Additional Power Losses In Low-Voltage Electrical Networks And Their Influence On People

Additional heat losses and their impact on the cable core insulation

Additional power losses in the network caused by unbalanced current are characterized by power loss increase factor $K_P$:

$$\begin{array}{}\text{(1.1)}& K_P=\mathrm{\Delta }{P}_N/\mathrm{\Delta }{P}_S=\mathrm{1}+K_{\mathrm{2}I}^{\mathrm{2}}+K_{\mathrm{0}I}^{\mathrm{2}}\left(r_{\mathrm{0}}/r_{\mathrm{1}}\right),\end{array}$$

where $\mathrm{\Delta }{P}_N$ - power losses in the network under unbalanced conditions; $\mathrm{\Delta }{P}_S$ - power losses in the network, caused by positive-sequence currents; $K_{\mathrm{2}I}$ - negative-sequence current unbalance factor; $K_{\mathrm{0}I}$ - zero-sequence current unbalance factor; $r_{\mathrm{0}}$, $r_{\mathrm{1}}$ – zero-sequence resistance and positive-sequence resistance, respectively.

For a three-phase four-wire line, as it is noted in F.D. Kosoukhov's work we will have (Kosoukhov, 1988):

$$\begin{array}{}\text{(1.2)}& K_P=\mathrm{1}+K_{\mathrm{2}I}^{\mathrm{2}}+K_{\mathrm{0}I}^{\mathrm{2}}\left(\mathrm{1}+\mathrm{3}{r}_N/r_F\right),\end{array}$$

where $r_N$, $r_F$ – active resistance of neutral and phase conductors of the line.

With the same cross-section of phase and neutral conductors of a 0.38 kV overhead line, the ratio for the line is $R_{\mathrm{0}}/R_{\mathrm{1}}=\mathrm{4}$. Then expression ( 1 ) is transformed as follows:

$$\begin{array}{}\text{(1.3)}& {\mathrm{К}}_{\mathrm{р}}=\mathrm{1}+{\mathrm{К}}_{\mathrm{2}i}^{\mathrm{2}}+\mathrm{4}{\mathrm{К}}_{\mathrm{0}i}^{\mathrm{2}}.\end{array}$$

Consequently, value ${\mathrm{К}}_{\mathrm{Р}}$ is more affected by factor ${\mathrm{К}}_{\mathrm{0}i}$, whereas factor ${\mathrm{К}}_{\mathrm{2}i}$ has much lesser influence on it.. Thus, if under the conditions of uniform electricity consumption (under the same load of phases) the neutral conductor temperature is not high, then under unbalanced currents in the neutral conductor there will be a current of considerable magnitude equal to three zero-sequence currents, which will greatly increase the power loss factor.

Impact of an insulated conductor heating temperature on the insulation quality

The installation of indoor wiring tends to upset the uniform distribution of single-phase loads, and probabilistic unbalance of currents leads to even greater additional heat losses. Normally, the indoor electric wiring is installed with the cables that have polyethylene insulation and aluminum cores with a cross-section from 1.5 mm² (lighting networks) to 2.5 mm² (socket network). According to (Wires and cables for electrical units on nominal stress up to 450/750 B inclusive, 2010; Rule of Devices of Electroinstallations (RDE), 2000), the admissible current for buried wiring does not exceed 14-20 A. However, the admissible current is the maximum current that can be carried by the conductor with a set cross-section, provided that the admissible temperature is not exceeded, i.e. for the balanced conditions. Cross section of the conductor should not exceed the established temperature. This means that the cross-section is selected when performing a condition K_P =1. However, the cross-section is always selected disregarding the zero-sequence flows in the neutral conductor in the 0.38 kV network. Thus, first of all, the cross-section of the chosen conductor is artificially underestimated, and secondly the rated parameters of the automatic the device protecting the line will also be chosen improperly since it is also selected based on the working current. Since the cross-section of the neutral conductor is not larger (and sometimes even smaller) than the cross-section of the phase conductor, the flowing three zero-sequence currents overheat the insulation which results in its melting and causes a single-phase short circuit. In the event that the automatic circuit breaker in the main section of the network is selected inappropriately (i.e. it is not checked for operation sensitivity under minimum short circuit conditions), the protection device fails to operate in case of a short circuit in some remote part of the network. As a result, fires take place in individual houses in rural populated settlements, cottage areas, urban flats, and in production rooms. The carried-out analysis of literature on the fire, shows that their main reasons are overloads and short circuits (Ward, 2015; Jesse, 2015; Zalok, Hadjisophocleous & Lougheed, 2009; Marxsen, 2017; Clapa, 2015; Walters & Hastings, 1998; Tsui & Chow, 2004; Hadjisophocleous & Chen 2010; Baratov, 1997; Harmon, 2017). However, none of the sources has ever considered the overload of a neutral conductor due to unbalanced phase currents as a cause for these conditions. Moreover, it is worth noting that the developed measures, methods and technical for decrease in asymmetry of currentse have never been considered as a tool for prevention of fires. Therefore, we are the first to suggest the use of balancers as a way to stabilize fire-hazardous conditions.

Experimental research of unbalanced conditions in operating 0.38 kV electrical networks

Over many years, we have been involved in the research into unbalanced conditions of distribution 0.38 kV networks in Leningrad and Irkutsk regions, Germany and Mongolia. More than 30 years, we have studied the unbalanced conditions in the outgoing transmission lines of a great number of rural transformer substations, what was noted in our publications for this period (Kosoukhov, 1988; Kosoukhov, 1985; Naumov, 2001; Naumov, Horenkamp & Schulz 2005; Damdinsuren, 2016). To exemplify, we can present some data on the results of measurements and calculations, for the Irkutsk region on one of the transformer substations with 4 outgoing transmission lines supplying residential-commercial load. The research was being done all the year round: in winter, spring, summer and autumn.

Based on an analysis of the data only for one (the first) outgoing transmission line, we can state the following. In winter, the value of the neutral conductor current in this line is 21.3 percent higher than the mean value of the currents in three phases. In spring, it is 26 percent higher; in summer, it is 35 percent lower, and in autumn – 30 percent lower. On the whole for the year, however, the value of neutral current is 8.1 percent lower than the mean value of current in the three-phase system, i.e. only by 1.3A. At the same time, an average value of the power loss factor for the same transmission line was 2.06, i.e. power losses under unbalanced operation of the transmission line exceeded the corresponding losses under balanced operation more than two times. Consequently, the value of the neutral conductor current in the conditions of sufficient current unbalance is comparable with the average value of current in the three-phase system.

Thus, a considerable increase in the neutral current of indoor cable wirings with the neutral conductor can be one of the main reasons for the fires in residential and industrial premises, posing a threat to the lives of people and animals.

An analysis of fires dynamics in the Irkutsk region

Let's pass to the analysis of the causes of the fires in the Irkutsk region. The analysis is carried out during 2000-2014 in the cities and rural settlements. On the published data of Head department of the Ministry of Emergency situations on the Irkutsk region (MDC Prevention of fire spreading, 1997). and also according to the data of Chief Administration of the Ministry of Emergency Situations of the Russian Federation in the Irkutsk region (Information letter of Chief Directorate of the MES for the Irkutsk region, 2014), “the total of the fires made 70246”, as shown in Figure 1 (Naumov, Ivanov, Podiyachikh, & Shpak, 2007). It should be noted that more than 20% from all number of the arisen fires fell to the share of those that were the result of violations of the Rules of devices and operation of electric equipment (further in the text “Rules”). From the considered number of fires as it is noted in (Naumov, Ivanov, Podiyachikh, & Shpak, 2007), largest number of fires was observed in 2002, what made 9%, and the smallest number of fires - in 2014, approximately near 4%.

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Change of the number of fires for a cause of infringement of Rules is presented in figures 2 and 3 .

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In the submitted figures ( 1 and 2 ) it is designated: 1) NFiC - the number of fires for the reason of fault in construction and manufacture of electrical equipment; 2) NIIEE .- the number of fires for the reason of improper installation of electrical equipment; 3) NIOEE .– the number of fires for the reason of improper operation of electrical equipment; 4) NAF .- the number of fires, the electric devices caused by misuse; 5) NIOEC .- the number of fires, the protection devices caused by the wrong choice in internal electroconductings; 6) N _OTHER .- the number of fires, the "Rules" caused by other causes of infringement.

The analysis of the submitted charts on figure 2 and 3 showed that the total of the fires for 2,3 and 4 reasons in 2014 are decreased. For example the number of fires for 2 and 4 reasons decreased more, than by 90% in urban areas and more than 60% in rural areas. The number of fires, the protection devices caused by the wrong choice in internal electroconductings practically remains invariable. And, at last, the number of fires for the 6th reason has a resistant tendency to increase. Apparently (figures 2 and 3 ) for urban areas such increase made more than 50% of charts, for rural areas - more than 60%. In figures 4 and 5 the temporary charts characterizing consequences of the fires in urban and rural areas are submitte. In the submitted figures it is specified: Ndead p. – the number of the died people; Ninj.p. - number of the people who were traumatized; Ndead cat. – number of the dead of animals; Ndead small cat - quantity of the died small animals.

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Apparently from the submitted charts, in the cities died and it is injured, respectively 583 and 774 persons, in rural areas 343 persons died, 187 people were traumatized. Besides, the number of the dead of animals in the cities and rural areas, was 231 and 262 respectively. As is seen from the analysis, the fires that occurred during the studied period led to drastic consequences: deaths of people and animals, and considerable material damage. Besides, the arising fires result in significant material damage. Apparently for a cause of infringement of "Rules" in city and rural settlements the material damage was the fires of figure 6 , respectively 570630 and 202147 thousand rubles.

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Development of the express balancing device

Our analysis of literature sources confirmed that one of the dangerous manifestations of electric current, from the standpoint of fire danger, is certainly its thermal effect. Conductors that draw the current exceeding the rated value begin to overheat. This can lead to an increase in temperature leading to ignition of the insulation, which can cause a fire. The temperature of the conductive core depends on the strength of the flowing current, the ambient temperature, the core diameter and conductor insulation, heat exchange with the environment, the resistivity of the conductor material, and the time of emergency operation. Moreover, none of the sources considers the overloading of the neutral conductor due to unbalanced phase currents as a cause of overload and short-circuit conditions. Along with this, any overloading leading to additional overheating of insulation in intra-apartment or in-house electrical networks and corresponding short-circuits are undoubtedly connected with the current increase in the neutral conductor. Therefore, the development of methods and technologies for balancing the operating conditions of electrical networks that feed the household load and their application in such networks will be the most effective way of preventing fires.

The main principle of this type of shunt-balancing device is the automatic control of its parameters in the function of the zero-sequence current. Each block of this device has the minimum resistance to the zero-sequence currents, which allows the zero-sequence currents to be closed in the section between the unbalanced load and the device connection place.

The use of balancing devices is a real way to reduce fire hazards because of a decrease in the power flows of zero sequence at the phase current unbalance (Naumov, 2001). Their installation at the incoming switchgears of the urban and rural premises will considerably decrease a power loss factor and, hence, prevent the most severe fire consequences. Thus, a decrease in the zero sequence currents flowing through the neutral conductor will make it possible to substantially reduce the power loss factor and decrease the probability of fire to a great extent. To achieve this, we propose using a balancing device with controlled parameters. The device can both decrease phase current unbalance and control zero sequence flows and, hence, minimize energy losses in the device itself in the conditions of minimum current unbalance (Naumov, Ivanov, Podiyachikh, & Shpak, 2007).

A balancing device (Fig, 7 ) for the three-phase four-conductor network with controlled parameters consists of six star-connected capacitors (1-6) and an inductance coil 7 that has an additional terminal 8. Through one end the inductance coil 7 is connected to a common point of the star and through the other end - to the neutral conductor N9. The points of the star of capacitors (1-6) are connected to the phase conductors of the network ( А , В , С ). In the first stage of power, three capacitors (1,3,5) and part of inductance coil 7 are connected. As the unbalanced currents and voltage increase, the second stage of power is connected and the power of the device rises. This is achieved through the connection of additional three capacitors (2,4,6) and full coil of inductance 7. The device is completely disconnected from the network ( А , В , С ) when the current in the neutral conductor N reaches the minimum value corresponding to the admissible value of the current and voltage unbalance. The maximum power of the device is determined by its parameters which can be calculated by the technique described in (Damdinsuren, 2016).

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