Instrument transformer dimensioning

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4th GCC CIGRE International Conference, Manama (Bahrain), 10.-12.11.2008 Conference Proceedings Report 06




Keywords: instrument transformers, current and potential transformer dimensioning, power system protection, secondary engineering,protection coordination

necessity of the new perspective for the instrument transformer dimensioning is accentuated.

1 Introduction
As the development of digital measurement and protection equipment has progressed over the last years, the criteria used for sizing the necessary instrument transformers have changed as well. Whereas in the past, due to the high burden of electromechanicalrelays, it was the rated power of the current transformers (CT) and potential transformers (PT) that was the crucial parameter. Nowadays, it is the transient performance of instrument transformers that has gradually become the over-riding influence within the digital world of relays, measuring and controlling devices. Firstly, due to paradigm change in the technology of the power system substations thetraditional usage of high VA-rated instrument transformers can become even dangerous both for themselves and for the secondary circuits and equipment connected to them. Secondly, the reduction of the switchgear dimensions, especially Gas Insulated Switchgear (GIS), leads to a reduction of the available instrument transformer compartments. That is the reason why the volume of the instrumenttransformers have to be optimized and adapted to the actual needs of modern measurement and protection equipment connected to them. This paper shows in structured form the state-of-the-art of the instrument transformer dimensioning. Thereby the physical behaviour and the standards regarding instrument transformers are shortly discussed and the

2 Current Behaviour



Inorder to understand the standards and give a background to them the physical behaviour of the current transformer must be shortly mentioned at first. The most important is the fact that a CT due to its physics always tries to draw such a secondary current Is through its secondary circuit that equalizes the magnetic flux Ψp or induction Bp excited by the primary current Ip (Figure 1). It means thateach current transformer is forced to introduce such a secondary current Is so that the secondary magnetic flux Ψs. linked with it equalizes at every point of time the primary flux Ψp.

Figure 1: Simplified equivalent of an ideal CT

Siemens AG, Energy Sector, Power Technologies International, Freyeslebenstrasse. 1, 91058 Erlangen, Germany

The primary core flux for sinusoidal quantities isgiven by eqation (1): Ip Φ p = w p ⋅ µ0 ⋅ µ r ⋅ ⋅ AFe (1) l Fe and the secondary core flux by eq. (2) respectively: I Φ s = ws ⋅ µ 0 ⋅ µ r ⋅ s ⋅ AFe (2) l Fe with

Φ = B ⋅ AFe (3) where B is magnetic flux density, AFe is the core crosssectional area, lFe is the mean length of magnetic path and wp, ws are the number of primary and secondary windings, respectively. For ideal conditions as shownsimplified in Figure 1, where winding resistance and leakage flux were totally neglected, one can write the equation for the core flux: Φ p − Φ s = Φ m = 0 or : Φ p / Φ s = 1 (4) Considering the relation in eq. (4) and using eq. (1) and eq. (2) one can write the following relation:
Ip Is = ws wp

the higher the primary current, the higher voltage must be induced to allow the secondary currentflow. In practice the construction of the CT for a simple design is close to the one presented in Figure 2, where the primary conductor is going symmetrically through the iron core. On this iron core there are windings wounded symmetrically over the core that build secondary winding of such CT-core. For the comparison to the simplified CT equivalent in Figure 1 the length of magnetic path lFe and...
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