ORIGIN
The concept of coordinated control began in the late 1950's as a means by which European designed once-through boilers could be adapted to American operating practice. The concern was that the perceived limited storage of once-through boilers could not respond to the rapid load swings needed to meet the tight control of area regulation and close frequency regulation that is the usual American operating practice. The concept of coordinated control was to keep the turbine and boiler operations together by providing them with a common demand signal. In this manner, the energy input to the boiler would be matched to the energy demanded by the turbine. Thus the expression "Direct Energy Balance." The initial designs were provided on six once-through boilers of Combustion Engineering/Sulzer Brothers design. The operation of these units was successful. In fact, the last of the operating units has only recently been modernized after over 25 years of service. Later, it was recognized that the Direct Energy Balance concept was equally useful on drum-type boilers. This discussion will cover the application of the Direct Energy Balance concept to drum-type boilers and how it has evolved into the present configuration.

ORIGINAL CONCEPT-REQUIRED OUTPUT
The original concept is shown in Figure 1. The increase-decrease pulses from the load dispatcher were integrated to produce a desired generation value that was limited by operator adjusted Maximum Generation, Minimum Generation, and Maximum Rate-Of-Change limits. The resultant value became the demand for both generation and firing rate. This signal was called Required Output. The Required Output signal was sent in equal measure, to the demand signals for generation, fuel input, and air flow, thus assuring that all three variables moved together. The direct, proportional action of the Required Output Signal was modified by three corrections.

GENERATION CORRECTION
First, Required Output was compared to actual generation and the resulting corrective action was added to the generation, fuel feed and air flow demands. Any generation correction was sent in parallel to the boiler inputs as well as the turbine valves.

THROTTLE PRESSURE CORRECT1ON
The second correction was from throttle pressure which was applied in a positive direction to the fuel feed and air flow demands and in a negative direction to the generation demand since the governor acts as a back pressure regulator when controlling throttle pressure.

GENERATION CONTROL
Generation demand consisted of the summation of three values. The first is the feedforward from Required Output. The second is the correction for generation and the third is the negative correction from throttle pressure. The summation of these three signals was compared to a signal which we called turbine valve position, but in reality was the cam shaft position of the mechanical governor. The generation controller pulsed the turbine speed changer motor to change generation.

FUEL FEED CONTROL
Fuel Feed Demand was the summation of four signals, as follows: First, Required Outputs as a positive feed-forward. Second, the positive correction for generation. Third, the positive correction for throttle pressure. And finally, a negative correction from excess air. This resultant value was compared to metered fuel input using flow meters for oil and gas and feeder speeds or other indices for pulverized coal. The fuel regulating equipment was controlled from this action.

AIR FLOW CONTROL
Air flow demand was the summation of four signals as follows: First, Required Output as a positive feed-forward. Second, the positive correction for generation so that air flow kept in step with fuel feed. Third, the positive correction for throttle pressure. And finally, a positive correction for excess air. This resultant value was compared to actual air flow and the air regulating equipment was adjusted. In the early 1960's air flow primary elements were seldom provided and air flow was inferred by measuring the pressure drop of the flue gas across the economizer. Air flow was often regulated by the Induced Draft Fans and furnace pressure by the force draft fans. This, of course, is in violation of current practice.

FREQUENCY BIAS
Theoretically, the system which has been described will cancel out any governor frequency correction. A speed change will move actual generation away from required output and the generation controller will seek to move generation back, thus canceling the effect of the governor action. To compensate for this effect, a speed correction which was called frequency bias, was introduced. The intent was to adjust this correction to match the droop characteristic inherent in the governor and therefore, bias required output in an equal amount. In actual practice, with mechanical governors, the governor seldom behaved in a uniform way and the frequency bias setting seldom matched the actual governor action. In fact, sometimes frequency bias would act and the governor would not.

ASSESSMENT OF THE FIRST GENERATION
The first applications of the Direct Energy Balance Concept proved that better generation control could be realized if the boiler and turbine were controlled by a common strategy. However, the initial operating experience revealed a number of problems. First, governor cam shaft position was not a reliable measurement of turbine valve positions, since it does not recognize valve points, governor deadbands or hysteresis. Second, it was found that the throttle pressure correction on the generation control tended to override the generation correction since, on a load change, they tended to work in opposite directions. This meant that generation response would be sluggish. In actual practice, the throttle pressure correction was set at minimum if not cut out entirely. Similarly, the excess air correction on fuel, which seemed theoretically correct, was found to introduce upsets in throttle pressure. As you might expect, it was set to zero in the field. In addition, inferential measurements of the fuel input by damper positions, feeder speeds etc. did not accurately measure the strategy input to the boiler, either quantitatively or dynamically. This required the throttle pressure controller and the excess air controller to correct any mismatch through their integrating actions. Finally, the use of a common demand for both turbine and boiler did not recognize that the energy storage of the boiler would change whenever load was changed. The integrating action of the throttle pressure had to supply all of this correction.

EARLY IMPROVEMENTS
The first improvements to the Direct Energy Balance are shown on Figure 2. The non linearities of the governor valve system were overcome by replacing cam shaft position with the pressure ratios signal. Pressure ratio is the ratio of turbine first stage pressure to throttle pressure and is an index of effective throttle valve position. The effect of pressure ratios is to treat the turbine valve complex as if it were a single, linear valve. Introduction of this measurement stabilized generation control and significantly improved unit response. The interaction of the throttle pressure on generation and the excess air correction on throttle pressure were dealt with in a positive manner. They were simply removed. The problem of variable heat value, mill lags and other non-linearities in the fuel delivery system was overcome on drum-type boilers by the introduction of the Heat Release Calculation. Heat Release infers the energy delivered by combining the change in energy storage, as determined by the rate of change of drum pressure, with the energy delivered to the turbine as determined by the turbine first stage pressure. Properly calibrated, this measurement is linear over the entire load range, recognizes changes in fuel heating value and fuel delivery, and dynamically recognizes the lags that are inherent in the operation of pulverizers. Finally the problems associated with changes in energy storage were reduced by introducing the rate of change of Required Output (ROR) into each of the demand signals. This rate signal could be adjusted to provide the increment of overfiring that is necessary to change the boiler storage when load is changed.- The results of these improvements were: Pressure Ratio and Heat Release linearized two critical measurements. This meant that the common loading signal could be matched to calibrated process feedbacks. The result was more stable operation, particularly during load changes. The introduction of Required Output Rate (ROR) provided the increments of energy storage needed during load changes, which in turn stabilized throttle pressure during and after a load change.

INTELLIGENT GOVERNORS
One of the most significant improvements in power plant control was the replacement of the mechanical governors with electronic and, later, digital devices, that linearized the valve operations, provided protective limits and overrides and in many cases, duplicated the generation control shown on the left side of this drawing. Since the turbine was selected well in advance of the boiler control system, it made little sense to duplicate the generation control functions that were already included in the turbine governor control.

THE THIRD GENERATION
As shown in Figure 3, we investigated the use of pressure ratio as a feed forward signal to firing rate demand. We determined that the ratio of turbine first stage pressure to throttle pressure accurately detected any movement of the turbine valves for whatever reason. When pressure ratio is multiplied by the throttle pressure set point, it becomes a linear and stable index of turbine energy demand and is therefore a valid feedforward signal for firing rate. This meant that full coordination of the turbine and the boiler could be achieved without a complex interface and without duplicating functions in both the turbine and boiler controls. The energy demand signal is compatible with any form of turbine control, from the most complex, to no control at all. The Required Output Rate signal was replaced by an equivalent signal which we called dynamic compensation which performed the same function in a simpler manner and the demands for fuel feed and air flow were simplified. In addition to eliminating the need for a complex turbine interface, this third generation control greatly simplified the internal self-balancing circuits that are needed when fuel feed and air flow were not in full automatic. One of the characteristics of conventional boiler control systems is that they overshoot and cycle on a sustained load change. This is because the throttle pressure controller and the fuel feed controller, act in cascade control and any sustained throttle pressure error causes the throttle pressure controller to wind-up. When variable throttle pressure operation was introduced, the amount of throttle pressure deviation, and therefore, the amount of wind-up became even more severe.

PRESENT GENERATION
We discovered the best way to eliminate the throttle pressure wind-up was to eliminate the throttle pressure controller. Figure 4 shows the present DEB configuration. Generation control may or may not be present, or it may be executed in the governor control. The fuel feed controller compares the calculated turbine energy demand with calculated boiler heat release. The operation of the fuel feed controller is shown on Figure 5. The heat release calculation, shown on the right, is balanced to the energy demand signal, shown on the left. If throttle pressure is less than its set point, the resultant ratio is a value greater than one. Since first stage pressure occurs on both sides of the equation, the equation can only be balanced by obtaining a positive rate of change of drum pressure. In other words, it is necessary to overfire to restore throttle pressure. Note that the controller can be balanced even if there is a throttle pressure error, as long as the amount of overfiring balances the relationship. As throttle pressure approaches its set point, the amount of overfiring is reduced. The configuration is self-dampening. When throttle pressure equals its set point, there is no overfiring and the boiler stabilizes at desired value without any overshoot and without any cycling.

BENEFITS
This stability of throttle pressure reflects itself in improved stability in generation. Which means that units with this configuration will more closely match generation to their demand. The heat release calculation is linear and reproducible over a 20:1 range. Many gas fired boilers operate stably below 5 % load. Since the heat release calculation represents actual energy input, the fuel/air ratio is more uniform, which, in turn, stabilizes steam temperatures during load swings. This is of particular benefit on coal fired boilers where pulverizer dynamics as well as uncertainties in the fuel heat value make it difficult to measure heat input with conventional means. Like Ulysses, we have been on an Odyssey of thirty years and hopefully we have returned home somewhat wiser and better for the experience. However, unlike Ulysses, we cannot afford to stay at home, for our world is a very dynamic one. We have developed correct feedforward signals and accurate feedback signals which have simplified, yet improved, boiler controls. The next improvements should be in the area of adaptive tuning and in more refined feedforward inputs.