POWER CASCADE

OBJECTIVE

This technical information details the new Power Cascade algorithm that can be selected within the OpenTherm page of the RegConfig Configurator.

CASCADE ALGORITHM FOR HEATING AND COOLING

The following image shows, within the OpenTherm page of the RegConfig, the position of the System Cascade Control Algorithm section and the Buffer Tank Compensation Temperature section:

The following image shows, within the OpenTherm page of the RegConfig, the position of the System Cascade control Algorithm section (blue) and the Buffer tank Compensation Temperature section (yellow):

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The cascade algorithm can be of three types:

• Parallel functioning: all OpenTherm generators enabled for heating and cooling are required to work simultaneously;

• Power cascade: this control can be applied to those generators that are able to return the power output to RegConfig as data point, as the function aims to keep the enabled OpenTherm generators on within a certain power output range;

• PID: stands for Proportional – Integral – Derivative. This last control allows you to switch on or off the enabled OpenTherm generators in relation to how close the temperature measured by a sensor (generally positioned in a buffer tank to which all the generators are connected) and the set point is.

Enabled OpenTherm generators are those generators that have a number (which can range from 1, highest priority, to 8, lowest priority) in the Generators section (highlighted in green) on the OpenTherm page and not OFF in the box corresponding to the “Heat/Cool” column. The number indicates the priority of the call, so in the image below G1 and G3 generators have priority 1 (they are called first), while G2 has priority 2 and G4 priority 3:

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Warning

Do not use G0 as it is the OpenTherm generator used with the DOT product, not with the REG system, therefore it is never considerate in the cascade logic.

POWER CASCADE CONTROL ALGORITHM

The aim of the Power Cascade Control Algorithm is to ensure that the system has a water temperature as close as possible to the set point calculated by switching on the required number of generators and trying to make them work as efficiently as possible. To do this, it is necessary to know the power supplied by the generators, the % of power corresponding to the maximum efficiency of the generator, and the water outlet temperature of the generator.

To define when to switch a generator on/off, the algorithm compares the minimum power supplied between the active generator (and not degraded by the system) with the PotUp and PotDown parameters (which will be illustrated below) after a settable minimum time has passed during which no change is made to the number of switched on generators, to allow the active generators to go full speed.

The addition or switching off of a generator must always observe the following 3 parameters:

• N. Start Generators: indicates the number of generators that are switched on starting from the condition “all OpenTherm generators are off”. For example, let us assume that the Start generators number is 2: you are in the condition “all OpenTherm generators are off” and the cascade algorithm results in only one generator switched on, but at least two generators are switched on at the same time. After switch-on (and therefore at least after the time indicated by “Stop between changes”) the algorithm may assess if the number of generators is appropriate to the demand of the system (and therefore leave it unchanged) or decrease or increase it; the number of generators at the start is useful if there are systems with many generators. Let us suppose we have a system with 8 generators: if the system starts with only one generator and then every “Stop between changes” (typically 3 minutes) adds another one, it will pass at least 21 minutes before the system goes at full speed; instead, if we set the initial number at 4, the system starts immediately with 4 generators, and then we decide if increase this number (and, in any case, in 12 minutes it would go to the maximum) or decrease it. 

• N. Minimum Generators: indicates the minimum number of active generators. If, for example, it is set to 3 and 3 generators are currently active, even if the cascade algorithm were to determine the need to switch off a further generator, none would be switched off; in another way, they would have never less than 3 generators on (Note: this means that the REG System enables 3 generators which can then decide to switch off the flame/compressor, because they determine that their water outlet temperature is satisfied).

• N. Maximum Generators: indicates the maximum number of generators that can be switched on at the same time. If 5 generators are enabled, but the maximum number is 3, this means that the algorithm can switch on maximum 3 generators, even though 4 would be needed. This choice has the direct consequence that the set point may never be reached or that may be reached but over a long period of time.

Once the number of start/minimum/maximum has been defined, it is necessary to define the following parameters:

• Stop between changes (s): the variation is the change corresponding to the switching on or off of an OpenTherm generator. The pause between two variations it the minimum time that passes between the switching on or off of an OpenTherm generator and its subsequent switching on (or subsequent switching off) of another OpenTherm generator. For example, if at time x a generator is switched on, for the time indicated in the Stop between changes box, the algorithm will not add or remove generators (even though the result of the calculation would require it) to give the switched-on generators time to get up to speed and see the effects on the system;

• PotUp [%]: among all the switched-on boilers (with flame and heating on), ignoring those that maybe are degraded and those that are not producing DHW, the lowest power output value (in %) is compared with PotUp: if higher, another generator is switched on. Usually set at 50%. If only one generator is switched on, the only power output will be compared with PoutUp. It must not be forgotten that any limits given by the number of generators start/minimum/maximum must be always respected, so for example if there is a system with 8 generators enabled, but the maximum number of generators is 6, the system will call the various generators in rotation, but it will never switch on more than 6 generators at the same time even though the algorithm would require it;

• PotDown [%]: among all the switched-on boilers (with flame and heating on), ignoring those that maybe are degraded and those that are not producing DHW, the lowest power output value (in%) is compared with PotDown: if lower, the first boiler that had been switched on is switched off. Usually set at 30%. If only one boiler is switched on, the only power output is compared with PotDown. Even in this case, the system can keep removing generators, but it will never drop below the start generator number, unless the heating demand from the zones stops;

• Gap [%]: represents the maximum power range that can be between the generator working less and the one working more, usually set at 8%. This parameter is useful to make the switched-on generators work at, more or less, the same power. So, if the minimum power expressed between the switched-on generators is 25% and Gap is 8%, all the generators are limited to 33% (=Maximum Power).

• Idle: this parameter (that means inactive) is the lower limit of the power limitation; a generator is never limited to a power lower than Idle. So, if the minimum power output between the switched-on generators is 5% and Gap is 8%, the maximum power would be 13%, but if Idle is 30%, then the generators will be limited at 30%. This parameter is essential when a new generator is switched on. Let us assume that we have switched on 2 generators that are both roughly 45% power. The minimum power output between the generators exceeds 50% (PotUp) and the system calls a third one, which starts at 0% power. If the Gap parameter is 8%, it means that between the generator expressing the minimum power and the one expressing the maximum power there can be a difference of at most 8%. So, if the third generator is at 0%, the others should be at 8%! To avoid this, a lower limit is put on the power of the already active generators, for example 30% (Idle). So, even if the generators that are switched on should stay at 8% power to conform to the Gap parameter, they will go to the set Idle value. 

MANAGEMENT OF DEGRADED GENERATORS

Two (non-mandatory) parameters are left to be set:

• Power degraded

• Temperature degraded

If left at zero, the algorithm does not manage degraded generators.

A generator is considered degraded if:

1. The stop time is passed (that is the conditions during the pause following the addition/removal of a boiler are not considered);

2. A generator is in heating or cooling demand (the DHW production is not considered);

3. The power to which the generator is limited is bigger than the power degraded.

4. The generator water outlet temperature is lower than the set point required of the generator less the temperature degraded (T water outlet < set point – temperature degraded, where the latter is a temperature delta K and not an absolute value). For example, if the set point is 40°C and the degraded temperature is 5 K, the generator temperature must drop below 35°C before it can be degraded, as long as the other four points also occur;

5. The generator power output is lower that the “Power degraded” (which is an absolute power and not a power delta). For example, if the power degraded is 20%, the power output of the generator must drop below 20% before it can be degraded, as long as the other four points also occur.

If a generator is degraded, the system excludes it from all the calculations shown (minimum power calculation, maximum power for comparison with PotUp, PotDown, Gap, and Idle).

The result of the settings made is displayed at the top right of the OpenTherm “Generator Demand Status” page:

• Enable/set Heating: if the square is ticked, this means that there is a request to the generators and in the rectangle on the right is indicated the required set point. For heat pumps, the set point is either heating or cooling. If the square is not ticked and the set point is zero, this means that no demand is coming from the circuits to OpenTherm.

• Enable/Set DHW: if the square is ticked, this means there is a request to the generators and in the rectangle on the right is indicated the required set point for the production of Domestic Hot Water. If the square is not ticked and the set point is zero, this means that no demand is coming from the circuits or the DHW page to OpenTherm.

• N. active generators (cascade): indicate how many generators have been activated for the cascade in heating or cooling (not for DHW production).

• The last rectangle at the bottom (yellow box) is the countdown of the Stop between changes in hundredths of a second.

Always referring to the image in the Generators section of the OpenTherm page:

note the following:

• Domestic Hot Water production does not respond to the power cascade algorithm;

• The cascade algorithm switches on the OpenTherm generators according to the priority entered.

The lower part of the OpenTherm page shows, for each generator, the result of the heating and DHW demands (red frame) and the feedback from the generators to the REG System (yellow frame):

Highlighted in red:

• The request for H = Heating, W = DHW;

• The required temperature in heating [°C];

• The heating power [%] is the upper limit and it is the greater between Idle and the lower of the active boiler power plus the Gap.

Warning

If the generator is used for DHW production only, both the required temperature and the power are at zero, the values that are passed in case of DHW production are those that are displayed in Generator Demand Status (top right of OpenTherm page).

The OpenTherm page can manage boilers via OpenTherm protocol. In yellow are highlighted the values transmitted by the boiler, closely linked to how the generator manufacturer has implemented the OpenTherm protocol on the boiler itself (always refer to the generator’s manual). By hovering the mouse over the various rectangles/squares, a tag appears with an extended description:

• Generators Status: indicates in which status the generator is (F = fault, H = heating, D = domestic hot water, F = flame, C = cooling etc.) If not implemented, no square will be ticked.

• Water outlet temperature (T outlet), Water inlet Temperature (T inlet), DHW Temperature [°C]: are the readings of the generator sensors if present.

• Pow.: power delivered by the generator in % (refer to boiler manufacturer’s manual).

• OT error: most frequent OpenTherm error (Ts = service request, Rr = remote reset enabled, Lp = low water pressure, etc.).

• OEM Error: indication of the number of the error, refer to generator manual for diagnostic.

• Reset: reset of the error, it only works if the boiler manages it; refer to the generator manual for diagnostic.

ROTATION

With equal priority, ignition follows the numerical order of generators for the first ignition, while for subsequent ignitions the generators are rotated:

So, in the example above at the first ignition the order will be:

G1 → G3 → G2 → G4

At the second ignition:

G3 → G1 → G2 → G4

Warning

Domestic Hot Water production does not respond to the power cascade algorithm.