DESIGN AND EXPERIMENTAL INVESTIGATION OF ROBUST CONTROL TECHNIQUES FOR RELIABLE AND SECURED OPERATION OF HYDRO GENERATING EQUIPMENT

Abstract

Hydro plant control system (HPCS) has gained a high level of attention in recent years in view of variable speed operation, digitization, and cyber-attacks on process controllers and measures suggested in recently released IEC Standard 62433 (2019): Security for Industrial Automation and Control Systems. Since 1900’s, synchronous machine based fixed speed hydropower plant (HP) has been installed in the European, American, and Asian continents, and now more than 140 GW fixed speed pumped storage power plant (PSPP) are operating in the world. Nowadays, adjustable speed HP is an emerging technology in a pumped storage system where it has several benefits. The first adjustable speed HP was commissioned in the early 1990’s, and till now, 18 such power plants are installed/under construction all over the world with a total capacity of 9425 MW. Synchronous machine based variable speed operation is not economical because high capacity power electronic converters (rating of the converter equivalent to the rating of the machine) are required. Therefore, Doubly Fed Induction Machine (DFIM) based variable speed operation has been gaining immense popularity due to its flexibility of operation below or above synchronous speed and lesser power converters (10-25% of machine rating), and high dynamic stability. In India, a 1000 MW variable speed hydropower plant consisting of four-unit (with voltage source converter based excitation control system) is under construction at Tehri dam, Uttarakhand. In DFIM, rotor side power converters control the real and reactive powers of the machine based on set points (reference) and feedback signals from various sensors. Grid side power converters control dc-link voltage and unity power factor. A comprehensive literature survey is carried out in the area of various control systems (hydro turbine control system (HTCS), excitation control system, control system of starting of pump, protection system, and dynamic braking) applied to fixed speed and variable speed hydropower plant (HPP). In addition, operational challenges for the different control systems have been discussed. In fixed speed HP, HTCS is responsible for delivering active power at desired grid frequency, whereas in variable HP, both generator control and HTCS are responsible. However, HTCS is well established in consideration of fixed speed (synchronous generator) where single input, i.e., guide vane opening (GVO), is controlled by single-output (speed/frequency). Nowadays, HTCS becomes more intricate insight of variable speed doubly-fed induction generator for compensating wide variation in water head. A complementary sliding mode controller (CSMC) is designed for GVO serving a 250 MW hydro generating unit. The performance of CSMC for GVO and its influence on generator characteristics are analyzed and compared with the traditional PID controller. The performance of CSMC is tested for three i | P a g e different scenarios as (i) startup process, (ii) transition from synchronous to subsynchronous, and (iii) transition from synchronous to super synchronous. It is observed from test results that dynamic performance is improved, i.e., overshoot and settling time (Settling time for startup process, synchronous to subsynchronous and super synchronous is reduced by 75, 35, and 15 seconds respectively in comparison to PID and overshoot is also reduced). Dynamic behavior of control circuit (excitation system) faults (speed sensor) of a 250 MW DFIM hydro generating unit, to be commissioned in 1000 MW Tehri PSPP, operating at generation mode, are discussed. In addition, the survivability status of DFIM under different types of speed sensor faults (speed encoder omission (SEO), speed encoder gain (SEG), and speed encoder saturation (SES)) are assessed based on performance measures. Further, control redundancy circuit is designed using second order sliding mode controller (SOSMC) based model reference adaptive system (MRAS) speed estimator. It is found more robust against machine parameter variation in comparison to PI based MRAS estimator. Dynamic behavior of 250 MW hydro generating unit during various hacked signals (speed reference and dc-link voltage reference) has been analyzed. Further, to protect the converters serving in hydropower plants from cyber-attacks, a parallel controller framework comprises the model predictive controller (MPC) as a digital and proportional-integral controller (PIC) as an analog controller is designed. To mitigate the adverse effects of unbalanced grid voltage conditions, a dual sequence proportional integral (PI) controller with synchronous reference frame-phase locked loop (SRF PLL) is designed for 250 MW hydro generating unit to control positive and negative sequence components of current separately. Further, the performances of single sequence controllers and dual sequence controllers are compared in terms of DFIM characteristics. In addition, a dual sequence controller with cascaded delay signal cancellation – phase-locked loop (CDSC-PLL) is designed which further, improves the responses of DFIM characteristics in terms of reduced overshoot and reduced settling time. An experimental set-up with 2.2 kW DFIM is developed in the laboratory to support the simulation results. Overall, the present research work shall be helpful to the project authorities/policymakers in hydropower engineering during the design stage of their future projects.