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