A distribution circuits may come in many types of different configurations with different circuit lengths and sector of use. But most of them share many common characteristics. Generally an electric feeder is one of the circuits or line out from the substation. No problem, I am going to give you a video link where you will find a grid distribution substation.
Primary Power Distribution:
The mains or mainline incoming feeder is the three phase backbone of the electrical circuit. The mains or mainline is generally a modestly of large conductor such as the 500 or 750 kcmil aluminum conductor cable. Utilities often designed 400 Ampere for the main feeder and often allow rating of 600 Ampere on emergency situation.
Ramifications from the mains are one or more laterals, which is also called taps, lateral taps or branch lines. These laterals can be single phase, two phases or three phases. Laterals usually have fuses to separate it from the mainline if they are any sudden fault.
But the most common power distribution system primaries are four-wire, multi grounded system. You will find three-phase conductors with a multi grounded neutral system. Normally the single phase loads use to served by power transformers which connected between one phase and the neutral system.
This neutral system acts like a return conductor and equipment safety grounding. For a single-phase line use to have one phase conductor and neutral but a two phase line system has separate two phases and a common neutral. You may find some distribution primaries which have three wire systems with no neutral like us. Because in my factory we are having, 33KV own distribution line without neutral.
You will find several configurations for power distribution systems. But most of distribution circuits or system is radial for primary and secondary distribution systems. These radial circuits has many type of advantages over all networked circuits like,
- Easy protection during fault current.
- Fault current is low over most of the circuit.
- Easy to control voltage.
- Easy to predict and control of power flows.
- Cost is low.
Distribution primary systems come in different types of shapes and sizes. All types of arrangements depend on the street layouts, total shape of the area will covered by the electric circuit, any kind of obstacles (like small River, Lakes) and where the big loads consumer are present.
Normally a common suburban electric layout system has the main feeder along a street with laterals tapped down side streets or into developments. But radial distribution feeders may have extensive ramification whatever it takes to get to the loads. For an express feeder serves the load concentrations for some distance from the electric substation. Three phases mainline use to run a distance before tapping loads off to customers.
Some circuits can come from one substation and a number of the circuits can have express feeders and some feeders use to cover areas which is close to the substation. But express feeders serve areas little bit farther from the substation. For more improved reliability, radial circuits are provided with normally open tie points to other electric circuits.
The circuits use to operate radially, but if any kind of fault occurs on one of the circuits then the tie switches allow some portion of the faulted circuit to be restored quickly. Usually this kind of switches are manually operated but some utilities use automated switches or reclosers to perform these operations more automatically.
A primary loop scheme is more reliable service and it’s sometimes offered for critical loads such as hospitals and some emergency sector. But the key feature is that, this circuit system is routed through each critical customer transformer. If suddenly, any part of the primary circuit is faulted then all critical customers can fed by re-configuring the transformer switches.
But the primary loop systems are sometimes used on distribution systems for some areas needing very high reliability. But in the open loop systems or design, where the loop use to left normally open at the some point and the primary loop systems have no benefits for momentary interruptions or any voltage sags. It’s operated in a closed loop system.
Faults on any of the electrical cables in the loop system are cleared in less than 6 cycles, which reduces the time duration of the voltage sag during the fault time. Now the advanced relaying similar to the transmission line protection is necessary to coordinate the protection and operation of the switchgear in the looped system.
The relaying scheme uses a transfer trip with permissive over reaching. Then a backup scheme uses directional relays and it will trip for a fault in a certain direction unless a blocking signal is received from the remote end. But the critical customers like hospital and some emergency service have two more choices for more reliable service where two primary feeds are available and primary selective and the secondary selective schemes both can normally fed from one circuit.
That’s means the circuits are still radial. In the time of fault on the primary circuit then the service is switched to the backup circuit.
Now in the primary selective scheme, the switching occurs on the primary and in the secondary selective scheme, the switching occurs on the secondary. This switching can be done manually or automatically and if there is even static transfer switches and it can switch in less than a half cycle to reduce any momentary interruptions and voltage sags.
Now a day the primary selective scheme is preferred mainly because of the cost associated with the extra transformer installation in a secondary selective scheme. Normally closed switch on the primary side transfer switch opens after sensing any loss of voltage. Its usually has a time delay on the order of seconds which is enough to ride through the distribution circuit’s normal reclosing cycle.
The opening of the switch can be blocked if there is an over current flow in the switch the transfer will also disabled if the alternate feed does not have the proper voltage. But the switch can return to normal through either an open or a closed transition in a closed transition where both distribution circuits are temporarily paralleled.
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