subject: 110kv And Above Voltage Network Should Be Three-phase Four-wire Electric Energy Meter [print this page] Abstract: 110kV and above the past, the power grid have adopted three-phase three-wire electric energy meter measuring electricity, it compared three-phase four-wire electric energy meter, whether active or reactive power will have a greater measurement error, the following description for the above two measurement 110kV Why use three-phase four-wire power meter measurement.
Keywords: 110kV high-voltage three-phase four-wire electric energy meter network
In the past, and 110kV power grid is more than three-phase three-wire electric energy meter by measuring power, it compared three-phase four-wire electric energy meter, whether active or reactive power will have a greater measurement error, the following measurements for the above two way explain why 110kV three-phase four-wire electric energy meter by measuring electrical energy.
1-phase three-wire power meter measurement Known by the electrical principle that any three-phase three-wire and three-phase four-wire circuit the instantaneous power is:
P = u a i a + U b i b + U c i c (1)
Type of u a , U b , U c - Phase voltage instantaneous value
I a , I b , I c - Instantaneous value of phase current
Average power three-phase circuit P = U a I a cos a + U b I b cos b + U c I c cos c (2)
Clearly, the instantaneous three-phase circuit (or average) power is equal to the phase instantaneous (or average) power of and, if symmetry has nothing to do with the three-phase circuit. Three-phase three-wire circuit is not
middle, there is no zero sequence current of each phase path, neutral current i n Zero, that is,
I n = I a + I b + I c = 0 or i b =- (I a + I c ) (3)
The type (3) into equation (1) to obtain:
P = u a i a -U b (I a + I c ) + U c i c
P = (u a -U b ) I a + (U c -U b ) I c
= U ab i a + U cb i c (4)
Its average power is: P = U ab I a cos (U ab , I a ) + U cb I c cos (U cb , I c ) (5)
Three-phase three-wire circuit, equation (5) and type (2) are equivalent, so you can save a list of current transformer and reduce the energy cost, often using three-phase three-wire three-phase three-wire power meter measurement circuit The active energy.
2 meter three-phase four-wire measurement of power
A long time, China's relevant departments of 110kV and above that the power grid under normal operating conditions, three phase, neutral grounding through the grounding transformer current tends to zero value, we adopted the three-phase three-wire power meter measurement. But in fact the power grid phase voltage, line impedance and load impedance is not symmetric, nor is it ground zero current in that
I n = I a + I b + I c 0
It has i b = I n - (I a + I c ) (6)
The type (6) into equation (1), obtained
P = u a i a + U b (I n -I a -I c ) + U c i c
= U ab i a + U cb i c + U b i n (7)
Type (7) and type (1) are equivalent.
3 measurement error between For neutral grounding of the power grid, using three-phase three-wire power meter measurement, by the formula (4) and equation (7) obtained three-phase three-wire power meter measurement errors
Or average This error is purely caused by the measurement method of three-phase circuits with symmetry change; and the power meter and transformer and voltage transformer secondary side error voltage drop error caused by circuit has nothing to do.
If the phase voltage equal to the difference of phase current does not exceed 10%, when U b And I n Angle equal to 0 (U b , In = 0 ) or 180 , by the equation (9) obtained the maximum estimated measurement error approximate formula that
In addition, in this case can not be used within the phase angle of 60 in the three-phase three-wire two-component cosine-type reactive power meter for reactive energy measurement, since it will have measurement error, that is,
Type of 0 - Zero sequence current I 0 And positive-sequence current I 1 Ratio, that is,
0 = I 0 / I 1 1,0 ? Now sequence voltage U 1 And zero sequence current I 0 Phase difference between that 1,0 = U 1 , I 0
1 ?? Now sequence voltage U1 with the positive-sequence current I 1 Phase difference between that 1 = U 1 , I 1
4 field test results 110kV substation and the four following users transformer neutral grounding of the ground current for the test, the test results shown in Table 1 (measured by the results of secondary data):
Table 1 transformer neutral grounding the ground current I n And three-phase three-wire electric energy meter maximum measurement error
