A progressive, forward-thinking mindset at a recently built semiconductor manufacturing plant in the Southwest results in electrical maintenance capabilities for 21st Century operations. Because products made here become outdated in only a few years, this thinking is a virtual requirement. Let's walk through this facility to see the results.
Redundancy features in electrical systems design. The electrical system design features dual-source supplies and multiple tie switches and CBs in a primary-selective/ secondary-selective network. With this scheme, a power outage is almost impossible. The design extends from a dual utility feed at 69kV and through dual 56MVA transformers, each of which can carry the entire million-sq-ft plant. Double-ended substations with twin primary 12.47kV switchgear and secondary-selective distribution supply 480/277V throughout the plant. This arrangement allows half of all switchgear, at any voltage level, to be taken off line for maintenance at any time because loads can be switched to the other half of the lineup. In addition, major power backup is supplied by three 2250kVA diesel generator sets that supply power at 12.47kV.
Numerous other features are incorporated into the design and operation of power, control, and maintenance systems. For example, harmonic analyzers are permanently connected in strategic locations. Also, 20kVA to 30kVA UPS systems backup emergency lighting, which is supplied from generator power. According to Salman Sabbah, P.E., senior electrical engineer at the plant, these solid-state UPS units are easier to maintain and more reliable than battery-pack fixtures.
Computerization maintenance. A state-of-the-art computerized maintenance program keeps equipment in top condition. The system consists of a main computer, several portable laptop units, and scan guns equipped with small screens. After all equipment to be maintained has been entered into the main computer software, a developed program automatically sends a preventive maintenance notice to the maintenance manager. He then assigns work orders for the scheduled inspection or service. An added feature is that a bar code is placed in a prominent location on each piece of equipment. A program, designed to correspond to the bar codes, provides a wealth of information, guidance, and instruction pertinent to the equipment. For example, it provides a maintenance procedure, troubleshooting guidance, and a signature recognition so that the person doing the maintenance is duly recorded.
Electrical maintenance procedures. The maintenance crew consists of both plant electricians and electrical/electronic technicians. Nearly all receive specialized training, such as infrared testing, vibration analysis, power-quality instruction, safety courses, and other specialized courses as well as on-the-job training.
Within the plant, the major emphasis for electrical maintenance is on annual infrared testing of all equipment. Infrared checks reveal overheated components and "hot-spots" within equipment. All irregularities are recorded on thermograms and video tape, and service is scheduled according to urgency.
Plant electricians also do infrared checks on motors, looking for hot spots. If any are found, in-plant vibration-analysis technicians pinpoint the problem. Sensors are installed on electrical system equipment and feeders. Operating parameters such as voltage, current, resistance, surges, dips, rpm, temperature, pressures, etc., are fed back to several locations where electronic technicians observe readings and adjust values for best equipment or system operation.
Maintenance test program
Sabbah points out that the most effective maintenance for electrical equipment is regular testing and operation of the equipment. Sabbah prefers to have all testing done by an independent test firm that specializes in test techniques for all types of equipment and systems.
Practically all testing of electrical equipment in the plant is done by Electro-Test, Inc. (eti), Phoenix, Ariz. This test firm, with headquarters in Danville, Calif., has facilities in several states across the country. Testing and service procedures follow those recommended by the interNational Electrical Testing Association (NETA), an association of testing firms that has developed standardized test procedures in an effort to obtain the highest degree of quality and professionalism in electrical testing and maintenance. Typical test procedures are described below and correlate with the accompanying numbered photos.
Photo 1. Test engineers unload instruments. Upon arrival at the plant, eti field engineers Ken Miller and Jim Barnhart begin unloading test instruments. On the rolling table is a DC high-potential (hi-pot) test set designed and built by eti. Inside the truck are additional test instruments to be used for maintenance tests. Included is an DC hi-pot test set, protective-relay computerized test set, digital low-resistance ohmmeter (DLRO), power factor (PF) test set, current-source power supply, power-quality data recorder, laptop computer, and turns-ratio set. Also on the truck are a technical library, test procedure manuals, small tools, and a full complement of safety equipment (blast suit, safety grounding clamps, hot sticks, testers, tapes, signs, locks, etc.)
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Photo 2. Vacuum CB low- resistance contact tests. This test is one of a series of tests being done on a 1200A, 15kV vacuum circuit breaker. It checks the resistance of the CB contacts when they are closed. Improper contact pressure, contact erosion, or uneven surface mating between stationary and moving contacts can increase contact resistance to an unacceptable value. To check this resistance value, a DLRO is being used by Barnhart. By injecting current and applying voltage, the internal measuring circuit of the instrument displays a digital direct reading in microhms.
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For this test, a 10A DLRO is being used. The contact resistance is measured between the load and line side of the closed contact and is compared with the values obtained across the adjacent poles. Any deviations of more than 50% are investigated. Generally, a resistance value in the range of 40 microhms is expected across the contacts on this specific type of breaker. If values are unacceptable, the vacuum bottles are checked. If a vacuum bottle is found to be defective, the entire bottle, which is a sealed unit, must be replaced.
Photo 3. Vacuum integrity test. This test verifies the integrity of the vacuum in each of the three vacuum contact bottles on the 1200A, 15kV CB. With the contacts open, an AC overpotential of 36kV is applied across each bottle for 1 min. The leakage current is not measured because this is a GO/NO-GO test.
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Barnhart is using an AC hi-pot test set designed and built by eti. Two external digital multimeters provide voltage and leakage current readouts. In addition to the contact resistance and vacuum bottle integrity tests, the following tests are conducted:
* Trip, close, trip free, and antipump function verification.
* Circuit breaker trip by operation of each protective device.
* Insulation-resistance (IR) testing, pole-to-pole, pole-to-ground, and across open poles at 2500V minimum.
Photo 4. Protective relay maintenance. Practically all types of simple and complex protective relays can be tested with this versatile digital test instrument. Miller first sets up his instruments for the test. Because he will be testing solid-state overcurrent relays, only a current power supply unit is being set up. The test instrument uses toggle switches to program the proper test current values. Digital displays provide current and time readouts. To allow for possible moving about during testing, Miller has placed a remote current-setting control unit (white box with an extension cable) on top of the test set.
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Maintenance procedures for overcurrent solid-state relays is done in two parts: Visual/mechanical inspection and electrical testing. The visual and mechanical inspection includes the following:
* Examination of the relay for physical damage.
* Inspection of cover gasket and cleaning of cover glass and case-shorting contacts.
* Checking for presence of foreign material or moisture.
* Checking for tightness of mounting hardware and tap plugs.
* Verifying that all relay settings are in accordance with existing trip settings, trip curves, and with coordination studies.
Photo 5. Recording data on laptop computer. Another important part of the maintenance program is data acquisition--records and history on each equipment and component. Instruments being used are a recording power quality data-acquisition and storage unit and a laptop computer. The laptop has been programmed with software specifically designed to work with the recorder and provides for the data-manipulation interface. The recorder can be connected to a phone line via an inboard modem, thus allowing transmission of the data to a remote off-site computer.
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The power quality data acquisition unit has a built-in hard drive to store data, and it will monitor voltage, current, harmonics, phase angle, power, frequency, and simple voltage disturbances. Power quality parameters are programmed via the laptop computer, and up to six graphs can be simultaneously displayed on the screen.
Photo 6. Motor overload retry tests. Sabbah (right) observes Miller, who is testing motor overload relays. Within the plant are motor control centers containing a variety of motor starters. These include across-the-line, reduced-voltage, and solid-state types, and range from Size 0 to Size 5 at 480V. Most starters have thermal-type protection.
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First, the test engineer turns off, locks out, and tags the appropriate disconnect. Then, after a double-check with a voltmeter, he begins his test. Using the current-source power supply, an appropriate current is passed through the relay contacts. This test set has a variable rheostat to control the test current and two external digital multimeters connected to the unit under test. One meter is used to observe test current being applied; the other checks voltage. A digital timer on the power supply permits observation of when the relay trips. The voltage and current values are recorded for the test report and for future reference.
Photo 7. Hi-pot testing of MV cables. A DC hi-pot test is being performed on cables feeding this 15kV, 1500kVA, oil-filled transformer. These cables are rated 15kV with EPR insulation at 133% level and shields. After the transformer has been deenergized, all disconnects opened, and all plant safety regulations and eti safety procedures carried out, test leads are connected to the "cable-end" terminals at each switch at each end of the cable run.
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Next, a high potential voltage is applied to each phase in at least five equal increments until the maximum maintenance test voltage of 49kV is reached. The DC leakage current is recorded at each step after 30 see, 1 min, and at 1 min intervals thereafter. The voltage is then held at the specified maximum test voltage for 5 min. Leakage current is carefully watched for any sharp increase in its slope (magnitude), which would indicate a problem. (Should this occur, the test would be terminated and plant personnel notified.) Then the conductor test potential is reduced to zero, and residual voltage is measured. All values are recorded.
When the tests are completed, safety grounding conductors are connected for a short time to adequately drain all insulation of any stored voltage charge.
The value of the test voltage for in-service cables of various ratings is specified in test manuals. However, the actual value selected depends on numerous site specific factors that must be taken into consideration by the test team. Also, the acceptable leakage current is also dependent on location and environmental factors.
The test set used is a 75kV DC hi-pot tester with two external digital multimeters to provide DC voltage and DC current readings. Relative humidity and ambient temperature are taken into account and recorded.
To assure maximum accuracy of test measurements, a constant voltage input to the test equipment is used. This assembly is a 12V battery bank and inverter to provide a regulated 120V AC source to the instruments.
In addition to a thorough visual and mechanical inspection, each cable receives a shield continuity test and insulation-resistance tests. All values are recorded.
Photo 8. Power factor testing of 15kV transformer.While the 1500kVA oil-filled transformer is deenergized and protected, eti field engineers begin an insulation power factor test, also known as a dissipation factor test. The test is being observed by Jean-Pierre Wolff, eti vice president. After the test engineer sets up the instrument, he connects leads to each low-and high-voltage bushing and to ground. Next, an AC voltage of 10,000V is applied to each bushing one at a time.
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In this test, the power factor value is determined by the ratio of charging current, which is measured in volt-amps, to dielectric loss, which is measured in watts. Power factor tests are an effective means of evaluating insulation integrity because the smaller the power factor, the better the insulator. Furthermore, it is a particularly effective means of checking insulation because dielectric losses to ground are measured, which allows detection of "poor" insulation even if it is located in a series of layered "good" insulation. The recommended power factor or dissipation factor of this 15kV power transformer should not exceed 2.0%
Maintenance and safety standards
A good source of reference for maintenance testing is a document entitled Maintenance Testing Specifications for Electrical Power Distribution Equipment and Systems. This document is published by NETA and is referred as NETA MTS-1993 edition. Another valuable maintenance reference is NFPA 70B, ANSI Standard Recommended Practice for Electrical Equipment Maintenance, 1994 edition.
An excellent safety reference is NFPA 70E, ANSI Standard Electrical Safety Requirements for Employee Workplaces, 1995 edition. The NFPA documents are available from, National Fire Protection Assn., 1 Batterymarch Park, PO Box 9101, Quincy, Mass. 02269.