«Abstract— Plant Operations personnel can avoid a forced shutdown by applying a predictive maintenance program to power cable and equipment systems. ...»
CONDITION ASSSESSMENT OF ELECTRICAL EQUIPMENT IN
Nagu Srinivas, DTE Energy Technologies
Dr. Oscar Morel, DTE Energy Technologies
37849 Interchange Dr., Farmington Hills, MI 48335
Abstract— Plant Operations personnel can avoid a forced shutdown by applying a
predictive maintenance program to power cable and equipment systems. However, the condition of an electrical power system, down to the individual component level, must be known before it can be scheduled for maintenance. A predictive maintenance program is beneficial for improving the reliability of medium voltage cable systems, transformers and motors. The Cable|wise diagnostic tool developed by DTE Energy Technologies had been used to assess the condition of such equipment. Cable|wise is an online, totally passive technique that utilizes radio frequency (RF) pulses, emitted by an operating cable system, to assess the remaining life of cable sections, splices, terminations, switchgear, transformers, and motors.
Nomenclature EPR Ethylene Propylene Rubber PD Partial Discharge PILC Paper Insulated, Lead Covered XLPE Cross-Linked Polyethylene PD Partial Discharge Introduction Power cable and electrical equipment such as motors and transformers must operate as long as possible within reliability and safety standards. Critical plant equipment must be monitored and maintained without sacrificing plant availability. A critical component is one whose failure could endanger plant safety, could cause an extended forced outage, or has a long lead time. An influence component is one whose failure would probably not result in an extended outage, would not endanger plant safety, and is unit specific. In a power production plant, cable and station main transformers are critical components.
Electrical systems do not last forever. At some stage, Plant Operations personnel must decide when to replace cable components, electrical equipment, or an entire electrical system. This is a difficult and potentially costly decision. Wholesale refurbishment of electrical plant is too costly to be practical economic option. Common practices used to schedule electrical system maintenance are as follows.
• Operate until the electrical system fails.
The least involved practice is to operate the electrical system without any preventive maintenance and repair when the cable fails. Replacement is scheduled when repair costs become more expensive than replacement.
• Replace based on specified failure rate or life span Replacing equipment when the failure rate reaches a pre-established level or replacing the cable system after a certain life span is still widely used. This assumes that cable and equipment age uniformly, which is not the case. Different segments of the cable system often age non-uniformly along its length. If Plant Operations has the means to identify only the components that need replacement, significant savings can be achieved.
• Conduct laboratory diagnostics Another way of assessing cable condition is to perform a laboratory evaluation on cable samples removed from field. The diagnostic tests conducted in the laboratory may reveal valuable information about the condition of the removed sample but the results have to be extrapolated to the rest of the system. High voltage DC (Hi-pot) and VLF tests are sometimes used to identify the cable components that are on the verge of failure. However, these are destructive tests.
• Perform Diagnostics testing.
Of all the diagnostic methods available today; condition assessment diagnostic testing provides the most detailed information about the performance of an electrical system, down to individual components. Condition assessment provides early identification of weak components of the cable system. It locates degraded components and determines the extent of degradation. This is essential to maintain system reliability. Cost savings can be realized by prioritizing the replacement of weak sections of a circuit. Condition assessment is often more expensive than other diagnostic methods. However, the value of the detailed test results and the savings achieved far exceed the cost of condition assessment.
DTE Energy Technologies Online Condition Assessment On line, in-situ testing to estimate future performance of operating cable systems and electrical equipment represents an advance in diagnostics technology for the cable industry. This advance is possible due to novel technology developed and patented by DTE Energy Technologies (DTECH), including advances in signal processing and interpretation. Of several diagnostic methods available today, the Cable|wsie condition assessment diagnostic testing is most effective, as it provides early identification of weak components of the cable system while the system remains energized. It can locate degraded components of the system and determine the extent of degradation. This is essential to maintain system reliability. On average an 80% cost reduction-n in cable and equipment replacement costs can be realized by DTECH condition assessment.
The overall objective of diagnostic testing, of course, is to identify defects that could cause a system failure, and estimate the time remaining before these defects progress to failure and cause an outage of the electrical system. The test should be economically justified and should not cause additional degradation to the system under test. Hence, testing performed at over-voltages are always of some concern.
Diagnostic test methods that detect partial discharges (PD), which are active during the time of testing, can only detect defects or imperfections that produce partial discharge greater than the sensitivity of the test method. Cable accessories such as splices and terminations are most likely to fail because of PD that causes degradation. For cables, not all degradation phenomena are associated with PD.
Power cables are used to supply power to plant equipment, such as large motors, auxiliary transformers, precipitators, and back-up diesel generators. These cables are rated from 2001V to 15 kV and are single conductor or three-conductor cables, shielded or unshielded construction. Before 1970 power cables were insulated with extruded dielectrics, such as XLPE, and butyl rubber. In some cases PILC cable was also used.
The majority of cable failures in an extruded cable system are related to water treeing, which fail the cable when they progress to electrical trees. Once a water tree progresses to an electrical tree, the time to failure normally is very short because the initiated electrical tree propagates rapidly through the already weakened dielectric. Thus, the only window for detection is during the conversion process. Under normal operating conditions, such conversion is caused by prolonged activity in cavities created in the water tree channel as the result of heat generation caused by ionic current.
In PILC cables, failures are commonly associated with moisture ingress, which normally fails the cable through thermal runaway. Moisture in PILC cables increases the dielectric losses resulting in localized heat generation that thermally degrades the paper insulation and normally leads rapidly to a cable failure. PD may only be present at advance stages of such degradation.
An integral method that provides detection, location and condition assessment of both PD and water content is needed to ensure reliable cable system operation. Any PD testing technique that requires a cable system or equipment to be isolated will require shut down coordination to switch off and isolate the equipment under test, and reroute power to other equipment not under test. This may not always be convenient or economical. Also, critical circuits remain in service while being tested. In contrast, the Cable|wise (DTECH) technique is capable of detecting and locating PD and moisture content while the entire system remains energized.
Note that measuring only the magnitude component of PD does not provide enough information to reliably assess the cable condition. The severity of the PD condition
depends on the:
• Insulation material in which PD occurs
• Environment in which the cable system is operating
• Type of defect producing the PD
• Location of the PD within the insulation wall.
Hence, any specific reported PD value diminishes in significance as that activity is further removed from the conductor shield.
DTECH Test Methodology - Cable As noted above, when cables and accessories age, the resulting changes do not take place uniformly along the system length. Hence, for any diagnostic tool to provide truly meaningful information, one must be able to assess the cable system by length. Nonuniform aging may be due to one or more of many factors; manufacturing issues, localized contamination leading to weak boundary layers at an insulation-contaminant interface, water migration to high stress sites, loose shields at discrete locations, microcracks produced by mechanical fatigue, and so forth. Of great significance is the exact location of the defect within the cable insulation wall.
Signal and partial discharge detection in the field is significantly different from partial discharge testing of extruded cables shortly after manufacture. The objective of the latter is to detect manufacturing defects (voids, shield-interface imperfections) as a result of the extrusion process. This testing is intentionally performed at an over-voltage. Testing in the field requires suitable sensors, a noise filtration system and signal detection and processing capability. The DTECH approach (Cable|wise) provides this at operating voltage, hence eliminating the need for a system shutdown and deleterious effects caused during off-line testing.
Due to defects, all cable components emit signals during operation, but the nature of those signals changes depending upon the cause (i.e., the defect type, as noted above); for example, loose shields yield different signals than do internal defects. The Cable|wise totally passive technique utilizes the (RF) emissions to provide an assessment of the remaining life of the cable sections, splices, terminations, and other electrical equipment connected to the circuit.
One challenge is to be able to measure such small signals and transmit the information, and another is to be able to interpret them (relative to type and location). DTE Energy Technologies has been able to perform this and relate the information from such signal detection to future reliability in operation. The signals measured are generally due to small conventional partial discharges; however, DTECH has been able to measure other age related signals also.
The signal developed during aging induces a current flow in the cable shield and conductor; this consists primarily of high frequency components of the signal. The magnetic field resulting from the current flow is measured. The DTECH method couples energy from the magnetic field of the signal pulse into the measurement system (Figure 1). The sensors, developed and constructed by DTECH, are designed to pick up the magnetic field and are capable of detecting signals in the high frequency range. The readings are taken at intervals of several hundred feet along the cable. This is preferred since, as noted above, cables age unevenly and knowledge of the aging condition of the system over discrete sections is desired. It is to be emphasized that this testing is performed while the system remains energized. Noise reduction is accomplished through signal processing in the frequency domain. The non-destructive test procedure is not limited by cable length, operating voltage, insulation type, cable construction, or branching of the system. A significant feature of Cable|wise technology is that it can distinguish between cable and accessory degradation activity.
The development of an extremely fast, affordable digital signal oscilloscope and waveform digitizer, and recent advances in signal processing and computer technologies have led to better understanding of the role and significance of individual partial discharges and related signals in degrading insulation systems.
Frequency domain testing1 Key to Cable|wise technology is Frequency domain testing. Frequency domain testing
has several major advantages over time domain testing, including:
1. PD can be detected, characterized, and located without having to trigger on the first pulse.
2. Since the frequency domain testing is usually carried out in service, and the PD is detected at various points along the cable, the cable between the point of detection and the cable termination acts as a high frequency filter which removes much of the noise which interferes with sensitive PD detection.
3. Since the PD detector is closer to the PD source and much of the interfering noise is filtered out by the cable, the bandwidth of the PD signal at the sensor can be used to judge location, and very sensitive PD detection is possible (Figure 5)
4. If the analyzer is triggered synchronously with the power frequency, the analyzer display becomes a phase/frequency fingerprint of the PD signal.
5. Because PD detection is undertaken in service, one can assume that any PD source which could be active is likely to be active. As explained above, if the cable is taken out of service to do off-line PD testing, the voltage must be raised to 2 pu in order to assure that all PD sources which could be active will be active.
Thus PD testing at 2 pu off-line is roughly equivalent to testing at normal operating voltage in service.
Since the signal magnitude cannot be used by itself to assess the significance of the detected signal, condition levels have been assigned to the signal information. A description of each level is provided below.
Level 1: The system is not degraded. No action needs to be taken.
Level 2: There is a small amount of aging related signals in joints and terminations. This amount of signal is normal and thus, no action needs to be taken. However, in extruded cables, retesting is recommended within the next two years.
Level 3: The system has a low probability of failure within the next two years. Consider retesting at a one-year interval.
Level 4: The system has a medium probability of failure within the next two years. Consider replacement.