The costliest piece of equipment in a substation is the Power Transformer. Therefore, a good maintenance program is required to keep these power transformers operating reliable. Over the past few years, the industry has observed a significant increase in lead times for new power transformers. Therefore, the maintenance and programs need to move from good to great. Communication is the key to make this happen.
In the past, a long list of tasks was thrown out to the test and maintenance crews. There was plenty of time and money for everything to happen. Now, in today’s world where outages have grown shorter and expertise has been lost, everyone needs to evaluate and agree on the scope of work for the given time and budget. There is not time to argue once the outage takes place or complain a task was not performed once the transformer is ready to be reconnected.
Communications is Key
The communication needs to happen up front. Communication will give a general understanding of Time allotted, Technical Requirements, and Budget Restraints, between all parties involved. To facilitate the effort, some type of work sheet is needed for documentation for the testing request and approval.
A work sheet allows Planners, Managers, Engineers, Test Technicians to budget and plan so the program achieves the maximum reliability at the lowest cost and highest availability for the transformer asset.
The Testing Program
A few test scenarios include: Initial test, Routine test (long outage – no budget restraints),
Routine test (Short timeframe and/or Budget constraints), Test after a trip or after a through fault. Since switching a transformer out of service may cause pressure on the power grid, it is important to look at other ways to determine the need for testing. Three examples include Thermography, Dissolved Gas-in-oil (DGA) and Oil Quality. Thermography is great for checking the cooling system and hot bushing terminal connections. Under the right conditions, the oil level of the transformer and bushings can be observed.
DGA is a great indicator of electrical fault type issues. Oil Quality test will spot issues like moisture ingress that will age the transformer prematurely. Once these online diagnostics have been performed, then the question that needs to be ask: “Are there other reasons to switch the transformer out of service?”. If the answer is just saying “The routine maintenance interval is about to expire”. Remember, switching the transformer out of service is costly and time consuming. This possibly could be avoided with good communication.
For Example, if there happens to be an outage or low load period, can the transformer have an abbreviated routine test before the routine maintenance interval expires. This could reset the routine test interval, save time and money.
Key Questions to Consider
- What is the reason for transformer testing?
- Unit Trip?
- Abnormal operating values (temperature, current, voltage?)
- Which test can we perform before taking the unit offline?
- What is the allocated schedule to return to service?
- What personnel will be involved in testing?
- Which offline test are critical to give us the needed information for a return to service?
An abbreviated routine test could be:
- DC insulation resistance
- DC resistance on Set Tap only for the (DETC) if the DGA shows no gassing issues
- Demagnetize the transformer to reduce inrush current when place back in service
- (Skip the Turn Ratio test – Without having a fault, how can the turns ratio change?)
- Insulation Power Factor
- (Skip C2 testing on bushings below 100kV)
- (Skip Hot Collar if the bushing’s level indicator shows normal oil level)
- (Skip the Lightning Arrester test – Infrared the arresters once back in service at nominal voltage)
- (Skip the Leakage Reactance test – Without having a fault, how can the impedance change?)
- (Skip the SFRA test – Without having a fault, how can the windings shift?)
- (Skip the DFR test – This is a Specialized Test)
Brief Over View / Summary of Each Electrical Test
Insulation Resistance
- 250 – 2500 VDC – Not to exceed the Windings Rating – i.e. Megger Test
- Quick Go / No Go test – Rule of thumb: Absolute Minimum of 1 Meg Ohm / kV
- Usually, the windings are up in the Hundreds of Mega Ohms or higher
Winding Resistance
- A DC current is injected through the specimen and the voltage is measured across the specimen
- Resistance equals dividing the voltage across the two voltage leads by the current
- Resistance must be less than +/-5% from factory test or +/- 5% of the average of all three phases
Winding Ratio
- Inject an AC voltage (typically 8 – 160 VAC) on the higher voltage winding
- Measure the AC voltage on the lower voltage winding to calculate the ratio Polarity is given by the test set
Insulation Power Factor
- Voltage (1kV – 10kVAC) is applied to the test specimen
- The test set calculates the power factor
- Modern test sets contain limit files for most major components and reports its condition
- Good power factors are generally less than 0.5%
- Deteriorated power factors are generally less than 1.0%
- However, initial test and trends are key drivers in the actual ratings reported
Leakage Reactance
- In general, this value should be similar to the transformer’s impedance
Sweep Frequency Analysis – SFRA
- Plots the excitation over a wide frequency range
- The purpose is to show defects like a buckled winding or coil deformation
- Very valuable as a base line and after a hard through fault when damage could have occurred
Dielectric Frequency Response – DFR
- Similar to performing a power factor test over a wide frequency range
- Can point out if high power factor test results are more related to moisture or other type of contamination
- Requires advanced analysis to truly understand the results
Key Actions to Consider
- Provide finalized testing activities to involved maintenance personnel
- Communicate baseline schedule to maintenance personnel
- Identify key role players who will decide to return unit to service or not after testing
Conclusion
A good Transformer Test Program can be transformed into a great Test Program with good communication. The sheet provided above is a starting point with a few extra notes and cautions to guide and track the process.
Mark B. Goff, MS EE PE ([email protected]) is a member of the T&D World Executive Insights Board and is responsible for multiple technical publications. Goff's experience spans a 34-year career at Tennessee Valley Authority (TVA). from field test engineer, systems engineer through to manager of the electrical energy conversion group in generation engineering, Goff built an effective transformer, breaker and infrared test program and maintenance support group. He currently is an engineering consultant for TVA’s Power Service Shop.
Cassandra H. Goff, MS EE PE presently works for Qualus as a relay protection engineer. Prior to joining Qualus, she was manager of the electrical balance of plant, generation engineering at TVA. For 10 of her 26-year TVA career, she was a relay protection engineer. She also has more than 14 years of experience working as a system engineer for the TVA transmission organization, working to develop an effective relay maintenance program and building talent to support the program. Also, she was an adjunct professor in the electrical engineering graduate program for the University of Tennessee at Chattanooga.
Collin Jay Rosson, PE, currently works for TVA at the Power Service Shops serving as a Production Engineer in the Transformer Services group. Jay has previously worked in the electrical design/construction field. Primary experience includes designing and implementing internal power distribution systems for chemical plants and pulp and paper mills. Jay is a registered Engineer in Training (EIT) and currently pursuing his Professional Engineering (PE) license.
