Power transformers are an essential part of bulk power transmission, allowing high capacity lines and cables to delivery energy at voltages used by commercial and residential customers. Transformer ratings are a critical part of evaluating the performance of a power transformer.
Some questions often asked during projects to assess transformer ratings are:
Q1 - How long does it take a power transformer to heat up once a high electrical load is applied?
The oil, core, case, and windings of most power transformers are massive and the cooling fluid may take several hours to reach a new steady elevated temperature after an increase in electrical loading. However, certain portions of the oil in ducts and the hottest parts of the winding itself may reach elevated temperatures relative to the cooling fluid in no more than a few minutes. Thus, because the hottest part of the winding usually determines both insulation degradation rates and the possibility of forming gas bubbles that might lead to dielectric failure, the rate at which the hot spot temperature increases is initially rapid followed by a slow increase as the fluid heats up.
Q2 - When a power transformer with multiple cooling modes is to be considered, do I need to enter data for all the cooling modes or is only the maximum cooling mode of concern?
It has been argued that thermal ratings need only be calculated for the maximum cooling mode since, if the transformer load gets near the transformer rating it will be running in the maximum cooling mode. However, running the program all the time in the maximum cooling mode will yield both hot spot and oil temperatures which are lower than actually occur under light load conditions and PTLOAD V5.0 will overestimate short time transient ratings possibly leading to an actual over-temperature occurrence.
Q3 - Where do the temperature rise parameters required in PTLOADV5.0 come from?
Transformer parameters such as the “temperature rise of top oil over ambient temperature at rated load” can normally be obtained from “heat run” sheets supplied by the manufacturer. Certain parameters such as the top oil and winding rise exponents, m and n, respectively, are supplied automatically by PTLOAD V5.0. Given measurements of top oil, simulated or directed measured hot spot temperature as a function of load and air temperature over an extended period of time, it may be possible to refine the parameters by parameter fitting as described in the PTLOAD V5.0 final report.
Q4 - EPRI has another program for transformer thermal ratings called DTCR which allows the real-time rating of power transformers. How is that different from PTLOAD V5.0?
There are three different PTLOAD thermal rating programs available from EPRI. PTLOAD V5 is able to perform traditional 24 hour rating “design” calculations which transformer engineers have used for many years to estimate peak load limits. It is also able, in the PLAN mode, to perform repetitive thermal rating for periods of weeks, months, or years using historical load and air temperature data files. This allows the user to study the frequency of occurrence of high transformer temperatures and the cumulative loss of insulation life for extended periods including the option of studying load growth effects. Finally, the PTLOAD module in DTCR calculates transformer ratings in real time based on continuous updates of air temperature and electrical loading. All the modules are capable of modeling the transformer using a traditional top oil model or the new bottom oil model both of which are documented in IEEE Standard 57.91-1995. Once the transformer parameters have been entered in one of the programs, the data can easily be transferred to the others.
Q5 - What is the advantage of running in real time?
Off-line calculations are based on maximum air temperature and load cycle data. Real-time rating calculations are based on actual loading and air temperature data and thus are typically higher than off-line rating.
Also, because of their large mass, power transformers have considerable heat storage capacity. Limited time thermal ratings calculated off-line typically assume conservative (i.e. high) pre-load conditions. Real-time thermal rating calculations use actual pre-load levels that are usually lower and thus real-time “limited time” ratings are usually higher than off-line ratings.
Q6 – What if I don’t know the parameter “hot spot rise over top
oil”?
This parameter is essential to the “top oil” calculation. It may be obtained from the power transformer manufacturer. If such data is not available, PTLOAD V5.0 provides a choice of two default calculations. The IEEE Standard method is consistent with IEEE Standard C57.91-1995 and is usually more conservative than the PTLOAD V4.1 method as suggested by Bill McNutt.
For additional information, please contact PDC's power transformer specialist, Tim C. Raymond.