{"id":121,"date":"2019-03-21T10:43:14","date_gmt":"2019-03-21T10:43:14","guid":{"rendered":"http:\/\/pb.ee.pw.edu.pl\/pb\/elap\/chapter\/__unknown__-2\/"},"modified":"2020-12-22T14:45:46","modified_gmt":"2020-12-22T14:45:46","slug":"__unknown__-2","status":"publish","type":"chapter","link":"http:\/\/pb.ee.pw.edu.pl\/pb\/elap\/chapter\/__unknown__-2\/","title":{"raw":"Exercise 5","rendered":"Exercise 5"},"content":{"raw":"
\r\n

MEASUREMENT OF ACCURACY LIMIT FACTOR OF CURRENT TRANSFORMERS<\/strong><\/p>\r\n

Introduction<\/strong><\/p>\r\n

The Current Transformer ( C<\/span>T)<\/span> is a type of \u201cinstrument transformer\u201d that is designed to produce an alternating current in its secondary winding which is proportional to the current being measured in its primary.<\/span> The principle of AC <\/span>CT is <\/span>based on <\/span>the magnetic coupling <\/span>principle. A typical <\/span>AC<\/span> CT consists of a toroidal ferr<\/span>omagnetic core, on which a copper wire of <\/span>N<\/span> turns is wound<\/span>. The CT has the bushing, rod type design. The primary winding consists of a firmly built-in <\/span>aluminium<\/span> or c<\/span>opper band, the end of which is finished either with a flag-shaped end terminal or with a terminating rod. The secondary winding consists of turns <\/span>N<\/em>. A typical AC CT arrangement is shown in Fig. 1. <\/span><\/p>\r\n

\"image\"<\/p>\r\n

Fig. 1. Basic circuit of the CT<\/span><\/p>\r\n

The conductor carrying the measured time-varying current<\/span> i<\/em>1<\/em><\/sub> acts as the primary of the current transformer. The toroid can be clamped around the current-carrying conductor. The <\/span>winding wound on the toroid<\/span> act<\/span>s as the<\/span> secondary of the <\/span>current transformer<\/span> i<\/em>2<\/em><\/sub>. The burden resistance <\/span>R<\/em>b<\/em><\/sub> is selected depending on the sensitivity th<\/span>at is required. For better <\/span>performance, current transformer cores are desired to have high permeability, high resistivity, low hysteresis and eddy current losses. The cores are made of a high gra<\/span>de <\/span>ferromagnetic alloys or magne<\/span>tically oriented transformer <\/span>sheets. On the outer side of <\/span>encapsulated core are wound <\/span>the secondary windings for output currents of 5 A or 1 A. All active parts of transformer are <\/span>encasted<\/span> into <\/span>e<\/span>poxi<\/span> resin.<\/span><\/p>\r\n

The current transformers are designed for normal operation, it means, the B-H curve dependence of iron core is linear. Under normal operation the difference between primary current <\/span>i<\/em>1<\/em><\/sub> and secondary current <\/span>i<\/em>2<\/em><\/sub> is given by their ratio and the magnetizing current can be neglected. The accuracy of CT in this case is very high, approximately 0.5%.<\/span><\/p>\r\n

Different situation is during short circuit conditions in power network, when the primary current of CT is several times higher than under normal operation. It causes the non-linear behavior of the current transformer. As it is known, the DC offset in the fault current following into protective core of current transformer can cause steel to saturate and produce a distorted secondary current. To know exactly a real primary current during fault operation is very important for the correct action of protection relays and also for the analysis focused on faults\u2019 identification and localization.<\/span> On Fig. 2 the real HV CT was shown.<\/span><\/p>\r\n

\"image\"<\/p>\r\n

Fig. 2. Example of a real HV CT<\/span><\/p>\r\n

Characterication<\/strong> of current transformers<\/strong><\/p>\r\n

An example of a nameplate of CT and its meaning was shown on Fig. 3. The nominal parameters can be found as:<\/span><\/p>\r\n\r\n