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 www.eit.edu.au Fundamentals of Power System Protection - Free Webinar 1 IDC Technologies and The Engineering Institute of Technology (EIT) The nuts and bolts of electrical power system protection www.eit.edu.au Key Topics • Need for protection • Characteristics and components of a protection system • Faults and protection • Earthing and its relevance to protection • Protective devices www.eit.edu.au Fundamentals of Power System Protection - Free Webinar 2 IDC Technologies and The Engineering Institute of Technology (EIT) 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 www.eit.edu.au 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. www.eit.edu.au Fundamentals of Power System Protection - Free Webinar 3 IDC Technologies and The Engineering Institute of Technology (EIT) 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. www.eit.edu.au 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. www.eit.edu.au Fundamentals of Power System Protection - Free Webinar 4 IDC Technologies and The Engineering Institute of Technology (EIT) 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. www.eit.edu.au 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 Fundamentals of Power System Protection - Free Webinar 5 IDC Technologies and The Engineering Institute of Technology (EIT) 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. www.eit.edu.au 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 www.eit.edu.au Fundamentals of Power System Protection - Free Webinar 6 IDC Technologies and The Engineering Institute of Technology (EIT) 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 www.eit.edu.au Fundamentals of Power System Protection - Free Webinar 7 IDC Technologies and The Engineering Institute of Technology (EIT) 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) www.eit.edu.au 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 Fundamentals of Power System Protection - Free Webinar 8 IDC Technologies and The Engineering Institute of Technology (EIT) 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 www.eit.edu.au 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) www.eit.edu.au Fundamentals of Power System Protection - Free Webinar 9 IDC Technologies and The Engineering Institute of Technology (EIT) 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. www.eit.edu.au 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. www.eit.edu.au Fundamentals of Power System Protection - Free Webinar 10 IDC Technologies and The Engineering Institute of Technology (EIT) 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 www.eit.edu.au 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 www.eit.edu.au Fundamentals of Power System Protection - Free Webinar 11 IDC Technologies and The Engineering Institute of Technology (EIT) 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) www.eit.edu.au 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 www.eit.edu.au Fundamentals of Power System Protection - Free Webinar 12 IDC Technologies and The Engineering Institute of Technology (EIT) 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 www.eit.edu.au 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 Fundamentals of Power System Protection - Free Webinar 13 IDC Technologies and The Engineering Institute of Technology (EIT) 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