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ELECTRICAL & COMPUTER ENGINEERING EXAM 3 QUESTION AND ANSWER
Grounding - are installed to reduce any current flow through a worker to an acceptable level by providing a low-resistance parallel shunt around the worker.
- Grounding controls the amount of current that will pass through a worker. Grounding conductors must be big enough to handle the available fault current at that location. bonding - must be installed so that a worker is kept in an equipotential zone. Within the equipotential zone, a worker must not be able to touch any two objects at different voltages.
- Bonding controls voltage to which a worker will be exposed. job briefing - a short informal discussion of the work to be accomplished and the safety measures to be incorporated. job briefing talk abouts - a. Hazards associated with the job b. Work procedures involved c. Special precautions d. Energy source controls e. Personal protective equipment and at least one job briefing talk before the start of a job every day three phase circuit - all 3 phases are connected, one delivers the other two act as a return path why three phase - three phase have a smoother delivery of power, power is constant, and three phase can carry twice amount of load then single phase
three phase - coils are mounted 120 degrees appart, Each coil produces AC and voltage, They reach peak values at 120 degrees, Essentially there are 3 circuits connected to form a 3-Ø circuit basic properties of a three phase circuit - Three-phase systems have either three or four conductors. The three phases are 120 degrees out of phase with each other (360 divided by 3). The fourth conductor is the neutral. wye - The wye, or star, connection is made by connecting one end of each of the phase windings together in a common node. Each phase winding has a voltage drop known as the phase voltage. The line voltage is measured from phase conductor to a different phase conductor. wye voltage - In a wye system, the line voltage is higher than the phase voltage by a factor of the square root of 3 (1.732). ELine = EPhase x 1. EPhase = ELine / 1. line current is equal delta - The delta connection is made by connecting both ends of each of the phase windings to an end of another phase winding. Each phase winding has a voltage drop known as the phase voltage.
Load Current - the normal current present when just the customer loads are drawing current from the system. (representative rural feeder 200 Amps) Reasons for grounding - Prove isolation-working on correct circuit, Protection from accidental reenergization, Protection from induction: static/voltage magnetic/current Grounding Principle- - controls the current around the worker, Reduce any current flow thru a worker at an acceptable level using low-resistance parallel shunt around the worker Grounds must be large enough to withstand fault current if circuit is/becomes energized bounding principle - limits the voltage exposure to the worker, Bonds must be installed so that a worker is kept in an equipotential zone Worker must not be able to bridge between a grounded circuit and any unbonded object not tied into the bonded network Voltage Regulators - Maintain the desired distribution voltage for feeders leaving substation, Buck when voltage gets a little too high •Boost when voltage gets a little to low •Special kind of transformer called an "autotransformer" because the primary and secondary share the same winding, Voltage regulators are installed at the distribution substation and further down the feeder to keep voltage within an acceptable range (+6% / -13%). Transmission Line Conductors - Selected for high strength to accommodate long spans Often use ACSR (aluminum conductor steel reinforced) Distribution Line Conductors - Selected for low resistance to limit voltage drop to an acceptable level
AAC (all aluminum conductor) is becoming more popular among coops triplex - 3 conductors 2 hot insulated conductors, may be all aluminum 1 neutral un-insulated wire, may be ACSR to support weight of other cables Underground Cables - Induced Voltage - If concentric neutral is grounded at both ends, ground current loops can be created Grounding the concentric neutral at only one end eliminates ground current loops Fault Current - is the very high current present when there is a fault on the line such as a ground fault or a phase-to-phase fault Load Current - is the normal current present when just the customer loads are drawing current from the system. Protective Switchgear - is equipment that can be operated (opened) when fault current is flowing through it. Examples include circuit breakers, fuses, and reclosers Isolation Switchgear - is equipment that can not be operated (opened) under fault current. Examples include air-break switches and disconnects. Some isolation switchgear is designed to operate under load current, but other isolation switchgear must be opened under no-load conditions Protective Relays - are devices that use information about the voltage and current curves on the line to make decisions as to whether to take some action, such as opening a breaker. Protective relays rely on potential transformers (PTs) and current transformers (CTs) to gather information from utility power circuits.
-4 opens -2 fast /2 slow -Clear transient faults without "permanent" outage, just blinks -What faults can the recloser see? Arc Suppression - -Insulating medium (oil, vacuum, SF6) -Air blast -Whip -Ultra-fast switch operation -Spring loaded -Pneumatics/hydrolics under high pressure -Fuses that Cause mini-explosion to expel arc Create high resistance in arc path Arc - -Ionized air becomes a conductor -Dangerous because -Releases lots of energy as light and heat -Current path moves unpredictably unlike normal conduction through a cable Caused by Faults (e.g., lightning) Opening a switch Conductor Properties - -Mechanical Strength -Ampacity -Voltage Rating
Ampacity - -Ampacity increases with cross sectional area -Ampacity increases as resistivity decreases -Skin effect decreases ampacity Current Rating of Overhead Transmission Circuit - Affected by: -Ampacity of conductor -Air temperature (affects sag Voltage Rating - Only applicable to insulated cables such as underground cables Material Choices for conductor and cables - -Copper (best conductor) -Aluminum -Aluminum Alloy -Steel (best mechanical strength)
- Potential difference allows current to flow
- Polarity for AC circuits change
- -North America current alternates direction 60 times a second -Cycles per second, (frequency) -Measured in units called Hertz -Symbolized by Hz
Current Lags Voltage - -With loads current lags voltage by 90 degrees inductive -These loads are typified by having a coil Current Leads Voltage - In capacitive loads current leads voltage by 90 degrees Typified by the presence of two plates which hold charge Capacitive loads- - paralleling conductors, does not generate heat or light, capacitors Inductive loads - motors and transformers, does not generate heat or light Resistive loads - do work, the amount can be measured by a kilowatt-hour meter Inductive reactance (Xl) - caused by loads such as motors, transformers, and any device with a coil Capacitive reactance (XC) - caused by paralleling conductors or capacitors Opposition - to current flow in capacitive and inductive loads is called reactance Impedance - is the combination of resistance and reactance in a circuit -Measured in ohms -Interchangeable with resistance, Ohm's Law The applied voltage and induced voltage are 180° out of phase - Two applications in a substation - Current limiting reactor
Shunt reactor Shunt Reactors - -Shunt Reactors and Series Reactors are used widely in AC networks to limit the overvoltage or to limit the shortcut current -Produces an inductive reactance which cancels out the capacitive reactance produced by the conductors Capacitance - The major source of capacitance in a power system is the lengthy paralleled conductors Electron Movement Capacitors - -A capacitor is charged by removing electrons from one plate and depositing electrons on the other plate. -Capacitors oppose a change of voltage Inductive and Capacitive Currents & Powers Cancel - -Customer loads tend to be inductive -Utilities place capacitors in parallel with customer loads to balance reactive power -Long transmission lines (2 long parallel conductors) have capacitance -Utilities introduce shunt reactors (inductors) to compensate Capacitive Reactance - Capacitors produce a counter-voltage that limits the flow of electricity Grounding procedure - -Portable Grounds -Earthing -Different with each utility
