Power Distribution Systems And Its Characteristics I

Power Distribution Systems and Its Characteristics I

Distribution System Characteristics The Electric Power System is usually divided into three segments, which are generation, transmission, and distribution. In a broad definition, the distribution system is that part of the electric utility system between the bulk power source and the customers’ service switches.

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Distribution System Characteristics

Distribution System A distribution system includes the following components 1. 2. 3. 4. 5.

Sub-transmission system; Distribution substations; Distribution primary feeders; Distribution transformers; Secondary circuits;

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Distribution System Some distribution system engineers define the distribution system as that part of the electric utility system between the distribution substations and the consumers’ service entrance.

Distribution Substation Distribution substations consist of an almost infinite number of designs considerations: „ „ „ „ „ „ „ „ „

Load density; High side voltage; Low side voltage; Land availability Reliability requirements; Load growth; Voltage drop Emergency conditions; Cost and losses

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Distribution Substation A Typical distribution substation arrangement.

Distribution Substation The voltage of the high side bus can be anywhere from 34.5 kV all the way to 345 kV and beyond. The average or preferred high side voltage level is approximately 115 to 138 kV. The average substation consists of two transformers with an impedance of approximately 10 percent (0.1 p.u.). The low voltage bus in a multiple transformer substation is usually split (contains a normally open breaker or switch) to alleviate circulating currents as well as reduce the short circuit current seen by the system.

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Distribution Substation Normally, Two or more feeders are connected to each bus through a feeder breaker. On smaller substations where short circuit levels are lower, a re-closer is sometimes used instead of a breaker. Short circuit levels at the terminals of the low voltage bus are generally kept at 12 000 amperes or less although there are many systems where much higher levels can be found.

Distribution Substation A typical substation may include the following equipment: „ „ „ „ „ „ „ „

Power transformers Circuit breakers Disconnecting switches Station buses and insulators Current limiting reactors Shunt reactors Current and potential transformers Capacitor voltage transformers

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Distribution Substation A typical substation may include the following equipment: „ „ „ „ „ „ „ „

Coupling capacitors Series capacitors Shunt capacitors Grounding systems Lightning arrestors/gaps Line traps Protective relays Station batteries

Substation Bus Schemes Selection of a particular substation scheme is based the following factors: „ „ „ „ „

Safety, Reliability, Economy, Simplicity, Other considerations,

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Substation Bus Schemes The most commonly used substation schemes include, „ „ „ „ „ „

Single bus scheme Double bus double breaker Main and a transfer breaker Double bus single breaker scheme Ring bus scheme Breaker and a half scheme

Single Bus Scheme

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Single Bus Scheme Advantages „ „

Lowest cost compared to other schemes Simple

Disadvantages „

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Failure of any circuit breaker results in shutdown of entire substation. Difficult to do any maintenance. Bus can’t be extended without completely de-energizing substation. Can be used only in places where loads can be interrupted or have other supply arrangements.

Double Bus Double Breaker Scheme

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Double bus Double Breaker Scheme Advantages „ „

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Each circuit has two dedicated breakers. Has flexibility in permitting feeder to be connected to either bus. Any breaker can be taken out of service for maintenance. High reliability.

Disadvantages „ „

Most expensive Lose half of the circuit for breaker failure if circuits are not connected to both busses.

Main-and-Transfer Breaker Scheme

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Main-and-Transfer Breaker Scheme Advantages „ „ „

Low initial and ultimate cost. Any breaker can be taken out of service for maintenance. Potential devices may be used in the main bus for relaying.

Disadvantages „ „

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Requires one extra breaker for the bus tie. Switching is somewhat complicated when maintaining a breaker. Failure of bus or any circuit breaker results in shutdown of entire substation.

Double Bus Single Breaker Scheme

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Double Bus Single Breaker Scheme Advantages „ „ „

Permits some flexibility with two operating buses. Either main bus can be isolated for maintenance. Circuits can be transferred readily from one bus to the other by use of bus-tie breaker bus selector disconnect switches.

Disadvantages „ „

One extra breaker is required for the bus tie. Four switches are required per circuit.

Double Bus Single Breaker Scheme Disadvantages (cont.) „

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Bus protection scheme many cause loss of substation when it connected to that bus. High exposure to bus faults. Line breaker failure takes all circuits connected in that bus out of service. Bus-tie breaker failure takes entire substation out of service.

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Ring Bus Scheme

Ring Bus Scheme Advantages „ „ „

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Low initial cost Flexible operation for breaker maintenance Any breaker can be removed for maintenance without interrupting load. Requires only one breaker per circuit. Does not use main bus. Each circuit is fed by two breakers. All switching is done with breakers.

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Ring Bus Scheme Disadvantages „

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For a fault during breaker maintenance period, the ring can be separated into two sections. Automatic recolsing and protective relaying rather complex. If a single set of relay is used, the circuit must be taken out of service to maintain the relays. Requires potential devices on all circuits since there is no definite potential reference point (These devices may be required in all the cases for synchronizing, live line or voltage indication. Breaker failure on one of the circuits causes loss of one additional circuit owing to operation of breaker failure relaying.

Breaker-and-a half

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Breaker-and-a half Advantages „ „ „

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Most flexible in operation High reliability Breaker failure of bus side breaker removes only one circuit from service. All switching is done with breakers. Simple operation; no disconnect switching required for normal operation. Either main bus can be taken out for service at any time for maintenance. Bus failure doesn’t remove any feeder from circuits from service.

Breaker-and-a half Disadvantages „ „

One and a half breaker per circuit. Relaying and automatic reclosing are somewhat involved since the middle breaker must be responsive to either of its associated circuits.

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Substation Location The location of the substation is dictated by the following factors: „ „ „ „ „

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Voltage levels, Voltage regulation consideration, Sub-transmission costs, Substation costs, The cost of primary feeders, mains and distribution transformers Other factors (e.g.

Substation Location However, to select an ideal location for a substation, the following rules should be observed: „

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Locate substation as much as closer to load centers (e.g. addition of load times distance from the substation is minimum). Locate substation such that proper voltage regulation can be obtainable without taking extensive measures. Locate substation such that it provides access for incoming sub-transmission lines and out going primary feeders, also allows for future growth. Locate such that it allows future expansion of the substation it self.

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Substation Location However, to select an ideal location for a substation, the following rules should be observed (cont.): „

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Locate substation such a manner that obey land use regulations, local ordinances and neighbors. Locate substation in such a way the number of customers affected is minimum by any service discontinuity. Other consideration, such as adaptability, emergency etc.

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