242 Power Plant Engineering

advantageous when the generator is connected to a relatively large transmission network, as commonly experienced in the western United States, where transmission lines running several hundred miles are not uncommon. Often, distribution pools have specific requirements as to the response characteristics of member utilities' plants. Selection of a higher response ratio than necessary does not pose any operation problems; however, an insufficiently rapid exciter can prevent the unit from remaining on-line when transient disturbances occur on the system. Except for long transmission systems, a response ratio of 0.5 is typically adequate for most installations.

8.7 STEAM FLOW CONTROL

The steam flow to a power plant steam turbine is controlled by the main steam control valves. These valves regulate the steam flow to the high-pressure turbine, that also controls the steam flow to remaining turbine sections. The operation of these valves in conjunction with pressure variation in the steam generator provides for four methods of controlling steam turbine generator output:

1. Throttling control, 2. Governing control, 3. Variable-pressure control, and 4. Hybrid variable-pressure-governing control.

These four methods of control and comparisons of the heat rates for each mode are presented in the following section. The control methods are also referred to as admission modes.

8.7.1 Throttling Control

Throttling control is the simultaneous operation of all main steam control valves at the same time. The main steam pressure is typically held constant at rated conditions and the control valves are all opened the same amount. The steam turbine output increases as the valves are opened and full load is reached when the valves are wide open. This type of operation minimizes the mechanical loadings on the steam turbine control stage as a result of the reduced pressure to the stage and the equal loading on all stage sections. At partial loads, throttling operation is the least efficient of all control modes because of the throttling process that reduces the energy available in the steam expansion process.

Depending on the turbine design, this mode of operation may also be referred to as full arc admission because of the steam admission to all portions of the control stage. The term typically is used in conjunction with the startup of a turbine. During the startup of some units, the control valves are wide open. Steam is initially admitted to the turbine by throttling the steam flow by use of bypass valves internal to the main steam stop valves. This flow control method is used up to 15% to 25% load. Above this load, the main steam control

valves are used to control the steam flow and the main steam stop valves are wide open.

8.7.2 Governing Control

Governing control is the sequential operation of the main steam control valves. Governing control varies the output of the steam turbine by increasing or decreasing the arc of admission of steam flow to the turbine control (first) stage. Each control valve feeds a particular section of the control stage, and the amount of arc in use is determined by the number of valves open. The valves are opened in a particular order that is determined by the allowable stresses on the control stage. Typically, a minimum of 25% of the control valves are opened simultaneously when initially starting a unit to minimize stresses on the control stage. When these valves are fully open, the remaining valves are opened in series. This control mode is also referred to as partial arc admission. Rated throttle conditions are used throughout the load range to the extent allowed by the steam generator. This control mode is more efficient than throttling control because the throttling process loss is minimized by reducing the number of control valves throttling at any one time.

8.7.3 Variable-Pressure Control

Variable-pressure control is steam flow control by varying the steam generator pressure with the main steam control valves in a fixed position. In this control mode, the main steam control valves typically are wide open. The steam flow to the turbine is controlled by the pressure of the main steam from the steam generator. Main steam pressure is controlled by the steam generator firing rate. The main advantage of variable pressure operation is that the turbine first-stage temperature remains relatively constant across the load range which shortens startup times and increases turbine rotor life. The disadvantages of variable pressure operation are poorer thermodynamic performance and limited load response capability. The lower main steam pressures of this mode result in less available energy than in the governing mode, but more than in the throttling mode. This reduced efficiency still exists on an overall cycle basis even when the reduced power requirements of the boiler feed pump (due to reduced discharge pressure) are included. The response time is limited to the rate the steam generator firing can be increased since the main steam pressure is controlled by the firing rate. In the other modes of control, steam turbine load can be increased more rapidly by opening the closed main steam control valves and utilizing pressure and thermal energy stored in the steam generator.

8.7.4 Hybrid Variable-Pressure-Governing Control

The hybrid variable-pressure-governing control mode uses the low-load high-temperature, no throttling loss advantages of variable-pressure operation and the thermodynamic and load response advantages of governing control. In addition,

Steam Turbine Generators 243

hybrid operation provides improvements in part load control stage efficiency. At low loads, some of the main steam control valves are wide open and steam flow is controlled by variable pressure operation of the steam generator. Increasing main steam pressure increases steam turbine load until the steam pressure reaches rated conditions. Load is increased further by maintaining the rated main steam pressure and sequentially opening the remaining main steam control valve as in the governing control.

8.7.5 Heat Rate Comparisons

A comparison of the steam cycle overall heat rates for the four control modes is shown in Fig. 8-19. The throttling control has the worst overall heat rate as a result of throttling losses. The variable-pressure control mode has a slightly better heat rate than throttling control, but still has poor performance because of the low steam pressures. The governing control has the best heat rate at higher loads because of the high steam pressures and minimized throttling losses resulting from throttling with only one valve at a time. The hybrid control mode takes advantage of the governing high load performance while reducing throttling losses and boiler feed pump power at low load with the use of variable-pressure operation. This hybrid mode results in the most efficient operation while extending unit life by maintaining high temperatures at low loads, reducing cycling effects.

Governing heat rates are typically plotted as a "locus of valve points," that is, as a smooth curve passing through the heat rate points where any valves open are wide open. The governing heat rate is shown by the solid line in Fig. 8-19. The actual heat rate curve is represented by a valve loop that incorporates the throttling losses associated with a valve throttling between full closed and full open. This actual heat rate is represented by the dashed line. As can be seen from the heat rate curve, the more control valves, the smaller the valve loop, the greater the possibility of operating without throttling, and the better the heat rate.

-THROTTLING CONTROL CONSTANT PRESSURE VALVE LOOP

- THROTTLING CONTROL VARIABLE PRESSURE

-HYBRID CONTROL VARIABLE PRESSURE

GOVERNING CONTROL CONSTANT PRESSURE

50% ADMISSION " ^ 700%

62.5% 75%

THROTTLE FLOW

Fig. 8-19. Control mode thermal performance curve. (From Westinghouse Electric Corporation. Used with permission.)

8.8 AUXILIARY SYSTEMS

Steam turbine generators require numerous ancillary systems to support their operation. These systems are normally supplied with the units. The function and scope of the auxiliary systems are described in the following subsections. Also included as part of the description of the turbine drains system are turbine interfacing recommendations and requirements with regard to water induction prevention. The design of the auxiliary systems varies by manufacturer. Although the information provided here is generic, it still may be more applicable to one manufacturer in some cases.

8.8.1 Turbine Seal System

The turbine seal system prevents seal leakage from pressurized packings of the turbine and valves, and prevents air leakage into zones under a vacuum. The basic concept of the seal system is shown in Fig. 8-20. The turbine shaft seals and the operation of the system are described below.

8.8.1.1 Shaft Seals. At the end of each turbine casing is a shaft seal (shaft packing) that prevents steam leakage to atmosphere and air infiltration into the turbine casing. The shaft seals consist of labyrinth-type packing rings with glands. There are two types of shaft packings: pressure packings and vacuum packings. Figure 8-20 shows a schematic of typical pressure and vacuum packings. Both seal systems incorporate labyrinth packing rings with glands and connections to two sources that have constant pressure. One source of constant pressure is the steam seal header, that is typically held to 3 to 4 psig (21 to 28 kPa gauge). The other source is the steam packing exhauster vent header, for that pressure is typically maintained at about 3 to 5 in. (746 to 1,244 Pa) of water vacuum.

The pressure packings are the shaft seals that seal against a positive pressure at full load. This type of seal is typically found on the exhaust end of high- and intermediate-pressure turbines, where the pressures can be on the order of 815 and 195 psia (5.62 and 1.34 MPa), respectively. The steam pressure is broken down to the steam seal header pressure in stages. At each stage there is a leakoff that discharges steam to a turbine extraction point or to the steam seal header. At full load, a portion of the leakage flow goes to the steam seal header. The remaining leakage steam flows to the condenser or a feedwater heater. During startup, when there is a vacuum condition throughout the turbine, the steam seal header supplies steam to the pressure packings to ensure a seal to prevent air inleakage.

The vacuum packings are the shaft seals that always seal against a vacuum. Steam is supplied from the steam seal header. A portion of this flows to the vacuum end of the turbine. The remaining portion of steam flows to the steam packing exhauster vent header.

8.8.1.2 Seal System. During startup and low-load operation, steam from the main steam supply system or auxiliary