Direct Digital Controller
60 Direct Digital Controller fw 4.013 – Ed. 01/2013
the dotted area reaches this value, therefore the regulator does not "block" the second stage and keeps
the first stage on. The second stage is maintained off in all of zone 14.
I. Point 15 and zone 16: because the second stage has not been "blocked", it is switched back on
immediately when the falling temperature of the water, escapes the differential band and zone 16 is kept
on.
J. Points 17 and 18 and zone 19: the situation is similar to that described in H.: the regulator switches off
the second stage of point 17, but in this case in the subsequent point 18 the dotted are that represents the
excess energy reaches the value set for the parameter “prevention integral”; therefore in point 18 the
regulator “releases” the second stage and switches off the first; it then maintains this state in zone 19. In
zone 19 the system is therefore once again in the situation of zone 1, described in A.; therefore all
possible subsequent evolutions of the system fall within the cases already described; for example,
in point 20 the first stage will be switched on again, as previously occurred in point 2.
It is possible to summarise the functioning of the regulation algorithm with the following rules:
• In a given moment, the regulator works with a certain number of stages released and the rest blocked.
• The first stage of the category with highest priority is never blocked.
• All blocked stages are always off; all released stages, except the last, are always on; the last released
stage is switched on or off when the temperature of the water, respectively falling or rising, exits the
differential band.
• A blocked stage is released (and switch-on) if the area that represents the energy deficit, calculated
starting from the expiry of “prevention time”, reaches the “enabling integral” (Figure 16) value.
• A released stage is blocked (and the previous stage is switched off) if the area that represents the excess
energy reaches the “prevention integral” value.
NOTE
• The release and block technique of the stages, based on the “enable and prevention integrals”, allow to
“centre” the power of generation adapting it to the load of the system. In a given load condition, the system
maintains some systems stably of and all of the others, except one, on. The remaining stage is switched
on or off alternately to regulate the temperature of the water, keeping it within the differential band. This
regulation adapts the generation power also when faced with fluctuations contained in the load. However,
successively, to significant load variations, one or more further stages are blocked or released in
succession, until the generation power is taken almost back into equilibrium with the load. At this point the
system goes back to regulating the alternate switch-on/off of an individual stage.
• The “prevention time” allows to consider the fact that the machines start to produce energy only after a
certain time from start-up. Starting from the moment that the regulator has switched a stage on, the
prevention time allows to wait that effect is produced on the regulation of this stage, before starting to
calculate the energy defect still present and potentially reach switching on another stage. Therefore the
values of this parameter are linked to the features of the various types of machines.
• As already mentioned previously, if several machine categories are present, on increase of the heat load,
the system first switches on all stages available in the highest priority category and then, if necessary,
passes to switching the stages on of the priority category immediately lower, and so on. When the heat
load drops the system switches the stages off in the reverse order. Each individual time, the regulator uses
the parameter values defined for the category with lowest priority that have at least one stage on; for
example, in the case of the system with two machine categories, the regulator uses the values defined for
the category for highest priority until only the stages of this category are switched-on. From the time that at
least one stage of the second category is switched on, it passes to using the values of the parameters of
the same. It goes back to using the values of the first category when all stages of the second are off again.
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