[Dsh21] District heating, transfer station, 1 heat exchanger

Plant example Dsh21 is a district heating substation with one heat exchanger.

Functions

Collects heat demand and controls to the corresponding setpoint for the secondary flow temperature
Primary return temperature maximum limitation
Return temperature differential limitation (DRT)

Plant diagram

Components

Valve (primary)
Pump (secondary)
Primary and secondary flow and return temperature sensor
Sensor for outside temperature

Components and functions

Components and functions of the plant example:

Component	Function	BACnet object
	Operating mode switch Switches the plant to: Auto \| Off \| On	[MI] Operating mode switch
	Manual operating mode selection Switches the plant to: Auto \| Off \| On	[MCnfVal] Manual operating mode selection
	Present operating mode Reports the present operating mode: Off \| On Plant operating modes and control sequence	[MCalcVal] Present operating mode
	Reason for present operating mode Reports the reason for the present operating mode: Exception \| Operating mode switch \| Manual operating mode selection \| Heating limit \| Heat request \| Scheduler	[MCalcVal] Reason for present operating mode
	Heat request Acquires and evaluates the heat request.	[ACnfVal] Offset for setpoint heating request [ACnfVal] Minimum setpoint heating request
	Sensor for outside temperature Measures outside temperature.	[AI] Outside air temperature
	Fault message expansion vessel Reports faults. Switches the plant off.	[BI] Fault expansion vessel
	Secondary pump	Pump (Secondary) (Folder)
	Command Switches on/off the pump as needed.	> [BO] Command
	Fault message Reports faults. Switches the plant off.	> [BI] Fault
	Pump run when the outside air temperature is low Pump always operates at low outside temperatures (frost protection).
	Kick function (pump kick) Prevents the pump from seizing during long idle periods. [KICKFNCT] Kick function
	Overtemperature detector Closes the primary valve. The secondary pump is switched on for two minutes. The pump is then switched off. Switches the plant off.	> [BI] Safety temperature limiter
	Secondary return temperature sensor Measures the secondary return temperature.	[AI] Secondary return temperature
	Secondary flow temperature sensor Measures the secondary flow temperature.	[AI] Secondary flow temperature
	Heat exchanger	Heat exchanger (Folder)
	Primary return temperature sensor Measures the primary return temperature.	[AI] Primary return temperature
	Primary flow temperature sensor Measures the primary flow temperature.	[AI] Primary flow temperature
	Valve (primary) Throttles the primary water volume to maintain the desired secondary flow temperature.	Valve (Primary) (Folder) > [Controller] Temperature controller
	Primary return temperature maximum limitation Controls the maximum primary return temperature. The setpoint is generated in a floating manner based on outside temperature. Limits the temperature controller for the secondary flow temperature to a minimum of e.g. 15%.	> [Controller] Limitation controller, maximum return temperature
	Return temperature differential limitation (DRT) Controls the maximum difference between the primary and secondary return temperature. For example, if the difference between the primary and secondary return temperature (DRT value) is exceeded during ramp-up, the control value closes and reduces the volume flow until the primary return temperature cools to the DRT value. Limits the temperature controller for the secondary flow temperature to a minimum of e.g. 15%.	> [Controller] Return temperature differential limitation controller
	Limitation active Indicates that the output of the temperature controller for the secondary flow temperature is limited.	> [BCalcVal] Limitation active
	Positioning	> [AO] Position

Functions and diagrams

Plant example Dsh21 is a district heating substation with one heat exchanger. Dsh21 can also be used as a precontroller with heat exchanger to hydraulically separate 2 heating systems.

Outside air temperature

The outside temperature [TOa] is used unfiltered and filtered.

Signal name	Description	Filter constant	Use
TOa	Outside air temperature	Unfiltered	Pump run when the outside air temperature is low
TOaEff	Effective outside air temperature	1h	Floating setpoint generation for maximum primary return temperature

Collect heating requests

The heat request can be gathered from different consumers. Binary demand messages and analog demand setpoints can be collected.

The program receives one function block each of type R_B that can be linked to a binary demand message via BACnet and an input block of type R_A for an analog demand setpoint.
[R_B] Read binary
[R_A] Read analog

Additional demand signals can be connected in parallel: Binary to one OR and analog to one MAX block.

Secondary flow temperature control and limitation controller

BACnet objects

Name	Description	Object type	Default value
Vlv(Prim)'TCtr	Temperature controller Controls the secondary flow temperature.	Controller	N/A
Vlv(Prim)'TCtr	The temperature controller acts as a PI controller (PID with derivative action time Tv=0) and compares the secondary flow temperature setpoint [PrSpTFlSec] against the secondary flow temperature [TFlSec] and calculates the valve position [Vlv(Prim)'Pos] on the output. The following controller variables are preset: - Controller gain: 2.5 [%/K] - Integral action time Tn: 120 [s]
Vlv(Prim)'TRtCtrMax	Return temperature controller for maximum limitation (option). Controls the maximum primary return temperature. The setpoint is generated in a floating manner based on outside temperature.	Controller	N/A
Vlv(Prim)'TRtCtrMax	The temperature controller acts as a PI controller (PID with derivative action time Tv=0) and compares the floating setpoints based on the outside temperature against the primary return temperature [TRtPrim] and calculates a limitation to the valve position [Vlv(Prim)'Pos] on the output. The following controller variables are preset: - Controller gain: 2.5 [%/K] - Integral action time Tn: 600 [s]
Vlv(Prim)SpTDiffRt	Setpoint return temperature differential	ACnfVal	6 [K]
Vlv(Prim)'TRtDiffLmCtr	Return temperature differential limitation controller Controls the maximum difference between the secondary and the primary return temperature.	Controller	N/A
Vlv(Prim)'TRtDiffLmCtr	The temperature controller acts as a PI controller (PID with derivative action time Tv=0) and compares the return temperature differential setpoints, e.g. 6 K against the temperature differentials [TRtPrim - TRtSec] and calculates a limitation to the valve position [Vlv(Prim)'Pos] on the output. The following controller variables are preset: - Controller gain: 2.5 [%/K] - Integral action time Tn: 120 [s]

An active limitation is displayed using the [BCalcVal] object 'Limitation active'.

In the event that one or both limitations are temporarily disabled (e.g. for a heat demand from the domestic hot water handling plant), it can be carried out at the ANDs for enabling the limitation controller. See figure below:

Plant operating modes and control sequence

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
Manual operating mode selection (3), resulting from an active (not equal to Auto) manual operating mode selection
Heat request (4)

There is a normal mode and local exception mode.

Normal operation

Sources for normal mode:

[MI] Operating mode switch
[MCnfVal] Manual operating mode selection
Heat request

In normal mode, components are simultaneously switched on and off as per the operating mode.

Exception mode

Sources for exception mode:

Fault to expansion vessel
Fault to secondary pump
Safety temperature limiter.

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

Start-up control and alarm handling

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:

Fixed switch-on delay (can be set on input pin [DlySttUp])
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.
Automatic acknowledge (2x) of possible pending alarms during a fixed switch-on delay.
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:

Automatic acknowledge (2x) after startup
Alarm acknowledgement with a value object
Alarm indication:
- For pending alarms On
- For unprocessed alarms flashing
Ability to connect the acknowledge button [BI]
Ability to connect to an alarm indication [BO], e.g. an alarm lamp through an output block [BO]
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

I/O data points

Name	Description	Type	Signal/connection	State text	Engineering unit
OpModSwi	Operating mode switch	MI	N/A	Auto \| Off \| On	N/A
TOa	Outside temperature	AI	LG-Ni1000	N/A	-70...70 [°C]
FltExpsVss	Expansion vessel fault	BI	NO contact	Normal \| Tripped	N/A
HExg'Pu(Sec)'Cmd	Heat exchanger pump (secondary) command	BO	NO contact	Off \| On	N/A
HExg'Pu(Sec)'Flt	Heat exchanger pump (secondary) fault Alarm function: Basic (Notification class 8) Reference value: Triggered Time deviation: 0 [s]	BI	NO contact	Normal \| Tripped	N/A
SftyTLm	Safety temperature limiter. Alarm function: Basic (Notification class 8) Reference value: Triggered Time deviation: 0 [s]	BI	NO contact	Normal \| Tripped	N/A
HExg'TFlSec	Heat exchanger secondary flow temperature	AI	LG-Ni1000	N/A	0...100 [°C]
HExg'TRtSec	Heat exchanger secondary return temperature	AI	LG-Ni1000	N/A	0...100 [°C]
HExg'TFlPrim	Heat exchanger primary flow temperature	AI	LG-Ni1000	N/A	0...100 [°C]
HExg'TRtPrim	Heat exchanger primary return temperature	AI	LG-Ni1000	N/A	0...100 [°C]
HExg'Vlv(Prim)'Pos	Heat exchanger valve (primary) position	AO	0...10V	N/A	0...100 [%]