Switching Loads with S7-1200 and SENTRON PAC3200 via MODBUS TCP (Set 24)
Power utilities charge industrial electricity customers not only for the active energy consumed (in kWh, "kWh rate"), but they also charge a price if a specified power limit (in kW, "price per kW") is exceeded. The power limit is defined as the average of the total electric power imported over a defined measuring period. If this power limit specified by the power utility is exceeded, an additional price per kW has to be charged for the entire accounting period (e.g., 1 month or 1 year, depending on the terms of the contract). To avoid this, large-scale electricity customers temporarily switch off dispensable loads. Now the task in this application is as follows:
Relative to the start of the respective measuring period, the total average power demand of all loads is to be permanently determined to forecast it for the end of the measuring period. If it is expected that the power limit will be exceeded, loads are to be temporarily switched off according to a priority list.
- The respective total average power demands at the end of the measuring periods are to be logged over at least the accounting period to furnish proof to the power utility.
For load control a SIMATIC CPU S7-1200 is used. SENTRON PAC3200 multi-function measuring instruments are used to measure the power or electrical energy of the loads. As the HMI a KTP600 operator panel is used.
The measured data at the power infeed point necessary for load control is acquired individually for each load by the SENTRON PAC3200 multi-function measuring instruments and added up in the PLC. The SENTRON PAC3200 devices transmit the data to the controller via "MODBUS on TCP". The switching-off of loads that is required when there is an imminent exceeding of the power limit is performed using the digital output of the relevant SENTRON PAC3200 device. The SENTRON PAC3200 devices receive the associated switching commands from the PLC via MODBUS communication.
The data logging is carried out with the S7-1200 data log function. At the end of the measuring period, the total average power demand relative to the measuring period is written to a csv-file in the load memory resp. to the memory card. Via the S7-1200 webserver you have access to the csv-file at any time.
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Load Management with S7-1200 and SENTRON PAC3200
SIMATIC STEP 7 Projekt
(STEP 7 BASIC V11 SP2, WINCC BASIC V11 SP2)
SIMATIC STEP 7 Projekt
(STEP 7 BASIC V12, WINCC BASIC V12)
Caution - The functions and solutions described in this article confine themselves to the realization of the automation task predominantly. Please take into account furthermore that corresponding protective measures have to be taken up in the context of Industrial Security when connecting your equipment to other parts of the plant, the enterprise network or the Internet. Further information can be found under the Entry-ID 50203404
Note about Industrial Application
This demo-application depicts a strongly simplified example of a peak load limiter and has not the grade of an industrial-compliant solution. Both the application example and the interaction of its respective components have not been tested for productive use in an industrial facility or production machinery.
For the implementation of an industrial-compliant solution for peak load limitation we strongly recommend the utilization of SIMATIC powerrate. The main advantages of this product over the depicted application example are as follows:
- Precise acquisition of the accounting-relevant total average power demand directly at the power infeed
- Reliable acquisition and long-term archiving of accounting-relevant energy data.
- Comprehensive parameterization of the load switching behavior (online editable priority list, minimum / maximum connect / disconnect times of the loads, switching of loads taking external interdependencies into account).
- Reporting tool providing consumption and cost center reports as well as duration curve analysis.
- Standard interfaces towards superordinate energy data management systems like SIMATIC B.Data.
- Industrial grade system quantity structure and scalability of the solution for the implementation of plant-wide energy data management and peak load limitation.
- Batch-related energy data acquisition and reporting.
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