A New Two-Wire Distribution System Concept

for Supplying Three-Phase Rural Loads

Juliane Cristina de Oliveira Fandi

Department of Electrical Engineering

Federal University of Triângulo Mineiro

Uberaba-MG, Brazil

José Rubens Macedo Jr, Senior Member

Isaque Nogueira Gondim, Member

José Carlos de Oliveira, Member

Geraldo Caixeta Guimarães Faculty of Electrical Engineering

Federal University of Uberlândia

Uberlândia-MG, Brazil

Abstract — Currently, the supply of electricity to rural districts

is realized, in almost all cases, through single-phase medium-

voltage overhead distribution lines. However, some of these

installations will eventually demand the use of more expressive

electrical loads, which makes the use of three-phase distribution

systems a necessity, thus bringing about the substitution of the

existing single-phase network. Consequently, the elevated costs

associated with the construction of a medium voltage three-

phase branch line, which in itself can imply miles of distribution

grids, makes the viability of investing in such a project for the

rural facility proprietor, as well as the local distribution utility,

very remote. It is within this context that this paper presents an

innovative, low cost solution to the problem, based on

electromagnetic arrangements (without the use of power

electronics), for attending to rural three-phase supply needs

through the use of a new overhead two-wire distribution system.

Index Terms — Rural distribution systems, single-three-phase

conversion, rural three-phase loads.

I. INTRODUCTION

The characteristics of electric energy consumers, along

with financial limitations for investments in rural electrification programs, have pushed electric energy companies to rely on the use of single-phase energy distribution systems. Some of the features associated with such systems are: low monthly consumption; low consumer density and few simultaneous maximum demands [2]. The use of electricity in the rural areas for production, besides the classic applications which do not supply driver loads, also covers the pumping of water for irrigation, the driving of farm equipment, milling processes, etc. [4].

The development of new techniques for irrigated agriculture, as well as the benefit of locally produced products, has placed upon the rural producers the need to increase their energy consumption, particularly when dealing with maximum demand. However, the use of the single-phase

system currently available, forces the rural agricultural producer to take on the limitations and peculiarities inherent to the system.

As it currently stands, a significant load increase to any rural consumer would depend upon the substitution of the existing medium voltage overhead network, for a new three-phase network, thus burdening the consumer to the extent that investments earmarked for increased production become unfeasible. However, the limitation associated with the load increase of rural consumers, lies solely upon the single-phase transformer and not the single-phase medium voltage branch itself, once that the minimal gauge conductors standardized by the various Brazilian distributors are sufficient to support load levels various times greater than the nominal current of standardized Brazilian single-phase transformers. In almost all rural facilities in this country, single-phase transformers of 5 or 10 kVA are found. One can still highlight here, that the quantity of rural consumer units with these characteristics had increased in more than 2 million installations between the years 2004 and 2010, with the launch of the program Luz para Todos (Light for Everybody), promoted by the Brazilian Federal Government [8].

Some modern techniques [1] [6-7] [12-14], based on power electronics, make the supply of three-phase loads possible from converters specially developed for single-three-phase transformation. However, the use of this type of device, in general, allows only a punctual service for particular three-phase loads as for example, electric motors. In other words, with this technology the number of converters necessary is equal to the number of electric motors that need to be supplied. In addition, the use of these converters does not promote an expansion in the local electric energy market, as the available power supply becomes limited to the existing single-phase consumer transformers.

The technique proposed in this paper is based simply on specific electromagnetic arrangements, without any need for embedded electronics, making not only the use of three-phase

loads possible from the available single-phase networks, but also the increase in the local electricity market, without the need of large investments for the project’s implementation. This therefore, becomes an innovative solution not seen in the national electricity sector as of yet.

II. THEORETICAL FUNDAMENTALS

The underlying goal behind this proposal is the three-phase

supply from two conductors, which is based on the availability of two energized conductors with angular displacement of 60 degrees between its correspondent voltages.

Therefore, if the existence of the single-phase medium voltage two conductor branch line (phase and neutral) is confirmed, then the first part of the proposal’s goal (availability of two energized conductors) can be reached with the transference of the neutral conductor connection to another energized conductor from the same three-phase feeder. Fig.1 illustrates the necessary procedures.

Figure 1. (a) Original derivation of the single-phase branch line and (b) derivation proposed for the new two-phase branch line.

In addition to the procedure indicated in figure 1, it will also be necessary to promote the substitution of the isolators of the original neutral conductor, which will now have a medium voltage level, and also to promote the elevation of the conductor support structure, as indicated in figure 2.

Given the above, the suggestion is that three-phase supply to rural consumer units can be achieved through the use of the existing single-phase branch line (phase and neutral), only making its adaptation to a new two-wire configuration. All the available conductors will be used and the only disbursement necessary will be the adaptation of the original neutral conductor support structure, including the changing of the isolators. Comparative to the costs associated to a new three-phase branch line, the costs associated with the new supply proposal are of little significance.

The second part of the underlying goal is relative to the angular displacement by 60 degrees between the two phase voltages, which can be achieved through transformer polarity resources. The polarity of a single-phase transformer depends primarily on how the turns are wound, which can result in additive or subtractive polarity. In the subtractive polarity, angular displacement does not exist between the primary and secondary voltages, resulting in an angular displacement of zero degree. On the other hand, when concerning additive polarity the angular displacement between the primary and secondary voltages, for the same phase, will be 180 degrees.

Figure 2. Adaptation of the original existing neutral conductor support structures of the new single-phase branch.

So, if one considers the addition of a single–phase transformer, in one of the phases, that with an additive polarity, the input and output voltages for this referred phase, would have an angular displacement of 180 degrees. Taking therefore, the phase voltage A as a reference and taking into consideration the installation of a single-phase transformer (additive polarity) in phase B, one will finally arrive at an angular displacement of 60 degrees between the resulting phase voltages A and B, as shown in the phasor diagram indicated in figure 3.

Figure 3. Phasor diagram illustrating the resulting voltages on phases A and B for the single-phase branch line.

Finally, the complete structure necessary for supporting the new supply service of rural three-phase loads, through a new two conductor distribution network, is presented in figure 4.

Figure 4. Diagram of the proposed system for supplying rural

three-phase loads.

One observes in figure 4 that there exists a need to use a

single-phase transformer with a turns ratio of 1:1 for the voltage adaptation of one of the phases of the new two-phase

branch line, as well as for the grounding of phase C in the primary side of the distribution transformer. Figure 5 shows the same structure in a more detailed form.

Phase A

Phase B

Phase A

Phase B

Cut-out fuse

Single-phase

transformer

(1:1 turn ratio)

DYg three-phase

transformer

Surge Arrester

Phase C grounding

Phase C

Figure 5. Schematic drawing of the network topology for the proposed system for supplying rural three-phase loads.

Once that the underlying goal for the foundation of the proposal of a two conductor supply to attend three-phase rural consumers has been submitted, it becomes necessary to give a mathematical confirmation of the proposal, which can be obtained quite easily by means of an unbalanced circuit analysis tool based on symmetrical components.

Therefore, taking into consideration the underlying goal of the proposal, the phase voltages available on the primary side of the consumer’s three-phase transformer are:

.0V = 1,0 0 AN pu ,

.0V = 1,0 60 BN pu

e .

V = 0,0 CN pu (1)

It is emphasized here that the phase C conductor is not

physically available in the new two-phase branch line. Therefore, the positive, negative and zero sequence voltages, on the primary side of a conventional three-phase transformer, with a grounded delta-star type connection and supplied by the voltages indicated in (1), will be given by:

.

0 0 0

.2 0

1

2 0.

2

1 1 1 1,0 0 0,577 301

1 1,0 60 0,03

1 0,0 0,577 30

V

V a a pu

a aV

(2)

The calculation for the resulting voltages on the secondary

side of the conventional three-phase transformer should take into consideration the angular displacement of +30 degrees for the positive sequence voltage, and –30 degrees for the negative sequence voltage, provided by the grounded delta-star connection of the three-phase transformer. So, the

resulting voltages on the secondary side of the three-phase transformer are:

.

0 0

.2 0 0 0

2 0.0 0

0,0 01 1 1 0,577 60

1 0,0 0 30 0,577 60

1 0,577 1800,577 30 30

an

bn

cn

V

V a a pu

a aV

(3)

Where Van, Vbn and Vcn are the phase voltages on the secondary side of the conventional three-phase transformer, with a grounded delta-star type connection.

Through the verification of (3), the resulting phase voltages on the secondary side of the conventional three-phase transformer make up a perfectly balanced three-phase system. The only caveat intrinsic to the process concerns the phase voltage amplitudes on the low voltage side of the three-phase distribution transformer, which are equal to 0.577 pu or

likewise to 1 3 pu. Therefore, to obtain line voltages equal to

220V on the secondary side of the consumer´s transformer, it will be necessary the use of a transformer with a nominal secondary voltage equal to 380V, which is already of a standard set by the electrical distribution utilities.

Once the algebraic developments relative to the new proposal for a two conductor distribution network are concluded, it becomes necessary to prove its effectiveness by means of computational simulations and also laboratory testing, which will be presented in the following topics.

III. COMPUTATIONAL EVALUATION OF THE PROPOSED

METHODOLOGY

For performing the computational evaluation of the

proposed methodology for providing a two-conductor three-phase system, the circuit in figure 6 should be used, which represents a two-phase medium voltage branch line, set aside for supplying a three-phase load. All the computational simulations were carried out in the time domain.

Figure 6. Three line diagram of the electric distribution system

being studied. (Note: SC = Short-Circuit power)

It should be highlighted the ground resistance in the circuit in figure 6 which will be analysed more specifically later.

Based on the regions indicated in figure 6, figure 7 shows the voltages on the connection point of the two-phase branch line (Region A) and on the primary side of the three-phase transformer (Region B).

Figure 7. (a) Voltage waveform results from the two-phase branch line

derivation point – Region A – and (b) voltage waveform results on the

primary side of the three-phase consumer transformer – Region B.

As shown in figure 7.a, the voltages for phases A and B for

the original three-phase feeder (from which the two-phase branch line is derived) have an angular displacement of 120 degrees. On the other hand, as an effect from the single-phase transformer installation (1:1 turn ratio) on phase B, and as already predicted in the analytic developments carried out during the last topic, the voltages of the phases A and B have an angular displacement of 60 degrees in the primary side of the three-phase transformer of the consumer unit (figure 7.b).

The current waveforms on the medium voltage side of the two-phase branch line, as with the return through the ground, are indicated in figure 8. All currents are indicated in pu on the basis of the nominal current load.

As already expected, the instantaneous currents on the medium voltage side of two-phase branch line are unbalanced. However, the associated imbalance amplitudes are much smaller than that given by the single-phase medium-voltage earth return systems, which are widely spread throughout Brazil.

Finally, figures 9 and 10 present the voltage and current waveforms on the secondary side of the three-phase consumer transformer, respectively. The amplitudes of the voltages are indicated in pu.

Figure 8. (a) Current waveforms on the medium voltage side of the two-phase branch line and (b) current waveform on grounding path.

Figure 9. Instantaneous voltage registered on the low voltage secondary side

(220V phase-phase) of the three-phase transformer of the consumer unit.

As can be seen in figure 9, in accordance with the

analytical development carried out in the previous topic, the voltage on the secondary side of the three-phase transformer of the consumer unit is perfectly balanced, considering the low voltage loads as also balanced.

In the same manner, the instantaneous currents registered in low voltage load are perfectly balanced, as shown in figure 10. All the currents are indicated in pu on the basis of a nominal current on the load.

Figure 10. Instantaneous currents registered on the secondary side of the three-phase transformer of the consumer unit.

In the event of unbalanced low voltage loads, the resulting voltage imbalance will be exactly the same as those obtained when considering conventional three-phase circuits.

The only caveat of importance and relevant at this point, concerns the impact of ground resistance (RGround) on the voltage imbalance, checked at the secondary side of the consumer three-phase transformer. Therefore, through the carrying out of new simulations for different values of ground resistance, the graph in figure 11 was obtained, which shows amplitude of the voltage imbalance in function of the different values of ground resistance.

Figure 11. Voltage imbalance amplitudes on the low voltage side of the

three-phase transformer considering different ground resistance magnitudes.

As indicated in figure 11, in practical aspects, the ground resistance value is of little significance in terms of the voltage imbalance amplitude checked at the secondary side of the three-phase transformer. Voltage imbalance levels with amplitudes of until 2.0% are perfectly acceptable at medium voltage levels [9]. The explanation for this result is extremely simple, once that the impedance of the three-phase transformer at the consumer unit is much greater than the system’s impedance equivalent seen from the medium voltage view point. And thus, the variations of ground resistance, when compared to the magnitude of the transformer’s impedance, present little influence to the voltage imbalance amplitudes.

In the next topic, the results for the various laboratory tests will be shown, which aim to provide definite proof for the proposed methodology, in the formation of a new two-wire distribution system for supplying rural three-phase loads.

IV. EXPERIMENTAL EVALUATION OF THE

PROPOSED METHODOLOGY

In order to carry out the evaluation of the proposal for providing three-phase supply from a two-wire distribution system, the circuit indicated in figure 12 was used, which considered a three-phase induction motor of the squirrel cage type, with ¼ CV rating and a nominal voltage of 220V. Both of the transformers used, the single-phase and three-phase transformer, present the same 1:1 turn ratio.

Figure 12. Schematic diagram of the experimental arrangement used for

performance analyses of the proposed methodology.

The instantaneous voltages obtained in the primary side of

the three-phase transformer, with a grounded delta-star type connection, are shown in figure 13. The results there indicated, considering the steady state condition on the induction motor, show themselves to be strongly coherent with those obtained by means of computational simulation (see figure 7.b). To help the comparison of the results obtained through practical experimentation in relation to those obtained by means of computational simulation; all results are expressed in pu. The current amplitudes will also be expressed in pu on the basis of nominal load current.

Figure 13. Instantaneous voltage results on the primary side

of the three-phase transformer – Region B.

The currents obtained on the primary side of the three-phase transformer are indicated in figures 14 (phases A and B) and 15 (ground path).

Figure 14. Instantaneous currents on the primary side of the

three-phase transformer - Region B.

Figure 15. Instantaneous current returning through ground path – Region D.

Once more, the currents obtained on the primary side of the three-phase consumer transformer are extremely coherent with the currents obtained by means of computational simulation (see figures 8.a and 8.b).

Figure 16. Instantaneous voltages registered on the secondary side

of the three-phase transformer (220V phase-phase).

As obtained in the computational simulations, the resulting

voltages on the low voltage side of the three-phase transformer, which service the induction motor, make up a three-phase balanced system as shown in figure 16.

Likewise, in accordance with figure 17, the steady state currents registered on the secondary side of the three-phase transformer (induction motor) are equally three-phase and balanced.

It is worth highlighting again that the currents are indicated in pu on the basis of nominal load current.

Figure 17. Instantaneous currents registered on the secondary side of the three-phase transformer (220V phase-phase)

Lastly, the performance of the proposed system was evaluated when a sudden and transient solicitation is made (under-load induction motor starting). The voltages and the currents, which result on the secondary side of the three-phase transformer, are indicated in figure 18.

Figure 18. (a) Instantaneous voltages and (b) instantaneous currents

registered on the secondary side of the three-phase transformer during the

period of the induction motor starting.

As can be seen in figure 18, despite the two-wire supply

system, the three-phase motor starting ran without anomalies, remaining the machine in normal operation until the end of the experiment.

In summary, therefore, the results obtained through the experimental procedures were found to be totally aligned to the results obtained in analytic and computational form.

V. ECONOMIC ANALISYS

Once that the performance for the proposed new distribution system has been proven satisfactory, when compared to the conventional systems, it is worth highlighting also the economic benefits which come with the new proposal. Table I presents the costs of adaptation of an existing single-phase branch line (with phase-neutral topology and 10 miles long) into the new two-wire system and those costs for the construction of a new conventional three-phase branch circuit.

It should be checked that the new medium voltage (MV) branch line arrangement for supplying three-phase rural loads, represents approximately 12% of the construction costs of a conventional three-phase branch line, once that the proposed topology uses the existing conventional single-phase structure.

TABLE I – CONSTRUCTION COSTS FOR MEDIUM VOLTAGE (13.8 KV)

DISTRIBUTION NETWORK USED FOR LOCAL DISTRIBUTION UTILITY.

The difference in cost indicated would be of tangible benefit to the consumer as well as electrical utility, the moment in which the 12% of costs, compared to the conventional three-phase network, is divided independently from any type of apportionment between distribution utility and consumer in relation to the investment to be made. In addition, in the specific case concerning the distribution utility, one has to still consider the load increase to be accounted for on an adapted single-phase branch line. In other words, taking into account the current single-phase branch line (phase and neutral) in use, the power restriction is found to be associated quite simply in the single-phase transformer and not the single-phase branch circuit, because the Brazilian distribution utilities, through stipulated rules, use conductors with a current rating much greater than the transformer rating.

VI. CONCLUSIONS

This paper presented a methodology for the energy supply of three-phase rural consumer units from a new two-wire medium voltage system based on electromagnetic arrangements which are simple and practical in nature. To meet this proposal, the single-phase branch lines currently used by electrical utilities can be easily converted over to two phase branch lines, at a relatively low cost, in order to promote one of the basic goals behind the proposed methodology.

The performance of the proposed methodology, which visualizes the use of a new two wire distribution system, for the supply of rural three-phase loads, was proven sound through the use of analytic, computational and experimental developments. Therefore, in scientific terms, the proposal presented good practical results, which are extremely promising. It is also stimulating the continuation of the project through the implementation of a pilot network in a real distribution system.

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