ANALYSIS OF SUB SYNCHRONOUS OSCILLATION IN HYDROPOWER SYSTEMS

Abstract

Hydropower is a leading energy resource amongst the world’s renewable energy capacities. According to the International Hydropower Association Status Report 2023, hydropower contributes 15.7% of total electricity production. To achieve proficient generation in the arena of the hydropower sector, a widespread design with recent trends and developments in technology is obligatory. Asynchronous machines are now extensively used in hydropower plants due to their flexibility in power generation, better energy efficiency at wide variation in water head, partly rated power converter interface, and intrinsic short-circuit and overload protection. Asynchronous generators of large capacity (e.g., 250MW) need further research to identify the amount of compensation in transmission lines in view of variable speed of operation. Sub Synchronous Oscillation (SSO) is generated in long distance transmission lines with the series compensation of certain amount. This SSO disturbs the constancy as well as security of the power system. The rotor side of the DFIM is typically linked to the grid through back-to-back power electronic converters, while the stator circuit is connected directly to the grid. As a result, it is considered a partial isolation from the grid and eliminates the requirements for synchronous operation of the turbine to deliver power to the grid. However, it is vulnerable to sub synchronous oscillations when connected to series compensated transmission lines or under weak grid conditions. For long distances, extra high-voltage transmission lines are used to transfer electricity from large-rated electric power stations to consumers for higher energy efficiency. To increase the power transfer capability, capacitor units are connected in series with the long transmission lines. However, SSO is a prevalent issue when DFIM is radially connected with the series compensated transmission lines. The events on the Texas Electric Reliability Council's electrical network, as well as the electrical networks of southwestern Minnesota, the United States, and Hebei, China, are examples of sub-synchronous oscillations that occur in wind farms with doubly fed induction machines. Based on literature and real events, it is understood that the research on SSO has been conducted extensively for wind power applications. However, the research conducted on SSO with DFIM-based large hydro units is limited, but it is essential in view of variable speed operations of the turbine and generator, multi-channel converters in the rotor circuit, and DC link capacitors.