Free Governor Mode of Operation

FREE GOVERNOR MODE OF OPERATION As a result of major electricity reforms, various state regulatory commissions and a central governing agency CERC, were formed. Central Electricity Regulatory Commission (CERC) is a regulatory body for introducing grid discipline and rationalization of the tariff in the country. CERC has proposed a Grid Code (IEGC) to maintain the Grid Frequency and thus prevent the frequent grid failures prevalent due to unnecessary pumping of power into the grid when demand is low and overdrawing by substations when supply is less. For various reasons like operational stability, constraints in the system etc., the power plant operators operate the plants under following modes, leading to fluctuations in the grid frequency. 1. Valve wide open operation 2. load limit controller 3. Frequency influence off mode 4. Change of droop setting 5. Dead band 6. Time delay in logic control. CERC has proposed Availability Based Tariff (ABT) to maintain the grid frequency within a range and thus improve the quantity and quality of power supply. Later CERC has given IEGC, which stresses upon Free Governor Mode of Operation (FGMO) to further limit the range of grid

frequency. Power Grid Corporation of India Limited (PGCIL) is trying to put all the Power Stations on FGMO. Availability Based Tariff (ABT)

Availability Based Tariff has been introduced to give incentives to the generation company to generate and pump in more power to the grid when the grid frequency is below 50 Hz and prevent them to generate more by imposing penalties in case frequency is above 50 Hz. Free Governor Mode of Operation (FGMO) Under FGMO, machines gets loaded / unloaded directly under the influence of grid frequency i.e. machine unloads when the grid frequency is more than 50 Hz and loads when grid frequency is below 50 Hz. The amount of loading / unloading with change of grid frequency is proportional to droop of the governor.  Successful implementation of ABT and IEGC will bring grid discipline and reduction in fluctuation of grid frequency.  CERC guidelines have helped PGCIL to bring the desired grid discipline. Advantages of FGMO  Due to operation of the machine on FGMO, off-frequency operation shall cease to occur. Thereby enhancing the reliability and availability of machines.  Reduction in fluctuation range of frequency will reduce the need to oversize the equipment for under frequency operation. Presently in

some cases the equipments are sized for operation at 47.5 Hz and now this can be limited to 49 Hz.  Longevity and better performance of electrical equipments.  Reduction in outage of equipment leads to reduction in maintenance cost an increase of productivity.  Reduction in auxiliary power consumption.  At lower frequency, reduction in performance of pumps, fans etc.,. At higher frequencies increase of stress and wear experienced.  Less chance of Turbine blade failures.  Avoids grid failures. Failure of Bearings due to oil starvation during grid failure can be avoided.  Higher operational availability.

INDIAN ELECTRICITY GRID CODE (IEGC) The following clauses of IEGC makes the operating stations to maintain the units under FGMO. Section 6.2 (e) All generating units, which are synchronised with the grid, irrespective of their ownership, type and size, shall have their governors in normal operation at all times. If any generating unit of over fifty (50) mw size (10 mw for Northeastern region) is required to be operated without its governor in normal operation, the RLDC shall be immediately advised

about the reason and duration of such operation. All governor shall have a droop of between 3% and 6% Section 6.2 (f) Facilities available within load limiters, automatic turbine run up system, turbine supervisory control, coordinated control system etc. Shall not be used to suppress the normal governor action in any manner. No dead band and / or time delays shall be deliberately introduced Section 6.2 (g) All generating units, operating at / up to 100% of their maximum continuous rating (MCR) shall normally be capable of (and shall not in any way be prevented from) instantaneously picking up five percent (%) extra load for at least five (5) minutes or within technical limits prescribed by the manufacturer when frequency falls due to system contingency Section 6.2 (h) The recommended rate for changing the governor setting, i.e. Supplementary control for increasing or decreasing the output (generation level) for all generating units, irrespective of their type and size, would be one (1.0) percent per minute or as per manufacturer’s limits. However, if frequency fall below 49.5 hz, all partly loaded generating units shall pickup additional load at faster rate, according to their capability Governor droop:

The droop characteristics of a governor helps in stable operation of the machine. The speed differential between full load and no load operation related to rated speed for a governor represented in percentage is called droop.

Droop = (3150-3000) /3000 *100 = 5%

The droop function of the governor helps in uniform sharing of loads during parallel operation. The setting of percentage of droop determines the change in load of machine during change in load on grid. Many power generating stations represented their operational problems for implementation of FGMO.  Large no. Of load cycling

 Forced unsafe operation: inability to maintain parametric variations of boiler, tg etc. Within safe limits  Off-design operation leading to  Life cycle impacts and prospective failures  Logistic constraints of multiple manual secondary intervention    

Affecting cycle heat rate Non suitability of certain old machines Slow response of hydraulic governor. Need for upgradation of controls.

A steering committee was formed by cea for examining the issue of free governor mode of operation in thermal units headed by member (technical), CEA. Modification in control logics of Electro Hydraulic Turbine Controller (EHTC) have been made by EDN,BHEL, Banglore to fulfill following requirements of FGMO as recommended by the steering committee. 1. Reference

frequency is floating depending upon the previous operating frequency between 49.0 Hz to 50.50Hz.

2. Load changes are made whenever the frequency changes from

operating frequency. This change is limited to + 5%. The load change so made is sustained for approximately 5 minutes (dynamic time delay). If the frequency varies during this period, dynamic time delay becomes zero and count starts again.

3. After this period the load value comes back to the original value in a ramped fashion.

4. For frequencies more than 50.5 Hz and less than 49.0 Hz load

change is as per droop characteristics and limited to +10%.

5. To take care of the load change, fuel firing is to be increased in the boiler system by giving a feed forward signal to combustion control system through Boiler Master control or Coordinated Master Control. The enclosed block diagram details the functional diagram of the frequency influence circuit to meet FGMO guidelines 1. Floating frequency set point: The speed actual signal is used to generate the floating signal. The time delayed set point generated by the actual value of speed. In the event of any change in frequency, the set point changes to a value equal to the new frequency (speed) value. This change happens in a slow ramp and the time taken is set to about 2 mins. 2. The proportional controller compares the set value and actual value

and generates a frequency influence signal with a proportional gain corresponding to droop setting. This proportional component is given in full magnitude at the beginning (limited to + 5%) and starts coming down as the deviation reduces on account of frequency set point slowly approaching the new actual value.

3. After a set time delay, an integrating circuit with the proportional error signal holds the frequency influence value. 4. The sum of proportional and integral values of frequency influence

(limited to + 5%) is added to the load set point to form the final load set point (Sigma Pr) going to load controller. This will increase the load signal.

5. After the set point settling down to the new frequency value, the error becomes zero. On sensing this, the integrator holding the integral part of the frequency influence is set to zero after a time delay. This process ensures that the increased value of load is held for a period of about 5 minutes. While the load is brought back to the original value a ramping circuit ensures that the load change is gradual. 6. In the frequency range beyond FGMO limits i.e. blow 49.0 Hz & above 50.5 Hz the frequency influence as per droop characteristic will become effective and drives the frequency influence signal to + 10% (this limit can be decided as per site conditions). 7. While the frequency influence acts on turbine load set point a signal is given to Boiler control to make changes in fuel firing with a feed forward action for a quick response. 8. The pressure controller of EHTC in Limit Pressure mode ensures that the load is reduced to maintain the boiler header pressure, in the event of a sudden pressure drop more than set limit value (10Kg/cm2 for 500MW unit).

 Modified control scheme, was tested on the simulator at corporate R&D of NTPC and was also tested successfully on the simulator at Simhadri project.  Modified control scheme was implemented in Simhadri unit-2 for live field trials. Results obtained are excellent and meet the objectives set in previous steering committee meet.  Effect of throttle steam pressure on load and dead band of ± 0.03 Hz were noted to be satisfactory.  Role of controls, interlocks and protections is very important in the operation of machine under FGMO. Presently supplied new machines by BHEL have state of art controls to meet operational requirements under FGMO. Retrofitting and up gradation need to be undertaken in the older machines.