breaker ratings • Widely adjustable characteristics • Built-in ground fault protection by default • Switching memory for obtaining exact thermal behaviour of protected equipment • Other protections (voltage, frequency and reverse power) • True RMS current sensing • Panel indication of load current and cause of tripping • Remote alarms and commands • Communication capabilities for power management www.eit.edu.au
Digital protection-ACB
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Digital protection-MCCB
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Features
• Long time delay (IDMT), Short delay (DMT) and instantaneous settings 2 • Selectable I .t feature • Optional ground fault protection with fixed 2 or selectable I .t feature • Ground fault with summation (internal input) • Alternatively with system neutral CT input www.eit.edu.au
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Typical protection curves
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Typical ground fault protection curves
a) Ground fault delay with b) Time setting-I2.t type fixed time setting www.eit.edu.au
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Extended protection
• Default current protections are overload, short circuit and ground faults • Special protections also available – Current/Voltage unbalance • Motor loads against single-phasing conditions – Over/Under voltage • Motor applications to prevent restarting after power interruption – Reverse Power and Over/Under frequency • Useful in cogeneration applications www.eit.edu.au
Other common features
• True RMS current sensing – Useful to avoid faulty operation of protection in circuits with high level of harmonic content • Panel indications – Uses a built in display on the protection device • Remote trip alarm and remote commands – Alarm of trips and remote command for CB on/off • Communication features – Ability to store and communicate circuit analog/digital parameters www.eit.edu.au
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Basic types of relays
• Electromagnetic relay – Attracted armature instantaneous – Induction disc type inverse time
• Static relay – Analog (discrete components) – Digital (Microprocessor-based)
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Timeline of relay development
Development of Development and electrical distribution 1930 wide use of Electro- 1960 networks mechanical relays
Development of Static Wide use of Static relays relays - discrete 1970 1990 - discrete components - components
Development and manufacture of microprocessor based relays www.eit.edu.au
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Attracted armature type relay
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Attracted armature type relay
• Simple construction • Instantaneous operation – No intentional time delay – Operation time less than 100 m. sec and usually around 0.05 sec – High speed relay operation measured in cycles – Applications in several protection schemes • High set over current protection, differential protection – Time delay obtained using a timer relay in cascade (if required) www.eit.edu.au
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Induction disc type inverse time relay
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CharacteristicCharacteristic of IDMTL relay
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Current Setting IDMT Relay • Plug setting : This adjusts the setting current by means of a plug bridge which varies the number of turns on the upper magnet
• This setting determines the level of current at which the relay will start or pick-up
• BS142 says - relay must definitely operate at 130% setting and definitely reset at 70% setting
• Normally the relay picks up at about 105% to 130% of its plug setting
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Time Multiplier Setting
TM setting : This rotates the tripping bar attached to the disc closer to or further away from the tripping contacts
Effectively moves the curve DOWN the axis
This curve shows the relay will operate in 3 seconds at 10 times the plug setting (with the time multiplier =1)
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Digital Relay advantages
Cost Flexibility Functionality that is not given by electro-mechanical relays Size CT burden low DC power drain low Improved sensitivity and speed With microprocessor relays any characteristic is possible www.eit.edu.au
IEC 60255 Inverse Curves t = k x β /( (I/I>)α - 1) where: t = operate time in secs. K = time multiplier I = measured current I> = set starting current α & β are constants for curve selection - Normal Inverse, Very Inverse and Extremely Inverse (and any other user defined curves) www.eit.edu.au
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Typical digital relay Schematic
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Static relay features • High set overcurrent with time delay – Closer settings due to absence of transient over-reach
• Breaker fail protection built-in – Impulse to a second trip coil or a back-up breaker
• Digital display of relay parameters and operating values
• Memorized information available after tripping and cumulative operational values
• Low auxiliary power requirement and burden on current/voltage transformers
• High input AC variation acceptable www.eit.edu.au
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Information from static relays
• Measurement data of current and voltage • Information stored by the relay after a fault situation • Relay setting values • Status information on the circuit breakers and isolators • Event information
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Integrated protection and control
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What is an IED?
• An electronic device that possesses some kind of local intelligence • IED in protection applications should have: – Versatile electrical protection functions – Advanced local control intelligence – Monitoring abilities – Capability of extensive communications
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Main functions of IED
• Protection • Control • Monitoring • Metering • Communications
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Communication
• Communicating all data in previous slides to/from Control center
• Facilitate remote control, monitoring and measurement
• Facilitate remote protection settings • IED forms the basis of modern substation automation systems www.eit.edu.au
Thank You For Your Interest If you are interested in further training, please visit:
The Engineering Institute of Technologies Online Certificate and Advanced Diploma programs:
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IDC Technologies 1, 2 & 3 day practical workshops, technical manuals, onsite training & International conferences:
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