Test and Interpretation of Test Results Kelman Profile P2 Test and Interpretation of Test Results

Kelman P2 Set-up

Main Display

Auxiliary Menu In the AUX Menu you can: Set the Display Level Set the Date and Time Set the Trigger Level Auxiliary Menu

To get the most accurate results this level should be set to as low a value as possible. The default setting is 0.2A. This should be ideal for most applications, however if there are unwanted signals superimposed on the dc current it may be necessary to raise the trigger level above the 'noise floor' to get reliable results. If the trigger level is too high, no record will be generated on breaker operation. Trigger Level

Testing a Circuit Breaker

Three Phase Testing Energized

Three Phase Testing De-energized

Runnig the Test

The Profile P2 will automatically correct for small offset signals from the dc current probe, however if the offset is too large the display will prompt the user to adjust the thumbwheel on the Probe until the display reads 0. Press OK to continue. The Profile P2 is now armed and ready to record the current profile when the breaker is operated electrically. Zeroing the DC Probe

The operation can be initiated in several ways, all of which will provide acceptable results. For example: Local push button control at the breaker. Remote control via SCADA. Manual trip initiation from protection relay(s). Electrical jumper at the breaker. Activation of the Test

What can we analyse from profiling a circuit breaker Detect if the circuit breaker operating time is outside limits due to the Trip/Close coil or main mechanism The state of ‘health’ of Close and Trip coil mechanisms The condition of the DC supply and associated wiring The presence of ‘sticky’ or faulty circuit breaker auxiliary contacts

Sample Test Result

List of the Parameters   Ltch: Time when the trip latch mechanism is released (ms) Bffr: Time when the striker pin reaches the end of its motion (ms) Acon: Time when the auxiliary contacts interrupt the trip coil current (ms) End: Time when the trip coil current reaches zero (ms) Mcon: Time of transition of the main interrupter contacts Ipk1: Amplitude of first current peak (A dc) Iplt: Maximum current during record (A dc) Vini: Initial voltage before breaker operation (V dc) Vmin: Minimum voltage during breaker operation (V dc)

The current draw of trip coil can be separated into two parts, the electrical and the mechanical as shown above; 0 to E: electrical, E to H: mechanical. The first part, electrical portion, is the time the coil armature is moving to make contact with the trip latch. This is seen by the hump that is produced in the DC current trace. If there is a problem with the coil or the trip latch, this will be seen as an extended time for the first hump to occur. The second part of the current draw, mechanical portion, is the time that the mechanism moves until auxiliary contact opens to drop out the trip coil. Poor bearings in the trip latch, for example, will produce an extended electrical portion of the travel. If there is a problem with the breaker mechanical assembly, this will be indicated by an extended amount of time after the trip latch is actuated. Poor lubrication of the mechanism, for example, will slow down the breaker and produce a slow second half of the DC current trace. The main contact time is determined by when current is no longer sensed by the AC clamp during the test. A slow breaker will be indicated by increased main contact opening times. The battery voltage drop during the test can indicate problems with the station battery, breaker wiring, or trip coils. Excessive amounts of voltage drop during a test will indicate that there is a problem in the DC system. Determining whether the Profile P3 traces are normal requires knowing the manufacturers specs for the breaker under test.

Theory of Trip Coil Profiling Normally the inductance in most electrical circuits is fixed Trip Coil is an electro-magnetic circuit which has inductance and resistance in the electrical part. As the plunger moves causing an increase in the inductance in the electrical circuit As the rate of change of inductance increases then the rate of change of current must decrease Theory of Trip Coil Profiling

When the protection relay operates its trip contacts connect the DC tripping supply to the trip coil. The first stage from zero to A shows the coil being energised and drawing current exponentially

The next stage from A to B shows the Striker Pin (Plunger) beginning to move and as a consequence the rate of change of current decreases

The next stage shows the current decreasing as the striker pin gains momentum it hits the latch bar which momentarily decreases its momentum and causes the current to rise before quickly starting to fall again. In some mechanisms this minor perturbance is barely perceptible as the latch bar only slightly alters the momentum of the striker pin movement.

The striker pin continues to move until it strikes the buffer at which stage the trip coil mechanism has completed its movement

After the striker pin has stopped moving, the trip coil draws current until it saturates.

After the striker pin has stopped moving, the trip coil draws current until it saturates. During this period the circuit breaker main contacts will open which then opens the auxiliary contact (G)

When the auxiliary contacts are open, the trip coil is the de-energised and the current quickly falls to zero. Please see a 3D demo of a trip coil operating along with the trip coil profile

The above examples are not exhaustive The above examples are not exhaustive. Each can be replicated in a test scenario, so it is possible to do this for any trip coil mechanism and obtain your own standard curve for normal and abnormal situations

TRIP COIL PROFILE ANALYSIS SECTION DESCRIPTION REASON FOR DEVIATION FROM STANDARD EXAMPLES Zero to A Energising of Trip Coil Problem with electrical characteristics of trip coil or supply voltage Circuit Breaker fitted with incorrect trip coil (should operate at 85% nominal) A-B-C Striker pin travels and contact is made with trip bar Restrictive forces impede travel of Striker pin Oil & greases providing a dashpoint action Insufficient lubricant Misalignment of coil C-D Striker pin making contact with trip bar & overcoming inertia of latch Increased resistance of trip bar/latch mechanism Incorrect adjustment of latching mechanism Inadequate lubrication D-E Striker pin, trip bar and latch move together. E represents point at which Striker Pin hits buffer The trip bar is restricted throughout its travel Misalignment of trip bar and striker pin guide Guide wear causing striker pin to make partial contact E-F Coil only operation. Coil current saturates (Iplt or Imax) Insufficient supply voltage Associated coil wiring F-G Coil stays saturated until it is de-energized by Auxiliary Contacts (ACon) G Coil is de-energized High impedance on supply voltage Fluctuations around G Poor lubrication of the mechanism Coil can’t saturate due to high impedance on supply voltage that causes voltage drop on system Fluctuations around G: Sticky or failing Auxiliary contacts Dirt on the Auxiliary Contacts The above examples are not exhaustive. Each can be replicated in a test scenario, so it is possible to do this for any trip coil mechanism and obtain your own standard curve for normal and abnormal situations

Increased Trip Latch Friction Breaker 736 is a good example of a breaker that has a slowing latch time. Notice the difference in the latch times where the past trip is about 7 ms faster than the present. The difference in the curves represents a trip coil that is taking longer to move the trip latch due to built up friction in the release mechanism. This affects the main part time as well, which results in a significantly slow trip. Another supporting observation is that both curves show the same time from latch point to ACon is 11ms. This insinuates that the problem is indeed happening before full mechanical motion of the breaker. Increased Trip Latch Friction

Breaker 756 is a case where the high main contact part time is only apparent in the first trip. The first trip MCON 3 time of 52 ms is well out of the acceptable range of 30 milliseconds. This is a lubrication issue on the main bearings. As you can see, the breaker is coming off latch at the same time on both trips. The second trip was taken immediately following the first, and no maintenance was performed. In this case, the breaker’s grease reconstituted for a much faster operation. Most likely the mechanism will have to be disassembled for cleaning and re-greasing. Also, the first trip shows the current staying high in the trip coil longer than is required which may eventually damage the coil. Main Bearing Problems

Dirt on the Auxiliary Contacts Current dip removed after cleaning aux contacts Current reduced in first trip This shows a first and second trip from a South Wales 11 kV circuit breaker. The current in the first trip is lower as shown in Fig (blue). On inspection it was discovered that there was dirt on the auxiliary contacts. Fig shows (red) the difference in the trip coil profile when the contacts have been cleaned. Dirt on the Auxiliary Contacts

This example shows the trip coil PROFILE caused by a semi-burned-out coil. The general form of the PROFILE is valid for both trip and close coils. Arcing within the coil as the insulation is breaking down causes the noise superimposed on the normal PROFILE.   The coil remaining energized for much longer than specified usually causes coil burnout. Since trip coils generally only conduct for a few milliseconds, continuous current can easily damage them. This type of fault is interesting for several reasons. The breaker operation (trip times and velocity) remains unchanged by this problem, until the breaker suddenly fails. Therefore a conventional timing test is unlikely to show this problem. The problem can be fixed by simply replacing the coil; however this ignores the likely cause of the coil damage. The root cause of this is probably a problem with the auxiliary contacts in the dc control circuitry. Burned-out Trip Coil

NO Trip