DC TOLERANCE OF CURRENT TRANSFORMERS AND ITS MEASUREMENT
Pavel Mlejnek, Petr Kaöpar · 2006
The measurement of energy is one of the most widesp read measurements in energetics. The modern electri city meters mostly use the current transformers for current sensing. Two m ain errors exist in the current transformers: the r atio error and the phase displacement. These errors indirectly depend on amp litude permeability of the ferromagnetic core. The ferromagnetic core may be exposed to the DC magnetic field in the real condit ion. It causes saturation of the core and consequen tly rapid decrease of the amplitude permeability. It is important to know wha t value of magnetic field causes this problem. This article deals with the definition of DC tolerance of current transformers and the method of its measurement. Finally nine sam ples of current transformers have been measured and their properties have been c ompared. The measurement of energy is one of the most wide- spread measurements in energetics. The electricity meters are installed in all households and in all building s that are connected to the distribution network. The modern e lec- tricity meters mostly use the current transformers (CT's) for current sensing. Two main errors exist in the c urrent transformers: the ratio error and the phase displac ement. These errors indirectly depend on amplitude permeab ility of the ferromagnetic core. The cores of the precisi on cur- rent transformers have amplitude permeability in th e order of tens of thousands to hundreds of thousands. This en- sures small ratio error and small phase displacemen t. On the other hand, even very small DC magnetization ca uses saturation of the core and consequently rapid degra dation of permeability and increasing the errors. One solution of this problem with the DC magnetiza- tion is using larger ferromagnetic core. Thus great er pri- mary current is needed to reach the same saturation inten- sity of magnetic field but large ferromagnetic core is more expensive and enlarges the proportions of the elect ricity meters. The smarter way of increasing the DC tolera nce is using ferromagnetic core with lower amplitude perme abil- ity, linear and very narrow hysteresis loop. Then s atura- tion intensity is moved to higher values. The drawb ack of this approach is a small increase of the ratio erro r and higher (but almost constant) phase displacement tha t can be compensated by the external electronic circuits. This article is not focused on the ratio error and phase displacement as such (see eg. (3,4)) but it deals w ith great error caused by DC component of measured current. This current occurs when the low resistance (eg. block h eater) is powered through a diode.