NAME :
RIOUAL
COUNTRY :
FRANCE
REGISTRATION NUMBER :
GROUP REF. :A2
Pref. Subject : 3
QUESTION N° : 3.4
QUESTION 3.4: How to reduce the risk related to inrush currents when energizing transformers ?
SYNCHRONOUS CONTROLLERS FOR THE REDUCTION OF
INRUSH CURRENTS AT THE TRANSFORMER ENERGIZATION
Context
EDF R&D has performed many studies involving the saturation of the magnetic core of transformers;
in the seventies, the aim was mostly the specification of the protective devices for insulation
coordination purposes, the determination of the overvoltages and the inrush currents occurring at the
terminals of those transformers, located on the EHV network and also step-transformers belonging to
the Production Division of EDF.
Thirty years later, one of the main objectives for EDF, which is widespread among other electric
utilities, is to keep those equipment on the network and increase their lifetime, with the help, if needed,
of adapted devices in order to reduce the stresses on those equipment.
Parameters influencing the amplitude of the inrush currents
The inrush currents are mostly determined by the saturation characteristics of the iron core, generally
derived from the (U,I) curve, with its linear part starting from zero, a few points given by the
manufacturers in the knee, and ending with its last slope; this last parameter is of major importance, a
flat saturation curve leading to high inrush currents. This is especially the case of small transformers,
or large transformers when technological aspects (windings in parallel) lead to low values of the aircore reactance (in p.u.) [1]. For air-core reactances ranging between 0.1 and 0.2 p.u., inrush currents
may reach values up to 7 In, where In is the maximum value of the nominal current. Those values may
range between 3 In and 5 In for moderate air-core reactance values, typically 0.4 p.u. The actual
tendency of reducing the losses in the iron core laminations, may also lead to highly non linear
saturation curves.
The value of the residual fluxes circulating in the iron core before its energization is also an important
point to consider. Residual fluxes are generated at the de-energization of the transformer, prior its
energization, leading to high values when the transformer is suddenly de-energized both sides, a few
second between the opening of the primary and secondary sides respectively. The final value of the
residual fluxes mainly depend of the capacitances of the circuit-breaker which operates [2], leading to
high values up to 0.7 p.u when these capacitances are small which is the case of MV circuit-breaker
[3], down to 0.3 p.u, for EHV circuit-breaker; this is the case of step-up transformers of pumped
storage power plants as an example, and described in figure 1 below:.
NAME :
RIOUAL
COUNTRY :
FRANCE
REGISTRATION NUMBER :
GROUP REF. :A2
Pref. Subject : 3
QUESTION N° : 3.4
Fig 1: Evolution of the residual fluxes measured on site (straight line) and those determined by simulations
(dotted line).
High values of residual fluxes may lead to high inrush currents at the transformers energization, the
total flux at the energization being the sum of two terms, the initial residual flux value and a term
depending on the closing times of the circuit-breaker poles.
Impact of high inrush currents on transformers
High inrush currents may lead to severe electro-dynamic forces inside the transformer, leading to
displacements in the transformer; it may lead to some damage in the transformer, if the amplitude of
those stresses are high.
Another point to consider is the repetitive stresses on the equipment, as it may occur in pumped
storage power plants, when transformers are energized several times a day; it leads to thousands of
energizations during their lifetime in the networks, this point happening in aged networks, which is the
case of most Europeans and North-American utilities. In that context, which is in fact quite new, the
reduction of the stresses, even moderate ones, is an important point to consider in order to keep the
equipment on the network for a longer time.
Means for the reduction of the inrush currents
In the past, the use of pre-insertion resistors in the circuit-breakers has been used to reduce the
stresses; initially applied for the reduction of the amplitude of switching overvoltages, in a first step for
waves travelling along open-ended lines, they have been used in a second step for the reduction of
the stresses at the energization of transformers; a very good efficiency, with values close to the surge
impedance of the line equal to 300 ohms, has been observed for the reduction of steep fronted
overvoltages; however, higher values up to 1 Kohms and inserted for duration higher than 10 ms, may
unfortunately not be sufficient to absorb the energy dissipated during the energization, with additional
inrush currents appearing when disconnecting the resistor after its use.
The traditional palliative devices, having an impact on the reduction of the currents and overvoltages,
may consequently be not sufficient. For transformers, the use of synchronous controllers based on the
measurement of the residual fluxes, located in the control cabinet of the circuit-breaker and leading to
specific closing sequences of the breaker poles, may be completely adapted. It implies in particular the
knowledge of the residual fluxes, which can be determined by transient tools, those studies requiring
specific data on the saturation of the magnetic core of the transformer.
The use of synchronous controllers at the transformers energization, having an impact for the
limitation of the magnetic fluxes, may reduce the inrush currents significantly. This has been applied
for the reduction of the inrush currents in the case of a 250 MVA step-up transformer in a hydraulic
pumped storage plant connected the 400 kV network [4], as described in figure 2 below:
NAME :
RIOUAL
COUNTRY :
FRANCE
REGISTRATION NUMBER :
GROUP REF. :A2
Pref. Subject : 3
QUESTION N° : 3.4
Fig 2: Overvoltages and inrush currents generated at the energization of a no-load 250 MVA transformer, the
synchronous controller being installed.
Up to know, it is however mandatory to operate the circuit-breaker several times a year, in order to
warranty their performance in repeatability.
Conclusion
The reduction of inrush currents at the energization of transformers is a major issue in existing and
aged networks, as they may reach high values and lead to repetitive stresses for certain operating
conditions. The use of adapted palliatives devices, like synchronous controllers, lead to very low
values for the inrush currents, and also the overvoltages, due to the limitation of the magnetic flux in
the iron core at the energization.
References
[1] M. Rioual, C. Crepy, Y. Guillot
« Determination of the air-core reactance of transformers by analytical formulas for different topological
configurations and its comparison with an electromagnetical 3D approach; discussions ». (GM-978-1-4244-6/09)
IEEE General Meeting, Calgary, 24-29 July 2009
[2] M. Rioual, Y. Husianicia
“Determination of the residual fluxes when de-energizing a power transformer & Comparison with on site tests”,
IEEE Power Engineering Society General Meeting (IEEE Cat. N° 05CH37686)”, San Francisco, Juin 2006.
[3] M. Rioual, J.C. Reveret
"Determination of the phenomena involved when de-energization step-up transformers for wind-farms: modelling,
residual fluxes calculation and validation by on site tests", 8th Internal Workshop on “Large-Scale Integration of
Wind Power into Power Systems”, 14-15 October 2009
[4] M. Rioual, H. Digard, X. Legrand, L. Martin, H. Ito, Y. Corrodi
“Field application of a synchronous controller based on the measurements of the residual fluxes for the
energization of hydraulic step-up transformers”, IEEE General Meeting, Detroit, July 2011, Paper n°2011
GM1021.
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