14480 Potsdam






IEC 60870-5-104, Modbus

Following the strong focus on technical communication equipment for renewable energy systems over recent years, we were very pleased about an order from our “traditional”, principle focus – technical telecontrol equipment for substations and distribution stations. This installation was to be carried out for Energie und Wasser Potsdam GmbH (EWP) in Substation South.


The starting point was the requirement to replace existing central terminal units and bay controllers because spare part procurement for the systems is now very complicated. The 110 kV double busbar and four 10 kV blocks were to be connected “anew” to the control centre using FO and IEC 60870-5-104 protocol during ongoing operation. Communication in the substation itself was also to be implemented over FO (multi-mode) on six separate sections. An FW-50 as a central terminal unit with a 19″ switch serves as a master instance.

The conversion of the bay controllers in the cells started on the 10 kV level. Isolation cell by cell was carried out for this. 20 outgoing feeds, two coupling feeders and four transformer cells needed to be connected up. For the former, a double command, 4 x double-point information, one current (1A) and 4 x single-point information were to be provided in each, and in the transformer cells, two double commands, 4 x double-point information, 3 x current (1A), 4 x voltage and 4 x single-point information each.

DSO cards with 1 out of n monitoring and measuring circuit test were used for command output. The electrical parameters required for operations are derived from the currents and voltages measured. FO links in accordance with IEC 60870-5-104 are used for data transmission to the control centre from the feeders. The FO are laid in four sections emanating out in a star shape. The substations for the 110 kV feeders are housed in standalone external cabinets. Each has their own FO link. Read and sent in addition to six double commands with return information are 3 x current (1A), 3 x voltage, 32 x single-point information, as well as the derivable measured values for active/reactive power, conductor-conductor, conductor-earth, zero voltage, power factor and frequency.

Interlock and control functions are implemented in all 110 kV substations. The automatic switchover unit for the transformers is controlled from the central terminal unit using a PLC program based on codeIT. The new systems were installed, set up and tested in close collaboration with EWP technicians. We appreciated very much the professionally structured yet straightforward working methods of the EWP team, especially in regard to the conversion during live operations. We were therefore able to replace all of the secondary level technology in the substation in just four weeks.

Digression – automatic switchover for transformers

Automatic switchover is required for two scenarios:

  1. It is used to enable the coupling in the event of a fault in dual transformer mode
  2. or in single transformer mode to switch to the intact transformer

Automatic switchover can be activated from the master display or on site from a button. Operational readiness is in this case reported to the control centre with IEC 60870-5-104 communication, and signalled on site by an illuminated button. A continual check is performed for whether the switch-on conditions are satisfied:

  • Status of Local/Remote switch
  • No circuit breaker or voltage regulator in fault or intermediate position
  • Protection fault
  • Arc protection and system protection – not actuated

Automatic switchover starts when one of the following signals is pending:

  • Diff protection OFF
  • Buchholz main vessel OFF
  • BH load switch OFF

Then a predefi ned program runs, thereby sending switching commands to the individual transformers and couplings to enable/disable switches. Every switching command remains pending until the corresponding return information is detected in the automatic switchover; only then is the next switching operation run. If switch return information is not detected within a defi ned time, an automatic switchover device fault occurs and it switches itself off. The relevant commands are reset and the fault is reported to the control system. Activating the Local/Remote switch during a running automatic switchover also results in this abort scenario. Following scheduled conclusion of all switching commands, the automatic switchover device switches off automatically and reports execution of the switching operations to the control system.

In order not to burden the communication paths unnecessarily between the feeders, we use a 32-bit switching signals over which a great deal of time-critical payload information can be sent with just one frame.