## 6 AC/DC Power Flow of a Multi-Infeed LCC-HVDC System  
The power system is a mesh network between generating plants and consumers. A power (load) flow study is a method for analyzing the flow of power numerically in a complex interconnected system. Power flow analysis computes various key parameters in a steady-state environment, such as real power, reactive power, voltage magnitude, and angle. Power flow analysis is very important for the planning and expansion of a power network. Load flow analysis gives the strength of a power system to supply power to connected loads. An adequate transformer tap position is selected to provide a scaled voltage to load. The power flow problem is highly nonlinear. Thus, numerical methods are employed to get a solution within acceptable tolerance. Before applying any technique, a number of unknowns are identified. Similarly, the buses are classified into slack bus, load bus, and generator bus. At the load bus, real and reactive powers are given. At the generator bus, real power and voltage magnitude are given. The slack bus is a reference bus with a known voltage and its angle. Power balance equations are derived to compute unknown parameters. There are a number of methods for solving nonlinear power flow equations, such as Newton-Raphson, Gauss-Seidel, fast decoupled, and holomorphic embedding. Among these methods, the Newton-Raphson method is very much famous due to its fast convergence ability and simplicity. The Newton-Raphson method uses the Taylor series to get approximate linear equations of a nonlinear power system by ignoring the higher orders. The Newton-Raphson power flow method has been used in a power system for a long time. Power flow analysis is essential in order to plan new systems and get the critical operating conditions of an existing power network [48]. With the introduction of a DC transmission technology, the conventional AC power flow term has turned into an AC/DC power flow. The two most important methods for solving an AC/DC system are simultaneous and sequential approaches [49]. In a simultaneous approach, the DC equations describing its power, voltage, and current relation, including control parameters, are solved in combination with AC system equations. The sequential approach solves both AC and DC systems separately by using interface variables, which link these two systems. The sequential approach is easy to program and is widely used in an AC/DC interconnected power system. In ref. [50], a unified (simultaneous) approach is discussed to compute the AC/DC power flow by incorporating reactive power devices. The purpose of an AC/DC power flow solution is to find unknown parameters for rated values of known parameters. The parameters for a DC system are extinction angle, converter tap position, AC bus voltage (interface variable), and AC bus voltage angle (interface variable). An application of a sequential approach is given in [51], and an application of a unified (simultaneous) approach is explained in [52]. An approach for AC/MTDC power flow is discussed in [53]. The method presented replaces a DC system with an equivalent voltage depending on the load and deploys the fast decoupled method to solve power flow equations. A load flow analysis for hybrid AC/DC systems with distributed energy resources is reported in [54]. The analysis uses a sequential approach with the Newton-Raphson method by incorporating converter losses. A sequential approach to the power flow of voltage-source-converter-based multiterminal DC systems is presented in [55]. The load flow using the Gauss-Seidel method of an integrated AC/DC network is discussed in [56]. The method uses a sequential approach with DC systems as the current injecting source at a converter bus. A solution to the AC/DC power flow problem of multiterminal HVDC systems with a sequential and Gauss-Seidel approach is discussed in [57]. A fundamental work in an AC/DC power flow is reported in [58]. The work provides insight information on both sequential and simultaneous techniques.