The function of relay and circuit-breakers in the operation of a power system is to prevent or limit damage during fault or overloads and minimize their effect on the remainder of the system. This is accomplished by dividing the system into protective zones separated by circuit breakers, each having its protective relays for determining the existence of a fault in that zone.
Thursday, January 12, 2012
Different types of faults in power system
The faults in a power system could be classified as symmetrical or unsymmetrical faults:
· Symmetrical faults:
They involve 3-phase short-circuit. Symmetrical faults are the least probable type of faults. Symmetrical faults represent the most severe type of faults i.e. maximum short-circuit current. Therefore the breaking (rupturing) capacity of circuit-breakers is determined by that type of fault.
· Unsymmetrical faults:
Comprise the following types of faults:
Power System Protection
The large amounts of capital invested in the power systems justify the importance of a reliable protective system. Protection is the art or science of detecting the presence of a fault and initiating the correct tripping of the circuit breakers (C.B). Importance of protection could be understood when a fault (or any other abnormal condition) arises in an electrical power system. That fault must be isolated as quickly as possible from all live supplies in order to:
Single Line Diagram and Layout of the Present 66/11 KV Substation
Studying the figure which presents the single line diagram of the 66/11 KV substation that feeds the residential area in this project shows that the substation consists of a sectionalized double bus bar system fed by eight 66 KV cables, two incoming from the preceding substation in the 66KV ring and two are outgoing to the next substation in the ring and the remaining four 66 KV feeder cells as reserve.
How the Maneuver between Busbars happens
If we want to make maintenance of operating B.B., or to avoid overloading conditions then we have to make maneuver between busbars. This is done by the following procedure:
Essential Civil Structures of an Outdoor Substation
The following civil structures are necessary in a conventional outdoor substation:
§ Towers of incoming and outgoing transmission lines. These are generally located outside the substation boundary, adjacent to the substation.
Substation Equipment
To do its task in a proper way, substations contain much equipment. The most important and common equipment in the transformers substations are the following:
1) Bus Bars
General Overview
The assembly of apparatus used to change some characteristics of electric supply is called a substation. Substations are important part of power system. The continuity of supply depends on the successful operation of substation.
A substation may be called upon to change voltage level or improve power factor or convert A.C. power into D.C. power. According to the service requirement, substations may be classified into:
66/11 KV Substation Layout
Substations function as the interface between the power plant and the electrical grid. The electrical power is produced at the power stations which are located at favorable places, generally quite away from the consumers. It is delivered to the consumers through a large network of transmission and distribution. At many places in the line of the power system, it may be desirable and necessary to change some characteristics of electric supply (voltage, A.C. to D.C., frequency, P.F.…etc.). This is accomplished by suitable arrangement called substation.
Transformer substations consisting of one or more transformers with associated switchgears, protective gear and control panels are required where transmission lines terminate or where power is transformed between two levels. Transformer substation equipment includes bus bars, transformers, H.V transmission lines and cables entrances, M.V feeders and switchgears.
Medium Voltage Network(MVN) Types
Medium voltage networks (MVN) are made up of switchboards and the connections feeding them. We shall first of all look at the different supply modes of these switchboards, then the network structures allowing them to be fed.
MV switchboard supply modes:
We shall start with the main power supply solutions of a MV switchboard, regardless of its place in the network
MV network structure:
We shall now look at the main MV network structures. The complexity of the structure differs depending on the level of power supply security required. The following MVN supply arrangements are the ones most commonly adopted
Distribution Transformer
This transformer is equipped to the ring main feeder through two units of switchgear (Load break switch, which can switch at light loads), then through a fused load break switch (which is cheaper than the circuit breaker) to protect the transformer from over current at fault time. This is known as Ring Main Unit (R.M.U).
Distributor
The second step on the network is the distributor. It is a sectionalized 11 KV busbar in two sections; the two sections are fed from two different 66/11 KV transformers in the same substation or from two different substations to make sure that the continuity of supply is achieved. In case of a fault occurring on the cables between the distributor and one of the transformers; this cable will be switched off from the network and the distributor will be fed from the other transformer.
The bar of the distributor will be fed from two different transformers through medium voltage with-drawable circuit breakers. There is one with-drawable circuit breaker on the bar called the bus coupler. This circuit breaker splits the bar in two isolated parts each part is fed from one transformer. In case a fault occurs; this circuit breaker will connect the isolated parts of the bar (after isolating the faulty feeder) to feed all loads on the bar of the distributor. This system is known as two out of three system (2/3 condition). The number of the outgoing feeders connected to the first part of distributor bar is equal to the number of the outgoing feeders connected to the other part. One feeder of the first part is connected to other one in the other part through ring main feeder to make sure that the continuity of supply is achieved. In case of fault; the ring main has a supply from one of the feeders coming from distributor. The standard cross section area of the feeder coming from sub-station is 3x1x400 mm2 for the 16 MVA distributors and 3 x240 mm2 for the 10 MVA distributors, these feeders are always double, and each pair came from different sub-station or from the same substation as mentioned above. The rule here in Egypt is that each pair of cables can carry the whole load of the distributor alone in case of loss of the other pair; that is the feeders are loaded by only 50% of their current carrying capacity in the normal conditions (when the bus coupler is opened). Loading the cable with only 50% of its ampacity is of course a much exaggerated rule from the economical point of view and we recommend that the feeder is loaded up to 70% of its ampacity.
The outgoing of the distributor has standard cables 3x240 mm2 for the 16 MVA distributors and 3x150 mm2 for the 10 MVA distributors. Each feeder in first part connected to another one on the second part and forms an open loop. Number of transformers in each loop ranges between 6à12 transformers.
Planning of Medium Voltage Distribution Network
In the pervious chapter we finished the design of the low voltage distribution network (also called secondary distribution network) and ends with the distribution transformers points. In this chapter it is desired to make a planning of medium voltage network (primary distribution network)
General overview of the distribution system:
In this section, we will take a quick overview on the main components of the medium voltage distribution network (sometimes called the primary distribution network).
Calculations of pillars and transformers
A residential area for population of 4,000 is divided into four parts. According to the population percentage occupying each type; our task is to:
1. Arrange their houses and service centers.
2. Arrange their supplying boxes so as to increase the reliability of the supply and also its continuity.
3. Connect the boxes to their distribution transformers.
Calculations in this part depend on trial and error concept, and there are many solutions. One of them is acceptable and the others are refused.
The calculation is done by the following steps:
1- Select the number of buildings to be fed by one pillar. Also select the number of feeders to feed these buildings
2- Find the diversified load of one building (from the chart or using diversity factor of 1.05à1.1) then multiply it by the number of buildings fed from the same feeder to get the actual load of the feeder.
3- Find the diversified load of one feeder by dividing the actual load of the feeder by 1.05à1.1, then multiply it by the number of feeders fed from the same pillar to get the actual load of the pillar. To check that the assumption is correct; the actual load of the pillar shouldn't exceed 80% of its ratings.
4- To select the feeder c.s.a, divide the load of the feeder by
5- Now that the load of the pillar is selected; it is desired to select the distribution transformer rating and the feeders to the pillars. Select the number of pillars to be fed from the same transformer. Also select the number of feeders to feed these pillars. Use the appropriate diversity factor to get the diversified load of the pillar then multiply it by the number of pillars on the same feeder to get the actual load of the feeder. Find the feeder current. It shouldn't exceed 70% of its current carrying capacity. Use the tables of the cables to get the appropriate c.s.a and the corresponding voltage drop of this cable. Notice that if more than one cable is to be laid in the same tunnel then an appropriate derating factor should be used.
6- To select the transformer, find the diversified load of the feeder then multiply it by the number of feeders from the transformer to get its actual load. It shouldn't exceed 80% of the transformer rating.
7- Selecting the route of the feeders is done by trial and error to fulfill the constraints mentioned before.
8- Finally the total voltage drop in the LVN is calculated. The design is good if the voltage drop is
9- The voltage drop = the voltage drop on the riser of the building box + the voltage drop on the feeder between the furthest building box and the distribution box + the voltage drop between the furthest distribution box and the transformer.
General Points to be Considered in Design
1- It is always preferred to put the distribution transformers in gardens as possible; yet the environmental constraints should be also fulfilled.
2- For buildings of flats we usually use the diversification chart since the load profile between buildings is not necessary to be the same so we can't take a certain figure to be the diversity factor. On the other hand, villas can be considered as loads of the same load profile since all villas' inhabitants do behave in almost the same manner which is somehow luxurious; thus in villas we usually take the diversity factor to be 1.05à1.1
3- Diversification is used for any node that supplies more than one node; i.e if the pillar feeds more than one feeder then to get the load of the pillar we consider diversification between these feeders. Same is done when considering distribution transformers and pillars.
4- Since feeders have the same load profile; we use diversity factor of 1.05à1.1
5- The locations of the transformers and pillars and the routes of the cables are chosen so that:
- The maximum voltage drop between any transformer and the furthest consumer is
- The crossing between cables should be avoided as much as possible.
- The routes of the cables should avoid street crossing as much as possible so that when maintenance in feeders is done we don't need to dig across the streets to get the cables out.
6- As we mentioned before the distribution boxes are connected in loops and so does the coffree of the buildings. Thus if two coffrees or two boxes of different loads are connected together then it is recommended that both have the same c.s.a of feeders which suits the one with the larger load. This is very important so that if a fault occurs on the box with larger load; the feeder of the other box can withstand the overload safely.
7- Low voltage fuses ratings are as follows: 2, 4, 6, 8, 10, 16, 20, 25, 32, 35, 40, 50, 63, 80, 100, 125, 160, 200, 250, 315, 400, 500, 630, 800, 1000 and1250 Amperes according to ABB pocket book(switchgear manual), 8th edition.
8- Standard ratings of pillars are 50 KVA, 100 KVA, 150 KVA, and 200 KVA
9- Standard ratings of distribution transformers are 300 KVA, 500 KVA and 1000 KVA. Yet it is recommend using of 500 KVA transformers instead of 300 KVA transformers as much as possible since the former is slightly more expensive than the other (only 1000 or 2000 L.E more) while it is better for the new developing areas to use it due to its larger capacity. Thus in this project we will try to use the 500 KVA transformer as much as possible.
10- Standard ratings of street lighting pillars are 150 and 250 KVA
11- Additional 25% spare equipments should be used in the design; i.e if the design shows the need of 4 cables then a fifth cable is added as a spare. In this project the extra equipments are not shown in the drawings but it is understood that they are found.
In the secondary distribution networks the c.s.a of the cables used shouldn't be less than 3General overview of the distribution system
In this section, we will take a quick overview on the main components of the low voltage distribution network sometimes called the secondary distribution network).
Distribution Box (pillar):
Distribution Transformer:
Building Box (Coffree):
Planning of Low Voltage Distribution Network
A realistic view of the power distribution systems should be based on "gathering" functions rather than "distributing" them since the size and locations of the customer demands are not determined by the distribution engineer but by the customers. Customers install all types of energy-consuming devices which can be connected in every conceivable combination and at times chosen by the customers. The concept of distribution starts with the individual customers and loads, and proceeds through several gathering stages where each stage includes various groups of increasing numbers of customers and their loads. Ultimately the generating stations themselves are reached through services, secondaries, distribution transformers, primary feeders, distribution substation, subtransmission networks, bulk power stations and transmission network as will be discussed in the next section.
In designing a system, distribution engineers may find a conflict between fulfilling the requirements of the electrical considerations and the economical considerations in the same time. A good distribution system is the one compromising both considerations in the same time as much as possible. An example of this conflict is the voltage drop on the feeders. For achieving good performance of the system, voltage drop should be eliminated in order to have a flat voltage profile. To achieve this we use cables of larger cross sectional area (c.s.a) in order to have smaller resistance. On the other hand, the economical considerations in some cases permits a certain range of voltage drop so as to fully use the used cables. Yet if the conflict between electrical requirements and economical requirements can't be solved; the priority is always for the electrical requirements since they represent the safe operation which is the main aim of the distribution engineer.
Another example on the conflict between electrical and economical requirements is to increase the service reliability for the critical loads, e.g. hospitals, computer and control centers, critical industrial loads. To do this some back-up systems such as emergency generators and/or batteries with automatic switching devices are used in such places. These extra equipments cost more money, yet the reliability of the service is more important in this case than any money.
In their system design decisions of the secondary distribution network, distribution engineers are primarily motivated by the considerations of economy, coppers losses in the transformer and the secondary circuit, permissible voltage drops and voltage flickers of the system. Of course, there are some other engineering and economic factors affecting the selection of the distribution transformer and the secondary configuration, such as permissible transformer loading, balanced phase loads for the primary system, investment costs of the various secondary system components, cost of labor, capital cost, inflation rates and other factors.
Types of lamps
The lighting design process in its most basic form entails identifying a task and then providing a light source that will provide proper quantity and quality of light for the task. The fixture protects the light source, connects it to the power source and distributes its light.
The light source is the actual light-producing component of the lighting system. It may operate simply as a lamp (incandescent/halogen) or as a lamp powered by a ballast (fluorescent and high-intensity discharge [HID]).
I-Incandescent Lamps:
Incandescent light sources are the cheapest light sources.
.
· Do not require a ballast
· It is based on the fact that current is passed through a filament, which heats until it glows
· Less efficacious light source
· Shorter service life than other light sources in most cases
· Filament is sensitive to vibrations
· Bulb can get very hot during operation
· Must be properly shielded because incandescent lamps can produce direct glare as a point source
· Require proper line voltage as line voltage variations can severely affect light output and service life
An example of incandescent lamps is given in figure 4.1
Efficiency of incandescent is 14 lumen/watt.
ii-Fluorescent Lamps:
These lamps rely on the gaseous discharge method.
· Require a ballast
· Low surface brightness compared to point sources
· More efficacious compared to incandescent
· Ambient temperatures and convection currents can affect light output and life
· Options for starting methods and lamp current loadings
· Requires compatibility with ballast
· Low temperatures can affect starting unless"cold weather" ballast is specified.
An example of fluorescent lamps is given in figure 4.2
Efficiency of fluorescent lamps is 46 lumen/wat
iii- Compact Fluorescent Lamps (CFL):
· It is a new and advanced lighting technology
· More efficient than incandescent lamps
· CFL use 70 - 75% less energy than their incandescent equivalents. When replacing a 100 watt incandescent lamp a 28 watt CFL is used.
· CFL last approximately 10,000 hours, which is 10 to 13 times the life of an incandescent lamp (expected life approximately 750 hours).
· Compact fluorescents are most cost-effective when used at least 2-3 hours per day.
· Although CFL may appear different than the common incandescent, they fit most standard fixtures found in homes today. The screw-in base is the same on both lamps.
· The typical incandescent lamp wastes 90% of the energy it uses, producing heat rather than light.
· CFL will provide the same amount of light (or lumens) at a fraction of the electricity used.
Designing the lighting system
To produce a new lighting system in a construction, it must be designed. The designer must determine the desired light levels for tasks that are to be performed in a given space, then determine the light output that will be required to meet those objectives consistently, taking into account all the factors that degrade both light output and light levels over time. Equipment must then be chosen and placed in a layout to produce the desired light distribution.
Requirements of a good lighting scheme
A good lighting scheme should fulfill the following:
- Provide adequate illumination.
- Provide uniform illumination allover the working plan.
- Provide light of suitable color.
- Avoid glare and hard shadows.
Lighting background
Importance of light:
Light is the prime factor in the human life as all activities of human beings ultimately depend upon the light. Where there is no natural light, use of artificial is made. Lighting increases production and reduce accidents.
Basic Definitions:
Candela
International unit (SI) of luminous intensity; term evolved from considering a standard candle, similar to a plumber's candle, as the basis of evaluating the intensity of other light describe the relative intensity of a source .
International unit (SI) of luminous intensity; term evolved from considering a standard candle, similar to a plumber's candle, as the basis of evaluating the intensity of other light describe the relative intensity of a source .
Candlepower Distribution Curve
A graphic presentation of the distribution of light intensity of a lamp or luminaire.
Illuminance (E)
The quantity of light (measured in foot-candles, Lux, etc) at a point on a surface.
Inverse Square Law
Formula stating that illumination at a point on a surface varies directly with the intensity of a point source, and inversely as the square of the distance between the source and the point; it illustrates how the same quantity of light flux is distributed over a greater area as the distance from the source to the surface is increased.
Light Loss Factor
The product of all considered factors that contribute to a lighting system's depreciated light output over a period of time, including dirt and lamp lumen depreciation.
Lumen
The international unit of luminous flux or quantity of light.
The international unit of luminous flux or quantity of light.
Luminaire
A complete lighting unit consisting of a lamp (or lamps) together with the parts designed to distribute the light, position and protect the lamps, and connect them to the power supply. This is sometimes referred to as a "fixture".
Lamp efficiency
It is the amount of output lumen per watt.
Lux (lumen/m2)
SI (international system) unit of illumination. One lumen uniformly distributed over an area of one square meter.
Mounting Height
Distance from the bottom of the fixture to either the floor or work plane, depending on usage.
Spacing to Mounting Height Ratio
Ratio of fixture spacing (distance apart) to mounting height above the work plane. Sometimes it is called spacing criterion. A normal range is 1 à 1.5
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