Class Notes - Year 4 - 2020-2021
Year 4

Lesson 402

Power Distribution Systems and Phase-Loss Monitors

Holding Circuit

IEC Manual 402

Distributed Generation

Also known as
On site generation
dispersed generation
Embedded generation
Decentralized generation, decentralized energy
Distributed energy
Fuel cell
If privately owned, typically include batteries
Excess can be sold back to utility

Electronic Phase Loss Monitors

Socket mounted
Undervoltage adjustment from 75% to 100%
Detects phase unbalance over 10%
Hard output contacts
Same protection as MPS
If 3 phase power at terminals 6, 7, and 8, NO contact closes
Will reset automatically if power is restored
Control circuit must be used
Line side
Load Side
Reversing circuit
MPS must be connected to line side of starter

Three-Phase Monitoring

Phase failure relay will
Increase motor life
Reduce costly repair or replacement
Minimize downtime
Increase safety

Types of Protective Relays

Two types
Current sensing
Voltage sensing
Advantages of current sensing
Not fooled by back EMF, which accompanies phase failure on motors
Can detect abnormal condition on either line or load side
Advantage of voltage sensing
Can detect abnormal condition even if motor is not running
Provides pre-start protection
Easier and cheaper to install
No CTs

Types of Failure

Phase failure
Phase Reversal
Voltage Unbalance

EMC5 28-1 - Electrical Power Distribution

Centralized Power Transmission and Distribution

Centralized power transmission and distribution
Power transmission
Process of delivering electrical power from power-generating plant to a substation
Power distribution
Process of delivering electrical power from substation to customer’s service entrance equipment
Systems can include:
Normally 12.47kV - 245 kV
Generating station transmission lines
Deliver power to transmission
Transmission substations
Step down to lower feeder voltage
Normally 4.16kV - 34.5kV
Primary transmission lines
Deliver power to:
Distribution stations
Heavy industry
Distribution substation
Step down to utilization voltage
Normally 480V - 4.16kV
Distribution Lines
From distribution substation , along street, to final step down transformer
Final step-down transformer
Step down to 480V, 208V, 120V
Can be installed:
On poles
On grade level pads
On underground vaults
Secondary is connected to service-entrance equipment
Transmission lines & towers
Carries large amounts of power at high voltage over long distances
Voltage varies depending on distance and current

Distribution Substations

A distribution substation
Outdoor facility
Located close to the point of electrical service use
Used for:
Changing voltage levels
Providing a central place for system switching
Monitoring protection
Re-distributing power
Receive and adjust voltage
Provide switching point for various connections
Provide safe disconnecting point
Provide location to take measurements and check operation
Have 3 main sections:
Primary switchgear
Secondary switchgear

Power Distribution Point of Use

Power delivered to residential, commercial, & industrial buildings
Connected to buildings via switchboards
Can be:
Lateral (underground)
Service point
Point of connection between utility and premises wiring


Power delivered here to building via transmission systems
Normally freestanding
Switchboard and switchgear used interchangeably
Often is the demarcation point
Can contain metering equipment
This section sealed like a meter
Remember 6 throws of the hand
May contains motor started and other control components

Panelboards & Branch Circuits

Normally wall mounted
Branch circuits
Wiring between final OCPD and load

Motor Control Centers

Motors require complicated control and protection circuits
Combines many functions:
Incoming power
Control circuitry
Overload protection
Overcurrent protection
Transformation of power
Combines individual control units into modular structure
Receives incoming power, delivers to control circuit & motor load
Examples of external control inputs to MCC:
Liquid level switches (float switches)
Limit switches
Examples of internal control components:
Control transformers
Centralized troubleshooting
Units can be switched out
Many pre-assembled configurations are available
Predrilled holes for additional items

Power Distribution System One-Line Diagram

Illustration of entire building distribution system

Feeders & Busways

Used where equipment changes are common
Factory elbows and fittings
Common length =10’
Two types of busways
Plug in
3 types:
Circuit breakers
Use NEMA designations


EMC5 28-3 - Power Quality Problems

Distribution System Components

Distribution lines

Quality Power

Within specified voltage
Capable of delivering enough current

Poor Quality Power

Voltage drops
Voltage unbalance
Voltage fluctuations
Current unbalance
Harmonic distortion

Single Phasing

Motor designed for 3 phase, but only operating on 2
One fuse blows
Mechanical failure in switching equipment
Lightning strikes one line
Motor will run in low torque applications
Will draw all current on only 2 legs
May not be apparent through voltage readings
Open windings act as secondary to transformer
Can be reduced with properly sized DETD fuses and heaters
Connected coils can become damaged, while disconnect coils remain in the best shape

Improper Phase Sequence

Change of sequence of any 2 phases (phase reversal)
Reverses motor rotation
Modifications or maintenance to system

Phase Unbalance

Occurs when power lines are out of phase
Occurs on 3 phase system when 1 phase loads are applied
One line carries more than the other 2
Unbalance causes lines to move out of phase, and are no longer 120 degrees apart
Motors will run at higher temperatures

AC Voltage Variation

High or low voltage
Motors operate at + or - 10% rated voltage

AC Frequency Variations

High or low cycles/second (Hertz)
Motors operate at + or - 5% rated frequency

DC Voltage Variations

High or low voltage
Motors operate at + or - 10% rated voltage

Voltage Surges

Higher than normal temporary voltage
Can be caused by lightning
First few motor windings will be burnt, while others appear normal

Voltage Unbalance

Voltage of three phases are not equal
One or two of stator windings become blackened
Voltage should not be more than 1%
Determining voltage unbalance
Read voltage between all 3 phases
Find average by dividing by 3
Subtract average from all 3 original readings
(Voltage Deviation / (Voltage Average x 100)) = Voltage unbalance

Current Unbalance

Currents on 3 lines are not equal
Current unbalance from overloading can cause voltage unbalance
Should not exceed 10%
Determining current unbalance
Measure current on all 3 lines
Add all together
Find average by dividing by 3
Find largest deviation by subtracting lowest measurement from average
Divide largest deviation by average and multiply by 100

Power Factor

Caused by motor loads that cause current to lag voltage
Can be correct by adding capacitor banks

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