TURBINE GOVERNING SYSTEM : COMMISSIONING & TROUBLESHOOTING AT 10X40 MW SHREE MAHESHWAR HYDRO

ELECTRIC PROJECT

SHANKAR SINGH PATEL, ENGINEER,

10X40 MW MAHESHWAR HEP The power house of Maheshwar HEP is equipped with ten units of 40 MW each with Kaplan turbine and matching hydro-generators along with BHEL make G-40 type of governor.

Governing system can be defined as a combination of devices and mechanisms which sense speed and power deviations and convert them to a servomotor stroke or gate position signal. Governing system is an important control system in the power plant as it regulates the turbine speed, power and participates in the grid frequency regulation. For starting, loading governing system is the main operator interface. Steady state and dynamic performance of the power system depends on the power plant response capabilities in which governing system plays a key role.

The load on a turbine (generating unit) does not remain constant and can vary as per consumer requirement. The mismatch between load and generation results in the speed (or

frequency) variation. When the load varies, the generation also has to vary to match it to keep the speed constant. This job is done by the governing system. Speed, which is an indicator of the generation – load mismatch, is used to increase or decrease the generation.

Following figure shows the basic architecture of the Governing system of Maheshwar HEP:-

Governing system controls the water flow to the turbine in response to the control

signals like speed error, power error. It is a closed loop feedback control system in which control action goes on till the power mismatch is reduced to zero.

The water flow through the hydro turbine is proportional to the gate opening in the

operating range. So when gate position changes, turbine flow changes and turbine power output also changes proportionally. Thus governing system changes the turbine mechanical power output. When the turbine generator unit is being started, governing system controls the speed precisely by regulating the water flow. Once the unit is synchronized to the power system grid, same control system is used to load the machine. As the connected system has very large inertia (‘infinite bus’), one machine cannot change the frequency of the grid. The gate opening changes either by changing the reference setting or by the change in speed (or frequency). This is called primary regulation. The reference setting can also be changed remotely by power system load frequency control. This is called secondary regulation.

Hydro units are very well suited for secondary regulation in view of their quick response capability and absence of thermal stress limits associated with thermal units.

Following is the System block diagram of the Maheshwar HEP governor system:-

The speed signal derived from Speed Signal Generators (SSG), which are coupled to the generator, is converted to DC signal, which along with DC signal from gate position potentiometer, is used for control functions. These analog signals are converted to digital form for implementation of PID controller function. Permanent and temporary droop functions are set in the processor. Then output of the regulation function block is fed to an analog output module, which, after further amplification by a booster amplifier, is fed to Electro Hydraulic Transducer (EHT).The polarity of the signal decides the direction of movement of the deflector servomotor control which controls needle servomotor and thus the inflow of water to the turbine. The control signals from the EHGC are wired to the Hydro-mechanical cabinet (HMC) and to the Runner control cabinet (RCC) which act as power amplifying servo units. The HMC operates the guide vane & RCC operates the runner mechanism.

The turbine governing system for Maheshwar HEP is capable of controlling the turbine at all power outputs under various operating conditions. This is achieved through electrical and hydraulic devices. These include: speed sensing and processing systems, hydraulic amplifiers, hydraulic pressure supply system and main gate servomotor. Hardware for an EHG system comprises redundant processor and speed sensing modules. Software for the governor consists of PID / temporary droop control, start-up/shutdown logic, speed relays, electronic limit, output limit according to head and synchronous condenser operation. The governing system controls the speed of the turbine (Prime mover) before synchronization and the output in MW (active power) after synchronization.

Commissioning Procedure of EHGC of Maheshwar HEP has been described as follows:-

Commissioning of EHGC at Maheshwar HEP S. No. Function Commissioning Activity

1) Checking of scheme As per Drawing 2) Insulation & HV test By Megger 3) AC/DC converter Apply input voltage & adjust o/p voltage 4) DC/DC convertor Apply input voltage & adjust o/p voltage

5) Regulation processor programming check Check set points,PID control & logics

6) Speed Sensor Connect square wave generator at I/P,check the O/P

7) Speed Relay Simulate over/under speed condition & check pick up/drop out

8) Parameter Check Check Permanent/Temp droop, damping limit,GV limit as per logic sheets

9) GV mechanical limit position transducer Check the O/P at different I/P values.

10) GV control -Power transducer Check the O/P at different I/P values.

11) Runner control -Power transducer Check the O/P at different I/P values.

12) Temperary droop mode Check the same as per documentation 13) Regulation Loop Check the same as per documentation

Troubleshooting Guide of Governor for Maheshwar Hydro Project:- Following approach for identification for the fault on Governing system proved to be very useful.