GB1563566A - Control system for a gas turbine power plant - Google Patents

Description

(54) CONTROL SYSTEM FOR A GAS TURBINE POWER PLANT (71) We, WEsTlNGHOUSE ELECTRIC CORPORATION of Westinghouse Building, Gateway Center, Pittsburgh, Pennysylvania, United States of America, a company organised and existing under the laws of the Commonwealth of Pennsylvania, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement; This invention relates to gas turbine power plants and, more particularly, to a control system for a power plant adapted to be connected to a power supply bus under "dead load" conditions in which loads connected to the bus are "unactivated".

In various areas of a power system, it may be desirable to provide a local generating plant to produce power for the area in the event the area becomes isolated from the rest of the system by a transmission line outage. In this way, the health and safety and convenience needs of the people served by the power system can be reasonably met during the outage.

Similarly, many large industrial plants such as metal processing plants have a need for local replacement generation in the event of a loss of the external power supply to such plants.

When an outage occurs, a plant which is to be brought on line to replace the lost generation often must be controlled to pick up or accept a "dead load", i.e. in a power system a load already connected to the lines made dead by the transmission line outage. Gas turbine power plants are well suited to providing replacement power because they can be started rapidly and they can provide relatively large amounts of power for replacement purposes for a part of the power system.

Residential dead load pickup by single gas tubine power plants has been performed previously in 1972 at the VEPCO Kitty Hawk plant in North Carolina. In such plant starts, the gas turbine control is triggered to accelerate the turbine to a speed above synchronous speed and the generator circuit breaker is closed to connect the dead load to the generator.

The load placed on the turbine causes the turbine speed to drop to near the synchronous speed value, and the turbine speed control enters an isochronous mode to meet the load demand and hold the bus to the desired operating frequency, i.e. 60 Hertz.

If multiple gas turbines are to be started for dead load pickup in the same area, it is desirable that they be operated in a coordinated way during the startup and load modes of operation. Further, simultaneous synchronization of two or more generators must be avoided since it can cause damage if the generator circuit breakers close when the associated generators are not matched in voltage magnitude, frequency of phase.

It is the principal object of this invention to provide an improved control system for a power plant adapted to be connected to a power supply bus during dead load conditions and including at least two gas turbinegenerator units.

With the above object in view, the invention resides in a control system for a power plant adapted to be connected to a power supply bus under "dead load" conditions in which loads connected to the bus are unactivated, said power plant including a first gas turbinegenerator unit and at least a second gas turbinegenerator unit, a first circuit breaker connected between said first unit and the power supply us and a second circuit breaker connected between said second unit and the bus said control system comprising first fuel control means for controlling the speed of said first unit and second fuel control means for controlling the speed of said second unit first and second means for closing said first and second circuit breakers. respectively, means for generatin a first signal indicative of the occurrence of a qcad load condition signified by a loss of voltage on the bus. means for operating said first and second fuel control means and said first and second circuit breakers in response to said first signal to accelerate said units to a higher than normal speed condition and to close one of the circuit breakers and subsequently operate the unit associated with the other circuit breaker to synchronize the other unit with the unit associated with said one circuit breaker and then close the other circuit breaker upon synchronization, and means for operating said first and second fuel control means to control the output frequency of the turbine-generator units and to share the load between said units.

The invention will become more apparent from tie following description of an exemplary embodiment thereof when used in conjunction with the accompanying drawings.

in which: Figure 1 shows a schematic diagram of a portion of a power system; Figure 7 shows a map of the area supplied by the power system portion shown in Figure 1; Figure 3 shows a schematic diagram of a power plant incorporating a control system in accordance with this invention; Figure 4 shows a more detailed functional block diagram for a control system employed in a power plant of Figure 3; Figure 5 shows a special surge limit characteristic employed in the system of Figure 4; Figures 6A-6E and 7 show various logic blocks employed in the system of Figure 4; and Figures 8A and 8B show certain turbine performance curves during dead load start operation.

More specifically, there is shown in Figure 1 a portion 12 of a power system 10 for which dead load start operation is to be provided in the event of an outage of line 14 and opening of the associated line breaker 16. A dead bus relay 18 senses a loss of voltage on the high voltage bus, and if the line breaker 16 is open generates a signal for automatic actuation of a pair of gas turbine generators 20 and 22 in an emergency power plant. A relay 17 trips the line breaker 16 on loss of line voltage, the interconnection to the line breaker 16 being omitted in the drawing.

As shown in Figure 1, the local load to be supplied includes Kitty Hawk estimated to be 4 to 10 MVA and Nags Head/Manteo/Hatteras estimated to be 7 to 16 MVA. If desired, line switches can be operated to sectionalize the system load after a line outage and thereby reduce the dead load presented during a start operation.

In Figure 2, there is shown a map of the area corresponding to the load shown in Figure 1.

It is known as the Outer Banks area of North Carolina, and electric power is provided to this area by the Virginia Electric Power Company.

Power is transmitted along a power line 24, and a section 26 of the line 24 is particularly susceptible to outages because of its bay location. The load is essentially residential in character. i.e. a significant portion of the load is motors for refrigerators, air-conditioners, freezers. etc. Accordingly. the load transient during a dead load start is typically about 2.5 times the normal running load. The load transient depends in part on how long the lines have been dead since, with increasing dead time, automatic temperature controls will cause more and more compressors to be turned on when power is restored. For the particular service area shown in Figure 1, it is estimated that the load increases about 25% within 25 minutes after an outage, and it is therefore desirable that the turbines be put on line within their start-up times, i.e. within about 10 minutes.

In Figure 3 there is shown a more detailed schematic diagram of the gas turbine power plants 20 and 22. Although other kinds of controls can be used in embodying the invention in the gas turbine power plants, there is in the present case employed the computer control described in detail in UK patent specification No. 1374809. Generally, respective electro-pneumatic fuel controls 24 and 26 vary the fuel flows to gas turbines 28 and 30 to control the turbine speed during start-up and synchronization and the turbine speed and load during load operation. The turbines 28 and 30 drive respective generators 32 and 34 which are connected to the load through circuit breakers 36 and 38.

Although a single computer system can be employed, each power plant is provided in this instance with a digital control computer system 40 or 42 which applies respective speed reference signals to the fuel controls 24 and 26. Respective speed sensors 44 and 46 generate actual turbine speed signals which are respectively applied to the fuel controls 24 and 76 in respective feedback control loops. Turbine speed errors are corrected by the controls 24 and 26 which make changes in the turbine fuel flows.

Operator panels 48 are provided for operator control and monitoring. The panels include manual start and mode select buttons which are coupled to the computers 40 and 42 through contact closure inputs. The dead bus relay 18 is also coupled to the computers 40 and 42 to permit the computers to automatically start up their respective turbines.

Each computer 40 or 42 includes a speed reference generator and surge and temperature limit control loops which normally place no constraint on the functioning of the speed reference generator. Each computer also includes logic controls related to mode selection and the coordination of the start-up of the two turbines. Generally, one turbinegenerator is made the lead unit and it is designated as the preferred unit. However, under certain start-up conditions detected by the computers, the lead can be shifted to the other turbine-generator and possibly back to the preferred unit.

Once the lead turbine-generator has reached the desired speed and its breaker has been closed, the other turbine-generator is synchronized to the line with closure of its breaker.

Thereafter, the lead turbine is operated by an isochronous control in its computer 40 or 42, i.e. a speed reference of about 105% rated speed is generated to hold the lead turbine at approximately 100who rated speed. The follow turbine computer tracks the lead computer speed reference in controlling the follow turbine-generator. The speed reference and the generator breaker status values are exchanged between the computers 40 and 42 for coordination purposes as shown in Figure 3.

More specifically, as shown for the preferred turbine-generator unit 20 in Figure 4, a dead load start is automatically initiated in the power plant by the relay 18 or by another trigger device such as an underfrequency relay (not shown) or manually by a panel pushbutton 50 after the line breaker 16 opens.

An arrangement like that shown in Figure 4 is also provided for the nonpreferred turbinegenerator unit 22 and accordingly it is not shown and will not be described except to the extent necessary to explain the total system operation.

A dead load mode control 51 in the control system for the turbine-generator unit 20 responds to the call for a dead load start and, after sequencing of auxiliaries and ignition, causes a speed reference generator 52 to generate a speed reference which increases under program control to a predetermined level, in this case 110% rated speed. Thus, the turbine is cranked, fired and accelerated to an overspeed condition with sufficient inertia to take up the initial load transient on dead load start.

The speed sensor 44 for the preferred turbine-generator unit 20 is connected to the fuel control 24 which develops an electrical speed error signal from the speed reference and the actual speed signals. A pneumatic signal is derived from the speed error signal to move the throttle valve and change the fuel flow and correct the turbine speed.

A selector 54 normally passes the speed reference to the fuel control 24, but the selector output can be limited by a temperature limit control 56 or a surge limit control 58 under abnormal operating conditions. To allow for the initial dead load transient on dead load start, a special surge characteristic is employed in the surge limit control 58 as shown in Figure 5. Thus, at 92% rated speed, the generator field breaker is closed and the special surge curve then is operative to limit fuel on the load transient as a function of compressor air inlet temperature so that the surge limit applied to the selector 54 allows the expected sudden fuel increase when the load transient occurs.

If limit control action becomes operative temporarily during startup, the startup time period simply becomes extended. Limit control action after dead load start causes a drop in bus frequency. Preferably, load rate limit action which is otherwise applied to limit the rate at which the computer is allowed to load the turbine is disabled so that a step fuel increase can occur when the dead load is applied.

When 107% speed is reached, the generator breaker 36 is automatically closed by a logic control 60 if a lead logic control 62 indicates the preferred turbine-generator 20 and its computer 40 have the lead and if a predetermined generated voltage magnitude condition is satisfied. On breaker closure, the dead load which has been connected to the power plant results in a load transient after which the speed reference is dropped to 106% rated speed and the turbine generator speed drops toward 100% rated speed. Load is picked up to the limit of turbine base load capability. Through the exchange of generator breaker status signals between the two control computers dangerous simultaneous generator breaker closings are avoided.

The turbine-generator unit 22 also responds to a call for a dead load start and its speed reference generator in the computer 42 causes the turbine speed to run up to 107% as the turbine-generator 20 is being run up to the same speed. As shown in the logic diagrams of Figures 6A and 6B, the lead logic control 62 for the preferred unit and the corresponding lead logic control for the nonpreferred unit function as part of the control system according to one embodiment of the invention and enable the preferred computer 40 to retain the lead if its turbine speed reference reaches 80% rated speed first, and thereafter the nonpreferred computer 42 is disabled from closing its generator breaker 38 onto a dead bus. On On the other hand, if the speed reference for the non-preferred unit reaches 80% rated speed first, its computer 42 is assigned the lead and the computer 40 is disabled from closing its generator breaker 36 onto a dead bus. The logic also causes a return of priority to the preferred machine after the nonpreferred machine takes the lead and fails to reach the target speed at which its breaker is to be closed on the dead bus or after the nonpreferred machine has its breaker closed and the preferred machine is later synchronized to the line.

The lead unit controls the generator breaker for sharing the dead load onto a dead bus and further controls the system frequency under iso.;i.ronous operation. The priority machine can control the line breaker 16, it takes control of resynchronizing to a live bus in the event the line breaker 16 trips while the machines 20 and 22 are operating into the whole system 10. Speed references and breaker status signals are exchanged between the computers to enable system coordination to establish priorities and designate the lead unit under changing conditions. The follow unit does not close onto a dead bus, rather it must be synchronized to the lead unit after the lead unit generator breaker has been closed on the dead bus. Further, the follow unit tracks the lead unit speed reference when both breakers are closed on dead load start operation and the line breaker has not been closed.

Immediately after the lead unit has been closed onto a dead bus, the follow unit senses the closure of the breaker 36 and it is automatically synchronized to the lead unit, i.e.

while the two units are both at about 108% rated speed and before the load transient is applied to cause a turbine speed drop.

For the purpose of synchronizing the generator 34, a synchronizer control in the computer 42 responds to external synchronizer circuitry and operates the speed reference generator to match the frequency, phase and voltage magnitude of the follow turbinegenerator with that of the lead turbine-generator. In the lead computer, a corresponding synchronizer control 64 responds to external synchronizer circuitry 66 and operates the speed reference generator 52 to synchronize the generator 32 to the bus when the computer 40 acquires a follow role assignment. A contact closure output (CCO) is generated for closure of the follow generator breaker when match conditions are achieved.

If the power system load to be borne is in excess of one machine capacity, the load can be sectionalized by breakers to provide assurance that one machine will not be overloaded in the event the second machine fails to respond. For example, a part of the total load might be designated as initial dead load on the bus, and after the two generator breakers are closed the remaining parts of the load can be sequenced onto the bus in a predetermined order.

Preferably, an isochronous control 68 for the lead turbine-generator becomes operative 6 minutes after closure of the breaker 36 on the dead load bus, and at that time and each minute thereafter causes a corrected speed reference to be generated to hold the turbinegenerator speed at 100% rated speed plus or minus 0.2% adjustable to plus or minus 0.1%.

In providing isochronous control, the block 68 compares actual turbine speed to the setpoint of 100% rated speed and generates a correction signal every minute. The corrected speed reference output causes the fuel control 24 to change the lead turbine fuel flow to a value which provides a correct actual turbine speed within the allowed tolerances.

After the breaker 38 is closed, the follow computer 42 uses a speed track control 70 to track the speed reference from the lead computer 40 and matches the speed reference output from the follow computer 42 to that of the lead computer 40. When the isochronous control phase of operation is reached, the lead computer 40 accordingly provides control over the generation frequency of the two generators by controlling the lead turbine speed with follow speed control action being provided for the follow turbine by the follow computer 42. If the nonpreferred computer 42 is the lead computer, a speed track control 70 in the preferred and follow computer may cause similar but reverse action to occur, but in this situation it is highly probably that the preferred unit has failed to respond and has been shut down.

If synchronization of the follow generator to the lead generator is unsuccessful, a threeminute time delay is initiated to allow new synchronization attempts while putting a hold on further loading. After the time delay, the loading sequence can be started by the system operator but the amount of applied load should be limited to the capability of the connected turbine-generator unit if the synchronization attempts for the second breake have failed. On final synchronization failure, the second unit is preferably shut down.

If both machines respond to a start signal and successfully load and the lead machine trips for some reason, the priorities are computer-checked and the computer associated with the turbine having its speed reference above 80% rated speed is assigned the lead machine priority. In the event either machine shuts down, the control logic preferably includes a single try logic function which prevents repetitive starting.

If desired after dead load start operation, one of the turbine generators can be released from dead load mode follow operation by a panel selector. In that event, the nonpreferred turbine can be operated at a preselected load level such as minimum load and the lead computer operates under isochronous control to take system load swings.

On an automatic start, selection is made of a normal start from the base load of the power system. While in the dead load mode, use of a MIN LOAD pushbutton provided for normal operations is blocked. This is because it uses load control and in an isolated system fuel changes cause speed changes rather than load changes.

If the line breaker 16 trips while the plant is functioning in a normal load mode and specifically while the turbines are on MIN LOAD, the resulting load may be less than the kilowatt reference in which case the unit goes toward 104% speed, or the load may be greater than the kilowatt reference in which case the turbine may trip on underspeed. Therefore, before selecting the dead load mode, the operator should check that the line breaker is open and that MIN LOAD iS NOT SELECTED.

Otherwise, the dead load mode can be selected as desired including during acceleration, holding at synchronous speed with the generator breaker open and after the line breaker opens while running to activate isochronous control.

Should the bus become activated after the dead load mode is triggered, the turbine will accelerate to 107% speed and the automatic synchronizer will bring it back down and synchronize it with the bus. When the generator breaker closes, the load rate limiter is out of service so that, if base load is selected, the unit quickly goes to base load as the speed reference is raised to 110%. Since the frequency should be within the deadband of the isochronous control, the isochronous control makes no corrections assuming a wellcalibrated speed measuring system. This type of operation should be avoided and the operator should push the dead load button to cancel the dead load start mode at the first opportunity.

Since either of both machines may be called upon to supply electrical power to the dead load, each must be capable of synchronizing the closure of the line breaker for return to normal line service when the reason for the line outage has been cleared. When the operator decides to synchronize the closure of the line breaker and both machines are operating, a push-button selection is made for automatic line breaker closure for the preferred turbine. Otherwise the operating turbine is selected.

To synchronize, the speed reference of the preferred turbine is controlled to cause matching of speed, voltage, and phase of the local generator(s) to those of the main power line. When matching occurs, a contact closure output (CCO) is put out to close the line breaker automatically.

In Figures 6A-6E and 7 there are shown more detailed logic diagrams for various logic controls employed in the system. Thus, block 50 of Figure 4 is further detailed in part of Figure 6A. This shows that the dead load pushbutton is active only in local control (43L) and that it is programmed to act as an alternate action pushbutton.

Block 51 of Figure 4 is detailed in the remainder of Figure 6A and also in Figure 6E.

When the dead load flip-flop whose output is labeled DLFF is set, the system is calibrated for dead load pickup. A list of the items which must be calibrated for dead load start is included in Figure 6A. When the dead load flip-flop is cleared, a lesser number of program changes are required to restore the programs to the normal condition and these changes are also listed in Figure 6A. The dead load flipflop is set by the dead load pushbutton or by loss of bus voltage indicated by signal 27B.

Signla 27B also triggers an automatic turbine start. The dead load flip-flop can be cleared by another pushbutton operation, closure of the line breaker 16 (signal 52LA) or by a signal called FREEZ which occurs on every turbine shutdown. Figure 6A also shows that a special surge curve described previously herein is used on dead load calibration after the field breaker closes automatically at a nominal 92% speed.

Another detail shown in Figure 6A indicates a computer contact is programmed to follow the position of the generator breaker and this contact is wired to the other computer so that each computer knows the position of the other computer's generator breaker.

Figure 6E further details block 51 of Figure 4. This logic indicates that under dead load mode control, if the other generator breaker closes and three minutes elapses and the one generator breaker fails to close, the turbine is shut down. (Master control L4 is cleared).

This timer can be blocked by the HOLDS signal which is one of the standard operator, selected sequence hold points holding the sequence up to a point prior to synchronization.

With reference to Figure 6B, the Lead Computer Determination logic has been previously described herein. Block 68 of Figure 4 is further detailed in Figure 6C which relates to Isochronous Control. If the dead load flip-flop is set and the generator breaker is closed and this is the lead unit, a six-minute time delay is activated which allows the customer's sectionalized load to be applied. At the end of the time delay and at one-minute intervals thereafter, the speed reference is changed an amount proportional to the difference between actual speed and synchronous speed giving, in effect, reset action to hold the speed within a small deadband around synchronous speed under normal load change conditions. In conjunction with isochronous control, Figure 6D further details the speed tracking control block 70 of Figure 4. This indicates that if the dead load flip-flop is set, and this is not the lead unit and that both generator breakers are closed, this unit's speed reference is set to match the other unit's speed reference to allow load sharing with the lead unit.

Finally, Figure 7 details the line breaker closing logic if provided. The usual provisions for manual close and trip of the line breaker from the turbine operator's console is omitted, with only automatic line breaker closure synchronizing and line breaker position indication being retained in this particular system. To initiate line breaker closure synchronizing, several conditions must be met: Closure of the generator breaker must not be being synchronized.

2. This computer must be the lead computer at the time.

3. Automatic synchronizing must have been selected.

4. The generator breaker must be closed.

5. The control must be in mode 2, the synchronizing mode.

6. The line must be live (2 independent voltage measurements must average above 90% voltage) and the dead load mode must not be selected.

If the synchronization of the closure of the generator breaker is automatically made and the above conditions are met, the control stays in the synchronizing mode and proceeds to synchronize closure of the line breaker. If the line becomes reenergized after the dead load has been accepted by the power plant, the control can be requested to synchronize closure of the line breaker by using the dead load pushbutton to clear the dead load flip-flop and control mode 2, the synchronizing mode, can be reentered by pressing one of the start pushbuttons.

In Figures 8A and 8B there are shown graphs which illustrate the performance of a single turbine on a dead load start operation.

Figure 8A shows the response of the throttle valve and pump discharge and nozzle fuel pressures to a dead load start. Figure 8B shows the delayed response of the exhaust control thermocouples in relationship to the blade path temperature. The time delays, inherent in the cycle temperature measuring ability during the transient pickup of several seconds duration, point to the need for the described means for limiting load and otherwise associated destructive fuel excursions.

WHAT WE CLAIM IS: 1. A control system for a power plant adapted to be connected to a power supply bus under "dead load" conditions in which loads connected to the bus are unactivated, said power plant including a first gas turbinegenerator unit and at least a second gas turbinegenerator unit, a first circuit breaker connected between said first unit and the power supply bus and a second circuit breaker connected between said second unit and the bus, said control system comprising first fuel control means for controlling the speed of said first unit and second fuel control means for controlling the speed of said second unit, first and second means for closing said first and second circuit breakers, respectively means for generating a first signal indicative of the occurrence of a dead load condition signified by a loss of voltage on the bus, means for operating said first and second fuel control means and said first and second circuit breakers in response to said first signal to accelerate said units to a higher than normal speed condition and to close one of the circuit breakers and subsequently operate the unit associated with the other circuit breaker to synchronize the other unit with the unit associated with said one circuit breaker and then close the other circuit breaker upon synchronization, and means for operating said first and second fuel control means to control the output frequency of the turbine-generator units and to share the load between said units.

2. A control system as set forth in Claim 1, wherein said operating means includes logic means for selecting one of said units as the lead unit and for shifting the lead between said units under predetermined conditions, and the first circuit breaker closed is the circuit breaker associated with the

Claims (10)

**WARNING** start of CLMS field may overlap end of DESC **. provisions for manual close and trip of the line breaker from the turbine operator's console is omitted, with only automatic line breaker closure synchronizing and line breaker position indication being retained in this particular system. To initiate line breaker closure synchronizing, several conditions must be met: Closure of the generator breaker must not be being synchronized. 2. This computer must be the lead computer at the time. 3. Automatic synchronizing must have been selected. 4. The generator breaker must be closed. 5. The control must be in mode 2, the synchronizing mode. 6. The line must be live (2 independent voltage measurements must average above 90% voltage) and the dead load mode must not be selected. If the synchronization of the closure of the generator breaker is automatically made and the above conditions are met, the control stays in the synchronizing mode and proceeds to synchronize closure of the line breaker. If the line becomes reenergized after the dead load has been accepted by the power plant, the control can be requested to synchronize closure of the line breaker by using the dead load pushbutton to clear the dead load flip-flop and control mode 2, the synchronizing mode, can be reentered by pressing one of the start pushbuttons. In Figures 8A and 8B there are shown graphs which illustrate the performance of a single turbine on a dead load start operation. Figure 8A shows the response of the throttle valve and pump discharge and nozzle fuel pressures to a dead load start. Figure 8B shows the delayed response of the exhaust control thermocouples in relationship to the blade path temperature. The time delays, inherent in the cycle temperature measuring ability during the transient pickup of several seconds duration, point to the need for the described means for limiting load and otherwise associated destructive fuel excursions. WHAT WE CLAIM IS:

1. A control system for a power plant adapted to be connected to a power supply bus under "dead load" conditions in which loads connected to the bus are unactivated, said power plant including a first gas turbinegenerator unit and at least a second gas turbinegenerator unit, a first circuit breaker connected between said first unit and the power supply bus and a second circuit breaker connected between said second unit and the bus, said control system comprising first fuel control means for controlling the speed of said first unit and second fuel control means for controlling the speed of said second unit, first and second means for closing said first and second circuit breakers, respectively means for generating a first signal indicative of the occurrence of a dead load condition signified by a loss of voltage on the bus, means for operating said first and second fuel control means and said first and second circuit breakers in response to said first signal to accelerate said units to a higher than normal speed condition and to close one of the circuit breakers and subsequently operate the unit associated with the other circuit breaker to synchronize the other unit with the unit associated with said one circuit breaker and then close the other circuit breaker upon synchronization, and means for operating said first and second fuel control means to control the output frequency of the turbine-generator units and to share the load between said units.

2. A control system as set forth in Claim 1, wherein said operating means includes logic means for selecting one of said units as the lead unit and for shifting the lead between said units under predetermined conditions, and the first circuit breaker closed is the circuit breaker associated with the lead unit.

3. A control system as set forth in Claim 2, wherein said output frequency control means includes an isochronous speed control which operates the fuel control means for the lead unit, and tracking means for operating the other fuel control means in correspondence to the operation of the lead fuel control means.

4. A control system as set forth in Claim 2, wherein one of said units is designated as the preferred unit, said logic means includes means for generating a speed setpoint and for sensing the speed of each unit and means for shifting the lead to the nonpreferred unit if its speed or speed setpoint exceeds a predetermined value while the preferred unit speed or speed setpoint is below the predetermined value.

5. A control system as set forth in Claim 4, wherein said logic means includes means for shifting the lead back to the preferred unit if its speed or speed setpoint reaches the predetermined value and the non-preferred unit fails to close its circuit breaker.

6. A control system as set forth in Claim 4, wherein said logic means includes means for shifting the lead back to the preferred unit after closure of the nonpreferred unit circuit breaker and synchronization of the preferred unit.

7. A control system as set forth in Claim 1, wherein said operating means comprises digital computer means employed to operate both of said fuel control means in response to speed signals from both turbines and status signals for both circuit breakers and said first signal.

8. A control system as set forth in Claim 2, wherein means are provided for generating said first signal when a further circuit breaker opens in response to a loss of voltage on the bus, and

slid logic means is operative in responsie to a second signal to operate the fuel control means for one of said units in synchronizing said one unit to the reestablished line.

9. A control system as set forth in Claim 2, wherein said operating means includes means for generating a speed reference and an actual turbine speed signal for application to each of said tuel control means, and means for causing th speed reference of the follow unit to track the speed reference of the lead unit after both circuit breakers are closed.

10. A control system for a power plant adapted to be connected to a power supply bus under dead load conditions substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.