Power System Engineering

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Power System Engineering

• ECE 0909.408.01
• ECE 0909.504.02 - Lecture 10
• 17 April 2006
• Dr. Peter Mark Jansson PP PE

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Admin. Items

• Leave 8.10 AM This Thursday
• PJM Interconnection
• Valley Forge, PA

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Aims for Today

• Chapter 5 Control of Voltage Q
• Parts 5.1-5.5, 5.7-5.9, 5.11
• Chapter 6 Load Flows
• Parts 6.1, 6.2, 6.6
• Chapter 8 System Stability
• Parts 8.1, 8.4, 8.8

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Voltage and Q

• Voltage difference between two points in electric
  system and flow of reactive power was given
  previously as

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Voltage drives Q

• In most systems XgtgtR and reactive power is
  determined then by ?V

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Power and Q Flow

• See Board
• Machine at A is in phase advance of B
• V1 is greater than V2
• Hence
• Power and Reactive Power flow from A ? B

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Power and Q Flow

• Machine at A is still in phase advance of B
• But now we adjust generator excitation so that V2
  is greater than V1
• Now
• Power flows from A ? B but Reactive Power flows
  from B ? A

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Power and Q

• Real Power flows are controlled by adjusting real
  power generated at any point in the system
  (leading to phase advance relationship) i.e.,
  steam flow to turbogenerator
• Reactive Power (Q) flows are determined by
  excitation to the generator and resulting voltage
  relationship between points A B

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Phasor Diagrams

• Phasor diagrams show that reactive power flow
  will be determined y the scalar voltages
  maintained within the system
• Hence Q flows from higher V to lower Vs

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Voltage Instability

• If reactive power support is insufficient for
  loads in a given part of the network voltage
  will fall
• Conversely, if there is an excess of reactive
  power voltage will rise
• Generating/Absorbing Q in system
• Synchronous Generators make/absorb Q
• Load Support capacitors change PF of load
• Lines (O/H, cable) and Transformers

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Synchronous Generator

• Used to generate or absorb VARS
• Typical 200 MW (85 PF) machine
• Has 45MVAr capability at full load
• Overexcited Machine (one with higher than normal
  rotor field excitation) generates vars (locally
  increasing voltage to export them)
• Underexcited Machine absorbs vars
• Main source of / - vars

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Lines O/H

• When fully Loaded
• Lines absorb Reactive Power
• For current I and Reactance X the vars absorbed
  are I2X per phase
• When lightly Loaded
• Lines can generate Reactive Power
• Shunt capacitance of lines can dominate their
  electrical characteristics on low current flows

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Lines Cables

• Whether fully or Lightly Loaded
• Always generate Reactive Power
• Shunt capacitance of these lines always dominates
  their electrical characteristics
• 275kV, 240 MVA cable
• Generates 6.25-7.5 MVAr per km
• 132kV 1.9 MVAr per km
• 33kV 0.125 MVAr per km

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Transformers

• Whether fully or Lightly Loaded
• Always absorb Reactive Power
• Why?

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Loads

• Inherent in power system are loads of time
  varying power factor
• Power factor of loads plays major role in
  determining required Q
• Example Load at power factor of 0.95 implies
  that we have a demand for 0.33 kVAr for each kW
  of real power delivered

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Voltage Control

• METHOD 1 Injecting Reactive Power
• Static Shunt Capacitors
• Static Series Capacitors
• Synchronous Compensators
• Current Compensation by series injection

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Static Shunt Capacitors

• Lagging Power Factor correction
• At source (factory, building, etc.)
• Should be switched into circuit, parallel with
  primary busbar, as inductive loads require
  (ON/OFF)
• Improving load power factor could alleviate
  entire reactive power problem on electrical system

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Static Series Capacitors

• Placed in series with line conductors
• Best when
• Total line reactance is high
• Voltage drop is limiting factor
• Can average voltage fluctuations in load
• Must be protected from short circuit currents
• Of little value if few load vars are needed

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Synchronous Compensators

• Synchronous motor operating without a mechanical
  load
• Depending on excitation level it can absorb or
  generate reactive power
• Cost of installation (OM) is high
• Flexibility for all load conditions
• Stored energy of rotating machine useful for
  riding through transient disturbances

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Series Injection

• Power Electronics
• High-power, high-voltage semi-conductor
  controlled devices
• Pulse turn-on, turn-off inverters
• Can inject voltage in in series with a line of
  any phase angle
• Sources transformers, battery pack, capacitor,
  etc.) external charging may be required

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Voltage Control

• MORE METHODS
• Tap Changing Transformers
• Combined Use of TCT and Q injection
• Booster Transformer
• Phase shift Transformer

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Voltage Collapse

• When loads are stable and mostly resistive
  collapse in voltage is unlikely as voltage
  decreases so does load
• However, most system loads are inductive and the
  loss of voltage leads to unstable responses from
  induction devices (motors stall, etc.)

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14 August 2003

• https//reports.energy.gov/
• U.S. Canada Power Systems Outage Task Force
  Causes of the August 14th Blackout

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Reliable transmission

• 7 principles
• 1. Continuously balance generation / demand
• 2. Balance reactive power to maintain voltage
• 3. Monitor flows to not exceed thermal limits
• 4. Keep system in stable condition
• 5. Keep system in reliable condition (n-1)
• 6. Plan, design and maintain to operate reliably
• 7. Prepare for emergencies

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Sequence unchanged

• Assessment
• Inadequate System Observation Loss of System
  Control and Data Acquisition
• Outages due to inadequate tree trimming
• Harding Chamberlain 345 kV 43.5 loaded
• Hanna-Juniper 345kv 87.5 loaded
• Star South 345kV 93.2 of emergency rating
• Once this situation occurred FE needed to
  voluntarily reduce load so as not to impact
  remaining system, due to their unawareness of the
  n-1 contingency breach they did not take this
  critical action

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Load Flows

• The steady-state solution for a power system
  given conditions is referred to as the load flow
• In most cases resistance of lines can be ignored
  with very limited impact on final solution
• Comprehensive mathematical models enable the
  simulation of large systems

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In order to simulate..

• We need a floating busbar to feed required
  losses into the system
• Load nodes specifying the complex power SP/-jQ
• Generator nodes voltage magnitude and power

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Load Flow Studies for

• Flow of MW, MVAr in branches
• Busbar voltages
• Effects of changing circuitry
• Effects of temporary loss of
• Generators
• Transmission lines
• Transformers or other key devices

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Load Flow Studies for

• Effect of injecting VArs
• Optimum system running conditions
• Minimizing system losses
• Optimum rating of and tap-range transformers
• Improvements
• Changing conductor size
• Additional generation
• Improving system voltage

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Why? A load flow study

• Check for unstable conditions
• Minimum loading
• Maximum loading
• Economic studies
• Optimize dispatch of units for economy
• Determine benefits of new lines, etc.

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Thursday AM

• Please dont forget to attend
• PJM Headquarters Visit 9-12
• 8.10 AM This Thursday morning