Trending is crucial. Condition monitoring of your electrical equipment provides the type of data that helps you anticipate and plan future maintenance activities.
By S. Frank Waterer, Schneider Electric USA, Inc.
Every facility manager should know what the monetary impact of an unplanned outage means to his/her respective operations. When an event occurs, however, the focus is usually on restoring power as fast as possible and at all costs. Often, the tangible and intangible costs are not accounted for during the event. The best way to avoid this financial impact is to reduce the risk of an unplanned outage, which requires time, effort, planning and the funding of an effective maintenance program.
A comprehensive maintenance and testing program should cover all of your electrical-power-distribution systems—regardless of manufacturer—to ensure that all equipment and components operate safely and reliably as originally designed and intended. The ultimate goal is to minimize equipment malfunction, power outages or interruptions to operations or service.
Approaches to electrical-equipment maintenance
Depending on the process, operating environment or age of equipment, companies may adopt one or more of the following maintenance strategies:
- Repair work conducted after a failure or breakdown, often completed as an emergency
- The most costly of all maintenance strategies
- Routinely performed maintenance, regardless of the equipment’s condition
- May be unnecessary, based upon condition of the equipment
- Specified list of inspections, cleaning, testing and part replacement
- Less expensive than reactive maintenance, but more costly than predictive maintenance
Predictive maintenance (or periodic diagnosis)…
- Scheduled, based on diagnostic evaluations
- Factors like equipment age, environmental stresses, criticality of equipment, utilization, etc., affect decisions concerning maintenance schedule
- Provides significant reduction in equipment-related incidents
- Least expensive of all maintenance strategies
An efficient maintenance strategy will identify the type and amount of maintenance to be performed as well as the frequency for maintenance activities on each piece of electrical-distribution equipment.
The value of predictive maintenance
Predictive maintenance (PdM) technologies enable companies to perform an effective amount of maintenance at an appropriate or practical time. Often referred to as condition-based maintenance, predictive maintenance tools monitor the conditions of in-service equipment, either continuously (online) or at periodic intervals. Since many predictive maintenance technologies are utilized on in-service equipment, disruptions to facility operations can be reduced. Having regular access to the current state of the equipment provides valuable information to determine when maintenance should be performed. From a cost-effective standpoint, it should take place before the equipment loses optimum performance.
Conditions that can be monitored on “in-service” electrical equipment include the following:
Specific PdM technologies
Power-system assessments. . .
Power-system assessments are often the first necessary inspections to be performed after the equipment was originally installed. Completed by professional electrical engineers trained in power-system analyses, power-system assessments provide for visual inspections of portions or all of the existing power-distribution system. Power-system assessments determine the present electrical and mechanical “health” of the electrical equipment and power-distribution system, and how long they will likely continue to function as originally designed and intended. Defects, deficiencies, deteriorations, hazards or weaknesses in existing electrical-power-distribution system installations are also identified as part of a power-system assessment.
Issues discovered as a result of the assessment are prioritized based upon one of four factors:
1. The safety hazard to electrical workers
2. The impact of the occurrence on key process elements
3. The probability of an occurrence
4. The ability to respond quickly to correct the negative effects of the occurrence (vulnerability)
Power-system assessments can be customized based upon the need to reduce risk to your facility from reliability issues, process disruptions, code violations and/or outdated workplace safety requirements.
Infrared (thermographic) inspections. . .
Infrared inspections use an infrared (IR) camera to detect anomalies not noticeable to the naked eye. In an electrical setting, infrared inspections identify hot spots that can be a precursor to equipment malfunction, which leads to unplanned downtime. Heat rise in electrical equipment can be the result of:
- Poor or loose electrical contacts or connections
- Unbalanced electrical loads
- Defective components
Having infrared inspections routinely performed may qualify a company for reduced insurance premiums.
Infrared Windows Enhance Safety
Since infrared inspections are performed while equipment is in service and under load, equipment covers typically need to be removed. Infrared windows installed in equipment panels permit permanent access for inspection of electrical components without disturbing operations or removing covers. These window are made of a glass-like material that is transparent to infrared rays and allows hot spots to be registered by a thermographic camera.
Online temperature monitoring. . .
Online temperature-monitoring technology provides 24/7 access to critical connection points where traditional thermography cannot be used. Continuous monitoring provides the means to evaluate the equipment’s current condition and detect abnormalities at an early stage. Conduction problems caused by loose connections or deterioration of contact surfaces result in a local temperature rise, which contributes to a deterioration of insulation properties and the reduction of the contact quality. Thermal runaways induced by conduction problems also deteriorate the insulating material and cause disruptive dielectric discharges, resulting in arcing faults.
How it works: During a planned outage, wireless temperature sensors are installed in low-voltage and medium-voltage equipment* in areas usually not accessible with an infrared camera. Utilizing wireless technology eliminates the need for special cables and provides lower installation costs than other types of online condition-monitoring equipment.
Temperature data is transmitted from the sensors to a nearby receiver via radio frequency signals. The receiver is connected to a computer via serial or Ethernet. The data can be collected, compiled, analyzed and reported by a dedicated software package or via a SCADA program with Modbus TCP connectivity. Desired or specific alarm set points can also be programmed.
(*Sensors can also be installed on equipment with high arc-flash ratings, allowing equipment condition to be monitored without a risk of danger to personnel or equipment.)
Insulating-fluid analysis. . .
This predictive maintenance technology measures the physical and chemical properties of insulating fluids within liquid-filled transformers and load tap changers. Since transformers represent a substantial investment in a company’s electrical-distribution system, they must be maintained properly to maximize their reliability and useful life. An insulating-fluid analysis can detect the breakdown of the internal insulating systems along with a variety of other potential issues. Common tests performed on electrical insulating fluids include:
- Clarity: Evaluates the clearness level and discoloration of insulating fluids and checks for suspended or settled particulates.
- Viscosity: Evaluates the density of the insulating fluid relative to the manufacturer’s product data sheets and inspects for breakdown or contamination
- Moisture Content: Increased moisture content can negatively affect the insulating properties of the insulating fluid and cause dielectric breakdown.
- Acid Number: As oil degrades, it produces charged by-products—including acids and hydroperoxides—that adversely affect the fluid’s insulating properties.
- Dielectric Strength: This is the maximum voltage that can be applied across the fluid without electrical breakdown or flashover. A substantial reduction in dielectric strength may indicate that the insulating fluid can no longer perform as intended.
- Power Factor: In transformers, a high power factor is an indicator that the insulating fluid has a significant power loss, usually caused by contaminants (water, oxidized oil or degrading cellulose paper).
- Dissolved Gas Analysis (DGA):Concentrations and ratios of certain gases in the insulating fluid may be indicators of operational problems with the transformer. Three DGA examples are:
- High levels of carbon monoxide in relation to other gases may be a symptom of thermal breakdown of cellulose paper.
- High hydrogen levels, in conjunction with methane, may point to a corona discharge in the transformer.
- Acetylene levels may indicate arcing within the transformer tank.
Possible issues can be discovered in time, and outages can potentially be avoided. In addition, an efficient approach to maintenance can be adopted and the optimum intervals determined for insulating-fluid processing, repairs or replacement.
Partial-discharge monitoring. . .
Partial discharge is a leading indicator of insulation breakdown and occurs in electrical equipment under high voltage stress, usually greater than 2000 V. Higher system voltage and greater amounts of leakage current correspond to a higher potential for damages or downtime. Specifically, partial discharge is a localized electrical discharge in an insulation system that does not completely bridge the electrodes. As insulation systems age, they become more susceptible to breakdown. Continuous monitoring provides early warning alarms to help prevent extensive damage to electrical equipment and downtime before it occurs.
Technology is available to detect and notify facility engineers or maintenance and operations personnel of impending insulation breakdown. Partial-discharge monitoring is applicable on equipment rated 5kV to 500kV, where the age of the insulation could be cause for concern. Specific equipment applications include:
- Metalclad switchgear and switches: 5kV and 15kV
- Motors and generators > 2.4kV
- Large power transformers and bushings
- Substation yard structure-mounted voltage transformers and current transformers
- Generator step-up transformers and bushings
How it works: Sensors are installed on equipment during a planned outage. They are wired to a monitoring unit, which can be tied into an existing SCADA system via open protocol (Web hosting is also available). The sensors monitor:
- Magnitude (mV or pC) - Size or volume of the deficiency
- Pulse Count (PS) – Number or growth of deficiency
- Intensity/Power (mW) – Destructive power of the partial discharge events
- Signature – Both phase and type of deficiency
Monitored results are evaluated and compared against an existing database. Trending data helps anticipate problems so that engineering and operations personnel can plan future maintenance activities.
Circuit monitors (CM) record data relating to system voltages, cur-rent magnitudes and power consumption. CMs also offer a full range of power-quality features to help facility managers and engineers understand where and when dangerous and destructive transients, sags and swells occur. These include:
- Waveform capture and wave shape analysis
- Disturbance recording
- Disturbance direction detection
- Transient analysis
Some circuit-monitor models can provide data on other utilities—including gas, compressed air, water and steam—to help management control operating costs. Whether they are addressing operational or power quality, circuit monitors have the ability to diagnose potential problems and minimize downtime.
Intelligent protective devices
Circuit breakers. . .
- Electronic trip units or separate monitors can provide intelligent information regarding an individual circuit breaker’s status, which includes:
- General condition: Identification, position, number of operations, cumulative interrupted currents, integrity of interrupting medium for medium-voltage (SF6 or vacuum)
- Mechanical condition: Operating times, charging time, travel-time curve, excess closing energy, wear of contacts
- Control circuit/auxiliaries: Supply voltage, supervision of trip coil circuit or sensors circuit
- Provides advanced planning ability as “condition-based” activity instead of routine maintenance
Motor control centers. . .
- Intelligent motor starters allow cost-effective monitoring of motor current and power:
- Motor control centers are upgraded and networked into a centralized or distributed control scheme.
- Digital display can be mounted on the door to minimize the need to open/close.
- Motor run time can be easily tracked.
- Intelligent monitoring functions include:
- Current: Thermal capacity, line currents, average current, ground current, motor temperature (sensor) and current phase imbalance
- Voltage: Frequency, line-to-line voltage, line voltage imbalance
- Power: Active and reactive power, power factor, active and reactive energy
- Electrical-signature analysis can detect:
- Rotor-bar damage
- Stator electrical faults
- Defective bearings
- Maintenance can be scheduled on an as-needed basis.
Typical Identifiable Root Causes of Motor Control Center Outages:
- Power Quality
- Voltage and Current Waveforms
- Harmonic Voltage Factor
- Voltage THD
Benefits of a predictive maintenance program
Industry trends point to reduced maintenance staffs and budgets coupled with aging electrical equipment. Predictive maintenance technologies offer the types of cost-effective solutions that maximize reliability and increase workplace safety. Incorporating them into a company’s maintenance strategy enhance’s the operation’s ability to successfully:
- Determine the operational status of equipment
- Evaluate present condition of equipment
- Detect abnormal conditions in a timely manner
- Initiate actions to prevent possible forced outages
In short, monitoring your equipment condition and analyzing trending data to help anticipate and plan future maintenance will help you save time and money. MT
(Note: Electrical equipment should be installed, operated, serviced and maintained only by properly trained and qualified personnel.)
Frank Waterer is an Electrical Engineering Fellow with Schneider Electric USA, Inc. In addition to his many years in various engineering and R&D roles with Square D, he served, among other things, as Chair of the PES/IEEE Committee responsible for the design, development and installation of IEEE Standards relating to all surge-protective devices.