Method of Calculation the Load Rates and the Unbalance Rates of the Current of the Phases in the MV/LV Network of the Sales and Service Center Lemba in Kinshasa

ABSTRAC T : In this article, we discuss the calculation of load and current imbalance rates, which is a steady-state study of the distribution network that consists of determining, for each subscriber station, the operating rate of the installed power transformer and the current difference between two phases of the MV/LV distribution station. Knowing the allowable currents of each low-voltage phase of the subscriber substations from the busbars as well as the rated current of the substations transformer, we can calculate, secondly, the average currents and load factors of the substations including the differences between the currents and their unbalance factors in the low-voltage distribution power lines. The mathematical equations of the electrical quantities of the power system facilitate the evaluation of the performance of the power transformer. These equations confirm that the variation in load ratio and current imbalance of three phases are closely related to the current draw of the allowable loads at each phase.


I. INTRODUCTION
Electrical energy is a very relevant factor for the development of society, its operation requires monitoring of current and voltage of phases in the distribution network [1] [2].However, an electrical network, even if it is installed according to the standards, is always subject to various disturbances in operation, such as unbalance of current and voltage of phases.Phase voltage unbalance refers to the capacity of a power system and the radius of the low voltage lines, for a given initial operating condition [3] [4].On the other hand, the phase current imbalance is said to be related to the way subscribers are connected to the different phases of the distribution network.We have observed several scenarios on the MV/LV electrical substations that supply an intolerable number distribution subscribers in the city of Kinshasa.Many of the substations operate with phase imbalances, which even influence the quality of electrical energy.These imbalances are due to the way subscribers are connected to the phases of the distribution network in the LEMBA area of KINSHASA.There is a voltage drop beyond the tolerable limit and the burning of low voltage cables.The operation of a three-phase network ideally requires that the voltage and current amplitudes are respectively equal on each of the three phases with an angle of 120 degrees [5] [6].The low-voltage network supplies mostly single-phase loads.Even if the distributor tries to distribute them evenly over the three phases, the variability of the consumption generates an unbalance phenomenon.Any voltage imbalance greater than 2% will cause equipment to overheat, making it necessary to oversize to avoid premature degradation [5][6] [7].A good load distribution, a static compensator and a judicious setting of the protections against current imbalance allow an optimal exploitation of the electrical network by the subscribers.These phase imbalances are solved by mathematical equations and simulated by Matlab software.This article consists in calculating the power transformer load rates and the phase current imbalance rates in the MV/LV distribution network of the LEMBA center in Kinshasa.Imax-écart=MAX (Imoy-  , Imoy-  , Imoy-  ) +   +   : phase currents.
The imbalance rate must be greater than or equal to 15% Utilization coefficient Ku Discounted Utilization Coefficient

I.3 Procedure for remediation of an electrical network
An electrical network which has within it a given number of electrical transformers of apparent power (Sn1), load rate (Tx1), Sn2, Tx2, ..., Snn, Txn must be reinforced for a given horizon, which is to limit the operating cost by maintaining the high level of reliability with constraints related to the environment.The total installed apparent power of the network to be remediated by the formula (8): =  1 +  2 +  3 + ⋯ +   (8) Formula (9) is used to determine the average load of the power system to be sanitized.
discharge transformer is determined by the formula (10) : The number of transformers to be installed on the network to be repaired is determined by formula (11) : The calculation of the total apparent power to the network to be remediated is determined by the formula (12).

III. UNBALANCE OF PHASE CURRENT III.1 Analysis of phase current gaps in MV/LV substations
It should be noted that during a fault, load shedding or overload of a feeder or a zone, subscribers can connect in a disordered way on one of the phases.And often, this creates overloads, burns cables or strips in a cabin.In our case, Lemba has at least 33 cabins that are supplied at 6.6 kV and 20 kV with different powers, 630 kVA, 800 kVA or 1,000 kVA.2), which varies from 1A (Kimafiki substation and Tuana3 cabin and Katanga cabin) to 72 A (Echangeur substation).The blue curve shows the current difference between phase two and phase three, which varies from 1A (Kiyimbi substation 1, Paka1 substation) to 303 A (Katanga substation).The green curve shows the current difference between phase three (3) and phase one (1), which varies from 5A (Luenda substation) to 304A (Katanga substation).

III.2 Analysis of phase current imbalance in MV/LV substations
So, during a load shedding we can calculate the behavior of the current in one of the phases and we will take the smallest and the largest value of the departure fed by the current 1 and 2 for the 32 cabins of CVS Lemba.The same for the current 2 and 3, and 3 and 1 whose result was the object of the simulation of the software MatLab and the result will be expressed in percentage (Tx%) as illustrated on the figure (5). .

Figure 5 : Analysis of phase current behavior in MV/LV substations
The purple graph shows the current imbalance rate between phase one and phase two, which varies from 0.1126% (Kimafiki substation) to 10.769% (Basanga substation).The blue curve shows the current imbalance rate between phase two and phase three, which varies from 0.13986% (Kiyimbi 1 substation) to 97.427% (Katanga substation).The purple curve shows the current imbalance between phase three and phase one, which varies between 0.01% (Kiyimbi 1 substation) and 79.349% (Katanga substation).We have plotted in Figure 13 the rate of unbalance of the phase current in the substations (Mohiya, Lulonga and Loange) as a function the number of substations at 20 kV.On the blue curve, the rate of current unbalance between phase one (1) and phase two (2) varies from 2.006% (Loange substation) to 4.032% (Mohiya substation).The curve in red color, current imbalance rate pattern, between phase two (2) and phase three (3) varies between 0.265% (Lulonga substation) to 3.01% (Loange substation).On the black curve, the rate of imbalance between phase three (3) and phase one (1) varies between 1.003% (Loange substation) to 5.65% (Mohiya substation).

Figure ( 1 )Figure 2 :
Figure(1) shows the analysis of the phase current in MV/LV substations.The blue curve shows the current of the first phase which varies between 156 A (Tuana12 substation) and 1052 A (Somida substation).The red curve shows the current of the second phase which varies from 145 A (Tuana12 substation) to 1098A.(Somida substation) The green curve shows the current of the third phase which varies from 125 A (Tuana12 substation) to 1025.(Somida substation).II.3.2Analysis of the average current of MV/LV substations

Figure 3 :
Figure 3 : Analysis of phase current behavior in MV/LV substations

Figure 4 :
Figure 4 : Analysis of phase current behavior in MV/LV substations

Analysis of MV/LV substations II.2.1 Analysis of the current in MV/LV substations Figure
1 : Analysis of the current in MV/LV substations