8/30/09 1 The “Physical” Layer of Energy Systems Randy H. Katz University of California, Berkeley Berkeley, CA 94720-1776 31 August 2009 Announcement!!! • Power Delivery System Tutorial – Alexandra von Meier, Sonoma State – 1000-1630 (lunch & afternoon breaks) – Room 250, Citris Building • This is a fantastic opportunity, and it is worth the effort to attend this even if you can only make a portion of the day • PDF of the Tutorial available on the course web site8/30/09 2 Physical Layer: The Network Analogy • PHY: first and lowest layer in the seven-layer OSI model of computer networking • Consists of: – Basic hardware transmission technologies – Fundamental layer underlying the logical data structures of the higher level functions – Most complex layer in the OSI architecture due to diverse hardware technologies • Defines means of transmitting raw bits rather than logical data packets over a physical link connecting network nodes – Bit stream grouped into code words or symbols and converted to a physical signal that is transmitted over a hardware transmission medium – Provides electrical/mechanical/procedural interface to transmission medium – Properties of electrical connectors, broadcast frequencies, modulation schemes, etc. • PHY translates logical communications requests from the Data Link Layer into hardware-specific operations to effect transmission or reception of electronic signals http://en.wikipedia.org/wiki/Physical_Layer Water-Electricity Analogy • DC circuit: voltage (V, volts) an expression of available energy per unit charge that drives electric current (I, amps) around a closed circuit – Increasing resistance (R, ohms) proportionately decreases I driven through the circuit by V http://hyperphysics.phy-astr.gsu.edu/HBASE/electric/watcir.html8/30/09 3 Basic DC Circuit Relationships • Ohm’s Law: I = V / R • Power Relationship: P (in watts) = V I • Energy: E = P * time (or P = E / time) • Voltage Law: net voltage change is zero around any closed loop (conservation of energy) • Current Law: net voltage change is zero around any closed loop (conservation of charge) http://hyperphysics.phy-astr.gsu.edu/HBASE/electric/watcir.html AC Power • More complex! • Instantaneous electric power in AC circuit is P = V I, but these vary continuously • Use average power: Pavg = V I cos φ – φ is the phase angle between the current and the voltage, V and I are the effective or rms values of the voltage and current – Aka cos φ is the circuit’s "power factor" http://www.ac.wwu.edu/~vawter/PhysicsNet/Topics/ACCircuit/SeriesLRC.html8/30/09 4 Reactive Power • Simple AC circuit w/source + linear load, current and voltage are sinusoidal • IF load is purely resistive: – V and I reverse their polarity simultaneously – Direction of energy flow does not reverse – Only real power flows • IF load is purely reactive: – V and I are 90 degrees out of phase: no net energy flow, i.e., peaks of voltage are centered at the times when the current crosses zero, and is half positive and half negative—"reactive power" • Practical loads have resistance, inductance, and capacitance, so both real and reactive power will flow to real loads – Power engineers measure power use as the sum of real and reactive power Q (V amps reactive or VAR) – Power factor: ratio of "real power" P (watts) to "apparent power" |S| (V amps), where apparent power is the product of the root-mean-square voltage and current http://en.wikipedia.org/wiki/AC_power Managing Reactive Power • What is Power Factor Correction? – Resistive, inductive and capacitive load – Most loads are inductive: transformers, fluorescent lighting, AC induction motors – Use a conductive coil winding to produce an electromagnetic field, allowing the motor to function, e.g., wind turbines • All inductive loads require two kinds of power to operate: – Active power (kwatts) - to produce the motive force – Reactive power (kvar) - to energize the magnetic field • Operating power from the distribution system is composed of both active (working) and reactive (non-working) elements – Active power does useful work in driving the motor – Reactive power only provides the magnetic field – High transmission losses along cables and transformers – Large users charged for both! • To reduce reactive power: – If an AC motor were 100% efficient it would consume only active power but, since most motors are only 75% to 80% efficient, they operate at a low power factor – Solution: capacitors8/30/09 5 Electrical Energy Distribution System http://en.wikipedia.org/wiki/Electric_power_transmission • Electric power transmission: bulk transfer of electrical energy to consumers – Allows distant energy sources to be connected to consumers in population centers • Power transmission network: – Connects power plants to multiple substations near a populated area – Distribution: wiring from substations to customers Electricity Networks • Early days: DC distribution – Difficult to scale up voltage to reduce current – Thick cables and short distances to reduce V losses – DC distribution limited to a small number of km • AC Distribution – Transformers @ power stations raise generator voltage – Transformers @ substations reduce voltage • High V reduces line current, hence size of conductors and distribution losses – More economical to distribute power over long distances • High Voltage DC (HVDC) http://en.wikipedia.org/wiki/Electric_power_transmission8/30/09 6 Proposed HVDC Lines • TransCanada's Chinook Initiative – Connect low-cost and renewable supplies to growing markets via long distance, (HVDC) transmission line – Major HVDC transmission line linking low cost, environmentally attractive fossil fuelled and renewable generation with growing loads in Nevada, Arizona, and California. • One project consists of a HVDC transmission line connecting Montana to Las Vegas. – Rated @ approx 3,000 MW, will cost US $1.2 - 1.8 billion. – Connect clean coal and wind generation resources in Montana with growing loads in southern Nevada, Arizona, and California. – Extension of the line to SoCal and/or Arizona is
View Full Document
Unlocking...