JOURNAL OF INFORMATION, KNOWLEDGE AND RESEARCH IN
ELECTRICAL ENGINEERING
IMPACT OF IEC61850 STANDARD ON VARIOUS PROTECTION SCHEMES WITHIN POWER SUB-STATION
1MR. VIRAL JOSHI, 2 PROF.VARSHA .A.SHAH
1 Department Of Electrical Engg, SV National Institute Of Technology, Surat
2 Prof, Dept Of Electrical Engg, SV National Institute Of Technology, Surat
,
ISSN: 0975 –6736| NOV 11 TO OCT 12 | VOLUME – 02, ISSUE - 01 Page 118
JOURNAL OF INFORMATION, KNOWLEDGE AND RESEARCH IN
ELECTRICAL ENGINEERING
ABSTRACT: IEC 61850 developed by International Electro-technical commission (IEC) is an approved International standard for communication within the substation. It has introduced many new features which can be used to build various applications for Protection of various equipments within the substation. This paper there for focuses on various possible Monitoring and control schemes based on GOOSE messaging facility available in IEC 61850 standard, its advantage/disadvantage over Hardwire conventional Monitoring and control scheme.
Keyword:- IED(Intelligent Electronic Device), Ethernet, Process Bus,IEC61850, GOOSE.
ISSN: 0975 –6736| NOV 11 TO OCT 12 | VOLUME – 02, ISSUE - 01 Page 118
JOURNAL OF INFORMATION, KNOWLEDGE AND RESEARCH IN
ELECTRICAL ENGINEERING
ISSN: 0975 –6736| NOV 11 TO OCT 12 | VOLUME – 02, ISSUE - 01 Page 118
JOURNAL OF INFORMATION, KNOWLEDGE AND RESEARCH IN
ELECTRICAL ENGINEERING
I. Introduction
The purpose of IEC 61850 is, to provide all the necessary specifications required to achieve interoperability between the equipment of an integrated substation automation system which includes the relays of different make manufactured at different platforms. In a substation automation system based on IEC 61850, cables and sensors (Like CT, PT) carrying binary or analog information between the individual equipment shall be replaced with a standardized Communication.
IEC 61850 will have an impact on protection in following aspects:
· The total architecture of a protection system will be changed as IEC 61850 provides communication services that can be used to exchange binary information between IEDs. This communication service can be used to implement the protection schemes between the relays and thus the wiring between the relays is reduced.
· The Hardware and Software design of protection equipment shall be change. With the introduction of IEC 61850-9-2 (for process buss) samples from the CT/PT signals are transmitted to relay via FO Network to one or more Protection Relays. With this New Technology the Relay hardware shall change considerably as it does not required to have Hardwire Interface I/O cards, Signal Processing Hardware like A/D Converter, multiplexer etc for processing of Analog signals.
· Approach towards testing of the protection system shall be change with Ethernet based Testing Facilities. Remote Automated Testing Facilities can be developed to get the benefit of the Ethernet based network.
· With the introduction of New and features an adequate training of the protection engineer is required.
Presently the industry is exploring methods for implementing the Sampled Values (SV) portion of this standard, which supports the transmission of sampled ac data (i.e. CT and VT signals) over fiber Ethernet-based local area networks. The chief technical issue is the configuration of a secure, reliable and purpose-driven architecture. This architecture must address the need for interoperability between devices of different purposes such as protection, metering, and communications, and devices of different designs, most noticeably in terms of sampling rate.
II. CONVENTIONAL SUBSTATION DESIGN
Conventional substations are designed for the high voltage switchgear with copper cables for all interfaces between primary and secondary equipment.
A typical conventional substation has multiple instrument transformers and breakers (Figure 1) associated with the protection, control, monitoring and other devices being connected from the yard to a control house with the individual equipment panels. The different types of cables described are used for the connections.
Figure: 1 Conventional Substation Design
As can be seen from Figure 2, cables are cut to a specific length and bundled, which makes any required future modification very labor intensive. This is especially true in the process of refurbishing old substations where the cables insulation is starting to fail.
Figure: 2 Substation Panel Wiring
Some of the problems of conventional Substation Design:
· The large amount of copper cables need to cover to provide the interface between the different devices exposes them to the impact of electromagnetic interference and the possibility for damages as a result of equipment failure or other events.
· The design of the conventional substation needs to take into consideration the resistance of the cables in the process of selecting instrument transformers and protection equipment, as well as their connection to the instrument transformers and between themselves.
· The issues of CT saturation are of special importance to the operation of protection relays under maximum fault conditions.
· Failures in the cables in the substation may lead to miss operation of protection or other devices. In some cases, such as an open CT circuit they represent a safety issue, especially when it occurs while the primary winding is energized.
· The induced secondary e.m.f. under these circumstances can be high enough to present a danger to people’s life and equipment insulation.
III. Substation Architecture Based on IEC 61850
A simplified diagram of the communications architecture of an IEC 61850 based substation protection system is shown in Fig. 3.
Figure 3: Simplified Communication Architecture for IEC 61850 Based Substation
It does not only define how data is communicated between functions in the substation, but also describes the functionality of the substation in an object-oriented approach. The concept of distributed functions is one of the key elements of the standard that allows for utilities to rethink and optimize their substation designs.
IV. Real Time Messages (Communication Services) in IEC 61850
IEC 61850 supports two communication principles.
1) The client/ server communication is based on MMS over TCP/IP and Ethernet. It is typically used for SCADA (Monitoring) communication and consists of a powerful set of services supporting self description.
2) The publisher / subscriber communication. It is intended for time critical information exchange between IEDs (Like Protection Services).
The publisher / subscriber communication consists of two services that have a major impact on protection: GOOSE (Generic Object Oriented System Event) and the transmission of sampled values. To reach proper performance avoiding unnecessary overhead, both message types are mapped directly to the link layer of Ethernet, i.e. the level 2 of the ISO/OSI model, as shown in Figure 4 below
Figure: 4 IEC 61850 proposed communication Stack
GOOSE messages can be sent out by every lED to signal information to other IEDs. The information is packed into a GOOSE dataset that typically contains binary status indications, but may also contain other data such as measured values. The limitation is that the dataset must fit into a single Ethernet packet (roughly 1500 bytes).
The GOOSE messages are broadcasted on the network; this means they are sent out without specifying a certain receiver and without confirmations. Any IED that needs information from certain IED must be configured to subscribe to this IEDs GOOSE.
GOOSE messages are event driven. When the content of the GOOSE dataset changes, then the GOOSE is immediately sent and repeated in short intervals to increase reliability in case of a packet loss. This provides constant supervision if the sending lED is still alive and if the connection is still present. This is a clear advantage over the classical hard wiring.
Sampled value messages are used to represent an analogue waveform with a stream of samples of that waveform. Sampled value messages are driven by the sampling. In principle, with each sampling, a new message is sent that contains the last sampled value. To optimize the network load, it is possible to collect several samples before one message is sent.
A receiver needs to be able to time correlate samples from different sources. This will be the case for any kind of differential protection. IEC 61850 has chosen the method of synchronous sampling. That means that all sources that sample data are synchronized. The sampled values are tagged with a counter value for the transmission.
Dedicated Ethernet types have been assigned for identifying the data packets Carrying GOOSEs and SVs, giving the network components the chance to treat this traffic in a special way, specifically supporting VLANs and priority tagging. Virtual LANs (VLANs) are logical sub-networks which are formed by restricting certain data to specific ports or network segments. This feature effectively supports the network load management. With priority tagging, the packets get a priority assigned according to their importance. The network components will forward the packets with higher priority first, thus building a "fast track" for GOOSEs and SVs.
Transfer Time for Different Messages:
Figure 5 shows the transfer time definition. Transfer time counts from “the moment the transmitting node puts the data content on top of the transmission stack up to the moment the receiving node extracts the data from the transmission stack”.
Figure 5: Transfer time definition according to IEC61850
Table-1 summarizes the transfer time requirements of the different SAS message types as defined in IEC 61850-5.
Type of Message / Transfer Time in (ms)Type-1 / 0-4
Type-2 / 1-100
Type-3 / 10-100
Type-4 / 0.1-10
Type-5 / 100-5000
Type-6 / 0.1-10
Type-7 / 1-1000
Table : 1 Transfer Time Requriement according to IEC61850
V. New PROPOSED Protection Schemes in IEC 61850
The publisher / subscriber services of IEC 61850 that have been explained in the previous chapter, introduce a new possibility for a standardized information exchange between IEDs that did not exist in the past, at least not based on a standardized communication. Using these services, many of the wires carrying analog or binary information that have been required between the different equipment in different bays may be replaced by a communication network. In many cases, that communication network already exists as station bus.
Figure 6: SLD for Typical Substation
Figure 6 Shows the SLD for Typical Substation, as it can be seen from the diagram the NS-1011A and NS-1011B are the two 33KV bus separated by the Bus-coupler NS-1011A/B. This Bus Feeds the Mother Unit Know as Crud Distillation Unit of the Refinery Area (CDU).
This Bus feeds total 8 outgoing feeders and 4 Bus rated at 6.6KV and 415V respectively. All Bus have standard 2 incomer 1 bus-coupler arrangement along with Auto-Transfer Scheme implemented in it. There are total 8 transformers connected to each out going feeders. The largest transformer is of capacity 20MVA with Resistive grounding on secondary side, and all the distribution transformers are of capacity 2.5MVA respectively. Depending on the criticality of the feeder and its voltage level different protection schemes are implemented in the system. Below are some of the Protection scheme proposed for this system.
A. OVERCURRENT REVERSE BLOCKING PROTECTION
One first example of using GOOSE is to implement substation wide interlocking within the bay level devices. With Reference to Figure 6 an interlocking function is implemented in such a way that Activation of Over Current Protection Function in Incomer breaker of Bus NS1012-A sends blocking signal to upstream breaker NS-1011A to avoid unwanted tripping (due to over reach of Relay at NS-1011A) of entire Bus(As NS-1011A is Incomer Breaker). Instead of hardwiring the Breaker position information from the Panel in NS1012-A to the Breaker Panel in Bus NS1011-A that information will be transmitted as a GOOSE message and distributed over the communication network. Multiple wires can therefore be replaced with one message over the communication network. If the same protection function if implemented through the conventional method large cable laying has to be carried out which is avoided by use of existing FO/Communication Network in IEC 61850 based system.
B. BREAKER FAILURE PROTECTION:
In a similar way Breaker Failure Protection can also be implemented as above to monitor the breaker position of down stream breaker i.e. NS 1012-A. Once the Breaker Failure Protection function has operated but if the Breaker has not opened at NS-1012A, GOOSE message can be transmitted over existing network to operate the up stream breaker without any need of additional hardwiring. If the same protection function if implemented through the conventional method large cable laying has to be carried out which is avoided by use of existing FO/Communication Network in IEC 61850 based system.
C. ADVANTAGE OF IEC61850 BASED SYSTEM:
1) Complete Hardwiring is removed for a particular Protection scheme as existing Ethernet network is used.
2) Continuous monitoring is possible since the connection can be monitered through Ethernet network same is not possible in conventional system
3) Modification in existing system is simple since it does not include any hardwiring.
4) Project Cost is reduced due to the fact that additional hardwiring is avoided due to the existing LAN Network within the substation.
D. DIS-ADVANTAGE OF IEC61850 BASED SYSTEM:
1) Loss of packets in existing Ethernet LAN may delay the operation of the Protection Function.
2) Dependability on Ethernet Network Switch, if Switch fails entire protection schemes would fail.
3) Additional Components are added in the Protection scheme vise Ethernet switch; LIU etc related to FO network, more the component more the chances of failure of the system.
4) Above scheme is considering the existing FO Network/LAN network within the Substations same if unavailable required high capital investment in terms of Ethernet Switches, Fiber accessories like LIU’s. Pigtails & Patch cords etc. Cost of this accessories may go up to 3 to 4 Lac Rupees depending on a make as against the cost of simple wire at 30 Ruppes per meter equaling to amount of some thousand of rupees within the substation.
VI. Summary
IEC 61850 is a communications standard that allows the development of new approaches for the design of new substations and refurbishment of old ones. A new range of protection and control applications results in significant benefits compared to conventional hard wired solutions. It supports interoperability between devices from different manufacturers in the substation which is required in order to improve the efficiency of microprocessor based relays applications and implement new distributed functions.