1. ECE 498HZ: Power Distribution System Analysis
April 14, 2016
ECE 498HZ: Power Distribution System Analysis
A Primer on Distribution System Protection
ECE Main Slide
Hao Zhu
Dept. of Electrical & Computer Engineering
University of Illinois, Urbana-Champaign

2. Announcements Homework 5 due next Tuesday, April 19
Tim Yardley from the Information Trust Institute will give a talk on the cyber-security aspects of energy delivery systems
Final exam:
7:00-9:00 p.m., Tuesday, May 10

3. The goal of distribution protection
Main objectives
Minimize the duration of a fault
Minimize the number of customers affected by the fault
Secondary objectives
Eliminate safety hazards as fast as possible
Limit service outages to the smallest possible segment of the system
Protect the consumers’ apparatus
Protect the system from unnecessary service interruptions
Disconnect faulted lines, transformers, or other apparatus

4. Overcurrent protection devices
Fuse: heated/destroyed under overcurrent
Circuit breaker: a switch to interrupt fault currents
Circuit reclosers: trips and recloses to clear/isolate faults
Line sectionalizers: similar to fuses but can coordinate with breakers/reclosers to “sectionalize” the feeder

5. Fuse cutouts < 34.5 kV Need a crew visit after one operation
Figures show a 7.2/14.4kV overhead installation
Fuses connected
to conductors
[S&C Electric,
Gonen’s book]
Fuses protecting
dist. transformers
[WikiCommons]

6. Minimum-melting time-current characteristics (TCC)
Minimum time versus current required to melt the fuse link
Another TCC is the total- clearing curve: plots the maximum time versus current

7. Power fuses > 34.5kV Higher ratings than cutouts
Typically for substation or subtransmission applications
Power fuses protecting
Substation transformers
[S&C Electric, Gonen’s book]

8. Automatic circuit breakers (CBs)
Unlike fuses, a CB can be reset (either manually or automatically) to resume normal operation conditions
Controlled by overcurrent protective relays (using current transformers)
Advantages: heavy-duty, good interrupting capability and design margins
Vacuum CBs, McGraw-Edison
[Gonen’s book]
Substation CBs
[WikiCommons]

9. Overcurrent relay TCC

10. Automatic circuit reclosers
Recall the Tamer’s talk on IntelliRupter
Unlike CBs, reclosers use instantaneous and
time-delayed trip/reclose operations
Lighter duty than CBs
Below link compares between the two
Advantages of reclosers: preventing transient faults; automatic power restoration
ABB, 3-phase reclosers

11. Recloser TCC
Reclosers have several curves related to the instantaneous and delayed operations

12. Automatic line sectionalizers
Working together with CBs or reclosers
For transient faults, sectionalizers allow upstream CBs or reclosers to clear them
If the faults persist, sectionalizers will trip to isolate the faulty circuit
This function similar to fuse cutouts
ABB, 1-phase sectionalizer

13. Type of faults
~75%–90% of the total number of faults are temporary (transient) in nature
Conductors contact with each other or ground due to trees, birds,..
The key is to minimize the duration for this type of faults
High-speed tripping, automatic reclosing mechanisms
Permanent faults require repairs by a crew
Burned-down conductors, removing tree limbs…
We want to minimize the number of customers under these scenarios
Isolating faulted areas using fuses and sectionalizers

14. An example scheme Transformers protected by fuses
Common practice to install fuses at the head of each lateral
Can clear faults by the end of lateral
Reclosers or CBs installed at the substation and along the primary feeder
Each has a protection zone

15. Fuse-to-fuse coordination
Fuse rating is selected based on in-series devices
Compare minimum-melting TCC of fuse A with the total- clearing TCC of fuse B
Use 75% of the min-melting for a safety factor of 25%

16. Recloser-to-fuse coordination
For transient faults, only reclosers activated (fuses not affected)
For permanent faults, fuses activated to isolate faulty areas
Sectionalizers can achieve the coordination without TCC comparisons