Transmission system is a series of interconnected lines that facilitate the bulk movement of electrical energy from a generating site to an electrical substation. This system suffers from unavoidable power losses and there is need to minimize these losses so as to improve the overall efficiency of the system. The existing methods with incorporation of Flexible Alternating Transmission System (FACTS) controllers suffer from inappropriate size and the exact location of these devices which could bring about appreciable reduction in real power losses. Hence, this research aims to reduce the total power loss by way of appropriate sitting and sizing of Static Synchronous Compensator (STATCOM) using Firefly Algorithm (FA) as optimization techniques.
In this research, a mathematical steady-state model of STATCOM Power Injection Model (PIM) was derived from one voltage source representation to form a new set of equations. STATCOM impedance was added into the admittance matrix of Newtow Raphson (NR) algorithm and new sub-blocks of Jacobian related to the STATCOM was formulated for power flow analysis. A MATLAB program was written to incorporate the STATCOM PIM model in NR Load flow algorithm and then optimized using FA. The approach was validated and implemented on IEEE 5-bus system and Nigerian 330 kV longitudinal transmission system respectively using the real power losses as performance metrics.
The results showed that at base load, the total active power loss for IEEE 5-bus system was reduced by 0.96 MW, which is 51.33% reduction after optimization. The best location and size of STATCOM device in the system after optimization is at bus 5 with shunt reactive power of 43.27 MVar. For 28-bus 330 kV Nigerian transmission system, power losses were reduced by 11.34 MW, which is 20.73% reduction after optimization. The best location and size of STATCOM device in the network after optimization are at buses 9, 13, 14 and 16 with values of shunt reactive power of 26.54 MVar, 43.74 MVA, 15.71 MVar and 6.14 MVar respectively. At peak load condition, power losses for Nigerian 28 bus 330 kV transmission system was reduced by 36.43 MW, which is 35.85% reduction after optimization. Also, the best location and size of STATCOM device in the network after optimization are at buses 9, 13, 14 and 16 with values of shunt reactive power of 40.25 MVar, 50.50 MVar, 58.40 MVar and 14.74 MVar respectively.
The results however demonstrated the applicability of the method. Therefore, optimization of STATCOM enhanced the capacity of the transmission system without expanding existing power system infrastructure.
1.1 Background of the Study
Sustainable energy supply play a key role in socio-economic development of many nations across the world. In recent years, geometrical demand for electricity in most developing countries of the world, caused by rapid growth in population, urbanization, industrialization and modern economy system, places a key challenge on power industry. As a result of continuous load growth which leads to power losses among others, unplanned constraints are placed on power transmission line (Ayodele et al. 2017).
Technical losses occur on power transmission lines, since the generating stations are always situated far away from load centers for several reasons such as environmental pollution, operational hazard and regulatory policies (Adegboyega and Onime 2013). The need to enhance the power carrying capability of the transmission lines in a bid to reduce real power losses and improve system voltage profile, that will guarantee power system security and reliability become imperative (Zhang et al. 2010). This situation warranted a review of the traditional power system concepts and its practices so as to realize a larger stability margin, greater operating flexibility and better utilization of existing power systems (Abido 2008).
However, many power utility expansion programs are being thwarted by land-use act and environmental constraints. In addition, building new transmission lines and electricity generating plants is capital intensive and involves many rigorous procedures such as licensing (Rajiv et al. 2007). A critical evaluation of the available options for optimizing existing power transmission equipment with high level of system integrity has pointed in the direction of power electronics devices (Sarita and Prateek 2015). Advancement in power electronics devices led to the development of flexible alternating current transmission systems (FACTS) devices. By design, FACTS devices are fast-acting, high power electronic devices endowed with advanced and reliable control mechanism (Natesan and Radman 2004).
FACTS devices are emerging technologies for providing reactive power compensation to mitigate issues of active power loss and voltage instability on existing power grid (Hingorani and Gyugyi 2000; Ajenikoko et al. 2017). It is vital to note that, the adequate supply and absorption of reactive power enhance the transmission line power transfer capacity (Hingorani and Gyugyi 2000). Recently in energy management practices, reactive power compensation via FACTS devices is one of the state-of-the-art methods for controlling and mitigating issues of power losses on the existing power grid (Sahu et al. 2012). Suffice to know that researchers are harnessing artificial intelligence techniques most especially the heuristic and meta-heuristics algorithms (Genetic Algorithm (GA), Firefly Algorithm (FA), Cuckoo Search Algorithm (CSA) and many others) to determine optimal sitting and sizing of these FACTS devices such that it will have overall improvement on system bus voltages.
Real power loss on transmission lines connotes revenue loss to the power utility companies. Its minimization, if unable to be completely eradicated, can be achieved via the use of FACTS controller such as Static Synchronous Compensator (STATCOM). STATCOM, being one of the most important members of shunt-connected FACTS devices, is a solid-state voltage source inverter coupled with a transformer and tied directly to the connected points such that the injected current is virtually in quadrature with the line voltage; consequently behaving as either inductive or a capacitive reactance at those connected points. Hence, it is more often used to enhance the performance of long transmission lines in this new age of power system (Kalyan 1998; Sirjani et al. 2012).
The core benefit of STATCOM over conventional shunt capacitor and Static Var Compensator (SVC) is that, its compensating current is independent of network voltage level of the transmission at the point of connection. SVC on the other hand, has constant current characteristics when the voltage fall outside the permissible range of limits. Shunt capacitor suffers from poor voltage regulation though its installation and maintenance cost is cheap when connected to the grid (Zhang et al. 2010; Sirjani et al. 2012).
Part of the problems on the Nigeria longitudinal system includes transmission losses, transmission capabilities inefficiency and its radial transmission nature which does not allow for system reliability. Additionally, problem of length of the transmission lines due to the long distances between generating station and load centre. This research therefore aims to minimize real power loss on Nigerian transmission system by suitable placement and sizing of the power injection model of STATCOM in the system.
1.2 Statement of Problem
The need for electrical energy supply is increasing on a daily basis and the electrical system is rather becoming more arduous to operate and less secured with unscheduled power flows that resulted in great losses which amount to imbalance between the generated power and power received at the consumers end (Sannino et al. 2003). An alternative approach in order to reduce these losses is to incorporate FACTS controller (STATCOM) on longitudinal transmission systems. However, there is the need to apply an optimization method capable of indicating the right sizing and sitting of this STATCOM controller in a longitudinal transmission system so as to reduce the system losses to minimum (Natesan and Radman 2004).
The optimal sitting and sizing of a STATCOM in a longitudinal transmission system can be quite challenging. In most cases, the main methods used for optimal sitting and sizing of a STATCOM controller in longitudinal transmission network are analytical and meta-heuristic methods. The formal are easy to implement and execute, but are computationally exhaustive and time consuming while the latter are very efficient for handling complex system, ability to attain global solution within shortest time and fast convergence in solving optimal solution (Adepoju et al. 2011).
This research therefore, employed a meta-heuristic technique, Firefly Algorithm for optimal sitting sizing of STATCOM power injection model on Nigeria longitudinal transmission system to minimize real power loss because past researchers had only installed STATCOM at buses where voltage magnitudes are low in which the issue of appropriate sizing and optimal placement were not considered for active power loss minimization.
1.3 Aim and Objectives
The aim of this research is to determine the optimal sitting and sizing of STATCOM on Nigerian longitudinal transmission system for power loss reduction using Firefly Algorithm (FA) as optimization technique.
The objectives of this research are to:
- perform power flow analysis on longitudinal transmission system using Newton –Raphson method.
- formulate the Newton-Raphson method with inclusion of Power Injection Model (PIM) of STATCOM and optimize the model using FA.
iii. validate and implement the results on standard IEEE 5-bus and Nigerian 330 kV longitudinal transmission system respectively using real power loss as performance metrics.
The Nigerian electric power system is a complex network faced with several operational challenges such as inadequate reactive power compensation, frequent system voltage instability and several cases of blackout in many parts of the country among others. Despite these identified problems, the demand for power is constantly increasing on a daily basis, since the place of uninterrupted energy supply in enhancing socio-economic activities and industrial growth of both local and foreign owned industries cannot be over-emphasized. Several preventive measures can be adopted, one of such is construction and installation of new power plants and new transmission lines, but the effect of environmental constraints and huge capital involvement render this option un-economical to implement. The need to enhance the present transmission network system so as to flexibly respond to generation and load patterns and minimize power loss become a promising option. This work therefore seeks to improve the present Nigerian transmission system by optimally incorporating STATCOM on the network with a view to minimize real power losses.
1.5 Scope of the Study
This work will only address the problem of joint optimal determination of location and sizing of STATCOM power injection model with the aid of Firefly Algorithm on longitudinal transmission system of Nigerian 330kV, 28-bus system and focuses on balanced three-phase system. Issues related with protection of transmission system are not considered in this work.