Design of Subtransmission Lines and Distribution Substations
Short Description
this is very helpful material for electrical engineers...
Description
Power Transmission and Distribution
Rizwan Khan University of Engineering and Technology, Lahore 1
Design of Subtransmission Lines and Distribution Substations
Rizwan Khan (Lecturer - UET LHE)
2
Design of Subtransmission Lines and Distribution Substations
Rizwan Khan (Lecturer - UET LHE)
2
Design of Subtransmission Lines and Distribution Substations •
Introduction
•
Subtransmission
•
Distribution Substations
•
•
Substation Bus Schemes Substation Location
•
Substation Parameters Derivations
•
Substation Application Curves
•
Percent Voltage Voltage Drop Formula Formul a Rizwan Khan (Lecturer - UET LHE)
3
Rizwan Khan (Lecturer - UET LHE)
4
Introduction The distribution system is that part of the electrical utility system between the bulk power source and customer’s service switches. This definition includes the following components of distribution system: –
Subtransmission system
–
Distribution substations
–
Distribution and primary feeders
–
Distribution transformers
–
Secondary circuits
–
Service Drops Rizwan Khan (Lecturer - UET LHE)
5
Introduction •
However, some distribution system engineers prefer to define the distribution system as that part of the electric utility system between the distribution substations and the consumer’s service entrance.
•
Proceeding figure shows a one line diagram of a typical distribution system.
Rizwan Khan (Lecturer - UET LHE)
6
Rizwan Khan (Lecturer - UET LHE)
7
Rizwan Khan (Lecturer - UET LHE)
8
Subtransmission •
The subtransmission system is that part of electric utility system which delivers power from bulk power sources, such as large transmission substations.
•
The subtransmission systems may be made of overhead open wire construction on wood poles or of underground cables.
•
The voltage of these circuits varies from 12.47kV to 245 kV, with the majority at 132kV voltage level specially in Pakistan. There is a continuous trend in the usage of the higher voltage as the result of the increasing use of higher primary voltages. Rizwan Khan (Lecturer - UET LHE)
9
Subtransmission •
The major considerations affecting the substation’s design are cost and reliability. However, the subtransmission system designs are: –
–
Radial System Improved Radial System
–
Loop System
–
Network System
Rizwan Khan (Lecturer - UET LHE)
10
Radial and Improved Radial System •
In the radial system, as the name implies, the circuits radiate from the bulk power stations to the distribution substations.
•
The radial system is simple and has the low first cost but it also has a low service continuity. Because of this reason, the radial system is not generally used. Instead, an improved type radial subtransmission design is preferred.
•
Proceeding figures are showing the radial and improved radial system diagrams. Rizwan Khan (Lecturer - UET LHE)
11
Radial System
Rizwan Khan (Lecturer - UET LHE)
12
Improved Radial System
Rizwan Khan (Lecturer - UET LHE)
13
Loop System •
In general, due to higher service reliability, the subtransmission system is designed as loop circuits or multiple circuits forming a subtransmission grid or network.
•
In this design, a single circuit originating from a bulk power bus runs through a number of substations and return to the same bus.
•
Proceeding figure is showing the distribution system with loop configuration.
Rizwan Khan (Lecturer - UET LHE)
14
Loop System
Rizwan Khan (Lecturer - UET LHE)
15
Network System •
•
It’s a subtransmission distribution system that has multiple circuits. The distribution substations are interconnected, and the design may have more than one bulk power source. Therefore, it has the greatest service reliability.
•
It requires costly control of power flow and relaying.
•
It is the most commonly used form of subtransmission.
•
Proceeding figure is showing the distribution system with network configuration.
Rizwan Khan (Lecturer - UET LHE)
16
Network System
Rizwan Khan (Lecturer - UET LHE)
17
Rizwan Khan (Lecturer - UET LHE)
18
Distribution Substations •
A typical substation may include the following equipment: –
Power transformers
–
Circuit breakers
–
Disconnecting switches
–
Station buses and insulators
–
Current limiting reactors
–
Shunt reactors
–
Current transformers
–
Potential transformers
–
Capacitor voltage transformers Rizwan Khan (Lecturer - UET LHE)
19
Distribution Substations –
Coupling capacitors
–
Series Capacitors
–
Shunt Capacitors
–
Grounding system
–
Lighting arrestors or gaps
–
Line traps
–
Protective relays
–
Station batteries
Rizwan Khan (Lecturer - UET LHE)
20
Distribution Substations
Rizwan Khan (Lecturer - UET LHE)
21
Rizwan Khan (Lecturer - UET LHE)
22
Substation Bus Schemes •
The electrical and physical arrangements of the switching and busing at the subtransmission voltage level are determined by the selected substation scheme.
•
On the other hand, the selection of a particular substation scheme is based on safety, reliability, economy, simplicity and other considerations.
Rizwan Khan (Lecturer - UET LHE)
23
Substation Bus Schemes The most commonly used substation bus schemes include: –
Single bus scheme
–
Double bus, double breaker scheme
–
Main and transfer bus scheme
–
Double bus single breaker scheme
–
–
Ring bus scheme Breaker and a half scheme
Rizwan Khan (Lecturer - UET LHE)
24
Single Bus Scheme
Rizwan Khan (Lecturer - UET LHE)
25
Single Bus Scheme •
Lowest Cost.
•
Failure of bus or any circuit breaker results in shutdown of entire substation.
•
•
•
Difficult to do any maintenance. Bus cannot be extended without completely deenergizing substation. Can be used only where load can be interrupted or have other supply arrangements. Rizwan Khan (Lecturer - UET LHE)
26
Double Bus Double Breaker Scheme
Rizwan Khan (Lecturer - UET LHE)
27
Double Bus Double Breaker Scheme •
Each circuit has two dedicated breakers.
•
Has flexibility in permitting feeder circuits to be connected to either bus.
•
Any breaker can be taken out of service for maintenance.
•
High reliability.
•
Most expensive.
•
Would lose half the circuits for breaker failure if circuits are not connected to both buses. Rizwan Khan (Lecturer - UET LHE)
28
Main and Transfer Bus Scheme
Rizwan Khan (Lecturer - UET LHE)
29
Main and Transfer Bus Scheme •
Low initial and ultimate cost.
•
Any breaker can be taken out of service for maintenance.
•
Potential devices may be used on the main bus for relaying.
•
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. Rizwan Khan (Lecturer - UET LHE)
30
Double Bus Single Breaker Scheme
Rizwan Khan (Lecturer - UET LHE)
31
Double Bus Single Breaker Scheme •
Permits some flexibility with two operating buses.
•
Either main bus may be isolated for maintenance.
•
Circuits can be transferred from one bus to the other by use of bus tie breaker and bus selector disconnect switches.
•
One extra breaker is required for the bus tie.
•
Four switches are required per circuit.
•
Bus protection scheme may cause loss of substation when it operates if all circuits are connected to that bus out of service. Rizwan Khan (Lecturer - UET LHE)
32
Double Bus Single Breaker Scheme •
High exposure to bus faults.
•
Line breaker failure takes all circuits connected to that bus out of service.
Rizwan Khan (Lecturer - UET LHE)
33
Ring Bus Scheme
Rizwan Khan (Lecturer - UET LHE)
34
Ring Bus Scheme •
Low initial and ultimate 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. Rizwan Khan (Lecturer - UET LHE)
35
Ring Bus Scheme •
If a fault occurs during a breaker maintenance period, the ring can be separated into two sections.
•
Automatic reclosing and protective relaying circuitry rather complex.
•
If a single set of relays is used, the breaker 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 cases of synchronizing, live line, or voltage indication. Rizwan Khan (Lecturer - UET LHE)
36
Ring Bus Scheme •
Breaker failure during a fault on one of the circuits causes loss of one additional circuit owing to operation of breaker failure relaying.
Rizwan Khan (Lecturer - UET LHE)
37
Breaker and a Half Scheme
Rizwan Khan (Lecturer - UET LHE)
38
Breaker and a Half Scheme •
Most flexible operation.
•
High reliability.
•
Breaker failure of bus side breakers 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 of service at any time for maintenance.
•
Bus failure does not remove any feeder circuits from service. Rizwan Khan (Lecturer - UET LHE)
39
Breaker and a Half Scheme •
One and half breakers per circuit.
•
Relaying and automatic reclosing are somewhat involved since the middle breaker must be responsive to either of its associated circuits.
Rizwan Khan (Lecturer - UET LHE)
40
Rizwan Khan (Lecturer - UET LHE)
41
Substation Location To select an ideal location for a substation, the following rules should be observed: –
Locate the substation as much as feasible close to load centre.
–
Locate the substation such that proper voltage regulation can be obtainable.
–
Select the substation location such that it provides proper access for incoming subtransmission lines and outgoing primary feeders and also, allows for future growth.
–
The selected substation location should not be opposed by land use regulations, local ordinances, and neighbors. Rizwan Khan (Lecturer - UET LHE)
42
Rizwan Khan (Lecturer - UET LHE)
43
The Rating of a Distribution Substation
The additional capacity requirements of a system with increasing load density can be met by: –
Either holding the service area of a given substation constant and increasing its capacity.
–
Or developing new substations and thereby holding the rating of the given substation constant.
Rizwan Khan (Lecturer - UET LHE)
44
The Rating of a Distribution Substation •
It is also customary and helpful to employ geometric figures to represent substation service areas as suggested by Van Wormer, Denton and Reps, and Reps.
•
It simplifies greatly the comparison of alternative plans which may require different sizes of distribution substation, different number of primary feeders, and different primary feeder voltages.
•
Reps analyzed a square shaped service area representing a part of, or the entire service area of a distribution substation. It is assumed that square area is served by four primary feeders from a central feed point, as shown in proceeding figure.
Rizwan Khan (Lecturer - UET LHE)
45
Rizwan Khan (Lecturer - UET LHE)
46
The Rating of a Distribution Substation
•
Reps simplified the voltage drop calculations by introducing a constant K which can be defined as percent voltage drop per kilovoltampere-mile.
•
Proceeding figure gives the K constant for various voltages and copper conductor sizes. This is developed for three phase overhead lines with an equivalent spacing of 37 in between phase conductors. Rizwan Khan (Lecturer - UET LHE)
47
Rizwan Khan (Lecturer - UET LHE)
48
The Rating of a Distribution Substation The following analysis is based on the work done by Denton and Reps.
Rizwan Khan (Lecturer - UET LHE)
49
The Rating of a Distribution Substation
•
Reps extends the discussion to a hexagonally shaped service area supplied by six feeders from the feed point which is located at the centre as shown in proceeding figure.
•
This figure is based on the fact that each feeder service area is equal to one sixth of the hexagonally shaped total area.
Rizwan Khan (Lecturer - UET LHE)
50
Rizwan Khan (Lecturer - UET LHE)
51
The Rating of a Distribution Substation The following analysis is based on the work done by Reps.
Rizwan Khan (Lecturer - UET LHE)
52
Rizwan Khan (Lecturer - UET LHE)
53
Substation Service Area Area with n Primary Feeders
Denton and Reps, and Reps extend the discussion to the general case in which
the distribution substation service area is served by n primary feeders emanating from the point, as shown in proceeding figure.
Rizwan Khan (Lecturer - UET LHE)
54
Rizwan Khan (Lecturer - UET LHE)
55
Substation Service Area with n Primary Feeders Assume that the load in the service area is uniformly distributed and each feeder serves an area of triangular shape. The differential load served by the feeder in a differential area of dA is,
Where dS = Differential load served by the feeder D = Load density dA = Differential service area of the feeder
Rizwan Khan (Lecturer - UET LHE)
56
Substation Service Area with n Primary Feeders
From the figure, the following relationship exists,
Therefore the total service area of the feeder can be calculated as,
Rizwan Khan (Lecturer - UET LHE)
57
Substation Service Area with n Primary Feeders
Hence, the total kilovoltampere load can be calculated as,
The percent voltage drop is given as,
Rizwan Khan (Lecturer - UET LHE)
58
Substation Service Area with n Primary Feeders
For n=1, the percent voltage drop in the feeder main is,
For n=2, it is,
Rizwan Khan (Lecturer - UET LHE)
59
Rizwan Khan (Lecturer - UET LHE)
60
Comparison of the Four and Six Feeder Patterns
For a square shaped distribution substation area served by four primary feeders, the area served by one of the four feeders is,
The total area served by all four feeders is,
The kilovoltampere load served by one of the feeders is,
Rizwan Khan (Lecturer - UET LHE)
61
Comparison of the Four and Six Feeder Patterns
Thus the total kilovoltampere load served by all four feeders is,
The percent voltage drop in the main feeder is,
The load current in the main feeder is,
Rizwan Khan (Lecturer - UET LHE)
62
Comparison of the Four and Six Feeder Patterns
On the other hand, for a hexagonally shaped distribution substation area served by six primary feeders, the area served by one of the six feeders is,
The total area served by all six feeders is,
The kilovoltampere load served by one of the feeders is,
Rizwan Khan (Lecturer - UET LHE)
63
Comparison of the Four and Six Feeder Patterns
Thus the total kilovoltampere load served by all six feeders is,
The percent voltage drop in the main feeder is,
The load current in the main feeder is,
Rizwan Khan (Lecturer - UET LHE)
64
Comparison of the Four and Six Feeder Patterns
The relationship between the service areas of the four and six feeder patterns can be found under two assumptions, •
Feeder circuits are thermally limited
•
Feeder circuits are voltage drop limited
Rizwan Khan (Lecturer - UET LHE)
65
Feeder Circuits are Thermally Limited
For a given conductor size and neglecting voltage drop,
Rizwan Khan (Lecturer - UET LHE)
66
Feeder Circuits are Thermally Limited By dividing the six feeders area by four feeders area,
By substituting the values, Therefore the six feeders can carry 1.50 times as much load as the four feeders if they are thermally loaded.
Rizwan Khan (Lecturer - UET LHE)
67
Feeder Circuits are Voltage Drop Limited
For a given conductor size and assuming equal percent voltage drop,
By dividing the six feeders area by four feeders area,
Therefore the six feeders can carry only 1.25 times as much load as the four feeders if they are voltage drop limited.
Riz
Khan (Le
UET LHE)
68
Rizwan Khan (Lecturer - UET LHE)
69
Derivation of the K Constant
Consider the primary feeder main shown in figure,
Rizwan Khan (Lecturer - UET LHE)
70
Derivation of the K Constant The phasor diagram is given as,
Rizwan Khan (Lecturer - UET LHE)
71
Derivation of the K Constant
When a lumped load is connected at the end of the main, the effective impedance is,
When the load is uniformly distributed, the effective impedance is,
When the load has increasing load density, the effective impedance is,
Rizwan Khan (Lecturer - UET LHE)
72
Derivation of the K Constant
From the phasor diagram,
The power factor angle is,
The power factor is a lagging one. When the real power P and the reactive power Q flow in opposite directions, the power factor is a leading one.
Rizwan Khan (Lecturer - UET LHE)
73
Derivation of the K Constant
Here, the voltage regulation is defined as,
The voltage drop is defined as, V b is the arbitrary base voltage
Rizwan Khan (Lecturer - UET LHE)
74
Derivation of the K Constant
From the circuit diagram,
Rizwan Khan (Lecturer - UET LHE)
75
Derivation of the K Constant
The voltage drop is defined as,
The complex power at the receiving end is,
Rizwan Khan (Lecturer - UET LHE)
76
Derivation of the K Constant
By putting the value of current in sending end voltage equation,
By putting the value of Vs in voltage drop equation,
Rizwan Khan (Lecturer - UET LHE)
77
Derivation of the K Constant
To determine the K constant, use voltage drop equation,
Rizwan Khan (Lecturer - UET LHE)
78
Derivation of the K Constant
Rizwan Khan (Lecturer - UET LHE)
79
View more...
Comments
