EFFECT OF ERRORS OF TRANSFORMERS

The decline in production in recent years has led to a decrease in the number of loads of power units, as well as lower consumption of industry, which in turn led to the emergence of a negative error in the automated systems of control and energy metering (AMR). The reason is the emergence of a negative error in the primary current and voltage sensors, which are used as the current and voltage transformers. This paper investigates the causes of errors and how to eliminate the undercount in the electricity metering systems.

The errors of current transformers (current and angular) are due to the magnetizing current, and calculated as [1, 2]:

current error

where Lm - the average length of the magnetic flux in the magnetic core, m; z2 - the resistance of the branches of the secondary current (impedance of the secondary circuit and the secondary winding), Om; f - Frequency, Hz; SM - a real cross section of magnetic circuit, m2; j - the loss angle and a - the phase angle between the secondary emf E2 and the secondary current I2, deg.;

angular error

The main influence on the values ​​of the errors of current transformers have their load current and the resistance of the secondary circuit. In terms of reducing power consumption by industrial enterprises Loading of current transformers are often no more than 5 - 15%, which leads to a significant increase in errors.

The limiting values ​​of current and angular errors of current transformers for measurement (according to GOST 7746-89) are listed in the table.

The results of calculation of the angular error and the current of current transformers type TPOL 600/5, the accuracy class 0.5, produced by (1) and (2) are shown in Fig. 1 and 2 (thin solid line - calculated curve, solid line - approximation). Type of approximating expression and criterion agreement of calculated and approximating curves shown in Figs.

To change the range (1 - 10% and 10 - 100%) primary current of the nominal value of the current mathematical models of the most common errors of current transformers are of the form:

The values ​​of the primary current I1 of the current transformer in the formula should be substituted as a percentage of the nominal value.

Studies conducted by the errors of current transformers in the NICE showed a reasonable convergence of theoretical and experimental results. In Fig. 3 shows the results of experimental studies such as CT TPLM10-200/5, the accuracy class 0.5.

Results of the study of current errors of various types of current transformers with primary nominal current of 75 - 600 A allowed the following conclusions:

1. in the range of the primary current of the nominal value of 1 - 25% of the current uncertainty has a negative sign;
2. with an increase in the absolute value of the primary current of the current error is reduced;
3. Experimental studies confirm the validity of the mathematical model of the current uncertainty of the current transformer;
4. account the current uncertainty in the current transformer metering will reduce the amount of unbalance in substations;
5. quantity of electricity allocated by consumers because of the negative current-current transformer error is underestimated compared to the actual value, so the current account of the error in the current transformer metering will more accurately assess the value of supplied electricity to consumers and gain some economic benefit, which will be evaluated further.

The second source of measurement error is a power transformer voltage.

According to [3, 4], the error voltage is defined as follows: DU = DUn DUh + (3)
where DUn - error voltage due to load current,% DUh - error voltage due to the current idle%.

Using the vector diagram, it is possible with sufficient accuracy to express the components of the error voltage transformer as follows:

where U2 - the voltage of the secondary winding of the transformer, V; Ia-active component of load current, reduced to the secondary winding of the transformer, A; r'1 - reduced resistance of the primary winding of the transformer, converted to the secondary winding, Om; I'p - reduced reactive component no-load current, reduced to the secondary winding of the transformer, A; x'1 - reactance of the primary winding of the transformer, converted to the secondary winding, Om; I2 - the load current of the transformer, A; r2 - the resistance of the secondary winding of the transformer, Om; cosj2 - PF load, rel. units.; x - inductance of the transformer, Om.

The angular error is defined as the voltage transformer

,
where d'x - angular error due to current idling; d'H - angular error due to load current.

The components of the angular error is defined as

;

The results of calculation errors in the voltage transformer is shown in Fig. 4 and 5. The main influence on the error voltage transformer has a secondary loading value I2.

The dependence of the error voltage from the transformer load factor of capacity (the ratio of the actual load of the secondary winding of the transformer voltage to the nominal value of the load) has the form

DU [%] = - 0,73 * Ks + 0.35,
where Ks - load voltage transformer secondary winding, rel. units.

The resulting expressions for the errors of current transformers and voltage transformers can increase the accuracy of the metering of electricity to the substations.

The effectiveness of the introduction of metering at the substation is dependent

• of the costs of implementing AMR;
• of the economic impact resulting from implementation.

Currently, records released energy and the calculation of the energy balance in most substations is by means of electromagnetic meters, excluding the errors of current transformers and voltage transformers.

Most voltage transformers run at startup of the secondary winding of more than nominal by several times, ie, a negative error. Most of the load voltage transformer up measuring devices connected to them, in particular, electromagnetic meters of active energy. For example, in a substation, "Sverdlovsk" set induction type meters SAZU-I670M, power consumption of which 4 watts. As a result of induction AMR meters will be replaced by e - PSCH-type, power consumption which is half - 2 watts.

In this case, the load factor of the transformer voltage is reduced by half to 1.1 values ​​and, hence, reduces the error of voltage transformer with a 1.15% to 0.5%. Reducing the error voltage transformer will increase the accuracy of accounting tempered consumers.

Accounting for the errors of current transformers, voltage and current metering system provides economic benefits. To estimate the economic impact of the introduction of AMR has been produced by the estimated calculation of the annual electricity consumption of the substation, "Sverdlovsk" taking into account the errors of current transformers and voltage. The calculation was carried out as follows:

1. According to available data for typical winter and summer day of the year (1997 and 1998.) Were calculated hourly values ​​of active power (taking into account the errors of current transformers and voltage) on the inputs and the supply line by the formula

   Rfakt = P * KI * KU,
   where P - power average values ​​determined from readings of electricity meters;
   KI - coefficient taking into account the error of electric current of current transformer, KU - coefficient taking into account the error of voltage transformer.

   KI = 1 - (Df _I / 100), KU = 1 - (Df _U / 100)
   where Df _I - current of current transformer error, Df _U - error voltage transformer.

    

2. We determined the power consumption for typical winter and summer days, taking into account the errors of current transformers and voltage (Wz.fakt and Wl.fakt) and without errors (Wl and Wz) for inputs and supply line:

   Wz.fakt Rfakt.z = S,
   Wl.fakt Rfakt.l = S,
   Wz = S Pi,
   Wl = S Pn.

    

3. Calculated value of the annual consumption of active energy inputs and the supply line from the formulas

   Wg.fakt Wz.fakt = Ns * + * Wl.fakt Nl,
   Wg = Ns Wz * + * Wl Nl,
   where Ns = Nl = 213 and 152 - the number of winter and summer days in the year.

    

4. The effect of the introduction of metering is determined by the formula DW = SWg.fakt - SWg,
   where SWg.fakt and SWg - annual consumption of electricity feeders with and without the errors of current transformers and voltage, respectively.

Perform an assessment of economic benefit for the two options.

1. When taking into account current metering errors of current transformers and voltage transformers reduce the errors due to low power electronic counters the effect will be:

   based on data for 1997
   DW = 331021094-326683013 = 4,338,081 kWh / year;

   based on data for 1998
   DW = 294647641-290512594 = 4,135,047 kWh / year.

   In monetary terms, the economic effect (E) is equal (at the cost of electricity 0.4 € / kWh)
   E = 1735 ... 1650 thousand rubles a year.

    

2. When only reduce the errors of voltage transformers due to a low-power electronic counters the effect will be:

   based on data for 1997
   DW = 328316428-326683013 = 1,633,415 kWh / year;

   based on data for 1998
   DW = 292196976-290512594 = 1,684,382 kWh / year.

   In monetary terms, the economic effect is (at the cost of electricity 0.4 € / kWh)
   E = 653 ... 674 thousand rubles per year.

In conclusion we can draw the following conclusions:

• reduce stress in a number of grid nodes, as well as reducing energy consumption of industry led to a negative error of a current transformer, and accordingly to undercount the commercial energy consumed;
• to eliminate the undercount consumption of electricity is necessary to introduce correction coefficients;
• account of the errors of current transformers in metering, as well as reducing the error voltage transformer due to the introduction of new electronic meters lead to a significant economic effect.