The Disconnector Only Operates on a No

The disconnector only operates on a no-load electrical circuit ( open-circuit operation)

Disconnector insulates a part of the electrical network

Making circuits, on load current interruption

Earthing switch

Closure avoids the appearance of any dangerous voltage

Due to operating error, inductive effect, overvoltage

The short circuit current decreases throughout the network

Rated voltage and breaking capacity ( short circuit withstand)

The higher the current the faster the fuse operates

Its always current vs time

Definite time-delay

Insulator ( dielectric ) between phases as well as between phase and earth

SF6 and GAS

IEC 62271-200 standard , switchgear in

Link between the bays is made by busbars.

Number of busbars in parallel is determined by the rated current flowing through the switchboard.

It is necessary to supply reactive energy as close as possible to the load.

4 types of neutral

1. Neutral earthed directly2. Neutral earthed via impedance3. Tuned neutral4. Neutral insulated from earth

SUBSTATION AUTOMATION

Here we will use three different terms: – Station Controller, the top level controller in a substation – Bay controller, the unit controlling a bay in a substation – Relay, at the lowest level controlling a single object

INTERLOCKING- Safe operation of switchgear

SWITCHING SEQUENCES- To ensure it is performed automatically in a correct sequence, ex feeder

LOAD MANAGEMENT- Automation of shedding and restoring of load, under frequency conditions feeders are disconnected

EQUIPMENT STATUS- Monitoring of SF6, transformer temperature , to see if the

Automatic voltage control

Synchronism

Tap position monitoring

Load & bus transfer

Load curtailment

Capacitor control algorithm

Substation maintenance mode

Fault detection

Sequence of event recorder

COMPONENTS IN A MODERN SAS

IED’s – for ex relay

Bay controller- A device that controls transformer , feeder

HMI

Communication Buses

RTU, IED, CT/VT, ISOLATORS, DISCONNECTORS, BREAKERS,

Merging unit- Take current and voltage inputs from IT/NCIT and sample, synchronize and merge all three phase quantities

IED is any substation device that can transfer current, voltage and control information through communications port

- Protection relays- Transformer bay protection and control, energy meter, disturbance recorder, - Bay controllers

The interconnection of the following equipments forms a Bay1. Line isolator with e/s and bus isolator2. Lightning arrestor3. Potential transformer4. Current transformer5. Circuit Breaker and 6. Carrier communication equipments

A real device is transformed to virtual device.

For ex a transformer bay is transformed to a logical device

The Power System and Substation Automation 119 services associated with this data model are defined in the Abstract Communications System Interface (ACSI). The following ACSI functions are listed by Karlheinz Schwartz [11]:

Protocols:

Read coil or digital output status (function code 01)This function allows the host to obtain the ON/OFF status of one or more logic coils in thetarget device.

Read digital input status (function code 02)This function enables the host to read one or more discrete inputs in the target device.

Read holding registers (function code 03)This function allows the host to obtain the contents of one or more holding registers in thetarget device.

Reading input registers (function code 04)This function allows the host to obtain the contents of one or more input registers in thetarget device.

AddressingT101 uses both link addresses and application addressesThis gives greater flexibility in routing messagesT101 has larger point address range, up to 3 bytes gives 16 777 216 addressesDNP3 uses link addresses only, no application layer addressesDNP3 link carries both source and destination addressesOverall, T101 has greater flexibility in its addressing system, both by including data linkand application level addresses, and through the use of variable address lengths. Thebenefit of variable lengths is that they allow savings on communications bandwidth whenonly small numbers of addresses are required.Data link communicationsT101 uses unbalanced and balanced (limited to point–point only)T101 does not support unsolicited messages on multidrop communicationsDNP3 uses balanced communications onlyBoth DNP3 and IEC 60870-5-101 support balanced or peer–peer communications,however, IEC’s balanced communications are limited to point-to-point configurations.This can be a significant limitation if a situation requiring a multidrop configuration iscontemplated. An example would be where there are a large number of outstationsconnected to a limited bandwidth channel such as a VHF radio link. In such a case pollingfor data may require an unacceptable bandwidth, and DNP’s support for multidropbalanced communications would be an advantage in implementing a reporting by exceptionsystem.Frame formatT101 uses FT1.2 frame; 8 bit checksum, length up to 255 byteT101 frames are fixed and variable lengthDNP3 uses FT3 frames; 16 bit CRC, length up to 255 byteDNP3 uses variable length onlyWhen the fixed length frame option is used under T101, a very short and simplemessage is created in comparison to DNP’s message. This reduces communicationsoverheads substantially.Application functions and data objectsApplication functions:T101 allows only one control point per message

T101 uses single character application acknowledgmentDNP3 allows control over multiple points in one messageDifferences between DNP3 and IEC 60870 309Data objects:T101 allows one type per messageT101 combines function and data types in type codeT101 data objects are oriented to substation communicationsDNP3 allows multiple data objects in one messageDNP3 uses separate function codesDNP3 has one function code per message, applies to all data objects inmessageThere are considerable differences between T101 and DNP3 in the application functionsand the data objects supported. The separation of functions and data objects in DNP3 providesperhaps greater flexibility, but also involves greater complexity.SecurityT101 relies on data link confirm before clearing eventsDNP3 requires application confirms before clearing eventsError checking is stronger in DNP3Both have select before operateWhilst it is the case that DNP’s error detection capability is stronger than for T101,whether or not this is significant would depend on the bit error rate on the communicationlines as well as on the length of messages. Because T101 messages tend to be shorter thanDNP3 messages, the overall effect may be not substantially different.InteroperabilityT101 has no official certification procedures or authoritiesThere are companies who provide testing to T101DNP3 has defined subset levels for IEDsDNP3 has defined conformance test proceduresDNP3 has defined certification authorities in North AmericaThe existence of testing procedures and authorities, combined with defined minimumimplementation levels are recognized as strong features of DNP3. However, whilst DNP3has established an early lead in this area, future developments may narrow this differenceas use of the T101 standard evolves.ComplexityIn a number of respects T101 is a simpler protocol and may operate in a simpler manner.Some examples of this are:No separate application function codesData objects are simpler, no variations as in DNP3Point addressing scheme is simpler than in DNP3Can be configured to have fixed length framesCan be configured to use unbalanced link layer transmissionsThis simplifies communications as collisions are avoided

Uses single-byte ACK transmissions on data link layerFT1.2 format is simpler (but gives less error protection)No transport layer and only one data type per message simplifies parsing310 Practical Modern SCADA Protocols: DNP3, 60870.5 and Related SystemsOn the other hand, it has been noted that T101 can have more low-level aspectsthat require configuration, which can increase difficulties during system integration. Also,although DNP3 is in a number of ways more complex, not all of the features have to beimplemented. The minimum implementation sub-sets for DNP3 confine themselves tolimited numbers of functions and addressing modes.EfficiencySome of the features of T101 such as the fixed frame length option, single characterapplication acknowledgments, and less rigorous error checking can result in smallermessages. However, T101 may require a large number of messages to send information, sosome of the benefit of reduced message overhead is lost. Without knowledge and analysisof the specific data to be predominantly carried on a system it is probably difficult to makeany conclusions as to whether one or other protocol will operate more efficiently.Where either protocol is to be carried over networks the message length will have abearing on efficiency, because the message will be encapsulated within additional data. Inthis case the smaller messages of T101 could lead to a loss of efficiency in this situation