Power System Fundamentals

1
Power System Fundamentals

• ECE 0909.402.05
• ECE 0909.504.05 - Lecture 6
• 4 March 2003
• Peter Mark Jansson PP PE MScEng

2
Aims

• Homework Review
• Problem 1-18
• Problem 1-19
• Sample EIT Test Problems (HW 3)
• Chapter 3 Transformers
• Chapter 4 AC Machinery Fundamentals

3
Tour Logistics

• Next Monday (10 March 2003)
• Worldcom Back-Up Power Facility
• Philadelphia (Center City)
• ALL meet here at 745 a.m.
• 2 cars needed

4
Chapter 3

• Transformers
• Benefits of Transformers (Completed)
• Types and Construction, The Ideal Transformer
• Transformer Efficiency and Voltage Regulation
• Transformer Taps
• Autotransformers
• 3-? Transformer connections
• Y-Y, Y-?, ?-Y, ?-?

5
Types and Construction

• Types
• POWER transformer
• Voltage sampling
• Current Sampling
• Impedance transformation
• Construction
• Core form
• Shell form

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Take it apart

• Cores are constructed of laminations electrically
  isolated from each other to minimize eddy
  currents
• Primary and secondary windings are wrapped on top
  of each other (in some smaller size units they
  can be wound bifilar with center taps)
• In larger units it simplifies insulation of Hi-V
  from core and reduces leakage flux

7
Names of Types

• POWER
• Unit Transformer output of generation steps-up
  to transmission voltages (110kV or higher)
• Substation Transformer lowering transmission to
  distribution voltage (2.3-34.5 kV)
• Distribution Transformer lowers distribution
  voltage to household or business levels (110,
  208, 240/480V)
• SPECIAL-PURPOSE Instrument Txs
• Potential Transformer samples hi-voltage for
  instruments
• Current Transformer samples hi-currents for
  instruments

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The Ideal Transformer

• Lossless device w/ Input winding and output
  winding

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Ideal Transformer and Phasors

• Magnitudes change, V I phase angles do not

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Power in the Ideal Transformer
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Real Transformer Efficiency

• In a real transformer current flows in the
  primary even when there is no load (or an open
  circuit) on the secondary
• Why?

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Magnetization and Core Losses

• Magnetization current the current required to
  produce a flux in the core of the transformer
• Core-Loss current the current required to make
  up for the hysteresis and eddy current losses

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Transformer losses

• Copper (I2R) losses in both coils
• Eddy current losses in the transformer core
• Hysteresis losses in magnetic domains of
  laminations
• Leakage flux that escapes the core and pass
  through only one winding which produce self
  inductance

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Transformer efficiency
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Transformer Taps

• Typical Installation (load usually disconnected)
• 5.0 tap
• 2.5 tap
• Nominal Rating
• -2.5 tap
• -5.0 tap
• Which has more or less turns in the secondary?

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Voltage Regulation

• TCUL tap changing under load transformer
• A.K.A. - the voltage regulator
• Uses built-in voltage sensing circuitry which
  automatically changes taps to keep system voltage
  supported
• Very common in modern power systems

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Autotransformers

• Sharing a common winding with either
• additional turns in series for a step-up
  autotransformer
• or a tap located before the terminal ends of the
  common coil for a step-down autotransformer

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3-? Transformation options

• Almost all major power systems are three phase AC
  systems
• Transformers that serve three phase circuits are
  constructed in one of two ways
• 3 single-phase transformers (3 cores)
• 1 three-phase transformer wound on a single
  three-legged core

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Preferred Option today

• Lighter, cheaper and slightly more efficient the
  three-phase approach is preferred.
• Older method used three units which allowed for
  individual replacement if there was a fault
• Many installations today still have three single
  phase transformers in operation

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Wye (Y) connection

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Delta (?) connection
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3-? Transformer connections

• Y-Y connection
• VLP / VLS a
• Very rarely used due to stability problems
• Y-? connection
• VLP / VLS ?3 a (phase shifts exist)
• ?-Y connection
• VLP / VLS a/?3 (phase shifts exist)
• ?-? connection
• VLP / VLS a
• No problems with unbalanced loads or phase shift