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Optimizing Isolated Phase Bus Maintenance with Novel Diagnostics

Optimizing operations and maintenance strategies for hydro projects is becoming ever more reliant on condition monitoring, which allows today's constrained budgets to be focused precisely where needed. One non-disruptive monitoring and assessment technique offers opportunities to improve performance.

By Brian Snyder

Condition-based maintenance is designed to prevent in-service failures and to focus maintenance on equipment as needed and when deterioration is indicated. This demands the clear identification of maintenance requirements to ensure that equipment is not over-maintained and consequently resources are not wasted.

Numerous types of on-line diagnostic methods are applied to electrical machinery and equipment, such as infrared and vibration analysis. Two on-line technologies that can be used to evaluate the condition of generator insulation are partial discharge analysis (PDA) and electromagnetic interference (EMI) diagnostics.

PDA is a time domain technique that measures and classifies electrical impulses resulting from insulation defects. Such events are assessed in terms of polarity, amplitude and frequency of occurrence, as well as on the power frequency phase relationship.

The second method, EMI Diagnosticssm, measures the precise frequencies and identifies radio wave energy that results from electrical activity at defects. The technique is possible because high-voltage discharges (partial discharges) and low-voltage arcing generate light, heat and noise, as well as ozone and radio noise. The resulting radio frequency spectrum, or EMI signature, of this radio noise is unique for each physical location and type of defect present within the electrical system being tested.

This approach can be used to test generators, motors, exciters, isophase buses, switchgear, transformers and power cables. Indeed, EMI analysis has been used for about 70 years to locate defects in power transmission lines that resulted in radio and television interference.

A non-invasive on-line survey technique, EMI can detect and identify both electrical and mechanical defects in a motor or generator and associated electrical systems while each power system remains in service. EMI analysis techniques can provide critical information useful for developing targeted maintenance strategies for power systems and can also identify equipment that does not need attention or repair.

In addition, unlike PDA, EMI Diagnostics is a system-wide as well as machine diagnostic technique. Because of this, more system component defects can be detected than can be generator stator problems, including many types of mechanical abnormalities.

The application of this technique to hydroelectric plant equipment started in 1980, and this servicing tool was first revealed as a fully developed product by Doble Engineering Co. in July 2007, branded as EMI Diagnostics.

Paul Griffin, vice president of Doble Power Services, explains that "EMI testing is a new diagnostic offering for Doble and is virtually unknown to the electric power industry. However, this is a tried and tested technology, developed over 30 years ago and proven with over 8,000 successful tests in the field on more than 500 different designs, 25 hp to 1,400 MW, with over 70 types of defects and conditions identified and verified."

According to Doble, EMI presents an opportunity for companies to dramatically improve their condition assessment and maintenance programs, allowing hydro plant operators to save money and time by identifying specific areas that require maintenance, repair or replacement. This allows strategic, proactive, planned decisions to be made well in advance of an upcoming scheduled outage. Doble says EMI Diagnostics can even provide maintenance recommendations on the very first test.

A split-core radio frequency current transformer is used to collect data on generator condition during electromagnetic interference diagnostics.
A split-core radio frequency current transformer is used to collect data on generator condition during electromagnetic interference diagnostics.

Collecting data

With this passive, totally non-invasive measurement technique, with no applied signal, data is collected without affecting operations. Data collection follows the international standard CISPR 16.

EMI data is sourced from the temporary installation of a single split-core radio frequency current transformer on a safety ground or around the neutral lead. The transformer used has a 12 cm window, and the frequency range is 50 kHz to 100 MHz.

There are no hot connections required to any energized conductor, and no interference with operations is required for data collection.

A single test location permits a global survey of the entire generator system.

Among the equipment that can be assessed using EMI are issues with generators, including: stator bar slot discharges from conductive coating erosion, stator slot side-packing erosion, loose stator wedging (loose stator bars), loose end winding ties, blocking and circuit rings, loose/broken stator sub conductors and dirt on stator end windings. In cabling and bus systems, EMI can also find contaminated/loose/cracked bus support insulators, loose bolted joint hardware, deteriorated enclosure insulation, foreign objects in the isolated phase bus enclosures, and defective potential transformer fuses and connections.

For example, an isolated phase bus (IPB) is the high-energy system that connects an electric generator to the grid through the main transformers. The IPB is usually a robust trouble-free system that demands little attention, and most designs are very reliable, providing years of service. Because years of trouble-free service is the norm, this system is often overlooked, with no inspections or maintenance performed on a scheduled basis. Furthermore, there are very few on-line tests available to detect problems, and off-line testing and inspection during resource-deprived outages are often not scheduled.

But deterioration of the IPB does occur, which can result in failures. Insulators can become cracked, contaminated or loose. Problems with the center conductor occur when connections to the transformer or generator are lifted and replaced with the bolts not properly torqued. Ventilation is a constant problem, and high moisture levels can result in insulator flashover and severe internal corrosion of metal parts. The enclosures of older designs have insulated sections, and this insulation deteriorates with time. All of these conditions can lead to in-service failures that result in high repair costs and weeks of lost production.

However, EMI Diagnostics are able to identify these various failure modes and can provide focused maintenance recommendations when applied regularly, say annually. Condition-based maintenance can then be applied and the risk of failures reduced.

Isolated phase bus designs

According to ANSI/IEEE, "An isolated phase bus or IPB is one in which each phase conductor is enclosed by an individual metal housing separated from adjacent conductor housings by an air space." (The bus conductor may be self-cooled or force-cooled by means of circulating air or liquid.) IPB is also governed by a related ANSI standard that also recognizes two variations of IPB design, namely non-continuous enclosure design and continuous enclosure design or minimum (external) flux design. The minimum flux IPB is the most common design.

The identifying features of the minimum flux design are the massive shorting plates between the three phase enclosures. This results in enclosure circulating currents almost equal to the conductor three-phase currents. The magnetic fields are opposing, and the result is very little magnetic field external to the IPB. This IPB design has many attributes but also a number of potential failure modes. For example, continuous enclosure IPB technology is used on all large machines and produces little by way of external magnetic fields. The largest are rated at 30,000 V and 30,000 A. The design offers excellent fault protection, while some designs are very reliable with minimum maintenance required. However, most designs use highly conductive aluminum for the center conductor and a stronger alloy for the enclosures. Aluminum will harden and crack from thermal cycling. Meanwhile, insulators can crack from vibration and bolted connections can become loose.

Visual and infrared (IR) inspections can locate some external defects, such as deteriorated enclosures. A walk-down and IR scan of the entire bus should be scheduled annually, at a minimum.

Figure 1 Enclosure Expansion Joint Cracking

Figure 1, on page 34, shows the cracking of an enclosure expansion joint after 30 years of thermal cycling. With the enclosure compromised, moisture enters the IPB and contamination on the insulators may result in a fault. Some of the circulating current that should be passing through the enclosure will now be forced to travel through the supporting steel, and overheating will develop.

In addition, the inside of an IPB must remain dry or the moisture and ozone combine to form nitric acid with resulting corrosion of all internal metal surfaces. High corona levels are the EMI pattern generated.

Maintaining reliable operations

The IPB is a critical high-energy system in a power plant that is usually very reliable and requires minimum attention. Inspections, cleaning and verification of suitability for service are necessary but often overlooked due to the reliability of the system. There are very few on-line investigative methods to detect deterioration inside an IPB, but EMI is one such that can determine the condition of the conductor, enclosures and insulators. Consequently, the application of EMI-based diagnostic tools to either new or mature IPB greatly reduces the risk of failure by providing information for condition-based maintenance of systems with detectable deterioration.

Furthermore, data is collected without affecting operations, and no design changes are necessary when using this totally non-invasive technique, which requires no applied signal to make its completely passive measurements.

The effectiveness of the technique is such that maintenance recommendations can be given with the first test, with trending numerous tests not necessary for data analysis.

With most electrical equipment, some 80% will require no maintenance in any one maintenance period. Identification of that 80% is very important for the allocation of resources to the equipment that does need attention.

Thus, EMI analysis techniques can provide critical information useful for developing targeted maintenance strategies and identifying equipment that does not need attention or repair. This presents an opportunity for companies to improve their condition assessment and maintenance programs and save money and time by identifying areas that require maintenance, repair or replacement. Thus, strategic, proactive, planned decisions can be made well in advance of a scheduled outage.


Internal problems with insulators and the conductor

Insulators can become contaminated from dirt and moisture or crack from vibration, and the end fittings may separate and the supporting hardware become loose. Infrared observations will not detect any of these problems and inspection of every insulator in the entire isolated phase bus (IPB) requires precious outage time while the IPB is out of service. However, electromagnetic interference (EMI) diagnostics can identify these conditions with a single test.

Case 1: Cap and insulator separation from vibration

Figure 2 EMI Signature for Defective IPB Insulators

Figure 2 shows the EMI signature for a generator before and after repairs were completed to the IPB. The IPB part of the curve is shown in frequencies above 10 MHz on the right of the figure. There was a 100/1 reduction in EMI amplitude of this generator after the six defective insulators were replaced.

This location had high floor vibration, and the IPB rested on the floor. Insulators were replaced every few years due to vibration-induced damage. The floor resonant condition was corrected.

Case 2: Broken IPB insulator near the generator

The data, from 1999, indicated the generator and IPB were in good condition. A test in 2004 indicated the IPB had developed a problem. An IPB inspection was requested and a support insulator was found broken. This is another example of the problem with placing these insulators in tension. Post insulators are designed for compression and cantilever loading only.

Case 3: Bolt failure

This small generator has just returned to service after an outage that included transformer testing. The signature above 10 MHz clearly indicated there were problems in the IPB. This system had not been tested before. One of the advantages of EMI diagnostics is defects can be identified by the first test. The EMI pattern indicated a small gap and a low voltage were involved. A loose connection was suspected, and a bus inspection was recommended. An inspection during the next outage located two bolts with missing heads. The remaining bolts held the connection tight, and overheating had not yet developed. This problem was unknown and would have resulted in failure after a few years of thermal cycling.


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