How the National Grid System Operates Chris Gorman Lead Account Executive Syracuse
2 Parts of the Electric System
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. 3
System Protection & Switching Switches are used to direct the flow of electricity. They are not used to clear faults. Fuses interrupt the flow of electricity when a fault condition exists. 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 ) 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. 4
Electric Faults
6 Animal Contacts
7 Recently Spotted in National Grid s Territory
8 Lightning Strikes Tree Contacts - Weather
9 Motor Vehicle Accidents
10 Equipment Failure
Distribution Line Reclose (Overhead lines) 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: 85% first attempt 93% second attempt 95% third attempt 11
Impact on Customer Facilities
Direct Distribution System Disturbances This is when a fault occurs on the feeder directly serving the customer. 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. 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. 13
Indirect Distribution System Disturbances This is when a fault occurs on an adjacent feeder within the same substation as the line serving the customer. Customer s on adjacent feeders may experience several brief blinks or dips as the substation attempts to restore power recloser attempts. 14
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. 15
Outage Prevention Installation of animal guards. System design: Proper spacing of conductors, installation of static wires and lightning arrestors Tree trimming. Preventive Maintenance. 16
A Few Notes Underground feeders (cables) usually do not attempt to reclose automatically. 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. Downtown (urban) networks have special and complex interconnections. 17
Ride-thru Capability In many cases, providing ride-thru capability on key equipment greatly reduces the impact of a disturbance on the whole facility. Power conditioners and UPS s (Uninterruptible Power Supplies) can be very cost effective and easy to install. Check ground fault main switches and computer controlled equipment for proper settings 18
Questions?