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.

2. FINAL OUTPUT OF THE POWER CONTROLLER :

As discussed above the final set point for the power controller is influenced by the Governor Free Control, Limiting Pressure Control and Droop Control (Frequency Influence).

3. MAIN ELEMENTS OF POWER CONTROL : 3.1. Power Setter (65P) :

Turbine output power is set in the following ways : • Power setting from the ATS

• Manual power setting via 7-65P (Raise or Lower P.B.) • Power setting from the unit coordinator control

• Automatic setting operation according to other conditions

The set value is indicated on the power setting indicator in the Unit Control Room, and the actual load is indicated on the output power indicator in the UCR.

Power Setting From The ATS :

This setting is done by supplying the initial setting (0%) command, and the 20% power command at ATS. When these commands are applied, the power setter increases or decreases to the target value according to the stroke time. The actual power under ATS is controlled at a change rate based on the starting mode by a secondary setter.

Manual Power Setting :

It is accomplished by use of the 7-65P (PB) installed in the UCR in case of not using the ATS system or voluntary setting by operator.

Power Setting From The Unit Coordinator :

On the coordinated mode (ALR mode) the demand signal from the unit coordinator is applied to the power setter.

Automatic Setting Operation According To Other Conditions :

 The power setter is returned to the lower limit value (0%) for restart when the turbine is tripped.  Automatic follow up of 65P. Power setter (65P) is automatically following the actual load during the

initial pressure mode operation or Run Back ON. 3.2. Power Change Rate Setter (65Pd) :

When the power controller is on operation, the excessive load variation is set by the power change rate setter (65Pd). The power change rate setting is done in the following ways :-

• Power change rate setting from the ATS. • Manual power change rate setting.

Power Change Rate Setting From The ATS :

This setting is done via the selected start mode at the automatic start up of the turbine.

When the starting mode is selected, the change rate setter is automatically set at the predetermined change rate for each mode. Power change rate of load down is also set automatically to 3% /min by ASD.

Manual Power Change Rate Setting :

This setting is done by 7 – 65Pd (raise or lower PB) in case of not using the ATS/ASD system or normal operation.

3.3. Limiting Circuit According To Wall Temperature Monitoring (WT) :

This system affects the secondary speed setter and the secondary power setter to limit the speed change rate and power change rate for keeping the temperature difference (the wall stress) within allowable limits. This wall temperature monitoring system (TSC) is installed in DCS supplied by BHEL, is capable of switching ON or OFF in the UCR.

3.4. Secondary Power Setter :

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

Power Change Rate Limiting :

The power secondary setter is activated by the power change

rate setter (65Pd) to determine the change rate. The signal of the limiting circuit via the wall

temperature monitoring (WT) also activates the secondary power setter to limit the change rate as necessary. The secondary power setter has a step component. When the wall temperature monitoring (WT) is not limited, the power variation ratio is as follows :

Sustained component : MW/min.

Step component : Approx. MW (two different setting values are applicable, one for loading and one for unloading).

The mutual relation between the power setter (65P) and the secondary power setter (65Pd) and via the wall temperature monitoring circuit (WT) is shown in the following Fig.

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

The quick balance is a function for automatically excluding the change rate limiting circuit. The integral time constant of the power change rate setter (65Pd) is decreased, practically this circuit is excluded. The conditions are as follows :

• Turbine trip

Power Setting Hold :

The power setting is held on the following conditions :

3.5. Electrical Load Limiter (77E) & ALFC :

Electrical Load Limiter can be used for sudden decrease of load during emergency conditions. Electrical Load Limiter works in association with ALFC or Automatic Load Limiter Follow Up Control which is activated above 82 MW. If ALFC is ON then the Electrical Load Limiter tracks the controller outputs (valve position) with a 10% higher value. This tracking is done at a certain rate controlled by the Load Limit 2ndry setter. During ALFC this rate is 33%/min. Thus this also ensures that no sudden increase or decrease of load changes take place under any abnormal conditions.

4. AUTO FOLLOW UP OF MECHANICAL GOVERNING :

This circuit makes the Speeder Gear to follow the Electro-hydraulic Governor at a value above 5% of the valve position of the E/H Converter. A function generator makes the 65M position with the EHC valve position.

Due to large variation of grid frequency at the commissioning period the auto follow up circuit was not functioning properly at Bakeswar and hence the normal practice here is to keep 65M at 100% . MHG droop curve changes as 65M is lowered and if there is a fluctuation in frequency it may so happen that MHG sometimes come into action which is undesirable. The disadvantage of this that if due to certain reason the EHG fails, the control valves will suddenly go to 100% causing accidents and damage in certain conditions.