Understand what is involved with keeping the lights on
By Mike Hensley
We test HV Breakers to make sure they are still operating up to par so that when they are needed to "protect," a power plant or substation for example... they will operate according to specifications.
Unlike the breakers in houses or offices, these HV Breakers located in substations are large, mechanical and carry a large amount of current, so, testing can be a challenge sometimes. The working principle for circuit breakers, small or large, is that of interrupting a specified current in a specified time before that current can cause damage to the electrical system.
Circuit breakers can be anywhere in the transmission and distribution scheme and can be anywhere from MV to HV Levels, and they all need to be tested.
According to the International Council on Large Electric Systems (CIGRE), the biggest problem in HV Breakers is "Failure to Trip".
Protection Systems are put in-place to detect all electrical faults or other abnormal operation conditions. These Protection Systems are coordinated to disconnect a minimum part of the power system in the event of a fault. Then, based on the system design, normal operation should be restored as soon as possible.
There are many things to consider when testing a HV Breaker, but safety is first and foremost:
- Wear proper PPE;
- Disconnect the breaker control circuit from test equipment before performing work on the breaker;
- Observe proper Polarity;
- Use touch proof connectors when possible;
- Connect Ground to Test Equipment;
- Always comply with local safety regulations;
- Always exercise care before operating a breaker to make sure the breaker is cleared of personnel before operating; and
- Make sure the breaker is grounded.
Causes of Breaker Problems
As with any electro-mechanical device, things can happen that cause problems in the substation. The mantra of most utilities in referring to breakers is "Thou shall not fail to trip".
But assurance that the breaker will operate when needed requires maintenance. Testing is essential.
According to some industry reports, breakers fail to trip or close about 50 percent of the time while other reasons account for the rest of the failures including component failures and operating without a command being received.
And, following the same industry reports, the operating mechanisms account for the majority of failures.
An ideal situation is a non-invasive method of testing. Using proper equipment newly measured values can be compared with specification values set by circuit breaker manufacturers. A series of tests can be initiated to compare with factory settings or previous test results or to create a footprint for future reference. When irregularities are found, further testing can be conducted to determine corrective action.
The strategy that is used can be varied. Here is a listing of the most widely used strategies.
Corrective Maintenance - This is probably the most costly form of maintenance because it waits until something happens and is performed in lieu of an actual maintenance "strategy".
Preventive Maintenance - This is performed based on the number of times units have been used, kind of like changing oil every 5,000 miles.
Periodic Maintenance - This is scheduled maintenance with specific tests and filed results.
Condition-Based Maintenance - A Maintenance flag is set
Predictive Maintenance - Service only when needed. As an example, this could be when monitoring vibration data and there is more vibration detected when compared to previous acceptable results, maintenance should then be scheduled.
Reliability Centered Maintenance - Same as Preventative Maintenance but also value / importance priorities added.
No matter which strategy is chosen, the same conditions should be used each time the breaker is tested. Use of high precision signal acquisition, with high measurement accuracy and a reliable method of storing of test measurement data, is highly recommended.
If, in this process, the required set-up can be minimized and the connection hook-up from the test instrument to the breaker, simplified, the tests and the evaluation can be completed faster.
Most Breaker Failures are Due to Lack of Maintenance
The three most important issues for breaker maintenance are:
- Contact Adjustment
- Neglect or lack of maintenance in the first place
Briefly, the most important thing for breaker maintenance is grease. ALL breakers use grease as a lubricant and over time, with the heat that is produced as the breaker carries is normal load current, grease dries out and breaks down and becomes less effective.
For the type of grease, the manufacturer's specifications should be followed. Most breaker manufacturers allow the use of Mobil 28 lubricant for both the current path parts and the mechanism parts.
DO NOT use WD-40. It evaporates quickly leaving the breaker without lubricant on moving surfaces. That is not a desirable situation.
Non-Invasive Functional Testing
Maintenance and inspection procedures can include any or all of the following test equipment:
- Breaker Analyzers
- Power Supplies
- Vacuum Testers
- High Current Source
The emphasis here is the Breaker Analyzer and its capabilities and some of the key parameters of measurement which are Time, Travel, Velocity, and Acceleration.
Timing of the contacts can include the Main Contacts as well as the Pre/Post Insertion Contacts.
Timing of the Contacts can be as simple as a single Break as with a Common Operating Mechanism Breaker or as complicated as 10-Breaks per Phase on a 765KV IPO Breaker.
Some of the other parameters that we should measure are Coil Current and Contact Resistance.
Coil Current can give an indication of sticky coils which could be a sign of impending failure of the coil, whether it is the Close Coil or the Trip Coil. The trace gained by performing tests on these coil currents should be used as a comparative guide to compare against any future test results.
The Contact Resistance can indicate how healthy the contacts are and their ability to handle the rated current that they are designed to handle. The maximum contact resistance should be verified.
Minimum current should be used according to the specification used. But the recommended levels are:
- 50A IEC 60694 and
- 100 A (ANSI)
This measurement technique is called a "Ductor Test" by some companies. Other companies call it a DLRO Test, which stands for Digital Low Resistance Ohm Meter. Whichever you call it is fine as long as you perform the test.
There are two types of Resistance measurements that can be made:
- Static Resistance Measurement
- Dynamic Resistance Measurement
Static resistance is measured to catch switchgear breakdowns caused by high contact resistances across bus bar joints, breaker contact points and isolators.
High contact resistance in circuit breakers occurs as a result of high current breaking operations. Modern networks are carrying increasing loads requiring improved contact resistance.
Static Contact resistance is measured by injecting a DC current through the breaker or device under test and measuring the voltage drop. Various standards exist:
- IEEE recommends 100 Amps
- ANSI states "at least" 100 Amps
- IEC requires 50 Amps DC
Some manufactures recommend 10 percent of the rated breaker current. The breaker would have to be in the closed state for these tests.
What is the benefit of using a higher current? At lower currents the measured values will be the same provided the resistance is linear vs. the current. The risk, under these circumstances is that low currents might result in very high resistance values in some instances. Such cases could be attributed to grease, corrosion, etc. on the contact surface or a polluted contact from rust products after several breaks at rated current. Reference values given by manufacturers are static values.
In one instance, I had a technician trying to make Static Resistance Measurements across a set of breaker contacts. He was taking the measurement across the bushings and was getting very erratic readings. After further investigation it was discovered that since this breaker was in close proximity to a cooling tower at a generation plant, the breaker bushings had significant oxidation and corrosion build-up. After cleaning the bushings thoroughly we were able to get good stable readings. We also used 100 Amps across that contact path to have enough current to burn through any residual debris.
Dynamic Resistance Measurement (DRM) is a test method used as a diagnostic and analysis tool. One of the main objectives of the DRM technique is to determine the length of the arcing contact. It is a comparative test using commissioning data as a baseline and as such should be compared with future tests as data is collected. The measurement is performed by injecting current through the breaker contacts (the breaker is in the closed position) and simultaneously monitoring the voltage drop as well as current flow during the operation of the breaker. With these two values, resistance can be calculated.
A resistance can be calculated by the appropriate software, and the operator is able to compare any of the measured entities against each other. Once travel motion is recorded, each resistance reading can be analyzed along a distance/time reference. This method can assist with the condition diagnosis of the contacts. A reference recording is selected as a comparison, and the new test is compared.
Looking at the typical resultant trace associated with a breaker being tested using DRM, the motion is monitored with an appropriate motion transducer (indicated by the white line in the graph). Where the white line crosses the blue lines very accurate calculations can be made of the length of the arcing contact by measuring the distance between where the blue line crosses the white line. All this is done without having to open the arcing chamber.
Single Ground System
Historically, breaker timers required the lifting of one side of the breaker from ground in order to test the breaker. This was because the tester did not have enough current to compensate for any ground current loss if both sides of the breaker remained grounded. However, the DRM test can be used as a timing measurement when it is not possible to disconnect both sides of ground connections to the breaker. This becomes important for Generator Circuit Breakers (GCB) at generating plants where to "un-ground" the breaker pole prior to the test and then re-establish a ground after the test would require many hours of labor.
This equipment can also be used in electrical systems with a single ground. Before connecting this unit, it must be verified that the high voltage ground and low voltage protective ground create a single protective ground with no measurable voltage potential existing between these ground systems. If a voltage potential is found between the ground systems, then local safety regulations must be consulted.
DualGround – Both sides grounded
This test can also be performed with breakers with the ground connections left in place on both sides of the breaker. This allows the technician to "work between the grounds." The most important advantage is improved safety but the method is also easier and will save time.
The amount of labor and number of tasks are reduced when the ground cable does not need to be disconnected and reconnected. Permission related work that may include paper work could be reduced or avoided. However, the local safety regulations must always be followed.
The only disadvantage to this method is that the measurement has a somewhat lower resolution.
Megger offers a new option for their TM1600 and TM1800 Breaker Analyzers that allows both the SRM and DRM measurements in a single box. It is the SDRM-202 accessory.
When testing with this new SDRM-202 Module, two channels can be tested up to 220A. This allows for making measurements on breakers with two breaks per phase. For 3-phase circuit breakers, three units would be required. Most users of this type of instrument make DRM measurements by testing only one phase at a time. The new SDRM-202 provides the "source" at the breaker chamber which enables it to have shorter leads than if the unit were sitting on the ground. This new method requires much less test time than conventional methods of testing the dynamic resistance of the breaker contacts using alternate high current sources.
The picture is of the SDRM-202 Module and how it would be connected to a breaker with two breaks per phase. The module itself weighs only three pounds, so it can easily be hung up close to the breaker bushings when performing this test.
Test result can also be stored for future analysis and comparison. By following these guidelines it is easy to maintain an effective list of breaker performance data.
About the Author:
Mike Hensley is an applications engineer in the technical support group at Megger. He earned an ASET from the University of Cincinnati and a BSEE from the University of Dayton (Ohio). Mike has held various positions in engineering and technical support including Software Support Engineer for DB Management and USA Technical Support Engineer with Programma. Mike joined Megger to help support the Programma products and integrate them into the Megger line.