3- laboratory work. Current transformers The purpose of the work : to study the basic information about current transformers, the vector diagram of the measuring current transformer, various types of constructions of current transformers (TT) and to conduct tests provided for in the technical operation rules. . Characteristics and vector diagram of current transformers Table 1 - accuracy classes and fields of application of TT Primary Accuracy current classes 0.2 0.5 1 %n 10 Tok angle ( ) according to ……. %() Field of application refuse 5 0.75 30 0.9 20 0.35 15 0.45 100-120 0.20 10 0.3 5 1.5 90 2.7 20 0.75 45 1.35 100-120 0.5 30 0.9 5 3.0 180 5.4 20 1.5 90 2.7 100-120 1.0 60 1.8 3 5 Margins of error 3.0 50-120 Energy and strength exact measurements (exact laboratory controls) Energy and power accurate measurement; 1st class gauges Energy and power measure; relay, 1st class counters Ammeters, pele, gauges connect 5.0 10.0 Class 0.2 current transformers are used for accurate measurements, inspections and research, they are equipped with electrical laboratories of power plants. Current transformers of classes 0.5 and 1 are installed in distribution facilities Class 3-10 current transformers are used for relay protection circuits, automation, where an error of 3% and above is allowed. In some schemes of relay protection and automation, special designs of TT, 1 for example, TTs with D magnetic core for differential protection and any other protection devices or P magnetic core TTs for relay protection are used. The accuracy class of TT depends significantly on the load on the secondary circuit. TT load is cos φ 2 or secondary at power S 2 and rated current I2 determined by the impedance of the circuit Taking the current as I 2 =5A, we get the following expression: The primary current also affects the accuracy of the ITT. The nominal accuracy class (see Table 1) is observed only at the limits of the primary current equal to 100-120% I1n specified by GOST. Figure 1 shows the TT connection diagrams. Figure 1. Connection diagrams of TT: a - full star scheme; b - incomplete star scheme; c - the difference between the two phase currents of the relay; d - the relay is connected to a three-transformer zero-sequence current filter Preparation for work 1 Study the modes of operation of TT from the necessary literature. 2 Prepare all necessary figures and tables. 3 Answer the control questions orally. 2 Order of work Enter the passport data of current transformers studied in the laboratory in Table 2. Table 2 - TT passport information Type Type Nominal voltage (kV) Nominal primary voltage (kV) 1 2 3 4 1 2 3 4 5 6 7 8 9 TPL-10 TZ TPFM-10 TNSHL-0.5 TKF-3 TZR UTT-6 I-54 UTT-5 No p/p For accuracy classes rated capacities 5 0.5 6 1 7 3 TT determining the coefficient of transformation. To determine the transformation coefficient TT, it is necessary to assemble the circuit according to Fig. 2. TT the transformation coefficient is checked for different primary currents (5-6 values). Readings are taken after 1 A, up to the limit value of the ammeter measurement of 5A. Enter measurement data in Table 3. 3 8 10 Figure 2 K - key; LATR - laboratory autotransformer RPO-250; T1 - TT of type TPFM10 or TPL-10; T2 - standard TT; A 1 and A 0 - ammeters 0 - 5 A Table 3. Measurement data (A) (A) T T is the transformation coefficient and secondary It is determined by the ratio of the currents in the coils of its primary Reading the magnetization characteristic of TT The magnetizing current of TT depends on the secondary circuit load e.yu.k. increases with increasing (Fig. 3). In the saturated part of its characteristic, the accuracy of TT decreases sharply. The availability of the magnetization characteristic or the voltampere characteristic is the basis for assessing the operability of the transformer. 4 Figure 3. Magnetization characteristic of TT TT assemble the circuit shown in Figure 4 to obtain the magnetization characteristic Figure 4. TT magnetization characteristic acquisition scheme Set the adjustment of the laboratory autotransformer to the zero position by turning it to the left. Run the circuit and record the voltage U 2 and the current I 0 in the secondary circuit of TT. Enter the measurement results in Table 4. 4 - Table CT magnetization properties (A) (A) 5 Based on the obtained results, construct the TT magnetization characteristic. 2.5 The report shall include: 1. Purpose of work. 2. Test plots, results and obtained graphs. 3. Analysis of the received data. Control questions 1. What is the function of TTs? 2. Connection schemes of TTs. 3. Why can't TT's secondary circuit be left open? 4. Errors of TTs. 6