Transmission and distribution grids are only as reliable as their weakest component. Changes in electricity demand, the effects of deregulation, aging infrastructure, the changing workforce, and business economics combine to cause an increased possibility of equipment failure.
Transformers are a key component for efficient and reliable power supply. Many transformers are equipped with a “mechanical switching device” that regulates voltage output. On-load tap changers (LTCs) have been used for decades, and a significant number of old units are still in use. Retrofitting to a transformer is not easy, and is costly. Mechanical maintenance is critical. Studies have shown that approximately one-third of transformer critical failures are the result of failed tap changers.
There are many methods of evaluating LTC condition, calculating useful life, and predicting the need for maintenance. Some plan maintenance on the basis of: “Tell me today what will fail tomorrow.” A combination of on-line and off-line methods can be used to help make this call. Off-line tests typically look at longer-term trends. On-line methods typically monitor and report short-term changes, and in some cases are useful in trending information. We will cover some new on-line monitoring options in this article. There are new mechanical parameters that can be easily reported via intelligent electronic devices (IEDs) and can be used in addition to other analytical methods.
There are many industry-wide performance factors that can be grouped into a few categories. These factors, when combined, help to determine the state of any equipment in the present and the future. Our goal is to identify the factors and especially the combination of factors, that can permit either safe continued operation or catastrophe.
Performance factor categories are defined as follows:
1. Population based, including:
- An older equipment profile;
- Equipment operated counter to design;
- Equipment beyond life projections; and,
- Equipment not properly maintained or upgraded in the past.
2. People based, including:
- Limited numbers of technical and maintenance staffing;
- Loss of technical experience;
- Complexity of the enterprise;
- Limited ability to analyze reams of data; and,
- Loss of knowledge during more frequent personnel turnover.
3. Design based (on newer units), including:
- “Value engineering”;
- Faster deliveries;
- Factory loading:
- Competitive pricing; and,
- Spec variations.
4. Operations based, including:
- Load over time;
- Load profile/increased switching;
- External operating conditions; and,
- Design vs. actual operating conditions.
5. Maintenance based, including:
- Outage time;
- Available Parts;
- Cost; and,
Being aware of the above is the first step in tackling the problem. The next step is resource allocation. At this point, the discussion should lead to necessary information, data collection methods, and the means to interpret.
Data collection can be limited or extensive depending on the equipment, the available technologies on-line and off-line, and the enterprise management system. You will only achieve positive results by selecting the right data and getting to the correct interpretation. A failure in any of the three steps leads to no gain, missed opportunities, and wasted resources (both financial and personnel). So the issue becomes one of choosing the right parameters. This usually involves a series of on-line diagnostics, inspections, off-line testing, operating history, design criteria, operating conditions and people experience. Algorithms and statistics are used to integrate.
Based on the complex process, inputs must be provided in a cost-effective and easy-to-interpret format. Data must be easily integrated, and useful to the methodology chosen. There is no one right way, but several that can yield similar results if chosen correctly. Documentation is imperative. Only 9 percent of companies have 100 percent documentation of maintenance procedures and less have reliable condition assessment records. Fifty-six percent of companies have no plan to capture people knowledge. Where do you stand?
In light of this, real-time analysis is important.
The goal in using an IED is to better pinpoint mechanical, electrical or control conditions, so that maintenance can be efficiently scheduled and not just timed, maintenance costs reduced, outage time minimized, and replacement postponed safely. Although time-based maintenance planning has a role in the power industry, studies have shown that 70 percent to 80 percent of substation equipment failures are still random. For example, line disturbances and lightning can cause more than 40 percent of failures. This occurs regardless of off-line analysis and condition assessments.
Improved business decisions can be made by using appropriate on-line analyzers and monitors. Deployment cost must be considered, including equipment installation and maintenance. Because of changes in the workforce, an IED should be simple to install, program, read, and integrate into the network.
An LTC is a complicated electro-mechanical switch that handles high current and voltage. Regardless of design, they depend on moving contacts and a mechanical mechanism. They are similar to switchgear/circuit breakers, and in most cases see more frequent use. Like other electrical apparatus, they cannot be inspected while on-line.
Typical failures are severe. Federal Pacific, Siemens, McGraw-Cooper and other common units cost between $25,000 and $50,000 for repairs. Retrofits are difficult and can cost upward of $100,000, plus labor and downtime. Availability of replacements and parts must be considered.
Failure mechanisms are generally grouped as:
- Erosion of Contacts (even on vacuum units)
- Contact Coking (leading to high resistance and overheating)
System operating conditions can cause increased usage and premature wear. Tap position history is often used as part of transformer hot-spot temperature calculations.
Level 2 Transformer Assessment includes loading calculations and functional testing of LTCs.
Utilizing a tap position sensor/receiver system for additional mechanical monitoring makes sense. The cost of the monitor, installation, programming and data acquisition are made easier by utilizing an existing piece of equipment. An enhanced position monitor can be justified on new installations, as well as upgrading existing units.
From a mechanical standpoint, LTCs will wear based on:
- Number of operations
- Alignment of contacts
- Contact integrity
- Dielectric quality
- Improper Control
- Mechanical Mis-alignment
- Improper or incomplete maintenance
These factors can be measured using several types of equipment. For example, INCON’s revised 1250B, the new 1250-LTC, can monitor several conditions by using the existing 1292 synchro transmitter to a greater extent.
This unit provides:
1. Absolute Position Feedback
By direct connection to the drive mechanism, the signal is always generated based on mechanical position and not other electrical signals or processor outputs. There is no deviation between actual and reported local or remote position indication. The converted synchro output is used for local and remote display, as well as for SCADA control feedback. Future voltage regulation changes can be better sequenced, and available remaining range of operation calculated. Tap changer performance and transformer condition algorithms can use the information. In addition the system will add data to onboard storage registers.
2. Tap Change Displacement
After updating LTC mechanical position, it registers the actual position of the LTC mechanism within the arc range associated with each tap. A deviation report, by exception, is generated, which shows the maximum deviation from centerline for each tap.
Based on this reporting (1/10 degree accuracy), drive mechanism performance, mechanical wear, braking performance, and post maintenance reset accuracy can be checked. Proper alignment leads to proper positioning of contacts. This in turn leads to reduced contact wear. Figure 1, shown on p. 36, is a pictorial.
Alarm settings can be made in 1/10th degree increments, depending on the unit. “Landings” within the Guard Band are OK. Variations greater than that can be alarmed as exceptions. Updates are registered each time the LTC moves to a new tap.
3. The Total Number of Tap Changes
In addition to other methods, including mechanical registers, the onboard IED register can be programmed to alarm, and to be polled locally or remotely, along with other maintenance information. The IED provides flexible communication methods to independently access this operating history for service and maintenance scheduling.
4. Number of Consecutive Tap Changes in One Direction
The IED registers the number of tap changes sequentially, in one direction. A limit can be set, to stop further movement in one direction, and requiring the LTC to reverse direction at least one step before continuing in the original direction.
This prevents transformer “run out”, and can help keep the transformer running in the efficient range, and with cooler transformer coil temperatures. It can also provide system controllers the opportunity to “search” for other regulated units to make the next adjustment. With automatic one step reset, the transformer can be moved in the original direction manually or automatically.
5. Number of Tap Changes “Up or Down to” Each Tap
The IED breaks down the total tap count, into the number of times a tap is reached from either direction. Registers are maintained for each side of each tap.
Contact wear can be better estimated on each tap based on the number of times that set of contacts are utilized. Just-in-time prediction of the need to replace a set or sets of high usage contacts can be made. This reduces the cost of spare parts and labor, as well as downtime. If there is uniform “wear” the LTC can be kept on-line longer.
6. Days Since First or Last Change to Highest or Lowest Tap
The IED provides a user-defined time stamp to identify the number of days, in 1/10 day increments, since the draghand hit high or low maximum deviation from neutral.
This aids in trouble shooting system run out conditions, control problems, and system control capability. The need to review reams of operating data to find “root cause” is eliminated, since the IED provides the days since it occurred. Based on the analysis, system control can be modified to improve equipment and system performance.
7. Number of Days Since Passing through Neutral
The IED can determine when the LTC leaves a position on one side of neutral, passes through one or more neutral taps and lands on a tap on the other side. The processor resets the time clock when this occurs.
This feature confirms that the reversing switches are exercised frequently to prevent coking buildup, contact wear, sticking or other performance issues. The unit can alarm if the time limit is reached. Via remote automation, manual remote control or local control, the LTC can be moved to clear the alarm and reset the clock.
8. Unstable or Loss of Transmitter Signal
Via mathematical algorithms, signal stability can be determined. An unstable signal can be caused by loose wiring, faulty components, power supply conditions or mechanical instability/vibration. The IED can report this condition.
The benefit is improved reliability of the position signal, and associated systems that depend on this signal. Mechanical vibration may be an indication of a problematic LTC condition.
9. Momentary Tap Change Relay Acknowledgement
The IED can be programmed to trigger a programmable momentary relay. The relay trigger time can have a programmed delay start time, and a programmable duration time.
This relay function can be used for absolute mechanical motion confirmation. Signals can be sent to RTUs for integration into maintenance and operating algorithms, as well as confirmation to controllers that commands have been implemented. It eliminates the need for separate switching devices commonly required on some control systems.
Seven of these nine features can be alarmed. Other than position, information is provided by exception, as opposed to a stream of constant data. The IED registers the data, and stores it for future polling set by user choice. The parameters are presented in an easy to read summary as shown in Figure 2.
Figure 2. Serial Data Dump Example
By the proper selection of on-line and off-line technologies, you can achieve:
- Reporting of critical LTC mechanical data;
- Reduced and shorter outages;
- More operations without an outage;
- Reduced maintenance and replacement cost;
- Increased reliability and extended life;
- Deferred replacement; and,
- Increased profitability
Consider the total cost of any option:
- IED Equipment cost
- Installation cost, including peripherals
- Instrument calibration, consumables and Maintenance costs
- Flexibility of data flow
- Ease of data interpretation
- Warranty and supplier’s customer support
The Ultimate goal is peace of mind.
About the Author: Anton G. Wagner is Business Unit Manager at INCON (Intelligent Controls).