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System Protection And Automation

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Question:

Discuss about the System Protection and Automation.
 
 

Answer:

Introduction:

IEC61850 is a standard integral to the application of modern networking technology for the purpose of automation, control and protection of electric power substations. It is a systems architecture, a modelling approach and a protocol. Beyond automation, this protocol was developed to simplify communication between several types of utility systems, as well as save on the cost of applying hundreds of different proprietary communication and information systems. Devices from different manufacturers required proprietary communication systems specific to them which presented a huge challenge where systems were composed of devices from several different manufacturers. The basic concept of automation dictates that there exists little or no human intervention. For this reason, it is necessary that various protection and control systems or protocols be integrated into the system. Supervisory control and data acquisition, SCADA, interfaces are also a necessity in an automated power system. The objective of IEC 61850 is to provide for seamless information integration across the utility enterprise using off-the-shelf products implementing these international standards.

As noted earlier, an automated system requires little or no human supervision, and since no system can be said to be perfect, errors are bound to occur. To correct such errors, human intervention is required. Some of the services provided by IEC61850 include event logging and reporting such that the system is capable of recording events as they occur and communicating the same to a manned node or human-machine interface. The feature is referred to as digital fault recording (DFR) and enables the user to quickly identify failure using built-in diagnostic tools. This is made possible by GSE, (generic substation events) a control model which provides a mechanism for data transfer within a network or peer to peer messaging. There are two different models of GSE namely GSSE (generic substation event) and GOOSE (Generic object oriented substation event). The difference between the two is that while the GOOSE message may include several data types such as analog, binary, and integers, the GSSE message is limited to support a fixed structure of binary event status data. The latter allows for multicast messaging meaning that the same message can be transmitted to several nodes at the same time using an Ethernet address known as a multicast address. In the occurrence that any data item known to the system is changed, a GOOSE message is said to be published and any device on the network can ‘subscribe’ to the data and use it as needed. This publisher-subscriber communication concept is used in favor of client-server communication which is considerably more time consuming since it must use the full seven layer stack. The IEC61850 system architecture consists of what is known as a station bus. This component connects all the devices within the system and defines the mechanism that enables multicast communication, GOOSE. One GOOSE message can replace hundreds of wires that would normally be used to enable inter-device functionality. These features allow the system to convey messages to a human operator who can take the necessary corrective actions required in any given situation. In this way, IEC61850 facilitates error recovery as well as system protection. It is also worth noting that the standard provides time-stamped data thereby conveying time specific information to the human operator.

 

Real time data is an invaluable asset especially in the case of automated systems. The reason being that the sooner data can be processed and transmitted, the sooner it can be acted on. Real time data means that data is made available to the recipient as soon as it is processed with little or no time delay. In a system such as that found in a substation whose coverage stretches over a vast scale, influencing hundreds of businesses as well as households, a small delay in resumption of normal service after an error may result in adverse effects. The IEC61850 supports real time data exchange leaving no room for delays on its part. It achieves this through MMS, (manufacturing message specification) which is a standardized protocol that supports requirements for various high performance services such as make or break connections between client and server, send an unsolicited read response to a client and get the definition of an object. IEC61850 also enables time synchronization across all the devices within the network. Time synchronization is realized through the Simple Network and Time Protocol (SNTP) which is specific to the standard. Since all devices are time synchronized, the data is time accurate. However, data can never be truly presented in real time due to transmission delays which are unavoidable.

Another service provided by IEC61850 is the process connection feature which allows information transfer between instrument transformers, protection equipment and circuit breakers. This transfer of information between devices is made possible by a core characteristic of the IEC61850 referred to as interoperability. Interoperability is the ability of a system to exchange information between intelligent electronic devices from different manufacturers. The standard is critical to process connection since this service requires timely information transfer. As discussed earlier, IEC61850 provides for real time transfer of data between systems as well as devices. The timeliness of the information has a direct impact on how long the protection function takes to react. The protection function must trigger as fast as possible to prevent further damage to the equipment in direct danger and by extension, replacement costs. IEC61850 also requires an Ethernet LAN connection between protective relays to enable communication. The high speed Ethernet connection is also able to exchange analog data between relays in the same message, so actual values of currents, voltages and power can be sent to other IEC61850 based IEDs.

IEC61850 is also used during planning of a substation. Various tests must be carried out to ensure the compatibility of various devices to each other and more so to the IEC61850 standard. One such test is the system performance test. Its purpose is to check the performance classes as defined in the mandatory part, IEC 61850 for the device in a system (system conformance) independent from a project, the IEDs may be tested in a test system with reasonable data traffic in the background . Another test is the factory acceptance test which has to prove that the system, as far as assembled in the factory, fulfills the given specifications of the customer. For this test, all parts must be compatible with IEC61850 and all SCL files must be present for the purpose of configuration and implementation. Not all parts may be available during the test and as such they must be simulated to ensure accurate results. The purpose of the tests is to ensure that all parts will work properly and to the customers specifications once the project is completed, and to avoid incurring any reworking costs. They also ensure that the system will continue to run seamlessly thereby continually providing protection for the individual devices and for the system as a whole. 

Time over-current is another protection feature integrated within IEC61850. This feature’s purpose is to protect the system from excess current and interphase faults . The feature also replaces traditional fuses which cannot be fully depended upon in an automated system. This is because once a fuse melts, the system cannot resume normal functionality until the melted fuse is replaced which also means that remote operation is not possible. Although fuses are the primary source of protection for distributed energy resources, another fault lies in the fact that one cannot control the time to trip. This creates difficulties in primary back-up coordination activities . The back-up protection to the fuse is provided by overcurrent relays at a feeder point. Overcurrent relays must also play a dual role in that they provide both primary and back-up protection. In the event that a fault goes undetected by the first set of relays, and the relays fail to isolate the device, an additional set of relays is opened. The second set of relays must not be opened unless the first set has already been opened. This requires what known as a wait state which is enabled by another feature referred to as Time Multiplier Setting (TMS). By increasing or decreasing this setting, one can vary the trip time appropriately such that the second set of relays are subjected to a time delay to allow the first set to open first.

 


SCADA interfaces are the only way through which data and information can be exchanged between the human component and the system. It is basically an input and output terminal which at its core is powered by IEC61850 in that the standard enables communication between the different devices including SCADA. A typical function of SCADA is the creation of alarm lists and event lists. SCADA enables automation and protection in a multitude of ways. First and foremost is that it receives data from the system in form of event logs and reports and conveys it to the human operator or protection engineer who acts on it accordingly. It is also the means through which the human operator manipulates data within the system when it is required. In this way, IEC61850 provides a link through which several operations are made possible. For this reason, it is also referred to as a human machine interface (HMI).

Another service provided by IEC61850 is the read/write feature which means that the system can give and receive instructions. This means that the system can react to various situations that were foreseen before or during system planning. The client can send commands to the server to modify its behavior by performing changes in internal data, change of parameter sets, analog set-points, enabling or disabling functions and so on. The system consists of intelligent electronic devices (IEDs) which facilitate the read/write feature also known as device control. IEDs implement control, but only after receiving instructions from a master computer. IEDs are also referred to as slave components. Device control means that the system can effectively trigger an action or stop one if required. This feature negates the need for human intervention when it comes to some common errors in the functionality of the system. This is, in itself, automation. A suitable example is the predefined quality flags which may force the system to report or take corrective action if possible where the quality of the power produced is below the required limit, perhaps in terms of stability.

IEC61850 provides object mapping and no manual mapping of equipment is needed. All devices can be viewed through a SCADA interface and all of them share a similar naming convention. Through a feature called self-description, devices and their details such as the model as opposed to data points are automatically generated for the user on the SCADA interface. Self-description also allows a device to tell its master what particular data it is going to report and enables the master to configure itself for a particular device. Object names can be retrieved from the IEC61850 automatically to effectively and efficiently report errors or events. Automatic object mapping shows the location of devices in relation to one another. This way, the user can identify devices with high precision in the event that some device requires repairing or replacement thus avoiding reworking and/or losses as a result of errors in the equipment ordered. Device configurations can also be implemented using SCL (substation configuration language) files. Devices from other suppliers use such files to document the messages. However, such devices require firmware changes to modify message configuration. Substation Configuration Language is a standardized method of describing substation power systems and device configuration. Substation Configuration Language is beneficial in that there is little conflict between equipment, devices and applications where one is changed for one reason or another. The user is only required to move a few files around such as SCL files and firmware files to configure new equipment. Configuration of an automated substation system, a task that would otherwise require a substantial amount of human labor, is automated and therefore saves greatly on time and money.

 


IEC61850 communications based protection and control systems can also be used to implement a lockout function. Traditional lockout relays typically serve to trip and isolate or keep out of service a given zone, especially during repair or inspection. The protection zone could be any single component within the system, for example a transformer, a bus, transmission line or a collection of equipment. This function protects both the personnel on site as well as the equipment undergoing repairs from suffering even further damage. Integrating a lockout feature with the IEC61850 communication protocol is a step into the future of protection and control systems. However, some core features required of a traditional lockout scheme must be present in any new lockout design. They are briefly described below.

  • Zone based lockout; this means that each protective zone that implements lockout must have its own lockout state. It must not be combined with other zones.
  • Local and remote indication; a means must be provided for operators to be able to determine the particular lockout in effect so as to determine the cause and remedy for each lockout.
  • Close inhibit; a breaker cannot be closed so long as any lockout is still in effect regardless of any lockouts applied to it that may have been reset.
  • Loss of protection system power; even if the protection and control system is de-energized, and then later re-energized, all the lockouts that were in effect remain in effect.
  • Single procedure reset; resetting a particular lockout only involves one procedure. All the equipment within the protective zone associated with a particular lockout is reset as a group when the lockout itself is reset.

Although the IEC61850 standard was initially developed for communication purposes within the substation, it can just as well be re-purposed to incorporate and automate traditional protection systems using GOOSE messages. GOOSE messages not only replace hard wires within the system, but can also monitor the health of virtual wires. The control model performs such checks to ensure that all protection protocols and devices are in good working condition if and when they are called upon to protect the system. GOOSE messages can do this because they are constantly re-transmitting in between certain intervals which the user can define. In the event that a message is received later than it was expected, the subscribing IED sends out a GOOSE alarm message notifying SCADA of the error. The IED then modifies its internal logic to address the error. This re-transmission feature also ensures that each message is delivered in the event that the first transmission is dropped or corrupted. Corrupted transmissions can be attributed to a high volume of Ethernet traffic or interference caused when control and power cables share a cabling tray with the Ethernet cable. The narrower the time window defined, the better. Reason being the channel may still fail in between transmissions where the probability of that occurrence is directly proportional to the time window set.  

Integration is the ability of computer based applications to interact with other systems in order to perform a useful function for the user. Integration is specific to the IEC 61850 standard. With this in mind, experts have realized a new frontier in substation communication. IEC 61850 is expanding outside the substation through communication between substations and control centers. IEC61850 communication, modelling and system engineering between stations are dependent on Ethernet communications. An Ethernet connection between stations can be established in a number of ways. One such way is through a direct wideband LAN interface. This involves the use of a LAN network possessing considerably high data transfer capabilities. That is, a large bandwidth in order to facilitate the timeliness of data. This option is disadvantageous in that it can only be applied over physically proximal networks. Another method, one that is perhaps better in terms of the scale that can be covered, is a tunnel that filters and directly passes packets over a Wide Area Network (WAN). WAN is a network spread over a large geographical area. WAN is composed of several LANs between which it transfers data. This is the only way it can facilitate data exchange between substations spread over hundreds and even thousands of miles. Integration, in conjunction with Ethernet enables the creation of a larger singular system which aids in coordination of protection and control services over a larger scale. A larger system controlling smaller systems under its umbrella also enables enhanced service delivery. In addition, with substations able to communicate between themselves, a higher level of automation is achieved.

 


Another application of IEC61850 lies in the start/stop feature. This feature is not standard to the IEC61850, but it can be integrated. As the name suggests, this feature can start or stop operations within a substation if or when the system deems it necessary. Through this level of automation facilitated by IEC61850, the system provides protection for itself in the event of an error.

In conclusion, IEC61850 avails several benefits in the way of cost reduction time saving and protection of power systems. It is the basis for all communication that occurs within a substation and is therefore critical to the protection and automation of the systems within a substation. The value of IEC61850 lies in the testing it allows and the standard that it provides with regard to communication between different components and devices within the system. Testing individual IEDs and their interoperability is critical to the success of any IEC61850 project. IEC61850 also considers future developments which is an important factor for any technological platform. It can thus be said to be future proof in that it follows the progress in mainstream communication technology. Through standardization of a communication protocol, IEC61850 saved several utilities from having to implement several different communication systems and also paved the way for other standards such as wind power and hydro-electric power. With such advancements it is clear that the same technology can be used as a basis to develop standards for other forms of distributed energy resources in the near future.

 

References

Y. Gao, "IEC61850-Based Analysis of Coal Mine Ground Transformer Substation Integrated Automation Communication System", AMR, vol. 619, pp. 199-202, 2012.

S. Yin, "Research on the Application of IBE in IEC61850 Substation Automation System", AMM, vol. 130-134, pp. 2805-2808, 2011.

H. Kirrmann, "Introduction to the IEC 61850 electrical utility communication standard", 2012.

I. MESMAEKER, "How to use IEC 61850 in protection and automation", 2016.

A. Ehsani Fard, "Using IEC 61850 Protocol in Automation Systems of High Voltage", Automation, Control and Intelligent Systems, vol. 4, no. 2, p. 28, 2016.

L. Rocca, P. Pinceti and M. Magro, "Can we use IEC 61850 for safety related functions?"Transactions on Environment and Electrical Engineering, vol. 1, no. 3, 2016.

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