[HGen21] Heat generation, cascade control, 2 boilers (burner modulating and 1-stage)

Overview

The following diagram is a simplified illustration of plant example HGen21.
Sequencing energy generators

Key

Step up/down (1)

Key

Important settings:

• Neutral zone (Nz) = 3 K
• Delay for up impulse (DlyUp) = 10 min
• Delay for down impulse (DlyUp) = 10 min

If the function is enabled, the higher value of the two temperatures in main flow 'TMnFl' and main return 'TMnRt' is selected as the controlled variable. If the measured temperature exits the neutral zone 'Nz' by setpoint 'SpT', an impulse "Step up" or "Step down" is sent over output 'StepUp' or 'StepDn' to power balance. The impulse is sent with a delay. The delays are determined using the settings 'DlyUp' and 'DlyDn'.

There are two cases:

• One or both temperatures drop. Pulse "Step up" is generated.
  Heat generation supplies too little power and/or flow in the consumer circuit is higher than in the generation circuit.
• One or both temperatures increase. Pulse "Step down" is generated.
  Heat generation supplies too much power and/or flow in the consumer circuit is lower than in the generation circuit.

To prevent continued switching prematurely, the evaluation logic is held after each impulse. The enabled heat generator returns its 'TraSta' to "Step up/down". The evaluation logic restarts once the transition state expires.
Step up or step down

Power control and compensation (2)

Key

Important settings:

• Automatic power control and compensation for a loss of power (EnCmpPwrShrtg) = 1 (see below)

Power control and compensation receive the request from "Step up/down" for more or less power as a result of the pulse "Step up" or "Step down". In this way, Power control and compensation" triggers a change to the following or previous line in the profile table.

"Power control and compensation" then checks whether the change in row in the "profile table" resulted in the desired increase or reduction to the currently produced output. It receives the feedback on input [PwrCap] on the power currently produced and compares it against the value in internal memory [PrPwrCap].

In the event that the desired result does not arrive, "Power control and compensation" changes the row until the desired increase or reduction to current power is reached.

Automatic power compensation in the event of a power shortage [EnCmpPwrShrtg]: Power shortage is detected based on input [PwrCap] and balanced by sequential steps, e.g. due to a loss of a heat generator due to a fault.
Power control and compensation

Profile table (3)

In the profile table, the sequence is determined for switching on the heat generators. Four different profiles can be created. The entries are entered in each profile so that total power of the enabled heat generators or stages increases as the row number increases.

Function block SELP_8MS determines the profile table.
[SELP_8MS] Profile and table value selection for 8 multistate values

Two profiles are prepared in the example:

Profile 1: "Winter profile"

Boiler 1 (100 KW) is a modulating boiler and is intended as the main boiler for winter operation.

Boiler 2 is switched on if:

• The power for boiler 1 is not sufficient
• Boiler 1 is in fault

Profile 2: "Summer profile"

Boiler 2 (50 KW) is intended as the boiler for summer operation.

It switches to boiler 1 if:

• The power for boiler 2 is not sufficient
• Boiler 2 is in fault

In the example, a parameter block is planned for the changeover of profile. A logic can be programmed for the specific project.
Profile table

Heat generator (Boiler 1 and 2) (4)

The heat generators enabled by the profile table report on available power [PwrCap] to power control and compensation.

They also report in the event of an active change of the power stage, the transient state [TraSta] to "Step up/down". No further "Step up" or "Step down" is triggered during an active transient state.

Key

Boiler 1 with modulating burner, 1-stage pump, maintain boiler return with 3-port valve

Operating modes: Off, On, Min.

Settings for power feedback:

Complete the power per stage on function block SELMS_R

1 Off = 0 KW

2 On = 100 KW

3 Min. = 60 KW

Settings for feedback of the transient state on the nested chart BoTraSta(1):

Components, sequence, and function in CMDSEQ_B (Bo1)

[CMDSEQ_B] Command sequence for binary

Start sequence

1. The pump switches on. It is further switched if an operating message comes from the pump on the input pin for CmdSeq(Bo1).
2. Control of maintain return with the valve and the burner switches on simultaneously (function AND).

Stop sequence

1. The burner switches off. It waits for 2 minutes (function Delay).
2. The valve is set to bypass and the pump switches off (function AND).

Boiler 2 with 1-stage burner, modulating pump, and shutoff valve

Operating modes: Off, On

Settings for power feedback:

Complete the power per stage on function block SELMS_R

1 Off = 0 KW

2 On = 50 KW

Settings for feedback of the transient state on the nested chart BoTraSta(2):

Components, sequence, and function in CMDSEQ_B (Bo2)

Start sequence

1. The shutoff valve opens. It is further switched after 30 seconds.
2. The pump switches on and its control is enabled. It is further switched if an operating message comes from the pump on the input pin for CMDSEQ_B (Bo2).
3. The burner switches on.

Stop sequence

1. The burner switches off. It waits for 2 minutes (function Delay).
2. The shutoff damper closes and the pump switches off (function AND).

Setpoint generation

Main pump group, consumer side

The example implements two main pumps, each with a fault message.

The changeover occurs:

• Every 168 hours regardless of operating hours
• Regardless of fault

The changeover is implemented with function block ROT_8.
[ROT_8] Rotation switch with 8 outputs

Setpoint generation and temperature control

Individual heat consumption transmits its heat request and forms the temperature setpoint for main flow as per this heat request. The setpoint is increased slightly and sent to the heat generators.

Key

BACnet objects for boiler 1

The setpoint for return temperature may not be exceeded and is specified by the boiler manufacturer. The setpoint for boiler temperature is derived from this setpoint. The setpoint for boiler temperature is limited to the minimum to a value derived from the setpoint for return temperature plus a difference of, e.g. 8K.

BACnet objects for boiler 2

Plant operating modes:

• Off (1)
• On (2)

The operating modes are reported on [MCalcVal] object 'Present operating mode'.

At the same time, the reason for the present operating mode is reported to another [MCalcVal] object:

• Exception (1)
• Operating mode switch (2), resulting from an active (not equal to Auto) operating mode switch
• Operating mode (3), resulting from an active (not equal to Auto) manual operating mode selection
• Heat request (4)

There is a normal mode and exception mode.

Normal mode

Sources for normal mode:

• [MI] Operating mode switch
• [MCnfVal] Manual operating mode selection
• [HReq (1)] Heat request

In normal operation, operating mode 'On', the main pumps are switched on and the logic for boiler sequence operation (step up/down, power control and compensation and profile table) is enabled. The logic for boiler sequence operation controls the individual energy generators using their function blocks CMDSEQ_B.
[CMDSEQ_B] Command sequence for binary

Exception mode

Sources for exception mode:

• Fire detection contact
• Faults to both pumps in the main pump group
• Fault to both boilers

In exception mode, all outputs are switched off directly at priority 5 and the present operating mode is set to 'Off'.

It is reported on input [RpdOff] from all CMDSEQ_B on the energy generators.

Start-up control (nested chart SttUpAlmHdl)

The following functionality is available for an automatic startup of the plant, e.g. after a loss of power and return of power:

1. Fixed switch-on delay (can be set on input pin [DlySttUp])
2. The plant remains off to ensure communications are reliable and cross-check references are triggered. 'Exception' is reported as the reason for the present operating mode.
2. Automatic acknowledge (2x) of possible pending alarms during a fixed switch-on delay.
3. Additional adjustable delay for staged plant ramp-up (can be set on input pin [DlyOn])

Alarm handling (nested chart SttUpAlmHdl)

The state of events on the plant is acquired by BACnet object 'Plant state'. The state is mapped on function block CMN_EVT.
[CMN_EVT] Common event

The following functions are available:

1. Automatic acknowledge (2x) after startup
2. Alarm acknowledgement with a value object
3. Alarm indication:
   - For pending alarms On
   - For unprocessed alarms flashing
4. Ability to connect the acknowledge button [BI]
5. Ability to connect to an alarm indication [BO], e.g. an alarm lamp through an output block [BO]
6. Ability to connect to other alarm handling functions on other plants, e.g. acknowledge button and alarm indication for multiple plants in the same control cabinet

Startup function and alarm handling for flat plants