Statcom integration in a power grid to enhance voltage stability

Received Jan 27, 2019 Revised May 1, 2019 Accepted Nov 24, 2019 To solve load growth of an existing electrical system, we at first inject the power in system by building new electrical plants as wind, solar or thermal. And secondly in year 2030, when the system is so meshed, some customers are far away at production stations. We propose the installation of FACTS devices to enhance voltage in stability margins of power system under study. The power flow analysis using Newton Raphson method was running on Matlab and PSS/E Simulator. The results of simulation without STATCOM show the most affected buses. To improve the voltage for all these buses, reduce the power mismatches and tranmit more ative power, we insert STATCOM. STATCOM is a shunt connected FACTS application which facilitate the fast voltage control, the reactive power control and reduce the harmonics in a power system. Keyword:


INTRODUCTION
The concern for good power grid management is not only to ensure that transits power are lower than the transmission capacity. It is also necessary to monitor several technical parameters, including the voltage drop [1], [2], [3]. The voltage must remain within an authorized range at any point in the network, in all foreseeable production and consumption situations. In this context, we propose an analysis for the current and evolving production system to satisfy the domestic demand of the 33 kV network and interconnected system. This analysis let us not only to find, but also to maintain a voltage profile in stability margins (between 0, 95 and 1, 05 pu). Another objective is, to propose a methodology for the management and control of transits power, in order to make the most efficient use of the system in a more suitable way [4], [5], [6]. To enhance voltage, we must inject reactive power (Q) at buses where the voltage is out constraint limit. Many reactive power compensation systems exist, e.g. conventional (shunt capacitor, series capacitor, reactor etc) or Flexible Alternatif Current Systems (FACTS). Our choice is focused to FACTS systems per comparison with conventional systems. Several types of FACTS currently exist and the choice of the appropriate device depends largely on the goal to be achieved [7], [8]. Thus, to achieve these objectives, we take the following four steps: • First step, a schematic diagram of the 33 KV loop network with inteterconnected system is given.
• Second step, the production and load forecasts are given at 2020 to 2030 years.
• Third step, a numerical model of Newton Raphson (NR) method is resolved and programmed on MATLAB environment and PSS/E simulator for two situations (without FACTS and with FACTS devices) [9]. The results of simulations will be accompanied by discussions. • The last step, a proposal is given on the behavior to be held for the optimal reactive power and the voltages control on an electrical distribution network. A conclusion will be given of the work. For the insertion of FACTS systems, it sought a stable electrical energy network which is capable even during a disturbance to provide the demand power. FACTS devices are defined by IEEE as "power electronic based controllers and other static equipment which can regulate the power flow and transmission voltage through rapid control action". In earlier days power system control was only based on generator control or capacity bank (Table 1) and there controlling ability on the transmission lines was little (neglected). Different FACTS (Table 2) controllers can influence these parameters to regulate the power flow in interconnected systems. STATCOM a shunt connected FACTS application can facilitate the fast voltage control, the reactive power control and reduce the harmonics in a power system. This is done while keeping constant and close the frequency to nominal value, the alternators speed rotation and the voltage magnitude in the various network buses [10], [11], [12], [13], [14], [15].

STRUCTURE OF SYSTEM 2.1. Structure of 33KV Loops of Nouakchott System
The single-line diagram ( Figure 1) represents the 33 KV part of Noukchott network and the transmission lines to Boulenoir and Nouadhibou cities. The lines (cables) data, the values of generated and loads grow are given in Tables 3 and 4. The electrical network consists of 13 transmissions lines, 8 generators (two wind power, two solar plants, three thermal plants and 1 Dual fuel) and 6 loads at bus 2,4,5,6,7 and 9 ( Figure 1).The active and reactive powers generated are given in MW and MVAR respectively. The voltage of each bus (i) is given in KV. The load bus is characterized by its active power P and reactive power Q. Therefore, (P, Q) are specified, while (V) is to be calculated. In this context, it is proposed for the North bus (1), to be slack bus. Finally, it should also be noted that a bus is numbered (i) and it is connected to (k) other buses such as those shown in Figure 1.

Given data of system
It is also proposed in Table 3, the active resistances(R), the lines reactances(X), bus voltage (V), the lengths (L) at each bus (i) connected to bus the (k). It is also given in Table 4, the projected generation and load forecasts data of studying system at year 2020 to 2030. Active power is given in MW and reactive power in MVAR. Table 3. Cable data of Figure 1 Table 4. Projected generation and load forecasts data [16], [17] Year

Numerical model of STATCOM
The Static Synchronous Compensator STATCOM is one of FACTS derivate family. It uses the forcing electronic power commutation (GTO, IGBT or IGCT). A STATCOM is controlled reactive power source and improve the transient stability of systems [18]. It provides voltage support by generating or absorbing reactive power at the point of common coupling without the need of large external or capacitor banks. The basic voltage source converter scheme is shown in Figure 2. The Thevenin's equivalent circuit of the fundamental frequency operation of the switched mode voltage source inverter STATCOM and its transformer is shown in Figure 2 [18].  (1):

Mathematical model of power flow with STATCOM
Where VSTC is STATCOM's voltage, ISTC is STATCOM's current, and ZSC is transformer impedance. The voltage injection bound constraint of STATCOM is given by equation (2) Where VSTC (min) and VSTC (max) are STATCOM's minimum and maximum voltages Equation (1) is transformed into a power flow expression for STATCOM and power injected at bus (i) by equations (3) and (4) respectively: Where SSTC is STATCOM's apparent power, I*STC is STATCOM's complex conjugate current, V*STC is STATCOM's complex conjugate voltage, and Y*SC is complex conjugate of short-circuit admittance.
The voltages in rectangular coordinates system are expressed as equations (5) and (6)  The STATCOM's voltage magnitude and angle are expressed as equations (7) and (8) respectively: The active and reactive power components for the STATCOM and bus i on the basis of equations (5) to (8) are respectively expressed by equations (9) to (12): Where PSTCis STATCOM's real power, QSTC -is STATCOM's reactive power, GSC -is shortcircuit conductance, BSCis short-circuit susceptance.
The Newton-Raphson set of linearized equations for power flow equations (3), (4), (9) and (10) obtained taken into consideration the modeling of shunt-connected STATCOM at bus (i) is given by equation (13):  Table 5 shows the results of NR without and with STATCOM. It shows the optimal STATCOM location in buses and shown it impact on the system. The voltage profile before and after STATCOM connected are shown in Figure 3(a). It demonstrates the voltage magnitude increased for the bus 2 at 0.66 (value out limit [0, 95; 1, 05 pu]) to 1.05 pu, bus 3 at 0.91 to 0.98pu, the bus 4 at 0.59 to 0.99pu, the bus 5 at 0.62 to 1.006pu, the bus 6 at 0.64 to 1.02pu, the bus 7 at 0.75 to 0.95 pu, the bus 8 and 9 are fixed to the values 0.98 and 0.97pu respectively.

RESULTS AND DISCUSSION
The voltage angle before and after STATCOM connected are shown in Figure 3(b), it demonstrates the voltage angles increased for the bus 2 at -9.01 to -20 degree, bus 3 at-1.9 to -4.1 degree, the bus 4 at -12.08 to -21 degree, the bus 5 at -11.02 to -20.5 degree, the bus 6 at -10.15 to -20 degree, the bus 7 at -5.97 to -12 degree, the bus 8 at -0.39 to -0.4 and bus 9 at -079 to -0.8pu.  Table 6 reveals the results of power flow balances. We can notice that the total generated minuce the total load power (demand) gives the total losses of active and reactive power respectively ( ). We can also observe the value of reactive power (QSh) needed to minimize power losses and improve the voltage. Table 6. Power flow balances of system [19], [20] Figure 4: (a) is active power losses; (b) is reactive power losses From Figure 4(a) there was a reduction in total active power losses from 599.5 MW to 575.9 MW, thereby improving the active power transmission lines. These results show that the STATCOM has the capability reduce active power losses on the power system. From Figure 4 (b) there was a reduction in total reactive power loss from 797.1 MVAR to 767.2 MVAR, thereby improving the active power transmission lines. These results show that the STATCOM has the capability to reduce reactive power losses on the power system. Table 7 shows the results of delta active power, delta reactive power, errors of voltage and the delta angle with and without STATCOM as well as corresponding iteration numbers. This table also demonstrates that the NR method is more efficient per comparison with others numerical methods as GS. The number of iteration is less (4 iterations per comparison with GS 10 iterations); the convergence time remains virtually fixed and precise even if solution is complexed by the Jacobian matrix. Table 7. Reached tolerance in 4 iterations Figure 7 demonstrates the comparison of the voltage errors variation (a) and the delta angle (b) with and without STATCOM connected to the system as a function of ieteration numbers. However, since the magnitudes of all the voltages and their angles are not known a priori, an iterative procedure must be used to estimate the bus voltages and their angles in order to calculate the mismatches. Moreover, Figure 8 demonstrates the relatioship of delta Pi and delta Qi with the iteration numbers. When this value stopped the variations, it said to have converged (Table 7).

CONCLUSION
The simulation of load flow program on the MATLAB and PSS/E Simulator using the NR method has been done. The main information obtained is the out limit of stability all buses voltage of system except the slack bus. After insertion of STATCOM the voltage for all buses has been improved to the stability limit.The power losses compared to the NR method without STATCOM are greater than with the STATCOM ( Table 6).In the end the expected disturbances and the power quality enhancement of the network in the horizon 2030 were attenuated by integration of STATCOM that is able to supply or absorb reactive power and to maintain the voltage to 1pu. The second novelty is to replace the conventional reactive power compensation by STATCOM. Optimal location of STATCOM can be found out using Genetic Algorithm and fuzzy logic. Economic Assessment of FACTS devices against other methods can be studied.