سایزبندی ترانسفورماتورهای قدرت در برنامه ریزی سیستم های قدرت
Abstract
1- Introduction
2- Benefit of a thermal criterion in planning
3- Thermal Criterion: Limits in practice
4- Linear thermal criterion
5- Conclusion Declaration of Competing Interest
Appendix A. Simplification of the thermal model
Appendix B. Linearization
Appendix C. Change of the considered thermal limit
References
Abstract
It has already been shown in the literature that power transformers may be more accurately sized by their thermal limits than by their rated power limit. In practice however, thermal limits are usually considered only in operations; but not at planning stage, where the more usual notion of rated power is used. This paper proposes a novel method to take into account (and benefit from) thermal limits directly at planning stage. This is made possible by quantifying separately the impact of each generator and load on the temperature of the distribution transformer. Decoupling the effect of individual generators and loads is achieved by linearizing and rewriting the analytical expression of the hot-spot transformer temperature. The practical value of the method is assessed using a real-world dataset, by estimating the increase in the hosting capacity of the considered transformer for additional generators and loads. Significant gains are obtained when the transformer is sized by generation, in particular when photovoltaic (PV) generators are involved.
Introduction
Distribution transformers and power (HV/MV) transformers are predominant assets in a distribution grid, and sizing them well is a crucial task for distribution system operators (DSOs). When replacing, reinforcing or adding a transformer, DSOs should find a compromise between the high capital cost of oversizing it, and the risks (in terms of endangering the reliability of electrical energy supply) of undersizing it.
The typical steps of a planning study aiming at sizing a transformer are the following. Some assumptions must first be made regarding future loading conditions, in order to generate a set of “extreme” scenarios for load and generation that will be used as stress-tests to size the transformer. For transformers sized by generation (not consumption), a typical example of such a scenario, currently used some DSOs, is to consider that all generators will output their rated power and that the load will reach its lowest possible value [1], while taking into account forecasts of the future evolution of load and generation.
After loading scenarios have been defined, one of the following two categories of methods may be used [2]:
• The first and simpler one is to simply define physical limits, typically on instantaneous active power but possibly on other criteria, and to size the equipment so that these limits are not violated in any of the stress-tests.
• And the second, more elaborate one, is to perform a so-called “lower cost optimization” that aims at finding an economical trade-off between a smaller transformer that will be heavily loaded and undergo accelerated aging and increased losses, and a larger one that will cost more but not suffer from increased aging and losses [3,4,15].