INTERFACING OF GOVERNING WITH AUTOSYNCHRONISER :

The EHG system is interfaced with the autosynchroniser. The autosynchroniser gives speed raise or lower command to the speed setter according to the beat frequency sensed by the autosynchroniser. The speed of the turbine increases or decreases according to the command.

4. MAIN ELEMENTS OF SPEED CONTROL : 4.1. Speed Setter (65F) :

Turbine speed can be set by the following ways :-

• Turbine speed setting from the ATS system.

• Manual turbine speed setting.

• Turbine speed setting by automatic synchronization system (ASS).

• Automatic setting operation according to the other conditions.

Turbine Speed Setting From The ATS System :

This setting is done by supplying a heat soak turbine speed command and a rated turbine speed command and target speed commands (pulses) at automatic start up of the turbine. When one of these commands is applied, the speed setting increases the speed up-to the target value according to the predetermined stroke time (refer to Table.1). but the actual turbine speed is increased at a change rate based on the starting mode by a secondary speed setter.

Manual Turbine Speed Setting :

There are two methods of manual setting. One is by target speed PB on the insert panel in the unit control room.

The other is by 7-65F (raise/lower by PB) in the unit control room which are used for the manual synchronizing operation and for the over speed test etc. The set value is indicated by the setter position indicator in the unit control room.

Turbine Speed Setting By ASS :

The signal from ASS is connected to the speed setter for matching the turbine-generator speed with the system frequency for the purpose of synchronization with grid.

Automatic Setting Operation According To Other Conditions :

• The speed setter is returned to the lower limit value (0 rpm) for restart when the turbine is tripped.

• When the turbine speed increase rate becomes under the preset value, the speed setter is run down to heat soak speed in order to avoid stopping at critical ranges.

• After closing the main circuit breaker, the power controller is turned on automatically and the speed setter is held at the rated speed.

4.2. Speed Change Rate Setter (65 Fd) :

The speed change rate setter limits the turbine speed

increase according to the turbine start mode. It activates the secondary speed setter. The speed change rate setting is done in the following ways :-

• Speed change rate setting from ATS.

• Manual speed change rate setting.

Turbine Speed Change Rate Setting From ATS :

This setting is done via a selected start mode at automatic start-up of the turbine. When the starting mode is selected, the change rate setter is automatically set at the predetermined hange rate for each mode.

Manual Speed Change Rate Setting :

The turbine speed change rate is set by selecting one of the predetermined change rates in manual mode.

4.3. Secondary Speed Setter :

A buffering action is performed so that the setting change of speed setter (65F) is kept within the allowable change rate of the turbine.

Speed Change Rate Limiting :

The secondary setter is activated by the speed change rate setter to determine the change rate. In the secondary setter, raise command from the primary speed setter (65F) is limited by the change rate from 65Fd and then the turbine speed is up to the speed given by 65F with the change rate given by 65Fd. The mutual relation between the speed setting (65F) and the speed change rate setting (65Fd) is shown in the following Fig.

Quick Balance (not in use of change rate limiting) :

The quick balance is a function for automatically removing the change rate limiting circuit. The integral time constant of the speed setter (65F) & speed secondary setter is decreased, practically this circuit is excluded. The condition is as follows : • Turbine trip (Emergency trip circuit switch – off ).

Turbine Speed Setting Hold :

The turbine speed is held on the following condition, but not within the critical speed. Either ON or OFF of HOLD function is selected, are used in order to block or release the speed secondary setter. This function is used just in testing of turbine and also used for power secondary setter.

4.4. Turbine Speed Increase Rate Monitoring Circuit (65F rundown circuit) :

In order to avoid interference with the turbine speed in the critical speed range, where a critical speed exists in the process of turbine speed increase, the turbine speed increase rate is

monitored, and when the increase rate is below a defined rate, the turbine speed setting goes to heat soak speed automatically and then the turbine speed is rundown to the heat soak speed

accordingly. This function is considered for the trouble of boiler pressure control fault, the bypass valve control fault etc.

4.5. Over Speed Test :

Over speed test is done by raising the speed setter manually (7-65F) after the over speed test “ON” is selected by key switch in the unit control room.

C.2. POWER CONTROL : 1. OPERATION :

The power controller ensures smooth loading of the turbine. It is active when the Generator Circuit Breaker (GCB) closes. Various features are incorporated in this controller which includes :

• Initial Load Concept.

• Governor Free operation.

• Limiting Pressure Control (LPC).

• Frequency Influence.

• Interfacing With Coordinated Master Control.

1.1. Initial Load Concept :

When Generator is synchronized the power set point automatically becomes 5% of the rated load i.e. 10.5 MW.

Since the turbine goes on power control with an initial load of 10.5 MW, this ensures that generator never goes on motoring which was one of the synchronization problems. It should be noted that this set point goes directly to secondary setter, hence no influence of rate is present.

1.2. Governor Free Operation :

In the Governor Free operation turbine responds directly with speed even though the turbine is on power control mode. In Bk.T.P.P. the Governor Free has a droop of 4.5%. The present dead band of this circuit is 0%, i.e.

loading and unloading of machine starts just as it crosses 50 Hz. However this is a variable quantity and can be changed as desired. The droop and the maximum load change can also be changed as desired. Presently the maximum load change from Governor Free operation is 50% i.e. 105 MW. The response of the load change can be controlled by proper setting of time constant for an integrator circuit.

When the grid frequency variation is large, the Governor Free operation is very difficult because of large variation of load changes.

1.3. Limiting Pressure Control :

If due to certain abnormal condition the pressure of the system falls, a control system reduces the load to a certain extent to control the pressure. This type of pressure control within the power control is known as Limiting Pressure Control. Here it should be mentioned that LPC can be switched on only if the turbine is on power control. In Bk.T.P.P. load rejection starts if pressure falls below 10% of the limiting pressure setting and total 21 MW load rejection take place when the pressure deviation becomes more than 20% of the set point.

1.4. Frequency Influence :

There is a proportional controller whose value depends on the speed deviation and which directly influences the power controller. The purpose of this controller is to decrease the load at high frequency. Here it must be mentioned that Governor Free operation does not have dead band and it is mainly a frequency controller. But the Frequency Influence circuit within the power controller restricts turbine loading at high frequency. Unlike the governor free operation this circuit always remains active when power controller is ON. Initially a dead band of 1.5 Hz was present but now this dead band has been increased to 3.5 Hz.

1.5. Interfacing With Coordinated Master Control :

Under CMC mode of operation the power controller becomes active and load set point is given by the unit load set point of the CMC circuit. The secondary power setter tracks the unit load demand set point. In this mode of

operation the rate of load change is determined from the CMC circuit and secondary power setter has no influence under CMC. Thus the turbine maintains load according to the demand from the CMC.