Contribution of DGs in the stability and voltage drop reduction for future MV network in desert regions

Received Oct 5, 2019 Revised Nov 9, 2019 Accepted Jan 23, 2020 Day by day, the integration of decentralized generation in medium voltage networks becomes more important during the last years and even in the near future. This increase causes, at the same time, several negative effects and rarely positive impacts on the stability of the network. Therefore, this work aims at analyzing the impact of ambient temperature on radial distribution network parameters’ e.g.: voltage drop and stability voltage level (index). Based on MATLAB program, different analyses of distributed generation (DG) insertion influence’s on voltage drop in the radial distribution feeder, as well as the influence of climatic conditions such as ambient temperature on network parameters. The Integration of Photovoltaic DGs in MV networks can play an important role in reducing the global warming effect (in voltage drop, and voltage stability index) especially in radial distribution feeder. Furthermore, it protects network’s parameters if its location and power are well selected.


INTRODUCTION
In the electricity sector the first step in understanding the impacts of climate change is based on knowing the certain terms such as increasing the ambient air temperature [1]. Around the world, environmental pollution has experienced a real acceleration in recent years; this acceleration has several forms such as the increase in heat of the gallop. In a desert region the heat can vary in normal cases between 5 up to 50° C, and on this basis several systems are installed especially those affected by the ambient temperature such as electrical energy distribution systems. The data bases used by the operators in the tables of facilities are drawn up on the basis of 25° C and 50° C. But this reality will change over time as a result of global warming, for example; the ADRAR region (desert) recorded a maximum temperature on July 9, 2018 which reached 65°C [2]. These new records recorded can seriously affect the stability of the electrical power distribution system in general, and the parameters dependent on the particular ambient temperature such as voltage drop.
Therefore, the electrical distribution systems have to be re-studied, and get an overall idea of the increase in ambient temperature impacts on the voltage stability of the electrical system, and the use of DGs as solution which also have impacts on stability. In this context, several studies focused on the integration impacts of the decentralized generation in the distribution networks, these impacts can take several aspects. Some researchers were carried out on DG technology and its impacts on the energy system. These impacts may concern network planning, losses, voltage, energy quality and reliability [3][4][5][6][7][8]. The large number of DG

PROBLEM FORMULATION
In this paper, the objective functions of ambient temperature to reconfiguration problem are voltage drop and voltage stability index which are calculated for each case in radial distributed systems.

Voltage drop (VD)
Voltage drops (VD) is the difference between the voltage at the source (or sending end) bus (Vs), which is assumed to be a fixed value, and the voltage at the load (or receiving end) bus (VL), which varies as a function of the load current (lL). [34]

Factors affecting voltage drop
One can classify the factors affecting the voltage drop in two categories. The first is dependent to load parameters that are; voltage, current and power factor, the second factor depends on line impedance i.e.; length and resistance. This last is linked directly to the material nature (copper, aluminum …), to the conductor cross-section and ambient temperature.

Voltage drop calculations
The determination of voltage drop value for lines inferior to 50 km in medium voltage network (U < 40 kV) is based on two formulas: First formula that gives the exact result: Where: VD is the voltage drop, I is the line current (A), R is the circuit line resistance (Ω), X: is the circuit line reactance (Ω), and is the load power factor angle (degrees) Vs; source voltage, VL load voltage. The voltage drops VD that is calculated as a line-to-neutral voltage drop (one-way), and the line-to-line voltage drop. And a second approximate formula that nearly gives the same values is: that is calculated for a single-phase system is 2VD; and the tine-to-tine voltage drop that is calculated for a three-phase system is √3VD. So, one can determine the percentage of the voltage drop by the following formula: [35] % * 100

Ambient temperature impact on voltage drop
The change in resistance as a function can be represented by the following formula: where: R1 is a conductor resistance in ohms (Ω) at temperature T1 in degrees Celsius (°C) R2 is a conductor resistance in ohms (Ω) at temperaturesT2 in degrees Celsius (°C) and k is the resistivity temperature coefficient at 0°C for the conductor; k= 234.5 for copper, Aluminum 228, lead 230, steal 202. [35] For T=T1 et R=R1 VD1= I*[R1*cos (θ) +X1.*sin(θ)] and for T=T2 , R= R2 So VD2=I(R2*cos θ +X1*sin θ) X1 is independent of ambient temperature. Therefore, the (7) becomes; and from (4) and (7) the percent voltage drop becomes voltage drop equal √3VD for three phase system So, the percent voltage drop for three phase system formula becomes; VD % √3 I R * cos θ X * sinθ * 100/V the (8) means that there is a relation between the ambient temperature and voltage drop variation.

Voltage stability index (VSI)
Voltage stability is the ability of a power system to maintain steady acceptable voltages at all buses in the system under normal operating conditions and after being subjected to a disturbance, a system enters a state of voltage instability when a disturbance, increase in load demand, or change in system condition causes a progressive and uncontrollable drop in voltage [36]- [38].

Voltage stability index calculations
Voltage stability index can determined in favor of this formula: when s is source, r is loads and R, X, (Z) impedance of line, P and Q is charges [39]. In reality there are other formula to calculate

Ambient temperature impact's on voltage stability index
The formula shows that voltage stability level has a big relation with the line impedance, voltage buses, and load power. The impedance value increases with increase of its resistance as the ambient temperature increases. This affect the voltage stability index in the different buses of the power system (see formula 5) So, formula 10 becomes:

METHODOLOGY AND SIMULATION
We will devise this part of simulation in tow devises, the first talk about the impact of ambient temperate in voltage drop value for case to IEEE 4-Bus Test Feeder, and who can limit its effects as shown in Table 1. The second simulation devise of take IEEE 33-Bus Test Feeder radial distribution system like example to effects of insertion of DGs on voltage stability index value, and selection of the good conditions to support voltage stability.

Fist case: IEEE 4-bus test feeder
To carry out a simulation of the distribution network during hot days, the 8th of June temperature model will be applied ( Figure 1) on the model IEEE 4 Node test feeder ( Figure 2).  Figure 3).
This figure shows that the ambient temperature variance influences the impedance of the overhead lines and finally over the entire distribution network. But there are two cases; a positive case that is attached to the ambient temperature decrease, and a second considered negative that is the case of the ambient temperature increase, it is considered by the growth of the percentage (%) of the voltage drop.

Advantage of DG in voltage drop reduction
In order to see the nature and the intensity of the temperature influence on voltage drop calculations and especially in the future medium voltage networks where connected DGs may be found. Based on that, the following application in two different seasons is taken. The first one in winter where the change in temperature from -5 °C up to 20°C, the second one in summer that varies from 20°C up to 60°C, and sometimes exceeds 65°C in some Sahara regions and sub-of the Sahara. Consequently, the study interval should be taken from -5°C up to 70°C. In the first case, an application is carried out on a simple network that contains a source, a transmission line MT and a load, the voltage drop for the bus 2 is therefore calculated (see Figure 4 (a)). In the second case, a DG source is connected to see how the changes made influence (see Figure 4 (b)). The DG contributes in the load supply with 50%. And Table below presents data of the different elements of the network studied.

Second case: IEEE 33-bus radial distribution
In order to confirm the impact of DG insertion and its location on the voltage stability index (level) under ambient temperature, an MV 33 radial bus distribution network ( Figure 6) is used. This distribution system contains 33 bus (nodes) and a 3.715 MVA, 2.3 MVAR, 12.47kV power source. For further data see the Appendix (A & B).
The measured voltage in the different bus bars under ambient temperatures is: -5°C, 25°C and 65°C respectively. These temperatures represent the two ends of T2 and 25 °C represents T1. The results obtained are presented in Figure 7. The figure shows the degradation of the voltage level along the line, especially in buses 18 and 33 under the effect of the ambient temperature increase. This also causes a decrease in the level of voltage stability index (see Figure 8). The future MV distribution networks are networks with high possibility of the DGs insertion in one or more points. This type of networks suffers from climate heating but has the advantage of being in areas of very high annual average solar radiation, so it is very advantageous to install DGs of photovoltaic PV types. In fact, it has firstly been dealt with this type of problem by trying integrating a PV type DG with a 0.5MW active power in different IEEE 33 bus bars radial distribution system, the results obtained are shown in Figure 9. The tests indicate that the two sites that are really improving are inserts 18 and 33.

DG advantage in voltage stability
In order to understand the impact of DG insertion or in a MV distribution system, the previous example in the two main points which give good performance in reducing the voltage drop will be used. Four photovoltaic type DGs with different sizes were chosen, see Table 3. These DGs supposed to generate active power only.   table 4. At the beginning the minimum and the maximum of the voltage stability index (SVI) for two different ambient temperatures 25 °C and 70 °C is measured, then the divergence min-max (SVI) for each scenario will be calculated. The results obtained are shown in Figure 10.
Three categories of scenarios can be distinguished; better, average and bad. The 10 and 11 scenarios are classified as best category that are related to the insertion of two small DGs (0.5 & 1MW) in the bus 18 and 33, the max of this category does not exceed 1 (SVI ≤1 stability), and the divergence min-max is minimum as well as the impact of the ambient temperature appeared almost zero. for the average category one can classify the scenarios 2,3,4 and 5 which are related to the insertion of a single small DG either in the buses 18 or 33, the max of SVI and almost 1, and the divergence Min-Max is under the initial value (without DG), the impact of the ambient temperature is minimum. concerning the bad category it is found the scenarios related to the insertion of one to the two DGs of large sizes (2 or 4 MW), which are the scenarios (6, 7, 8, 9, 12 and 13), the max usually exceeds 1 (SVI ≥ 1 instability) and the divergence Min-Max is greater than the initial value, and the impact of the ambient temperature is remarkable. From these results it can be noted that the insertion of the small DGs at the end of the branches plays a significant role in the stability of the distribution networks and limits the impact of the ambient temperature on the installation.  Figure 10. divergence min-max of Stability voltage index to differencts scenarios for 25°C and 70°C

CONCLUSION
The electric energy distribution system is usually exposed to external impacts such as the ambient temperature that affects the internal stability parameters of the electric system that is usually voltage. For future electric distribution networks, the climate change effects should be taken into consideration during the phase of installation and especially for the calculations of the transmission lines impedances by keeping the ambient temperature superior or equal to 75°C. The insertion of the DGs remains a good means to fight against the impact of these temperature peaks that often touches the Sahara zones. The DGs insertion in a radial distribution network and its impact on voltage drop and on voltage drop indices under the effect of the difficult climatic conditions (extreme hot temperature zones) has been studied in this paper.
It has been shown that DGs can play a very important role in the improvement of the voltage stability index, as well as limitation of the voltage drop for future electric distribution networks, selection of the size, number and the insertion points of the DGs that well reduces of voltage drop and give a good stability. The impact of the ambient temperature on voltage drop (VD) and on voltage stability indication (VSI) that appears minimal in the different chosen scenarios will become more important in the most complex electric distribution networks and especially during peak days. According to the study, the optimal position where the DGs should be inserted in the radial distribution networks is at the ends of the branches as the 33 buses case been studied. The latter deals with the insertion of two photovoltaic type DGs with medium or small dimensions. This has shown a good efficiency in terms of stability for radial distribution network and minimized considerably the increase of the ambient temperature impact.  Load11  11  45  30  Load12  12  60  35  Load13  13  60  35  Load14  14  120  80  Load15  15  60  10  Load16  16  60  20  Load17  17  60  20  Load18  18  90  40  Load19  19  90  40  Load20  20  90  40  Load21  21  90  40  Load22  22  90  40  Load23  23  90  50  Load24  24  420  200  Load25  25  420  200  Load26  26  60  25  Load27  27  60  25  Load28  28  60  20  Load29  29  120  70  Load30  30  200  600  Load31  31  150  70  Load32  32  210  100  Load33  33 60 40