Download How the National Grid System Operates

How the National Grid
System Operates
Chris Gorman
Lead Account Executive
Syracuse
Parts of the Electric System
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Parts of the Electric System
1. Generating Station: Produces Electricity.
2. Transmission Lines: Move electricity great distances at
high voltage. Typically 115,000+ volts.
3. Transmission Substation: Redirects power delivery and
provides switching/fault protection.
4. Distribution Substation: Lowers the voltage for delivery on
local streets and provides switching/fault protection.
5. Distribution Lines: Delivers electricity to neighborhoods.
Typically lower than 15,000 volts.
6. Service Wires: Connects individual buildings to the grid.
Typically lower than 480 volts.
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System Protection & Switching
Š Switches are used to direct the flow of electricity. They are
not used to clear faults.
Š Fuses interrupt the flow of electricity when a fault condition
exists.
Š Relays sense specific conditions on the electric system and
instruct breakers and switches to open or close. (over current,
low voltage, under frequency, single phase, etc…)
Š Breakers can perform the function of both switches (control
the flow of electricity) and fuses (fault clearing). The operational
part of a breaker is fully enclosed - some are oil filled. Breakers
are more compact than switches and open/close based upon
instructions provided by relays.
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Electric Faults
Animal Contacts
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Recently Spotted in National Grid’s Territory
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Lightning Strikes – Tree Contacts - Weather
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Motor Vehicle Accidents
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Equipment Failure
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Distribution Line Reclose (Overhead lines)
Š A significant number of faults on overhead lines are
temporary, Therefore, utilities attempt to automatically
restore power – typically 3 attempts are made: 15
seconds, 30 seconds and 45-60 seconds. Line are
restored as follows:
Š 85% first attempt
Š 93% second attempt
Š 95% third attempt
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Impact on Customer
Facilities
“Direct” Distribution System Disturbances
Š This is when a fault occurs on the feeder directly serving
the customer.
Š If the fault occurs on the main ‘back-bone’, customers
on the feeder will experience up to three reclose attempts
and maybe a total loss of power for an extended period of
time.
Š If the fault occurs on a ‘side-branch’, a fuse may isolate
the fault. Customers on the branch circuit will lose power
and other customer’s on the feeder will experience a brief
‘blink’ or ‘dip’ – maybe one reclose.
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“Indirect” Distribution System Disturbances
Š This is when a fault occurs on an ‘adjacent’ feeder
within the same substation as the line serving the
customer.
Š Customer’s on adjacent feeders may experience
several brief ‘blinks’ or ‘dips’ as the substation attempts
to restore power – recloser attempts.
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Transmission System Disturbances
In most instances, an event on the
transmission system results in a widespread
‘blink’ or ‘flicker’. In rare cases, electric service
may be interrupted to some customers.
Problems are usually isolated automatically and
quickly. Most customers only see disturbances
during the initial event.
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Outage Prevention
Š Installation of animal guards.
Š System design: Proper spacing of conductors,
installation of static wires and lightning arrestors…
Š Tree trimming.
Š Preventive Maintenance.
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A Few Notes
Š Underground feeders (cables) usually do not attempt to
reclose automatically.
Š All electric utility systems operate in a similar manner.
When a fault occurs, it will take a certain amount of time
for the system to sense the problem and take corrective
action.
Š Downtown (urban) networks have special and complex
interconnections.
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Ride-thru Capability
Š In many cases, providing ‘ride-thru’ capability on key
equipment greatly reduces the impact of a disturbance on
the whole facility.
Š Power conditioners and UPS’s (Uninterruptible Power
Supplies) can be very cost effective and easy to install.
Š Check ground fault main switches and computer
controlled equipment for proper settings
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Questions?