Methodology of electric power quality support in distributive electric networks

Fedotov Evgeny Alexandrovich

Kazan State Power Engineering University

Russian Federation

1 INTRODUCTION

The adaptation of Power systems to the market relations, the growth of energy use by non-industrial consumers and also the mass deployment of the digital control systems caused the increased demands to the reliability and quality of the power supply in the distributive networks. Its whole length in Russian Federation exceeds the 70% of the total lenght of all transmission lines that shows the quantitave structure of the non-industrial consumers coverage.

The regional distributive networks of 6-10 kV fundamentally differ from the urban networks in their significant extension and in their great number of dispersion loads. For the number of feeders are typically the several tens of connections where the total lines length achieve 20-30 km. A significant feature of fuel transfer stations is their usage for connections of consumers located in rough country.

In the power supply systems of this regions can be used mid-range transmission lines with the 40-50 km length because of the high voltage networks absence, that makes practically impossible to transfer the energy in compliance with required quality values.

The experience of oilfield transmission lines operation shows [1] that the main problems of power supply interruption are the short circuits on overhead transmission lines; voltage drop on overhead transmission lines of 6-10 kV that feed the industrial consumers (their length can exceed 10 km where the load is over than several megawatt that is caused by voltage loss of 15% and more), as result the end consumers have the voltage on the lower level what complicates the starting and self-starting of electric motors.

In the electrical networks of 10 kV voltage the loads don’t have active character.

In fulfilment of standard requirements for reactive power compensation the load angle tangent on the buses of 0,4 kV should not exceed 0,35 and on the buses of 6-10 kV should not exceed 0,4 (the contribution of the transform inductance is considered). Thus, in reactive power compensation it’s necessary not to ignore the reactive component of the load and particularly in the absence of compensation devices.

2 Methodology

Let us view the transmission line, fig.1, where is installed booster transformer with transformer coefficient k. Let us present the parameteres of the transmission line after booster transformer to the main substation center. If the booster transformer is not installed it is possible to take k=1, than . This impedance represents the load impedance.

Than

. (1)

Fig.1. The principal circuit (a) and its equivalent circuit (b)

Using the formula (1) for the transmission line impedance exclusion from the subsequent equations. By the installing of the booster transformer we have

, (2)

where is the targeted transformer coefficient.

The formula (2) connects load voltage before the booster transformer installing with the transformer coefficient of the transformer booster at the planned level of the targeted voltage .

As far as in the formula (2) are included the complex quantities, and only the modulus of desired voltage is given, we come to the following equation with regard to transformer coefficient

(3)

Evidently, it is necessary to clarify the influence of the voltage phase on the transformer coefficient value for the next calculations.

Let us assume that

,

where δ – angle between the voltage vectors in the main substation center and on the load.

Further we evaluate the possible range of angle δ changes depending on the load parameters and on the transmission line, but let us to continue the derivations. The expression (3) gains the following form

After transformations we’ve got the following equation

.

We come to the following formula that connects the transformer ratio of the booster transformer k and the load voltage before booster transformer installation:

.

Solving the obtained equation relative to the voltage we find

(4)

3 RESULTS

In the figure 2 are listed the sets of characteristics built after the expression (4) where for the construction convenience the X-axis and Y-line are exchanged: the variable is the voltage.

The angle δ is limited by value of cosδ=0,90. It matches the load up to 4 МW where the length of transmission line is 30 km what can’t be achieved for the transmission lines 10 kV.

Fig 2. The required transformer coefficient of booster transformer for the providing of desired voltage level on the load with regard to the fase change between the voltages

I, II, III – characteristic sets corresponding to targeted voltage level values: I – α=1,05; II – α=1,00; III – α=0,95;

for the each set the sequence of the curve is the same and complies with the different values of angle δ :

1 – cosδ=0,90; 2 – cosδ=0,93; 3 – cosδ=0,95; 3 – cosδ=0,98; 5 – cosδ=1,00.

From the obtained results the influence of the voltage fase change should be taken into account because otherwise the understated values of transformer coefficient come out.

So for the set I fig. 2 we take that (curve 3). May the original voltage was 0,94 о.е. As it’s seen on graphs the required transformer coefficient is k=1,17. At the same time if the influence of the voltage fase change would be neglected and to mean that , we find k=1,13.

The obtainded difference in booster transformer values is essential.

The maximum increase of booster transformer voltage installed in three phases is 15% [2].

The obtained results show that for the achievement of required voltage level is necessary to install two booster transformer sets in two phases.

4 CONCLUSION

1. The installing of the booster transformer on the transmission lines in regional distributive networks helps to provide the standard values of the electric power for the consumers without transmission lines reconstruction.

2. It is necessary to choose the transformer coefficient of booster transformer taking into account the parameters change of the transmission line head part and the phase change of the voltage on the ends of transmission line.

3. The inventive method for definition of targeted transforer coefficient of booster transformer allows to exclude from the calculations the transmission line inductance and use as key parameter the voltage load that exist before installing of booster transformer.

References

[1] Baranovsky I.D., Khuchev J.V., Аbеuоv R.B. About problems of alternative control devices efficiency in electric power supply / Russian conference «Electric Power: produce, distributive and use». Tomsk, 2008. - PP. 47-49.

[2] Perinsky Т.V., Rodionov О.S. Automatic voltage control operating experience of station in distributive electric networks 6 – 10 кV. – Electro, vol. 3, 2009. – PP. 34-35.

Eugeny Fedotov received engineering degree in electrical engineering at the Kazan State Technical University in 2000 and PhD degree in technical since at the Kazan State Power Engineering University in 2003. Currently he is a Dozent of Electric Power Plants Department of Faculty of Electrical Power Engineering and Electronics of Kazan State Power Engineering University. He teaching interests include power plants, electric part of power plants, electromagnetic transients in power systems. His research interests include provision of quality of the electric power in the regional distribution networks; ensuring the reliability of electric power systems; work sustainability in regional distribution networks; research electromagnetic transition processes in synchronous machine; simulation synchronous machine with thyristor excitation system in Power System.