Boilers have enormous thermal mass and are relatively slow to react. Turbines are nimble and quickly answer an operator’s command. Coordinating an entire plant requires an intimate knowledge of both systems and selecting the right logic tools to bring them together.
The front end, in the jargon of the power plant controls engineer, consists of the boiler master and turbine master. As explained in Part I of this two-part series, the operator’s window into the control system is referred to as a station or master, and it provides the operator interface for a given control loop. Access to that loop is typically from a switch or hand station located on the control panel in older plants or, more commonly, the operator’s keyboard in plants fortunate enough to be equipped with digital controls.
The best case is when both the turbine and boiler masters are in the distributed control system (DCS). But this is not always the case. We often find that only the boiler controls have been upgraded. In such cases it is important that the DCS be able to interface with the existing turbine controls if you want to take advantage of the DCS’s full abilities. Options for tuning the entire plant are limited with a DCS that includes the boiler master but lacks a communications link with the turbine controls.
Boiler Control Options
Boiler tuning is something of a balancing act. Feedwater enters the boiler through a series of low- and high-pressure steam heaters into the drum. The water then journeys through the water walls of the furnace and absorbs heat until steam is formed in the main steam drum.
This steam then enters the main steam line and passes through a series of superheaters and desuperheaters until it finally ends up at the turbine governor and/or stop valves. The boiler controls the turbine throttle pressure by modulating the boiler-firing rate. This means that the amount of fuel and air that is going into the furnace is increased or decreased depending on whether the turbine requires more or less steam pressure.
There are four usual modes of operation in the world of drum boilers: base mode, boiler-following mode, turbine-following mode, and coordinated control (Table 1). Each of these operating modes is described in the following paragraphs.
Table 1. Options for plant boiler control. Source: Tim Leopold
In general, the boiler master will be either in auto or manual control mode. The turbine is another matter. Turbine controls generally have a number of stand-alone loops — such as megawatt, pressure, valve position, or speed — which are control loops that do not respond to the DCS turbine master. If the turbine controls are not looking at the front end, then as far as the front end is concerned, the turbine is in manual control. For our purposes, "auto" under the turbine master heading in Table 1 means the front end is controlling the turbine governor valves.
Base Mode. In this mode, there is no automatic response to changes in main steam or throttle pressure or megawatt setpoint by the front-end controllers. An operator’s steady hand is required to make the final boiler control adjustments. The turbine might be in one of its own stand-alone loops, but the turbine master has no control of the plant. Many plants operate in this or a similar mode prior to upgrading their turbine controls to a DCS.
Boiler-Following Mode. In this mode of operation, the boiler master is in automatic and the turbine is not. This is an automatic control loop, controlling steam pressure. Depending on the boiler, it can be well controlled. Generally, this is the loosest of the three typical automatic front-end modes of operation (Figure 1).
1. Loaded questions. A typical boiler-following response following a setpoint change. Source: Tim Leopold
This is one of those loops that uses the dreaded derivative gain. The proportional gain is normally pretty high, the integral action slow, and the derivative is absolutely a must. The real keys to tuning the front end are a few simple concepts. For example, don’t add to an upset; that is, don’t have any of your gains disproportionately high. We use the derivative because we are trying to anticipate the steam pressure deviation.
The feedforward signal is an important part of this control loop and is often referred to as target steam flow. Target steam flow is the measured steam flow multiplied by the ratio of throttle pressure setpoint to throttle pressure. Typically, there is a function generator designed such that 0% to 100% of the input signal is proportional to a 0% to 100 % output signal. The nicely dynamic nature of the ratio helps the boiler master move in the right direction. Additional "kickers" may also be available. One option is a throttle pressure setpoint kicker that adds a little to the feedforward signal if the setpoint is changed. The derivative action of the controller also acts as a kicker.
Turbine-Following Mode. In many ways, this is my favorite plant operating mode, because it is the easiest to tune. It also offers a good strong safety net to operators in times of crisis. In turbine-following mode the boiler master is in manual and the turbine master is in automatic mode. The turbine master controls throttle pressure by modulating the turbine governor valves. Megawatts are then produced in the generator and pushed to the grid as a function of the boiler load.
Compared to the slow and sometimes lumbering response of the boiler, turbine response is usually fast and agile. Proportional gains are usually moderately large, and the integral action can be quite fast. Although adaptive tuning is possible, there usually isn’t the need for this; many units use only one value for the proportional and/or the integral gain. Also, the need for a feedforward is minimal. The turbine governor valves operate as one large pressure control valve that can easily control throttle pressure when the control loops are well-tuned.
Turbine-following mode is also a favorite among operators. If the plant is in coordinated mode, and the unit starts to go out of control for almost any reason, operators simply have to put the boiler master into manual. Immediately, the controls will automatically default to turbine-following mode. The valves open or close, as necessary to control the main steam pressure. Meanwhile, because the firing rate has steadied, the boiler controls will soon settle out.
Figure 2 plots the data taken during start-up of a 320-MW power plant. At the lower left corner you can see where the valve transfer occurred. The valve transfer is a process in which the turbine, upon start-up, transfers control from the stop valve to the governor valve. There are actually two sets of valves in the main steam line before the turbine: the main stop valve and the governor valves. The next interesting point on this figure is the area that I call the "disturbing delta." There was a long period, during this load ramp, when the difference (delta), between the throttle pressure and the throttle pressure setpoint was virtually constant (the purple and green lines at the first vertical white dotted line). When we expect the controls to act one way, and they do not, it’s time to investigate.
2. Under control. Taming a control loop that switched out the integral control on a load ramp. Source: Tim Leopold
During a change in unit load demand, in coordinated control, it is common practice to decrease the integral action of the boiler master controller to zero until the load ramp is finished. This strategy was used in all of the turbine and boiler master controller modes. This is a case where more is definitely not better; there was a touch of feedforward, based on boiler demand, substantial proportional gain, and no integral gain when I looked at the logic. Tuned as it was, the error signal between throttle pressure and throttle pressure setpoint will never go away.
I tried to tune out the error without success. Although the error decreased, as shown in Figure 2, we soon discovered that the tuning was not robust under all operating conditions. We then downloaded the necessary logic modifications (the second white vertical dotted line), causing the unit to drop out of turbine-following and into base load mode, and then back again. When the logic modifications were made, from that point on (the third white vertical dotted line) you can see good control of the throttle pressure. This is how a well-tuned turbine-following mode should operate.
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