Intelligent Systems in Power System

Information for:

Security Assessment

Fault Analysis

Power System Protection

Objective

To demonstrate a small Knowledge-Based System using Matlab to automate the decision making process for protection device coordination in distribution systems.

Background

As mentioned earlier, distribution automation is becoming important to the power industry today. Each utility is competing to provide better quality and reliability to its customers. By using automation, we can improve the speed of protection coordination and, at the same time, increase the accuracy of the result.

The next section discusses the basic logic of power system protection which is the foundation for the Knowledge Based System module for protection coordination.

Basic Coordination rules

Basic distribution protection rules are:

Assume the fault is temporary for a certain amount of time.
Lock out for permanent faults
Try to isolate as small a number of customers as possible

The next figure shows the conventional definition of protective devices based on the location.

Now for this project we only use two protection devices to give the basic idea of how to implement a Knowledge Based System for protection coordination. Those devices are a fuse and a recloser.

Fuse

A fuse is an overcurrent device with a circuit-opening fusible member directly heated and destroyed by the passage of overcurrent in the event of an overload or short circuit. The time current characteristics of a fuse are represented by two curves: the minimal-melting time curve and the total-clearing time curve.

The difference between the curves is the arcing time within the fuse.

Automatic circuit reclosers

An automatic circuit recloser is an overcurrent protective device that automatically trips and recloses a pre-set number of times to clear temporary faults or to isolate permanent faults. It also has provisions for manually opening and reclosing.

Automatic circuit reclosers can be set for different operation sequances, e.g.,

2 instantaneous trip and reclose operations, followed by 2 time-delay operations prior to lock-out;

1 instantaneous trip-and-reclose operation, plus 3 time-delay operations;

4 instantaneous trip-and-reclose operations, plus 1 time-delay operation;

4 time-delay operations.

Possible recloser locations:

In substation as the primary feeder protective device.
On some line at a distance from the substation. This will sectionalize long feeders and prevent outages of the entire feeder when a permanent fault occurs near the end of the feeder.

Coordination Of Protective Devices

Proper coordination of protection devices helps to eliminate service interruptions due to temporary faults, to minimize number of customers affected and to locate the fault.

The following data will be needed:

Feeder configuration diagram
Location of protective devices
Time current characteristics
Load currents
Fault currents

Fuse-Fuse Coordination

The “damaging time curve” is defined as 75% of the minimal-melting time curve. To coordinate fuses A and B, the total clearing time curve of B must be lower than the damaging time curve of A within the desirable coordination current range.

The 25% margin takes into account some operating variables such as ambient temperature or preloading.

Recloser-Fuse Coordination

For permanent faults, fuse C should clear the fault. The recloser is set to trip for temporary faults before any of the fuses can blow. The recloser then recloses the circuit.

Temporary faults where a < current < b
Then recloser instantaneous operation clears the fault. The fuse does not blow.

Permanent faults where a < current < b
Then the fuse will blow first. Coordination in range [a,b].

Consider a recloser is programmed for 2 instantaneous operation followed by 2 delayed operations (20 cycles each).Practically, the fuses will heat up during the operation as illustrated below:.

To incorperate the temperature effects: use the fuse damage curve which is 75% of the original MMTC.

The coordination limit is [a',b']

Maximal coordination current b’ is found by intersection of

Fuse damaging time curve (75% of minimal-melting-time curve), and

Maximal-clearing time curve of recloser’s fast-trip operation 2A (2 fast trips)

The minimal coordination current a’ is found by intersection of:

Total clearing time curve, and

Recloser curve 2A+2B (2 fast trips and 2 delayed trips).

The next section will lead to the Knowledge Based System module.

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