Method to measure phase capacitance of electric network. RU patent 2510034.

FIELD: electricity.

SUBSTANCE: method is described to measure phase capacitance of an electric network with isolated neutral, which includes alternate measurement of short circuit currents in each phase and differs by the fact that additionally angles are measured between vectors of short circuit currents and vectors of voltages that arise during short circuits on the neutral, using which they calculate phase capacitances.

EFFECT: method increases accuracy and eliminates errors when determining phase capacitances of an electric network.

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The invention relates to the electrical engineering and may be used to measure capacitance between phases and the case (or land) in any three-phase power networks, for example in the ship.

The known method of measurement of capacity (RU №143141 A1, G01R 27/26, publ. 01.01.1961), consisting in oscilografului process of discharge of the vessel through some resistance by using conversion scheme that converts the time constant in the capacitance value.

The disadvantage of this method is its inapplicability to measure the capacity in the network without de-energizing that impairs its use.

There is a method of measuring capacitance on the hull of the ship electric power systems (RU №2028633 C1, G01R 27/18, publ. 09.02.1995), namely, that powered-down system, include switching devices, check the absence of galvanic connections with the case, short circuit between the three phases of the main switchboard, after which include AC bridge between any convenient point of current-carrying parts of the system and the housing and produce measurement of total system capacity relative to the housing.

The disadvantage of this method is its inapplicability to measure capacity in the electricity supply without failures, and absence of the possibility of measuring the capacity of the individual phases.

The closest one is the way of definition phase capacities of electrical system relative to the housing (Ksenofontov A.P. Protective devices ship and shore installations fishing industry / Apostophes, Wasactually, Vlastnosti. - M, 1984. - 255 S., str), which is the closest analogue, which consists in the sequential closure phases of the mains supply on the housing through the ammeter and further calculation of phase tanks on the measured single-phase currents short circuits on the case.

The disadvantage of this method is that the calculation conducted either under the simplified expressions that lead to a considerable error, or by the nonlinear system of equations with multiple solutions and require the use of numerical methods, that does not preclude errors in the determination phase of the tank outlet.

The task of the invention consists in increasing the accuracy of determining the phase-capacity electrical outlet through the use of an additional option that allows you to simplify the calculations conducted after measurements.

To solve the set tasks in a certain way, including alternate dimension currents earth fault of each of the phases, it is proposed to measure the angles between the vectors circuit currents and vectors generated when circuit voltage neutral, which is known way to calculate the angles between the vectors circuit currents and vectors leakage currents neznakomyh phases, using which to calculate the phase of capacity at the following expression:

where C A , C B , C C - values capacities between each phase and housing (or land); V - angular frequency network; U c - RMS voltage network; α A , α B , α C - angles between short-circuit current and the leakage current lagging phase; β A,? B,? C - angles between short-circuit current and the leakage current advance phase; I A , I B , I C - effective values of currents closure phases.

To be attached to the application graphics the materials represented:

- figure 1 is a diagram of measurements by the method when used to determine the angle between the vector of the short-circuit current and the voltage vector neutral wattmeter with winding voltage, included between neutral and housing;

- figure 2 - scheme of measurements during the implementation method when used to determine the angle between the vector of the short-circuit current and the voltage vector neutral wattmeter with winding voltage, connected serially between each of neznakomyh phases and housing;

- figure 3 - vector diagram of currents and voltages at the closure of one of the phases.

On the attached schema, the following notation:

1 - ammeter; 2 - power meter; 3 - winding of electrical equipment; 4 - phase network capacity; 5 - electrical system; 6 - vector current circuit; 7 - vector leakage current through lagging phase from abridged; 8 - vector leakage current through a quicker phase; 9 - the voltage vector neutral; 10 - voltage vector lagging on phase after closure; 11 - voltage vector on advanced phase after closure; 12 - vector EMF abridged phase; 13 - vector EMF lagging phase; 14 - vector EMF advance phase.

Increase the accuracy and simplicity capacity is due to the fact that thanks to the measurement of the angle between the vector of the short-circuit current and the voltage vector neutral't determine the angles between short-circuit current and the leakage currents neznakomyh phases. This significantly simplifies the calculation of the phase tanks network, which consists in the computation of the linear system of equations has a unique solution.

Figures 1 and 2 show variants of connection of measuring devices at realization of a method. In the version shown in figure 1, for measurement ammeter 1 included between phase and housing, and wattmeter 2 included current windings in series with the ammeter and winding voltage between neutral network and housing.

In the version shown in figure 2, ammeter 1 included between phase and housing, and wattmeter 2 included current windings in series with the ammeter and winding voltage before switching between one neznakomyh phases and body, then switched between the second of neznakomyh phases and housing. In the future, both readings wattmeter be added up and divided by three.

On the measured values of current, active power on the well-known expression, you can determine the angle between the vector of the short-circuit current and the voltage vector in neutral with the closure of the first phase:

where P i - active power with the closure of the first phase; U i - phase mains voltage; I I - fault current i-St phase (fundamentals of theory of circuits. Textbook for universities / Gwice, Paintin, Aventail, Swistrack. - 4-e Izd. - M, "Energy", 1975. - 752 S.; str).

As can be seen from the vector diagram shown in figure 3, the vector current of the circuit forms a triangle with vectors leakage currents each neznakomyh phase 7 and 8. With vectors leakage currents neznakomyh phase 7 and 8 always shifted towards the voltage vector abridged phase angles

. On this basis the angles in this triangle can be found as follows: between the vector of the fault current and vector leakage current lagging phase 8

and between the vector of the fault current and vector leakage current advance phase 7

Presenting the module of the vector current of the circuit as a sum of projections on him vectors leakage currents neznakomyh phases, which in turn linearly depend on the capacity of these phases, you can create a system of linear equations for circuits of each phase. The solution of this system will be the capacity of each phase.

The example of the method is shown in figure 1 and figure 2. Phase network capacity have the following meanings: a C A =2 IFF C B =8 international film festival; C C =4.5 MF. Voltage 230 C. mains Frequency 50 Hz.

Instrument readings when measuring for the version depicted in figure 1:

ammeter - I A =1.133 A; I B =0.596 And; (I C =0.947 A;

wattmeter - P A =34.36 W; P B =24.54 W; P C =58.9 W.

The calculations made by expression (2), give the following values:

& Phi; A =80.8 degrees; & Phi; B =77.48 degrees; & Phi; C =109.1 degrees.

From expressions (3) and (4):

α A =20.8 degrees; α B =17.48 degrees; α C =49.1 degrees;

β A =39.18 degrees; β B =42.52 degrees; β C =at 10.89 degrees.

Substituting these values into equation (1), we obtain the following values of phase containers: a C A =1 IFF C B =8 international film festival; C C =4.5 MF.

After the measurements according to the scheme shown in figure 2, we get the following readings:

Ammeter - I A =1.133 A; I B =0.596 And; (I C =0.947 A;

wattmeter - P AB =132.52 W; P AC =-235.6 W (closure phase);

P BA =-132.52 W; P BC =58.9 W (closure phase);

P CA =235.6 W; P CB =58.9 W (closure phase).

The sum of readings of the meter, received at the closure of each phase, divided into three:

P A =34.36 W; P B =24.54 W; P C =58.9 W.

As you can see, the original data calculation coincide with the previous variant of the method. As further calculation similar to the one above, then the result is the same.

The method adopted for the nearest equivalent, leads to the next.

The results of simpler expressions C A =2.3 international film festival; C B =8.29 international film festival; C C =4.37 international film festival. The maximum error of determination of containers is 14.48%.

Used instead of the simplified expressions system of nonlinear equations has two solutions: 1 - matches the valid values of phase tanks (C A =2 IFF C B =8 international film festival; C C =4.5 MF); 2 - non-compliant with the real values of phase tanks (C A =3.65 IFF C B =0.12 international film festival; C C =3.87 ICF), which can lead to the error of definition phase tanks.

Thus, it is seen that way solves the problem of exact definition phase tanks power and eliminates the possibility of errors.