Protection & Control Relaying Schematics
This technical article explains the AC/DC schematic representation of the protection and control systems used on power networks. This includes AC schematics and DC schematics and diagrams that prominently feature relaying.
There are other equally important types of drawings that are not the subject of this article including logic diagrams, data tables and single line diagrams, wiring diagrams, data communication schematics as well as those single line diagrams that do not significantly address relaying.
Contents:
- AC Schematics
- Instrument Transformers
- Voltage Transformers (VT) or Potential Transformers (PT)
- Current Transformers (CT)
- Protective Relays
- Metering Functions
- Instrument Transformers
- DC Schematics
- Common Practices
- Unique Standards
- DC Schematics and the Microprocessor Relay
- DC Schematics and IEC 61850 Station Bus
1. AC Schematics
AC Schematics, which are also called AC Elementary Diagrams or Three Line Diagrams,will show all three phases of the primary system individually.
Examples of this can beseen in Figures 1, 2 and 3. Similar to the one-line, the location of all significantequipment will be shown. Bushings are identified on circuit breakers and power transformers.
The drawing will also include equipment continuous thermal ratings, circuitbreakers in amperes, transformers in MVA. An example of this transformer informationcan be seen in Figure 2.
Detailed connections to all equipment utilizing AC inputs willbe shown as well. These detailed connections often include terminal numbers. Theexample figures do not include all terminal numbers for the sake of readability.
Example A –AC Schematic
Example B –AC Schematic
Example B – AC SchematicContinuation
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1. Instrument Transformers
Voltage Transformers (VT) or Potential Transformers (PT)
The AC schematic will show the point on the high voltage system where each VT isattached and provide both primary and secondary connection details for each of thephases.
Details will typically include winding ratios, number of primary and secondarytaps, polarity marks, nominal voltage ratings, and winding configuration (e.g. delta,grounded wye). If secondary fuses are used, their location and size would also be shown.
It is also common practice to include secondary wire names, for example P1, P2, P3, andP0 for the three secondary voltages and neutral of a grounded wye source as shown inFigure 1. This may be used as a source to supply protective relaying and meteringequipment.
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Current Transformers (CT)
Multi-ratio current transformers are commonly used for protective relay applications. CTlocation, full and connected ratios, polarity, and winding configuration (e.g. delta or wye)will be indicated on the drawing.
Nominal secondary current rating (commonly either 1Aor 5A) will also be shown along with secondary wire names, for example C1, C2, C3, andC0 for the set of CTs connected in a wye configuration on Figure 1.
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2. Protective Relays
Protective relays that are applied to monitor changes in the AC system will be shown onthe AC schematic connected to current and voltage transformer secondary outputs.Diagrams must show detailed connection information that follows manufacturerrecommendations to insure correct operation.
If a circuit is protected with a number ofsingle function devices (these would typically be electromechanical relays) it is importantto show current and voltage connections to each of the elements that make up these relays.
This connection to individual current elements can be seen in Figure 3 between the 50/51TBU and 51/87TP coils in the AC Current Elementaries. This detail should include terminal numbers, polarity marks, and any other importantinformation that pertains to the AC inputs. This will provide valuable informationregarding input quantities specifically used by the relay elements as well as informationabout directional sensitivity (if applicable).
When microprocessor relays are used, internalrelay program parameters will determine how secondary input quantities will be measuredas well as the directional sensitivity of specific elements. Additional information will berequired on this drawing if it is needed to detail exact functions that are in use.
Another vital function of the AC schematic is to show how the AC current and voltagecircuits can be isolated for testing. Detailed information regarding the wiring andoperation of these test switches are included in these schematics and an example can beseen in the lower left corner of Figure 1.
Here current test switch 6 TC clearly shows theterminal point number and what each test switch does on operation. For example, testswitch 1-2, when opened, will short the circuit from point 2 to point 4.
This level of detailis needed to insure that testing can be done easily and avoids errors while testing.
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3. Metering Functions
Metering information generally required for utility operations may include voltage,current, power (both Watts, and Vars) as well as other values. Present microprocessorrelays are often capable of providing this information with acceptable accuracy.
Discreetmetering devices including panel meters and transducers are often no longer required.
Ifmetering functions are to be included in a microprocessor relay these functions could beindicated on the AC schematic drawing or even the single line diagram. This is one placewhere the effect of microprocessor relays on schematic representation can be seen.
Withthe use of these relays to perform metering functions, it is no longer necessary to carefullydetail all the transducers needed to perform the same functions.
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2. DC Schematics
DC schematics, often referred to as elementary wiring diagrams, are the particular schematics that depict the DC system and usually show the protection and controlfunctions of the equipment in the substation. It should be noted that sometimes the controlfunctions are supplied by AC and are included in the elementary diagram (refer to Figures 6 and 8).
One example of a DC schematic is a circuit breaker control schematic thatshows the tripping and closing of the circuit breaker whether from controls or protectivedevices as well as the alarms for the circuit breaker.
Examples of typical elementarydiagrams are shown in Figures 4, 5, 6, 7, and 8.
Electric utilities have used elementary wiring diagrams to show their designs for manyyears. As the experience in using these drawings grew, practices common throughout theindustry arose while at the same time utilities developed many standards regarding thedetails of the elementary wiring diagram that works best for them.
Since the details inthese standards often differ slightly but significantly from utility to utility, it is importantto understand the standards when reviewing these types of drawings.
As utilities havegone through some corporate changes through the years such as merging variouscompanies, the selection of a common standard can often be a challenging process.
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Common Practices
There are a number of common practices that are seen in DC schematics. If complexity ofthe system requires it, the devices controlling the equipment, like the two relays featuredin Figure 4, may be shown on one drawing.
The equipment being controlled would showup in another drawing, such as the switcher in Figure 5 below.
The DC circuit is usually shownwith the positive bus closer to the top of the page and the negative bus closer to the bottom. The general layout of these drawings is that the DC source is usually shown atthe left end of the drawing and the initiating contacts are shown above the operatingelements.
For example, in Figure 5, when the contacts labeled 51/87TP close and the89/a contacts are closed then positive DC at the top is connected “down” to the trip coil(TC) and the switcher is operated.
There are also functional similarities with the AC schematics. Similar to AC schematics,the DC schematics will include the rating for circuit elements such as fuses, heaters andresisitors.
For example, in Figure 6 we see that FU-1 is rated at 20A, that HTR2 is ratedfor 300W at 240V and that a 7500Ω resistor is needed when connected to 250VDC.
Andjust like the AC schematic, the location of test switches are shown in detail so outputs andinputs can be isolated for testing.
Refer to Figure 5 and the test switches for the outputsof relays 87TP and 50/51TBU.
Figure 6 provides examples of the transition that schematics make between the functionaldesign and the physical design. Near the center of the figure is the number 13 just abovethe text “79 “NLR21U”.
Note that 13 is repeated to the right next to the contact labeledR2 and to the left next to the contact labeled C1. The repetition of 13 is not needed on thisschematic to communicate that all these points are electrically the same, this fact can beeasily seen in the drawing.
However, 13 is also used in the physical design shown in thewiring diagrams.
Terminal blocks will be marked with this number and in this application, it is an indication that all the points are electrically the same and can beidentified by the same 13 on this schematic.
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Unique Standards
Figure 6 provides examples of standards that have been developed regarding the detailsof the design. For instance, the black triangles and diamonds throughout the drawinghave specific meanings regarding the disposition of the wires.
They symbolizetransitions from one location to the next. Care must be taken to appreciate the differencebetween the black triangle symbol used to indicate transitions and the black trianglesymbol used in the lower right to indicate a diode.
Other examples from Figure 6 of unique standards include the use of the symbol ~ forohms and the use of the circle with a line through it for terminal points. Though thesesymbols may be explained in a key somewhere on the drawing this is not always the case.
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DC Schematics and the Microprocessor Relay
Today a new challenge is occurring as utilities have moved from their traditional designsusing electro-mechanical relays to designs using microprocessor relays and advancedcommunications systems.
The basis of the problem is the design of the protection systemhas moved from a hardware based system to a software based system with little experience in the best methods to document these designs.
Documentation of the logic inmicroprocessor relays adds one level of challenges and the advent of schemes that use relay to relay communication connections and protocols, such as IEC 61850, add anotherlevel of challenges.
As with traditional designs, utilities will continue to document the hardware connectionon the elementary wiring diagram. Since microprocessor relays are so powerful andflexible a new emphasis rises to show not only what the protection design is but also whatit is not.
In other words, the documentation needs to capture the IED resources availableif the design ever changes and new resources (IED inputs and outputs) are required.
Relay I/O
Oneuseful table generally included on a DC schematic or one-line would be a table of theinputs and outputs on a microprocessor relay, indicating which were being used (labeledwith associated function) and which were available. This table is handy in tying therequired functionality of the settings and logic to the physical wiring and the settings of the relay.
This table is shown in the right of Figure 7. Another approach, shown inFigure 8, is to show all the available relay inputs and outputs in a graphical form on onedrawing.
The connections showing the output and input contacts will be shown on the schematicdiagrams but the issue remains how one documents what is happening within theprogramming. Several alternatives will be presented that have worked for other utilities.
One of these alternatives may appear to be the best choice or a combination ofapproaches may work. It is also noted that these alternatives are not all-inclusive and abetter idea may be developed. The alternatives that will be briefly discussed includehardware only documentation, software shown as part of the traditional elementarydiagram, and showing a logic diagram on the elementary.
The first approach is to document only the hardware that is connected on the relay. Inaddition to showing the specific contacts that are used in the design, labels could be used that may show small details regarding the contact such as ‘51’ for overcurrent relaycontact.
The major issue with this approach is the possible lack of sufficient informationregarding the design. For simple designs, the label of the contact may be enough but ifthis approach is chosen for complex designs, then additional documentation will need tobe supplied.
One option would be to include more information on the relay setting sheetor some other type of documentation that goes with the relay.
The additional documentcould include a verbal description of the relay logic that allows one to understand whenthe contact will operate. Logic diagrams could also be used as the additionaldocumentation to show how the design is developed.
One advantage to this approach is itkeeps the elementary wiring diagram simple for those who do not require the details. Forthose that need the details, they can obtain it from the additional documentation.
Anotheradvantage to this approach is the flexibility improvement for most organizations. Thechanges of elementary wiring diagrams often require a process of obtaining approvalsthat hinders making changes often.
One of the advantages of using microprocessor relaysis the ease in changing the design if improvements can be made. If no wiring changes aremade, then the use of additional documentation or setting sheets to document the changesis often less strenuous than changing elementary wiring diagrams.
The second alternative is to show the details of the logic in the form of the elementarywiring diagram. So, similar to the wiring diagram, if logic is using an “OR” functionthen the variables are shown in parallel. If an “AND” function is used then the variablesare shown in series.
The difficulty in this alternative is distinguishing between hardware connections, which feature physical contacts, and the logic, which depicts logical outputsas contacts. Therefore it may be beneficial to use different colors or line types forsoftware logic.
One other alternative is to use logic diagrams on the elementary wiring diagram. Logicdiagrams are a graphical display that shows what is happening in the logic of the relay orcommunications system.
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DC Schematics and IEC 61850 Station Bus
Early applications of relay protocols provided engineers with basic tools for substationautomation, but they were often limited in functionality. Several are proprietary and forthat reason, one must reference the vendor manufacturer’s relay manuals for schematicrepresentation methods.
IEC 61850 differs from other standards/protocols because itcomprises several standards describing client/server and peer-to-peer communications,substation design and configuration, and testing.
IEC 61850 provides a method for relay-to-relay interoperability between IEDs fromdifferent manufacturers. With the open architecture, it freely supports allocation ofC37.2 device functions.
What makes it unique is that the station bus described by IEC61850 operates digitally over a secure Ethernet based network sending protective relaymessages called Generic Substation Events (GSE) or Generic Object Oriented SubstationEvents (GOOSE) between relays (and other IEDs) on that network.
Because of thisfeature, it eliminates most dedicated control wiring that would normally be wired fromrelay-to-relay (i.e. a trip output contact from one relay to the input coil of another relay).
Due to this digital communication between relays, a typical DC schematic diagram aloneis not an adequate method for describing the system.
Therefore IEC 61850 GOOSEmessages (signals) are best represented in a point-to-point list or spreadsheet format (e.g.a bus differential relay would subscribe to all associated feeder protection relays on thatbus, or a main-tie-main relay group would subscribe to each other to perform breakerinterlocking).
This point-to-point (publisher/subscriber) list is not maintained by acomputer on the Ethernet network, but instead a protective relay engineer uses a SystemConfigurator software tool to program each IED to subscribe to each other depending onthe protection scheme.
It is possible for one IED to communicate the same protectivemessage to several other IEDs at the same time. The IEDs, once programmed tocommunicate with each other, will manage the messages they have been programmed toreceive and transmit.
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Reference // Schematic Representation of Power System Relaying by Power System Relaying Committee IEEE Power Engineering Society
