Por: Marcelo Paulino e Guilherme Penariol
Presented in APAP2011 - The International Conference on Advanced Power System Automation and Protection (Shangai-China)
1 Introduction
The Brazilian Electric Power System consists of an extensive and complex system of interconnection due to the great centers of generation and power consumption. In the case of transmission lines, the system of protection with teleprotec-tion schemes are designed using IEDs (Intelligent Electronic Devices), one at each end of the transmission line and a communication system between them.
In addition, technological developments require changes, which in a conventional installation the exchange of data between IEDs are performing by hard wiring connections for analog signals, binary signals, control of switching equipment signals and alarms (digital inputs and outputs). Besides the greater amount of material used, the number of inputs and outputs was limited to the available hardware terminals. Using IEDs and Ethernet, especially with the advent of the IEC61850 standard, the analog and digital signals by hard wires are replacing by data on communication networks. The requirements for testing and validation of IEDs and integrated communication networks of IEDs have different approaches and require appropriate testing tools.
The test tools must be prepared to evaluate the individual components of the system, as well as performance of the combined operation of the different functions enabled in the system tested. When the protection and automation system has a communication structure, validation of the system must provide the individual testing of IEDs and the communication system together. This paper discuss the testing and commissioning principles that evaluate the functionality and features of protection and control by individual IEDs testing and systems testing. The tests evaluate all the protection system and associated communication. The application of these tests is possible using a method to test the complete system.
The work shows application examples, including those involving the coordinated work of technical teams of different companies, highlighting the complexity of planning, coordination and execution of tests, not only by technical difficulties.
Approaching the communication systems applied to pro-tection devices, the work focuses on the existing protocol in IEC61850 standard that enables new ways of performing tests in IEDs of a substation on the substation configuration language and communication architecture. While this reality isn´t yet fully implemented, an alternative architecture for application in remote test systems based on IEC61850 is discuss in this paper. The main advantage in IEDs and systems remotely testing is the fact that the expert could be far from the test site eliminating the wait time travel to the location of the problem, improving service performance.
2 Protection, Automation and Control System with Communication
The simple or complex protection, automation and control systems use communication technology where the functions may be locally at the substation or can be arranged and activated in different levels of hierarchical systems. This function can be started by monitoring elements, based on the change of equipment status or alteration of system parame-ters. All communication interfaces used in the system can be based on different protocols and can use different types of communication links. The IEC 61850 standard has great importance due to the significant benefits achieved by the use of messages point-to-point high speed. They are applied under different elements of the protection, automation and control scheme
Knowledge of the system functional hierarchy and the interfaces between components plays an important role determining the requirements for functional testing of devices and distributed applications [1].
The protection IEDs are not the only component of a fault clearing system. They are responsible for detecting a fault condition, determining its cause and the decision of the specific measures to eliminate or reduce their impact on the electric power system and its components. However, they are not the only elements that require testing to ensure the safety of the system in different operating conditions. If we analyze the components of a typical fault clearing system, the potential failure of each individual element may lead to failure of an operation or undesired operation under normal conditions or at the border between the normal operation and the failure. In particular, the communication system used by the protection and automation devices.
If the protection system uses communications between the two ends of a protected transmission line, the components of the communication link between the IEDs are also part of the fault clearing system that need to be e considered when determining the requirements for the validation tests. Last, but not least is the protection device itself, with its binary and analog input modules, measuring and protection functions and relay outputs. When discuss the requirements for protection, automation and control system testing, we need to do that within the context of the fault clearing system and not as isolated devices. At the same time, we must analyze the reasons for the test, since it will determine what kind of tests should be performed and what kind of test equipment and tools will be most suitable for testing.
3 IEC61850 Communication Requirements
The IEC61850, Part 5, defines the communication require-ments for functions implemented in several substation au-tomation system levels and device models [2]. The functions are established and determined in accordance with the tasks to be performed at the substation, such as monitoring, control and protection. The IEC61850 data model specifies automatically all data required to describe these functions. These data make a mandatory minimum list of signals that can be controlled with the help of Logical Nodes, LN. The data required for configuration of distributed functions are contained in the specification of each function. The reason, how and why these data are reported by IEDs is outside of the specifications of features, but within the proposed solution for suppliers and system integrator. The IEC61850 standard defines the data model of communication. The manufacturer of IED makes the protection function. The description of these functions and nomenclature of logical nodes can be obtained by reading the text of the standard on the part 7-4 of IEC 61850 [3].
3.1 Data Model and Communication Model
Typically, the communication between two systems uses a set of rules that define the type of messages and the order they should be changed. This set of rules is known as a protocol. When the communication requires a large number of protocols, they are grouped into these features together and forming a layer and the set of layers form a stack.
The main purpose of a protocol (or protocol stack) is do that the systems (or equipment), even though they have different internal architectures, speak the same communica-tion rule and thus be able to exchange information. As described earlier, the IEC 61850 standard specifies the set of functions and protocols. The protocol in the standard IEC 61850 defines two models: the data model and communication model.
The data model defines functions, logical nodes, which model the functions of control and automation of a substation that have attributes and services following the technique of object orientation. The standard leaves the paradigm of physical equipment and specific functions (LN) that are hosted on hardware (IED).
The communications model of IEC 61850 is based on the concept of abstract definition. The Abstract Communication Service Interface (ACSI) performs the interaction between the data model and communication model. To become part of the real world is need to map these services on an actual protocol. The ACSI is responsible for performing the mapping for the protocol ISO 9607 (MMS) and IEEE 802.3 [3].
3.2 Communication Services Mapping
The part 8.1 of IEC 61850 defines the specific service map-ping (SCSM) for the protocols ISO 9506-1 and ISO 9506-2 (MMS) and ISO / IEC 8802-3 [5] and uses the philosophy of client - server. This type of acquisition is usually held to meet the data acquisition systems (SCADA) of control centers or other functions that do not require time requirements. Clients send service requests and receive confirmations of service that was processed on the server. A customer can also get a direction of the report in the server. The protocol stack being used by mapping a specific communication service communicates all service requests and responses. There are several services explained in part 7-2 of IEC 61850 standard that are mapped to communication protocols and profiles that do not make use of ISO 9506 (MMS, due to information critical time constraints [4]). In accordance with Section 2.7, services are mapped into four different combinations:
•Client / Server: ACSI core services using MMS pro-tocol suite;
•Time Sync: it performs time synchronization protocol with the Simple Network Time Protocol - SNTP
•Sampled Values (SV): sampled values of current and voltage transmitted in the network
•GOOSE (Generic Object Oriented Substation Events) and GSSE (generic substation event status): GSE class messages (generic substation event)
In the OSI model only the first two layers, physical and data link, are common to all services and their messages being that the physical layer using data from the Ethernet type. In order to allow for different coverage requirements in the communication system of the substation, the messages defined in part 5 of the IEC61850 standard are classified according to performance.
The performance classes are set according to the functio-nality required in the substation. The IEC 61850 standard specifies two types of message applications to meet critical time requirements: GOOSE and GSSE. The GOOSE message handles large amounts of data arranged in data set (data blocks) and the GSSE message handles the state of substation organized in pair of bits.
Each IED in communication network need to determine the sender of the message and the received data is of interest. This features a message as multicast ant the GOOSE message sent and received can be used for different devices on the network, or to a recipient only. It is also performed filtering by a MAC address to increase the reception performance of multicast messages.
In the case of sampled values, defined in the IEC-61850-9.2 LE, each message has samples of current and voltages, both three-phase. Since the goal is to eliminate any latency in the transmission of the message, the communication is of the type Publisher and Subscriber. In this type of communication, there is no time spent setting up connection or to start negotiating a communication section. The sender publishes the messages and users subscribed capture the message that has an interest.
3.3 The Merging Unit and Process Bus
The process bus set out in Part 9-2 of IEC 61850 specifies the use of digital connection between protection IEDs, con-trollers or meters that are going to level. This promotes the digitization of the connection between these IEDs, current and potential transformers. This connection uses the Ethernet network and information is exchanged using the data link level (OSI Model, Layer 2). The digital connection of the process bus uses devices called Merging Unit to perform the interface function between the CT / VT conventional or unconventional and the level of IED allocated in the bay level. The voltage and current from the CT / VT are processed (sampled), they are generated, and distributed output values called analog sampled value, compliant and standardized according to IEC 61850-9-2.
4 Universal Test Device Requirements
A test device should allow an appropriate test, fitting the requirements of protection and communication system to be tested by simulating the characteristics of the substation and the electrical system. This should have the following func-tions:
•Simulation of analog current and voltage to the IED tested and sampled measured values (SMV) simula-tion according to IEC 61850 -9-2;
•Simulation of digital signals that represent the changing status of the breaker and the remote control signals, such as traditional outputs for IEDs and si-mulation of GOOSE messages;
•Analysis of binary events and GOOSE messages with monitoring and recording time of data from the IEDs under test to evaluate their performance. The test device should be able to map different messages from different manufacturers IEDs at the same time;
•Simulation of power net, with configuration tools that allow the user to configure the test equipment for the needs of validating systems and IEDs tested and provide the sending GOOSE messages to multiple IEDs included in the protection system;
•Simulation of testing process with test tool that allows flexible configuration of test sequences required for the validation of the IEDs under test, with the application of currents and voltages corresponding to states of pre-fault, fault and post-fault, steady or transient.
•Devices with time synchronization that meet the re-quirements of the system under test.
The development and implementation of equipment and automation systems based on the IEC61850 standard require a new generation of specialized test equipment as well as methods for functional tests for the various system compo-nents.
4.1 Complete Test Set Design
The full test of the functions of the process bus and station based solely on communication (IEC 61850-8-1 or IEC 61850 9-2) will be held similarly to test in individual IEDs. The main difference is that this case will test multiple devices with virtual simulators or analog outputs. The simulation environment of the substation and the system will require the simulation of multiple Merging Units based on IEC 61850-9-2 and other IEDs (interface IEC 61850-8-1). Distributed functions performance assessments will be based on the test system subscription component for the GOOSE messages from different IEDs involved. If these devices have hard wired relays output, their operation should be monitored to evaluate system under test performance. If necessary, compare the results using communication network and the data obtained with wiring connection. Figure 1 show a simplified block diagram of this test system.
Figure 1 Simplified block diagram of a complete test system
5 Functional Testing with Communication Interfaces
The functional testing procedures can be separated into several categories. They are related to the complexity of the functionality of individual devices. These devices are used at different hierarchical levels of the system, and have different types of distributed functions implemented.
From this point of view presents the test methods most commonly used:
•Functional element testing
•Interoperability Test
•Integration and system testing
5.1 Functional element testing
The objective of the functional element test is to determine if the element under test has the behavior expected under different conditions. System testing, on the other hand, verifies that the complete system performs adequately from an external point of view. The functional elements in a sys-tem test are considered units; in other words, the smallest system components with a visible interface and defined behavior.
Functional tests must be performed with test equipment capable of simulating analog signals (currents and voltages) and Samples Values or SVs) carried in the network. It should also be able to perform the subscription of conventional binary signals and GOOSE/GSSE messages carried out through the Ethernet network and issued by diverse IED's simultaneously, independently of the manufacturer. Figure 2 illustrates a simplified block diagram of an example test system.
Figure 2 Simplified block diagram of test system.
The objective of this test is to illustrate a test scenario that permits remote tests of IEC 61850 based IEDs and systems, also allowing for remote analysis of results. The main advantage in performing such remote tests is that specialists do not need to be dispatched to the location where the device under test is found, therefore saving travel time to the substation, reducing equipment outage times, and consequently increasing the availability of the equipment and protection system.
For this test, an OMICRON CMC 356 test set with the capability of injecting 10 simultaneous analog signals (4 voltages from 0-300 V and 6 currents from 0- 32 A) was used. This equipment is also capable of simulating and subscribing to GOOSE and GSSE messages in an Ethernet network, as well as carrying out conventional binary signal measurements through hardwired relay contacts. The device under test was an AREVA IED type P443, configured with distance protection settings. Figure 3 illustrates the test arrangement.
The test sites chosen were Camaçari (state of Bahia) for the specialist observation point to coordinate and control the remote test, and the city of Recife (state Pernambuco), 800 km away from Camaçari, as the location of the IED under test. All tests were performed with the local and remote users connected to the CHESF corporate intranet. Figure 3 illustrates the test setup, including IP description and location of each network element used.
A distance function was parameterized on IED with the time of zone 1 as instantaneous and the time of zone 2 in 200 ms. The test equipment was set to inject a fault of awareness as to the performance of the first protection zone. His proce-dure involved the injection of a fault of realization described, repeated 40 times. The remote operator, located in Camaça-ri-BA, performed the first group of 40 tests. The operator, located near the test equipment, made the second group of 40 tests using the same test setup. Figure 4 shows the results of the tests performed with the oscillographic of the voltage and current signals and the response of IED.
The results showed no significant differences between values for the IED performance of remote test and the local test. This occurred because the test equipment made the reading of the response of IED and not by the computer that was running the test. This computer serves only as an inter-face for the user to access the readings by the test equipment. It is imperative that the test equipment to behave like one IED and has an GOOSE Data Set to subscribe and simulate the GOOSE messages. Under these conditions, the location of the operator or user responsible for testing does not influence the test result.
Figure 3 Test scenario between Recife-PE and Camaçari-BA [7].
Figure 4 Test Result - OMICRON NetSim [7].
5.2 Interoperability Test
An interoperability test of a device is needed to take this product before it is accepted for use in a PAC system. It is intended to verify the correct behavior of any device when integrated as part of a system. This is done to ensure that this device interoperate correctly with other devices already used in the system. These existing devices are not necessarily the same manufacturer of the new device or technology.
These interoperability tests should be limited to the simulation that will result in the sending and receiving different types of messages required for distributed applications between devices such as individual GOOSE messages of the IEC 61850. Interoperability testing is conducted in the laboratory early in the process of designing the system and is used to ensure that all devices are able to correctly change the types of messages used. The following is an example of interoperability testing in a test station mounted by company CHESF, Hydroelectric Company of São Francisco, Recife-PE. It is a facility prepared to put into operation several IEDs, with a switch and Simulated Bay [8]. In the example described were parameterized and monitored relays Siemens 7SJ62 and Areva P442. To raise awareness of the protection scheme were applied simultaneously three channels of voltage and six current channels (analog or SV) in the system. During 500 ms, were applied signals of pre-fault, and then were applied and kept for signs of fault in the point of a load (IED P442). The IED P442 sends the trip to the breaker via GOOSE message. With the continuation fault for more than 2 seconds, the P442 sending the start breaker failure for IED 7SJ62, also by GOOSE message, and this in turn acts instantly sending tripping via conventional wiring for circuit breaker between the bay 1 and 2 .
The test set up is shown in Figure 5 was designed with the OMICRON CMC356, and subscription allows simultaneous mapping of analog signals SV and messages over the network, as well as binary signals GOOSE messages and block diagram. In the first test, the IED Areva P442 receives signals from analog voltage and current. In the second test is the IED P442 replaced by another unit, where the current and voltage signals are received through the network signal SV. The used test system, the OMICRON CMC356, could map and subscribe both analog and SV messages over the network, as well as binary signals and GOOSE messages.
Figure 5 The test set up [8].
The time of action of the tested system, with analog or hybrid system are shown in the following figures, showing that the systems were approved.
Figure 6 Table of measurement of time of operation of the IEDs - only with analog signals [8]
Figure 7 Table of measurement of time of operation of the IEDs working with SV and analog signals [8]
5.3 Integration and System Testing
Integration testing is performed to ensure that the individual components of the system not only interoperate correctly, but is also used to ensure compliance with the requirements of performance according to the specification of protection, automation and control system. In this case, they will include tests of the devices at both ends of a communications link. The procedures and tools used are in the category of end-to-end testing and test subsystem.
In transmission lines protection systems, one must stand the test of IEDs at each end of the transmission line and the communication interface, simulating the real conditions of operation of any system of protection. This method consists of testing end-to-end, synchronized by global positioning system (GPS) or IRIG-B signals, using transient signals generated by network simulation software or obtained by network storage systems, such as disturbance recorders. The particular fault is applied to the IEDs on the ends of the transmission line, synchronously; as a result can check the full functionality of any protection scheme.
To perform the test are used test sets to secondary injection, usually used to perform routine and commissioning testing with all kinds of relays, shown in Figure 8. Therefore, this procedure is able to test the complete system as a unit, i.e., test the relays in the transmission line ends and the communication link with a test simulating actual conditions of operation of protection system.
Figure 8 Testing schemes of end-to-end in transmission line [9]
6 Conclusion
The protection and control functions based in communication interfaces require a different approach and particular tools settings to test individual system components, as well as assessing the performance of functions distributed throughout the communication system.
Functional testing should be performed with test equipment capable of simulating analog voltage and current signals and sampled values (SV). They must also subscribe and simulate conventional binary signals and GOOSE messages in several IEDs simultaneously, independent of the manufacturer.
Interoperability testing must be performed to verify the correct behavior of any device when integrated as part of a system, regardless of manufacturer or technology of the devices that make up the system. It is imperative that the test system used is able to simulate and record the information simultaneously to all devices tested
Integration and system testing should include testing of individual devices operating on the communications links to ensure the attendance of performance requirements in accordance with the specification of protection, automation and control system.
7 References
[1]APOSTOLOV, A, PAULINO, M. E. C. “Testes de Sistemas de Automação de Subestação Complexos Baseados na IEC 61850”, In: Simpósio de Automação de Sistemas Elétricos, VII SIMPASE, Salvador - Brasil, 2007.
[2]INTERNATIONAL ELECTROTECHNICAL COMMISSION. IEC-61850, part. 5: communication requirements for functions and device models. 2003. IEC 61850-5:2003(E)
[3]─. IEC-61850, part 7-4: basic communication structure for substation and feeder equipment – compatible logical node classes and data classes. 2003. IEC 61850-7-4:2003(E)
[4]─. IEC-61850, part 7-2: basic communication structure for substation and feeder equipment – abstract communication service interface (ACSI). 2003. IEC 61850-7-2:2003(E)
[5]─. IEC-61850, part 8-1: specific communication service mapping (SCSM) – mappings to MMS (ISO 9506-1 and ISO 9506-2) and to ISO/IEC 8802-3. 2004. IEC 61850-8-1:2004(E)
[6]─. IEC-61850, part 9-1: specific communication service mapping (SCSM) – sampled values over serial unidirectional multdrop point-to-point link. 2003. IEC 61850-9-1:2003(E)
[7]Paulino, M. E. C., Carmo U. A. “Solução de Arquitetura para Testes Automatizados a Distância de IEDs Utilizando Equipamento de Teste”, In: Simpósio de Automação de Sistemas Elétricos, VIII SIMPASE, Rio de Janeiro - Brasil, 2009.
[8]Paulino, M. E. C., Carmo U. A. e Lellys D. "Validação de Sistema de Proteção e Controle com Implementação Completa da Norma IEC61850 – Teste de Interoperabilidade com Barramento de Estação e Barramento de Processo" in Proc. 2010 IEEE Power Engineering Society Transmission and Distribution Conf., São Paulo, SP, Brazil, 2010.
[9]Paulino, M. E. C., Aoun G. M. “Teste Ponta a Ponta em Sistemas de Transmissão de 500kV Utilizando Sinais Transitórios Sincronizados por GPS”, In: 5th Latin-American Congress: Electricity Generation and Transmission – São Pedro – SP - Brasil; 2003.
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