Substations in high and medium-voltage power networks include primary devices such as electrical cables, lines, bus bars, switches, power transformers and instrument transformers, which are generally arranged in switch yards and/or bays. These primary devices are operated in an automated way via a Substation Automation (SA) system. The SA system includes secondary devices, among which Intelligent Electronic Devices (IED) responsible for protection, control and monitoring of the primary devices. The secondary devices may be hierarchically assigned to a station level or a bay level of the SA system. The station level often includes a supervisory computer that includes an Operator Work Station (OWS) with a Human-Machine Interface (HMI) and running a station-level Supervisory Control And Data Acquisition (SCADA) software, as well as a gateway that communicates the state of the substation to a Network Control Centre (NCC) and receives commands from it. IEDs on the bay level, also termed bay units, in turn are connected to each other as well as to the IEDs on the station level via an inter-bay or station bus primarily serving the purpose of exchanging commands and status information.
A communication standard for communication between the secondary devices of a substation has been introduced by the International Electrotechnical Committee (IEC) as part of the standard IEC 61850 entitled “communication networks and systems in substations”. For non-time critical report messages, section IEC 61850-8-1 specifies the Manufacturing Message Specification (MMS, ISO/IEC 9506) protocol based on a reduced Open Systems Interconnection (OSI) protocol stack with the Transmission Control Protocol (TCP) and Internet Protocol (IP) in the transport and network layer, respectively, and Ethernet and/or RS-232C as physical media. For time-critical event-based messages, such as trip commands, IEC 61850-8-1 specifies the Generic Object Oriented Substation Events (GOOSE) directly on the Ethernet link layer of the communication stack. For very fast periodically changing signals at the process level such as measured analogue voltages or currents section IEC 61850-9-2 specifies the Sampled Value (SV) service, which like GOOSE builds directly on the Ethernet link layer.
SA systems based on IEC61850 are configured by means of a standardized configuration representation or formal system description called Substation Configuration Description (SCD). An SCD file includes the logical data flow between the IEDs on a “per message” base, i.e. for every message source, a list of destination or receiver IEDs, the message size in terms of data set definitions, as well as the message sending rates for all periodic traffic like reports, GOOSE, and SV.
Substation Automation (SA) systems include a number of basic SA functions for protection, control and monitoring of the substation, and which functions relate to individual pieces of primary equipment or to entire substation bays. In addition, higher-level applications are provided, which involve at least a station level operator HMI and/or the connection to a remote operation place or network control center by means of a gateway. Known applications are used as operator support or for automating handling of emergency situations within the station. They require operational information from more than one piece of primary equipment, even from more than one bay, and hence are termed “inter-bay”, “station-level”, or “distributed”. In addition to a possible primary functional or operational goal, a configuration of such inter-bay applications therefore also relies on the dynamic switchyard configuration or topology, as well as the basic SA functions used to gather data from the switchyard and execute commands on it. Known inter-bay functions are station-level interlocking, station and bay level switching sequences, transformer parallel control, transformer auto close functions, as well as any other kind of load transfer and/or shedding.
Conventionally, inter-bay functions are engineered or implemented on top of an existing SA system, the latter providing the process state information and some means to control the process. This engineering is done manually, by instantiating the needed function blocks and connecting them signal-wise to the existing SA system.
In the context of the present disclosure, a load transfer function or application is understood to involve and coordinate substation primary devices in order to maintain, following a failure or emergency, a supply of power to the loads connected to the substation in a sustainable way, e.g., without overloading the substation equipment that is not affected by the failure. By way of example, a transformer auto-close function transfers or assigns, in case of a failure of a coupling transformer, the load previously supplied via the failed equipment to other coupling transformers. In simple cases, where only two in-feeding bays or in-feed related bus bar sections exist and where the relations between load and transport capacity are known in advance, it is straightforward to determine which circuit breakers need to be closed to transfer the load. However, in more complex single line configurations an analysis of the current or instantaneous state, based on a current state of a plurality of switching elements, is needed to determine the circuit breaker to be closed in a particular fault case.
Load transfer functions can be triggered by failures of in-feeding or transporting parts, and are used in different variants in the substation automation area, often in medium voltage systems for industrial processes. One example is the high speed in-feed transfer, where two bus bar segments each have one or more in-feeds, feeding the load at the appropriate bus bar segment. A failure at one of the in-feeds leads to a protection trip disconnecting this in-feed. The resulting drop of frequency due to the overload triggers the closing of the bus section between the two bus bar segments, thus sharing the remaining in-feeds with all loads. In this simple topology, no topology analysis is necessary—the drop of frequency signals an overload situation, which then leads to a load transfer to the one still healthy bus bar segment.
Another example is the transformer auto close function, where two pairs of transformers coupled at the HV side feed two different bus bar segments at the MV side, to each of which approximately half of the total load is connected. Any three out of the four transformers can sustain the sum of all loads. Therefore in case of a transformer trip due to some protection functions, the load of the affected transformer pair, together with the remaining transformer of the affected pair, has to be transferred or assigned to the unaffected transformer pair.
In case of a simple bus bar scheme the logic to decide if a situation allows a load transfer and which circuit breakers to close for the load transfer is relatively simple. In more complex bus bar situations a complex logic is required, the implementation and testing of which is specific for each project or single line diagram, as well as error prone due to the complexity. Finally, at network level, as opposed to substation level, e.g. in distribution automation, topology based function implementations are used for load balancing, load shedding or system reconfiguration, based on an implementation specific topology representation. However, in this case, the configuration input of the correct network topology is likewise far from being trivial.
According to the patent application EP-A 1191662 an engineering wizard for an SA function can automatically generate the data flow between IEDs and a function configuration, based on known switchgear parameter values and function block allocation to the switchgear as obtained from a Substation Configuration Description. In particular, the configuration of a first SA function involves allocating this function to a primary device and an IED. Following this, a primary device model and a topology model are used to automatically determine second, more basic, SA functions, of which data or procedures are specified by the first SA function during operation. If specified, the corresponding communication links between the first SA function and the second SA functions are determined automatically on the basis of a communication model representing communication means.
The patent application EP-A 2088444 proposes that protection, measurement and control IEDs in a substation compute, according to interlocking rules or physical principles as well as a knowledge of the dynamic topology of the substation, for every switch they control if that switch may be operated safely, in contrast to a conventional and separate programming of the interlocking logic for each IED. To this purpose, the IEDs have access to the substation electrical topology, to the real-time information generated by other IEDs, and to the rules for interlocking. The disclosure takes advantage of a standardized Substation Configuration Description (SCD) of the substation for which the SA system is intended, as well as of a standardized description of the implemented device functions or capabilities of an individual IED. In particular, the substation topology is available from the substation configuration description (SCD file in IEC 61850 format), the real time information about the position of switches and line voltage/current can be read over the IEC 61850 protocol and the rules are available in script form. This concept applies both to simulated and real devices, and greatly increases system testing possibilities by supporting an efficient configuration of a simulation.
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