DESCRIPTION

SYSTEM LEVEL TESTING FOR SUBSTATION AUTOMATION SYSTEMS

FIELD OF THE INVENTION

The invention relates to the field of Substation Automation (SA) systems for substations in high and medium voltage power networks. More particularly, it relates to the implementation and testing of interlocking functionality in an Intelligent Electronic Devices (IED) of an SA system.

BACKGROUND OF THE INVENTION

An electric power system comprises a power transmission and/or distribution network interconnecting geographically separated regions, and a plurality of substations at the nodes of the power network. The substations include equipment for transforming voltages and for switching connections between individual lines of the power network. Power generation and load flow to consumers is managed by a central Energy Management System (EMS) and/or supervised by a Supervisory Control And Data Acquisition (SCADA) system located at a Network Control Centre (NCC). Substations in high and medium voltage power networks include primary devices such as electrical cables, lines, bus bars, power transformers, instrument transformers as well as switching devices (circuit breakers or disconnectors) interconnecting bus-bars and/or bays (feeders). These primary devices are operated in an automated way via a Substation Automation (SA) system responsible for various substation-specific tasks such as controlling, protecting, measuring and monitoring. The SA system comprises secondary or process devices, also called digital relays, interconnected in a SA communication network, and interacting with the primary devices via a process interface. The secondary devices are generally assigned to one of three hierarchical levels, which are (a) the station level including an Operator Work Station (OWS) with a Human-Machine Interface (HMI) as well as the gateway to the Network Control Centre (NCC), (b) the bay level with its devices for protection, control and measurement, and (c) the process level comprising e.g.

electronic sensors for voltage, current and gas density measurements as well as contact probes or position indicators for sensing switching device states and transformer tap changer positions, as well as actuators controlling the drive of a switching device or tap changer. At the process level, intelligent actuators may be integrated in the respective primary devices and connected to a bay unit via a serial link or an optical process bus. The bay units are connected to each other and to the devices on the station level via an inter- bay or station bus.

In more detail, a Protection, Control and Measurement (PCM) secondary device, also known as an Intelligent Electronic Device (IED) in the IEC 61850 standard, and controlling a particular switching device, operates on the base of signals from attached sensors for switch position, temperature, voltage, current etc., signals from other IEDs indicating the state of their controlled elements and signals from the supervisory system. Conversely, an IED generates signals to act on its switching elements, to communicate its state to other IEDs or to inform the supervisory system. These signals are either hard- wired or transmitted as network messages, for instance according to IEC 61850-8 or IEC 61850- 9-2 messages.

In its protection function, a PCM IED monitors the state of a substation or of a part there of and autonomously opens an assigned circuit breaker in case it detects a potentially dangerous situation such as overload. In its control function, the PCM IED executes commands from the supervisory level, in particular opens and closes assigned switching elements.

In a "select before operate" sequence, an operator may reserve a switching device for operation and ask the PCM IED, by way of a switching request, to execute a particular close or open operation on a particular switch. The assigned PCM IED may then accept or refuse such a command depending on the electrical state of the attached lines in order to prevent a hazardous or damaging operation, such as connecting a live bus bar to earth. This safety mechanism is called interlocking.

Conventionally, the logic implementing the interlocking is programmed as Boolean expressions in tabular, code or function chart language on each PCM IED individually during the engineering phase of a substation project, which requires both time and considerable experience as well as a perfect knowledge of the substation topology. Parts of the latter are "compiled" and comprised in the function chart type logic on the PCM IEDs.

This engineering process is normally done on the base of a fixed substation topology, and requires substantial changes in case, for instance, of an extension of an existing substation.

Any custom-made SA system is required to undergo system verification and validation. The supplier of the SA system is expected to demonstrate to the customer the correct coordinated operation of all parts in all possible scenarios, as well as the expected quality or performance like throughput and timely response also under high load. As the operation of a particular PCM IED depends also on signals that are generated by other PCM IEDs, such signals have to be properly prepared in order to reproduce all expected states of the substation. A large range of tests allowing to manipulate the signals generated by other

IEDs, hereafter called system level tests such as System Integration Tests or System

Verification Tests (Factory Acceptance Test (FAT), Site Acceptance test (SAT)) have thus been devised.

The aforementioned system level tests are generally performed in a test environment or test rig, in which a number of IEDs are installed. However, due to the sheer number of

IEDs necessitating an increasingly complicated test rig, and due to cost and space limitations, not all the IEDs of a particular substation are installed for a FAT. Accordingly, the extent and coverage of the test configurations is limited, and in particular, interlocking functionality as detailed above is only partially testable in the factory. The tested IEDs represent only a typical part of the substations and are only given access to a minimum number of signals necessary as input to their respective interlocking logic.

According to the patent US 4767941, the implementation of interlocking functionality can be automated provided that the IED knows the actual topology of the substation, i.e. the actual switching states of all switching devices, and an exhaustive set of rules for interlocking operation. Decoupling the topological configuration of the substation from the interlocking rules allows updating each of them independently and thus increases flexibility. The signals that indicate the switching state of the associated switching devices are supplied via dedicated data buses and data links to a centralized data acquisition and processing unit. By evaluating the signals the actual topology of the substation is inferred, and based on the interlocking rules, a release pattern is determined and stored. The latter indicates a release or blocking property for each switching device, i.e. if a specific switching operation request command is to be accepted or refused.

DESCRIPTION OF THE INVENTION

It is an objective of the invention to facilitate and extend system level testing of interlocking functionality for Substation Automation (SA) systems. This objective is achieved by a method of testing interlocking functionality of an Intelligent Electronic Device (IED) as well as by an Intelligent Electronic Device (IED) according to claims 1 and 7, respectively. Further preferred embodiments are evident from the dependent patent claims.

According to the invention, a first IED implementing interlocking functionality to be tested, also termed Device Under Test (DUT), prepares an interlocking response indicating a release or blocking property, i.e. whether or not the first IED intends to open or close a first switching device that is assigned to and controlled by the first IED. At the same time, a testing device which may be an industrial computer with appropriate processing power, and which may itself be considered an IED in the sense given above, independently determines an interlocking status for the first switching device. The interlocking status is then compared to the response from the first IED. In case of discrepancy between the two, the existence of an error is concluded and reported. In order to facilitate and render more flexible this testing process, the testing device is given information about a dynamic or present topology of a part of a substation comprising a neighbourhood of the first switching device. This information in conjunction with a set of pre-established, generalized interlocking rules enables the first testing device to perform interlocking decisions on behalf of the first, or any other, switching device, and requires considerably less configuration efforts as compared to a programming, at the testing device, of function chart type logic for each of the respective switching devices. In other words, the interlocking functionality or logic of the first or any further real IED to be tested is simulated or emulated by the testing device in a scalable manner of which the benefit increases with the number of IEDs.

In a preferred variant of the invention, the testing device stores information in digital form about a static substation topology, and generates itself information required for obtaining the relevant dynamic state of the substation from the static substation topology. Hence, in a first step, the testing device is given a static topology or structure of the substation in a standardized description or format, such as, for instance, the substation section of a Substation Configuration Description (SCD) file according to the substation

communication standard IEC 61850, indicating in particular the role of the first IED in the substation. In a second step, during the actual testing, information about a status of at least one further switching device in the substation other than the first switching device assigned to the first IED is exploited. Likewise part of the SCD file is information about a Configured IED Description (CID), i.e. a standardized description of implemented device functions of the first IED to be simulated. This information is advantageously used by the testing device in order to properly assign or link data from the first IED to various primary devices of the substation.

In an advantageous embodiment of the invention, the first IED receives a switching request directed to the first switching device and responds with a command response message comprising the interlocking response. This message is transmitted to, received and evaluated by the testing device. In other words, the testing device is responsible for both the simulation of the interlocking behaviour of the IEDs under test as well as for the subsequent comparison of their interlocking response and the interlocking status. At the same time, the SA communication network, i.e. the correct addressing and exchange of network messages, can be tested as well. As any change of switch state at one IED also influences the interlocking state at further IEDs, even if assigned to other bays, the testing device may, by means of the command response messages from the first as well as further real IEDs, obtain and compare with its own simulation a coherent and time-resolved interlocking response of the entire substation.

In further preferred variants, the testing device prepares the information about a status of at least one further switching device in the substation on behalf of the first IED by means of suitable network messages and/or binary signals. The network messages are transmitted over an SA communication network to the first IED, and are preferably Generic Object Oriented Substation Events (GOOSE). On the other hand, the binary-type copper signals are generated by the testing device itself, or a dedicated signalling device connected to the testing device, and transmitted via dedicated copper cables that are hardwired to the first IED. In a real Substation Automation system, such copper signals report the state of the further switching devices that are directly wired to terminals of the first IED, and may even originate from a bay other than the one comprising the first switching device. In order to correctly generate and transmit the binary signals, the testing device has to be aware of the logical signal name allocated to the binary signals at the terminals of the first IED, and this information may be provided to the testing device by means of the

private wiring section for non-IEC 61850 signals of the Substation Configuration Description (SCD) file mentioned above. By way of example, CBR.2=X.1O1/X.1O2 indicates that terminal 101 and 102 receive the double indication status of switching device CBR.2. It has to be noted that the copper based binary signals may also carry other information, such as a sufficient SF ₆ pressure in the first switching device. The first testing device may generate the signals corresponding to a plurality of further IEDs, and emulate an entire bay of the substation or even the whole substation.

Aspects of the above preferred variant are not only applicable to simulated IEDs emulated by or embedded in a testing device, but to real process devices or dedicated Protection, Control and Measurement (PCM) IEDs providing or implementing interlocking functionality for a switching device in a substation of an electric power system. Hence, a real IED is provided with and stores, in digital form, information about a static substation topology as well as an exhaustive set of interlocking rules that enables the IED to perform interlocking safety. In a first step, for instance as part of an engineering process, the IED is given a static topology or structure of the substation in standardized description or format, indicating in particular its own role in the substation. In a second step, during testing or regular operation of the SA system, messages indicative of a status of at least one further switching device in the substation are generated by a second IED of the SA system and transmitted over a SA communication network to the IED. Preferably and in terms of the substation communication standard IEC 61850, said standardized description is a Substation Configuration Description (SCD) file, while said network messages are Generic Object Oriented Substation Events (GOOSE). Since the SCD file is large, an extract from the SCD file comprising all information needed for topology-based interlocking can be communicated in a compressed form to the IED. During operation, the IED evaluates the static topology and the actual switch states to produce a dynamic topology of the substation, based on which, together with the interlocking rules, a switching request from the supervisory level (operator, remote control or local human interface) received via MMS with Select/Operate service is processed and accepted or denied. Preferably, the IED is wired, via its binary I/O terminals, to a conductor or process bus connection, via which it receives binary signals indicative of switching status. The IED, in this case, is provided with information about these binary signals required for obtaining the relevant dynamic state of the substation from the static substation topology, and in

particular the assignment of the terminals as, for example, comprised in the private wiring section for non-IEC 61850 signals of the Substation Configuration Description (SCD) file.

Implementation as well as a later adaptation, e.g. to a changed static topology, of the interlocking functionality of Intelligent Electronic Devices (IEDs) of a Substation Automation (SA) system is considerably simplified by the above aspect of the present invention. Furthermore, the preparations for processing a switching request directed to the switching device assigned to the IED are advantageously carried out upfront, generating suitable release patterns to be consulted upon reception of a switching request, and to be cyclically refreshed at suitable intervals of e.g. 5-10 ms corresponding to the reception of the most actual switching state information from the neighbouring IEDs.

It has to be noted that a real IED comprising a "topology engine" as above may likewise be the object of an interlocking test as described previously. In this case, the outcome of the IED and the testing device is identical, so any discrepancy between interlocking response and status has to be due to an incorrect "cabling". The latter may include addressing errors when configuring GOOSE messages, or erroneous wirings for the binary signals.

In a summarizing embodiment, the present invention 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 invention 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) and the real time information about the position of switches and line voltage/current can be read over the IEC 61850 protocol, while the interlocking rules are available in script form as disclosed in US 4767941, as expert system rules, or in any other suitably coded form. This concept applies both to simulated and real devices, and greatly

increases system testing possibilities by supporting an efficient configuration of a simulation.

The present invention also relates to a computer program product including computer program code means for controlling one or more processors of a testing device connected to the communication network of a Substation Automation system and configured to test an interlocking functionality of a first Intelligent Electronic Device (IED) itself adapted to control a first switching device of a substation of an electric power system. When executed, the computer program makes the testing device execute the steps of

- determining, based on a dynamic substation topology representative of at least a part of the substation as well as based on interlocking rules, an interlocking status indicating a release or blocking property of the first switching device, and

- comparing the interlocking status with an interlocking response prepared by the first IED.

BRIEF DESCRIPTION OF THE DRAWINGS The subject matter of the invention will be explained in more detail in the following text with reference to preferred exemplary embodiments which are illustrated in the attached drawings, in which:

Fig.l shows an excerpt of a single line diagram of a substation, and Fig.2 schematically shows a testing environment. The reference symbols used in the drawings, and their meanings, are listed in summary form in the list of reference symbols. In principle, identical parts are provided with the same reference symbols in the figures.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Fig.1 shows a single line diagram of a part or section of an exemplary substation at an assumed voltage level of e.g. 110 kV, as well as exemplary Substation Automation (SA) and/or secondary equipment. The model of a switch yard at single line level contains the topological respectively electrical connections between primary equipment. The substation comprises a double bus bar configuration with two bus bars 10, 10' feeding two bays 11, 11 ' via disconnectors 110a to 110'b. Each bay comprises a circuit breaker 111, 111 ' and an earthing switch 112, 112'. The corresponding excerpt of the substation automation system depicts, in broken lines, a communication network 20 and two IEDs 21, 22, which both

host logical nodes of class CSWI (switch control). Each logical node is allocated to one of the aforementioned switching devices 110a to 112' as indicated by the thin broken lines in Fig.l. An Operator Work Station 23 is also depicted.

A simple aspect of interlocking prevents the disconnectors 110 and circuit breaker 111 to close as long as earthing switch 112 connects the attached lines to ground. Hence, in order to prevent a hazardous or damaging operation, such as connecting a live bus bar to earth, a corresponding command would be refused. Likewise, disconnector HOa is not to be opened while disconnector HOb is being closed, i.e. interaction between two switching processes is to be avoided. Fig.2 shows a test environment or test set-up according to the invention together with a first IED 21 under test. The latter is connected to the SA communication network 20, as is a testing device 30 with dedicated processing means for simulating or emulating the IED 21 under test (indicated by simulated IED 21 '). The testing device also simulates further IEDs that may or may not be physically present in the test environment. The testing device 30 includes a Topology Engine (TE) module 31 for inferring a dynamic substation topology and a memory for storing Interlocking Rules (IR) 32. The TE module 31 is provided with at least an excerpt of the SCD file 33 of the substation and comprising the static substation topology, and creates a dynamic topology based there upon. In accordance with the latter, network messages are generated and transmitted over the SA communication network 20 to the first IED 21 under test. The response of the latter is monitored and evaluated to conclude whether or not the IED 21 operates as expected.

A signalling device 34 or remote controlled signal generator is shown external to and in communication with the testing device 30, wherein the former may also be a part or module of the latter. The 31 generates binary signals representing sensors or switch status information, these simulated process signals are applied, via dedicated copper wires, to binary input terminals of the IED 21 under test.

LIST OF DESIGNATIONS

10 bus bar 11 bay

110 disconnector

111 circuit breaker

112 earthing switch

SA communication network first Intelligent Electronic Device (IED) second IED operator work station testing device topology module interlocking rules

SCD file signalling device