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11. STEAM TURBINE SPEED CONTROL

11.1 The Principles Of Governing

During operation of a Turbine-Generator Unit the Load carried by the Generator may vary over time. In order to respond to changing System Load demands the amount of steam directed to the Turbine must be varied in proportion to each demand. The function of a governor is to provide rapid automatic response to load variations.

Steam to Turbine

Manually OperatedThrottle Valve

Generator Turbine

Variable Load Condenser

Throttle valve setting manually adjusted following each speed reduction due to Load increase

Turbine Speed versus Load Characteristic for each throttle valve setting

0 Turbine Load Max

Figure 55 Turbine Speed-Load Characteristic for Single Turbine withManual Throttle Control

Consider a Turbine-Generator operating with the most basic form of manual throttle control. As Load is increased the turbine speed will drop due to the increased electrical output demanded for the same steam input. On sensing the

decrease in speed the operator will manually increase the throttle valve

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opening to increase steam flow and restore the turbine to the correct speed Figure 55 shows a hypothetical Speed-Load Characteristic for such a Turbine-Generator. Each time the throttle valve is adjusted the turbine settles at a new speed-load characteristic, if left on a single setting the turbine speed would fall as load was increased in line with that shown on the graph (Figure 55). For every new setting of the manual throttle valve there would be a new speed load characteristic each approximately parallel to each other.

While manual operation may be suitable for a turbine operating under steady load condition the response of an operator controlling the turbine manually is not sensitive enough to cater for a constantly varying load. An automatic control system is required that can both sense changes in turbine speed and make appropriate adjustments to the steam flow to the turbine in order to return the turbine speed to the required set point.

Throttle Valve PositionControlled by Governor

Steam to Turbine

Generator Turbine

Variable Load Condenser

Throttle valve automatically adjusted following each speed reduction due to Load increase

A % Droop

CB

0 Turbine Load Max

Figure 56 Droop Curve for a Turbine with Flyball Governor ControlledThrottle Valve

A simple flyball governor is connected to the turbine through a secondary drive. As the turbine speed increases the speed of the governor also increases proportionately. The increased speed causes the flyballs to swing out further with increased centrifugal force and in so doing operate a mechanism to close in on the throttle valve setting, reducing steam flow to the turbine and reducing speed. As speed decreases the opposite effect is achieved.

In Figure 56 a simple flyball governor has replaced the operator manually controlling the turbine speed. The flyball governor will be more responsive to speed variation and adjustments will be made far more frequently than in the case of the operator.Speed is regulated within a narrow band with A and B being the bounds of the upper and lower speed limit (The speed band between A and B is shown magnified in the figure for emphasis, however in practice the bandwidth is so small that it is usual to consider the two lines A and B as coincidental forming one line C as shown)

The smaller the speed deadband (between A and B) and the smaller the slope of the governor speed-load characteristic, the more sensitive the governor.

The drop in speed from no load to full load expressed as a percentage of the desired or no load speed is referred to as the governor “droop characteristic”.

All governors of machines, which are to operate in parallel, should have some droop for reasons of stability and the droop should be identical if they are to share load in direct proportion to their capacity. This ensures stability and is desirable when two or more turbines are operating in parallel.

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Throttle Valve PositionControlled by Governor

Steam to Turbine

Generator A Turbine

Condenser

Throttle Valve PositionControlled by Governor

Steam to Turbine

Generator B Turbine

Common Load Condenser

L1B = 0

Generator A placed on line and partially loaded to L1A

L1A

Generator A Speed-Load

L2 L2A

Generator B Speed-Load

L3B L3A

0 Turbine Load Max

Figure 57 Synchronising and Loading Two Turbo-Generators in Parallel

11.1.1 Turbo-Generators Operating in Parallel

Two similar Turbo-Generators (A and B) fitted with simple flyball type governors, each with a slightly different speed-load characteristic, are to be placed in service and operate in parallel.

Turbo- generator A is placed on the line first and partly loaded to point L1A.

Turbo-generator B is then placed in service and synchronised to Turbo-generator A (represented by the No Load point L1B on Turbo-generator B Speed-Load Characteristic).

The synchronisation of B to A can only take place at this one point. At any other loading on machine A it would be impossible to synchronise B with A.

If machine B was placed in service first, then machine A could not be synchronised with it.Once the two machines are synchronised they must operate atthe same speed if they are to share load. Each will act either as a generator and generate power, or a synchronous motor and absorb power. If turbine A was to run faster than turbine B then turbine A would supply power to the system load and power to generator B causing it to rotate at the same speed as turbine A. The division of load between the two machines can be determined from Figure 57 Machine A Load is given as theintervals L1A, L2A and L3A, Machine B as 0 at synchronisation, L2B

and L3B

respectively.No other division of load for each speed would be possible.

The simple flyball governor has several limitations:

The Load demanded of the generators determines the point on the speed-load curve at which the machine will operate.

The system frequency must change with load

It is not possible to add or remove a generator from service without departing from the standard frequency

The synchronisation of further units to the system would need to be done in an order dependent on individual speed- load characteristics

11.1.2 The Speeder Gear of a Turbine Governor

In order to maintain the system frequency constant and at the same time allow load variation to occur, it is necessary to be able to compensate for the loss of speed experienced with increasing load and the speed increase which accompanies load rejection. To achieve this a device is fitted in conjunction with the governor which effectively changes the speed-load characteristic of the turbine in such a way that speed effectively becomes independent of load. The device is known as the speeder gear.

Figure 58 shows a turbine flyball governor fitted with speeder gear. The flyballs move out under centrifugal force as the speed increases against the restraining action of Spring A located between the flyballs. An addition adjustable Spring B connects the speeder gear to the governor linkage.

It is not possible to make adjustments to the flyball spring while the device is rotating, however, the adjustable spring B attached to the speeder gear tends to govern the movement of the sleeve X in conjunction with spring B. With the operation of the linkage to the governor valve the effects of spring B and spring A are additive.

The overall effect of altering the tension in spring B is the same as altering the tension in spring A of a governor which had no speeder gear, that is, to shift the speed load characteristic to a new position approximately parallel to the original position.

Flyball Restraining Spring A

Moveable Sleeve X

Shaft Movement transferred to Throttle Valve Control

Driven from Turbine Shaft

Handwheel

Spring B

Fixed Nut

Clutch

Speeder Gear

Motor

Figure 58 Flyball Governor with Speeder Gear

11.1.3 Load Sharing Between Units Fitted with Governors HavingSpeeder Gears

Once units are fitted with speeder gear governor control frequency and load control becomes variable and Load sharing between generators is variable rather than tied to a single speed-load characteristic.

In Figure 59 lines A and B represent the speed-load characteristics of two machines (A and B) operating in parallel, with speeder gear compensated governors. Operating at initial speed X1, the load on each machine is given by the intervals LB1 and LA1.

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Generator B Initial Speed-Load Characteristic

Generator B Speed-Load Characteristic afterSpeeder Gear Operation

LB1

LB2

0

LA1

LA2

Turbine Load

X2 X1

A

B2

B1

Max

Figure 59 Flyball Governor with Speeder Gear

The speeder gear on machine B only is operated to increase its speed to X2 the machine will adopt a new speed-load characteristic B2. The governor setting on machine A remains constant

Because both machines are synchronised to each other the speed of machine A will also rise to the new value X2.

In increasing speed machine A must lose a portion of its load

Machine B now carries a higher load LB2

The addition of a speeder gear to turbines governors in a combined system allows the load sharing between units to be controlled by the operating staff so that the load on any particular machine can be reduced to zero in order to take the machine out of service. By a similar arrangement it is possible for any machine to be synchronised with the rest of the running system and hence machines can be placed in service in any chosen order. Further, the system frequency can be controlled.

11.1.4 Relays

In all but the smallest turbine, it is necessary to use some means of amplifying the power of the governor in order to maintain a small sensing and control device and yet still have the motive force to position large sized throttle valves. The devices used as amplifiers are known as relays.

The most common type of relay uses an oil system employing a pilot valve and a power piston. There are two types of these relays in use:

double

acting single

acting

Figure 60 shows a primary relay of the double-acting type, when A is raised by the governor, C is held stationary by the fixed volume of oil above and below the piston and B consequently raises the pilot bobbin, the pressure forces on which are balanced. Oil is thus drained from the bottom of the power cylinder and the piston moves down under oil pressure. There is no further motion of A and the pilot valve is reset to its neutral position. Since the pilot valve begins and ends in this position the lever may be regarded simply as having its fulcrum at B. The high pressure oil is always connected to the centre of the pilot bobbin to avoid the need for glands.

Figure 60 Double Acting Relay