Modular Multilevel Converter-based HVDC System Under Fault Conditions and Fault Detection Fahad Khan PG Scholar, Dept. of EE, Integral University, Lucknow India Isarar Ahamad Professor, Dept. of EE, Integral University, Lucknow India Abstract— Electrical energy in modern times is required in almost all commercial and industrial purposes. But in the case of HVDC systems the faults and protection mechanisms are not as developed as are for AC systems. This paper presents the fault analysis for a Modular Multilevel Convertor in HVDC system and a fault detection system, using MATLAB. Different DC & AC faults are analyzed. And the fault detection system is tested for robustness. DC & AC faults are different in nature which allows us to differentiate between them. Keywords— Modular Multilevel Convertor, HVDC, Fault detection System I. INTRODUCTION For many years electricity is generated/produced in AC form. Then this electricity is transmitted and distributed to customers also in AC form. For transmission to long distances its more beneficial to convert it to DC and then transmit the power. Although the initial cost is high in HVDC systems but for longer distances they are less than those of AC transmission systems and also lower losses when compared to AC systems. Moreover, HVDC causes less impact on the environment than High Voltage AC. HVDC is used for around 700 KM in case of overhead lines and 40 KM in case of underground lines. HVDC systems can also be used to connect two different grids operating on different frequencies. Also, grids in different countries/continents can be connected together to increase grid stability and reliability. Modular Multilevel convertor (MMC) has many ad vantages flexible expandability configuration without the need for transformers, higher reliability, redundancy, low switching losses, high efficiency, low harmonic distortion in the output which allows for omitting bulky filters at the AC side and reduction of DC link capacitance. They also have some disadvantages like complex overall structure, circulating currents, complex control strategy, voltage fluctuation at low frequency. Different types of faults may occur on both the AC and DC side of the MMC and therefore we need to study them and devise a fault detection system which can detect these faults. Also we have take into account the presence of different devices like STATCOM, UPFC and SSSC have on the impedance calculations. A simple relay is accurate enough for small, simple systems but when these systems contain high power devices, the distance relay has a tendency to under reach or over reach. The impedance increases/decreases due to the presence of these devices which will impact the working of traditional relay. For example, a device absorbs reactive power, then the measured impedance will be greater than the actual and it will operate in under reach or in another case if the device connected delivers reactive power, then the impedance measured will less than original and the relay will go to overreach. Different fault types that are possible in a system are: 1) Positive pole to ground fault 2) Positive pole to Negative pole fault. 3) Single line to ground fault on the AC side 4) Double line to ground fault on the AC side 5) Triple line to ground fault on the AC side. II. LITERATURE REVIEW This section deals with MMC topology and the components used in a three-phase MMC. It consists of a DC input terminal, an AC output terminal and different converting submodules with one leg for each phase. There are two converting submodules in the upper arm and lower arms also contain same submodules which are connected together with an inductor to filter high frequencies components from the arm current. MMC’s are capable of bidirectional power conversion. Fig. 1: General configuration of Three phase MMC The SM or sub-module is an important part of the MMC. They can be classified as: 2 level SM (single source) and multilevel SM topology (multi-source). 2-level Topology: in the past 20 years many different topologies have been proposed. The most widely used topology is the half bridge SM due to its simplicity and low cost. It’s comprised of 2 switches with diodes that are anti parallel coupled with a capacitor of floating type. The voltage in SM can be made zero depending on the state of the capacitor. It is for this reason sometimes this SM is also referred to as chopper. III. OPERATING PRINCIPLE Inside a 3 phase Modular Multilevel Convertor, every phase is divided in to two parts and each part has N no. of International Journal of Engineering Research & Technology (IJERT) ISSN: 2278-0181 http://www.ijert.org IJERTV10IS060389 (This work is licensed under a Creative Commons Attribution 4.0 International License.) Published by : www.ijert.org Vol. 10 Issue 06, June-2021 836