Power System Engineering

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Power System Engineering

• ECE 0909.408.01
• ECE 0909.504.02 - Lecture 6
• 27 February 2006
• Dr. Peter Mark Jansson PP PE

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Aims

• Leave 8.00 AM This Thursday
• PJM Interconnection Worlds Largest Utility
  Interconnection System (Valley Forge, PA)
• Reading through Ch. 4
• Key concepts/definitions
• The Per Unit Method of analysis

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Concepts

• What is electrical system?
• The complete network generators, loads,
  transformers, switchgear and control equipment
• What does Load mean?
• Device or group of devices that consume power,
  power required from a given supply line, power
  passing through a line or machine.

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Concepts (cont.)

• What is Grounding?
• Connecting a device or conductor to the main body
  of the earth, assuring the resistance (impedance)
  is below a prescribed resistance value.
• What are faults?
• A malfunctioning of the electrical network
  usually due to short-circuiting of two conductors
  or live conductors connecting to ground.

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Concepts (cont.)

• Outage?
• Removal of a circuit from service (either
  deliberately or inadvertently)
• Busbar?
• Electrical connection of zero impedance joining
  several items, such as lines or loads. Often
  made of actual copper or aluminum buses.

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Concepts (cont.)

• 1st contingency?
• A reliability term used to describe how the
  system or part of a system can remain in service
  with the loss of the first most critical piece of
  equipment (Major transmission line, major piece
  of equipment, etc.)
• 2nd contingency?
• See above (but for the next most critical piece
  of equipment out of service)

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TD system

• Transmission the bulk transfer of power via
  high-voltage links between major generators and
  load centers
• Distribution the conveyance of power to
  consumers via lower voltage networks or radial
  feeds

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T,D G

• Typically generators are centrally located
  (multiple units at each station) interconnected
  to key load centers and transmission networks
• Typically transmission is a network loop of bulk
  power electrical lines.
• Typically distribution (in rural areas) is
  radially fed

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Why Interconnection?

• Economic to operate large generators 24-7-365,
  they must be connected to a broad base of loads
• Spinning reserve can be provided at the system
  level as opposed to at each generating facility.
• Multiple paths through network to load centers

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Rural Distribution

• Predominantly overhead system
• 12kV, 3 phase (radially from substation)
• Pole mounted transformers (5-200kVA)
• Fused protection (manually replaced)
• Located near roads (for easy service)
• 10-12 miles in length
• 80 of faults are transitory flashovers
• Reclosers increase reliability

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Typical recloser operation
Open
Closed
? Fault Incident Begins
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Urban systems

• Predominantly underground system within duct
  banks
• 12kV, 3 phase (ring buses, networked from
  substation)
• Pad mounted transformers (5-200kVA)
• relay protection (more automatic operations)
• Located under roads (for easy service)

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System planning

• Generally plans are made 2-5 years ahead to
  assure that system equipment is not overstressed
• Loads can be forecasted based upon consumption
  trends and building permits / construction cycles
• Demands placed on equipment are taken / recorded
  regularly and loads in excess of 70 of capacity
  are id for potential upgrades

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Global differences

• Voltages selected for td have their origins in
  history and geography
• 60 and 50Hz are the two frequencies used globally
• 60 Hz in the American continents and Japans
  northern islands and 50Hz in the rest of the world

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Loads

• Industrial, commercial and residential
• Load factor (LF) (total energy used / total time
  to use it average demand) / divided by peak
  hourly demand
• Industrial ? highest LFs
• Residential ? lowest LFs

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Real Reactive

• 3-? balanced loads

Where V line-to-line voltage I line current ?
phase angle between load current and load
voltage
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Real Reactive

• 3-? balanced loads

Where Vph phase to ground voltage Iph phase
current ? phase angle between current and
voltage
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3-? transformers

• Three-legged core type common
• Reluctances not identical
• Wound core in steel tank filled with insulating
  oil
• Oil acts as electrical insulation and cooling
  medium to remove heat from losses

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3-? transformers

• Low voltage winding over leg, high-voltage wound
  over low voltage winding
• Core has steel laminations insulated on one side
  to reduce eddy losses
• Voltages across windings are phase voltages and
  are related by turns ratio
• Secondary windings are 30o out of phase with
  primary windings

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autotransformers

• Single winding per phase
• Secondary is tapped off primary winding directly
• When modest changes in voltage are required (/-
  50)
• Significant winding reduction
• Higher fault (short circuit) currents
• Secondary and primary in phase

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harmonics

• Harmonics often exist in 3-? systems (created by
  non-linear loads)
• No triple harmonics exist
• Penetration of harmonics into the power network
  assume effective reactance for nth harmonic is n
  times the fundamental

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Reactive power Gen

• VArs can be thought of as either being produced
  or absorbed in an electrical circuit (inductive
  loads RjX absorb capacitive loads R-jX
  supply them)
• Synchronous Generators can produce or absorb
  VArs
• Overexcited field supplies vars -jX
• Underexcited field absorbs vars jX

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Reactive power Other Equipment

• vars can be produced in an electrical circuit by
  -jX
• Overexcited synchronous machines
• Capacitors
• Cables
• Lightly loaded overhead lines

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Reactive power Other Equipment

• vars will be absorbed in an electrical circuit by
  jX
• Underexcited synchronous machines
• Induction Motors
• Inductors
• Transformers
• Fluorescent Lighting
• Heavily loaded overhead lines

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Reactive Power

• Q is absorbed by a reactance of XL? equal to I2
  XL where I is current
• And note that at a node

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Reactive Power

• A capacitance with a voltage V applied produces
  vars equal to V2 B where V is the voltage and B
  1/Xc
• And note the following

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The Per-Unit System

• Actual values / base or ref values
• Helps to estimate across device types
• Reduces 3-? calculations
• Help with digital calculations and system modeling

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Example 1

• A d.c. series machine rated at 200V and 100A has
  an armature resistance of 0.1 and field
  resistance of 0.15?. The friction and resistance
  loss is 1500W calculate the efficiency when
  operating as a generator.