t = C . N
P . K I . K U
2. For 3-phase meters, when testing each element, the power input to each element is equal to:
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In there:
P P
n n
P- Total circuit power
n- Number of elements in the meter
From there, we can deduce how to calculate the nominal standard time for each element in the meter. It is important to note that when testing each element, the current is applied to that element while the voltage must still be sufficient at all elements.
3. Corresponding to the rated load capacity, the divisions on the wattmeter scale may not be even. In this case, to avoid parallax error, it is allowed to increase the current to achieve an even value of the wattmeter and thus the standard time must be recalculated corresponding to the rounded capacity.
- Example: Calculate the rounded power value in the following case: 1-phase electric meter 220V, 5 A
Watt meter has parameters:
Voltage scale 240 V Current scale 5 A
Total number of wattmeter scale divisions 150 divisions
a. Rated power input to the meter at power factor C os =1
P n =UI C os =1 =220.5.1=1100 (W)
b. Wattmeter division value
Cw= 240.5 8 (W/rev)
150
c. Wattmeter reading corresponds to rated power
W= P n
C w
1100 137.5
8
(line)
We round up to 140 marks.
In this case, the current must be increased to achieve a reading of 140 bars on the wattmeter scale and the standard time must be recalculated to correspond to the rounded rated power of 1120 W.
4. The number of revolutions of the meter disc should be selected as an even number and a set of 10 for counting, so that it will be convenient for calculation and implementation during testing at low load modes and at power factors other than 1.
When testing by the time power method, the meter error is calculated according to the following formula:
In there:
t 0 t
t
.100(%)
t 0 - Standard time calculated according to rated value.
t - Actual time counted on the measuring device corresponding to N rotations of the meter disc.
5. Due to the different notation of each country that produces the meter, when calculating the constant C of the meter, attention must be paid to the unit of measurement written on the meter dial.
To improve the productivity of testing, reduce the time for calculating and processing measurement results, the following data should be calculated in advance:
- The wattmeter readings correspond to the load values and power factors that need to be checked.
- The number of disk revolutions selected for testing must be chosen so that the time must be over 30s.
After determining the nominal standard time, the wattmeter reading corresponds to
100% load value at power factor C os =1. We continue to calculate the wattmeter reading
and the number of revolutions of the disc for other values of the load and at other coefficients of phase shift.
Example: Determine the error of an active power meter.
3 1 0 0 V ; 2 5 A ;1 K W h = 2 5 0 0
ring
To check, people use 2 wattmeters with the following scales:
- Voltage 150 V
- Current 5 A
Total number of scale divisions: 150 divisions
Wattmeter reading at the time of inspection W 1 = 34 lines; W 2 = 56 lines
The counted time of 20 rotations of the meter disc is 63.4 s.
a.Calculate the meter constant
C 3600.1000 1440 (WS/Rev)
2500
b. Calculate the division value of the watt meter
C 150.5 5
(W/line)
w150
c.Nominal standard time
t 0 =
C . N
1440.20
64( S )
C w (W 1 +W 2 ) (34 56).5
d. Meter error
t 0 t .100 64 63.4 .100 0.94%
t 63, 4
II. Direct comparison method with standard meter
This is a simple and easy method to implement and has many advantages, the most prominent of which is eliminating the impact of unstable power sources. However, in our country, due to the small number of standard meters, the application of this method is not yet popular. During the implementation process, attention must be paid to the phase order of the power supply.
meter specified in QTKĐ 19-1994. While checking the starting time and ending time, the number of revolutions of both the standard meter and the test meter must match exactly. In the process of calculating the error of the test meter, the error of the standard meter in the load values must be noted.
Determine the error of the meter according to the formula:
C . NC 0 . N 0 .100(%)
C 0 . N 0
In there:
C and C 0 - constants of the test meter and the standard meter
N and N0 - the number of revolutions of the dial of the meter and the standard meter. If the standard meter is electromechanical, to simplify the calculation of the error, the above formula can be shortened to:
100 A 0 . N .100(%) ( A 0 . N ).100(%)
In there :
A . N 0
A . N 0
A and A 0 - Number of revolutions of the test meter and standard meter corresponding to 1KWh.
N and N0 - number of revolutions of the test meter and standard meter, countable
during the test
In case it is necessary to extend the measuring limit of the standard meter, it is necessary to use the standard T 1 with higher accuracy level, or use the T 1 equipped in the meter test table. The way of the meter head on the test table is similar to the watt meter head. But the error formula will be:
In there :
(
A 0 . NA . N 0 . K T 1
1).100(%)
A and A 0 ; N and N 0 – same as above K T1 – Transformation coefficient T 1
Example 1: Determine the error of a 3-phase meter measuring active power with the following technical parameters:
3 100 V ; 2 5 A
Test meter: 1KWh=1750 turns Standard meter: 1KWh=1000 turns
Number of revolutions of the meter disc recorded during the meter check period: 10 revolutions
Standard meter: 5.73 rounds Error of test meter;
( 1000.10
1750.5, 73
1).100(%) 0.27(%)
Example 2: Determine the error of a 3-phase meter measuring reactive power with the following technical parameters:
3 100 V ;3 5 A
Meter test: 1KVARh=2500 cycles
Standard meter: 1KVARh=750 cycles
Number of revolutions of the meter disc recorded during the test period: Test meter: 20 revolutions
Standard meter: 6.05 rounds Error of test meter:
(
750.20
2500.6, 05
1).100 0.83(%)
When using electromechanical standard meters, the purpose is to simplify the calculation and error handling work, increase the productivity of testing and correction. Based on knowing in advance the technical parameters of the testing meter and the standard meter, we can create a pre-calculated table for all types of meters; corresponding to a number of rotations of the testing meter that have been set and based on its accuracy level,
Calculate in advance the limited range of the standard meter's number of revolutions, corresponding to a set number of revolutions of the test meter, and based on its accuracy level, calculate in advance the limited range of the standard meter's number of revolutions, in which the test meter will achieve the accuracy level. The method is as follows:
1. Calculate the number of revolutions of the standard meter when the test meter rotates N revolutions and has an error of 0.
A 0 . N 0
A . N . K
1 .100
Hence:
0 TI
N A 0 . N
TI
0 A . K
2. Based on the accuracy level, calculate the allowable error limit of the meter through the number of revolutions of the standard meter:
N 0.
A 0. N .
A . K
TI
In which - accuracy level of the meter
3. Pre-calculate the range of rotation limits of the standard meter so that the test meter achieves the correct level.
For example: A single-phase meter with accuracy class 2 has the following parameters: U=220V, I=10A, 1KWh=600 turns
Standard meter with accuracy class 0.6 has the following technical parameters:
U=220V, I=5A, 1 KWh=900 cycles
a. Calculate N 0 when the number of revolutions of the meter is 10 revolutions and has an error of 0. Choose K TI = 2.
N 900.10 7.5 (turns)
0 600.1
b. Calculate the allowable error limit corresponding to the accuracy level of the meter 2
7, 5.2% 0.15 (round)
c. Calculate the range of the standard meter's rotation limit for the meter to be tested.
precision level 2
N 0 (7.5 015) turns
Thus, when testing the above meter at 10 revolutions, for the meter to reach accuracy level 2, the number of revolutions of the standard meter must be within the limit of 7.35 to 7.65 revolutions.
III. Long-term loading method
This method is often applied at inspection stations or test points to check a large number of meters at the same time. With this method, the standard for testing can be a standard meter or a meter with small error. If the power source is stable, a standard wattmeter and stopwatch can be used.
All standard meters are hung on the test table and energized for a long time, tested in turn at different load modes, at each load mode, the index on each set of test meter numbers corresponding to the beginning and end of the current time. The current time at each load mode can be arbitrarily long but is enough for the smallest engraved drum of the set of numbers to rotate 2 times.
Based on the amount of electricity recorded, calculate the meter error according to the formula:
In there:
WW 0 .100
0
W 0
W- electric energy recorded by the meter
W 0 - electric energy recorded by the standard meter or equal to Pt if using the standard of wattmeter and stopwatch.
0- standard meter error.
CONCLUSION
To complete this graduation project, I am extremely grateful for the dedicated guidance of teacher Chu Dinh Khiet and all teachers and friends.
This is the first time I have been exposed to and familiarized with design research. Due to limited knowledge and experience, there are still many shortcomings in the design calculation process. Therefore, I really hope to receive guidance and help from the teachers who approved the design topic and all the teachers in the defense council.