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ELEN 3524: Electrical Transmission and Distribution

1. Overview of Distribution Systems2. Load Characteristics

Jayson Bryan E. Mutuc, REE, RMEBSEE-PUP

MSEE, Major in Power Systems - MIT (on-going)

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Overview of Distribution Systems

Electric energy is the most popular form of energy because it can be transported easily at high efficiency and reasonable cost

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Electric Industry Structure

The bulk generation of electricity in the Philippines is produced by power generating companies or IPP’s (Independent Power Producers)

The transmission system of the Philippines is handled by the National Grid Corporation of the Philippines

Distribution of electrical energy to customers is handled by distribution utilities such as MERALCO and Electric Cooperatives

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Modern Power System

• Generation• Transmission or Sub-transmission• Distribution• Loads or End user

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Modern Power System

Figure 1.1 Electric Power System Structure

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Modern Power System

Figure 1.2 Modern Power System Structure

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Generation

One of the essential components of power systems is the three-phase ac generator called synchronous generator or alternator.

Definitions from RA9136• Generation of Electricity

The production of electricity by Generation Company• Generation Company

Any person or entity authorized by the Energy Regulation Commission (ERC) to operate a facilities used in the generation of electricity.

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Transmission

The purpose of an overhead transmission network is to transfer electric energy from generating units at various locations to the distribution system.

Voltage Level: 500kV, 230kV, 115kV(Mostly MERALCO franchise areas), Visayas and Mindanao grid: 138kV, 230kV

Definitions from RA9136• Transmission of Electricity

Refers to the conveyance of electricity through the high voltage backbone of the system

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Distribution

Distribution system is the part that connects the distribution substation to the consumers service-entrance equipment.

Voltage level: 115/66 or 69kV(subtransmission), 34.5/19.92kV, 13.2/7.62kV, 460/265, 230

Definitions from RA9136• Distribution System

Refers to the system of wires and associated facilities belonging to a franchised distribution utility extending between the delivery points on the transmission or subtransmission system or generator connection and the point of connection to the premises of the end-user.

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Distribution• Distribution of Electricity

Refers to the conveyance of electric power by distribution utility through its distribution system.

• Distribution UtilityRefers to any electric cooperative, private corporation,

government owned utility or existing local government unit which ha an exclusive franchise to operate a distribution system.

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Loads or End-User

Loads of power systems are divided into industrial, commercial, and residential

Voltage Level: 69kV, 34.5kV, 13.8kV, 460V, 230V

Definitions from RA9136• End-User

Refers to any person or entity requiring the supply and deliviery of electricity for its own use

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Voltage Class

Voltage class is the term often used by equipment manufacturers and power companies to identify the voltage that the equipment will be connected to

Voltage Class Voltage Category

Under 600 V Low Voltage

601-7200 V Medium Voltage

15 kV – 138 kV High Voltage

230 kV- 750kV Extra High Voltage

Above 1,000,000 Ultra High Voltage

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Distribution System ConsiderationIn determining the design of distribution systems, three broad classifications of choices need to be considered: 1. The type of electric system: dc or ac, and if ac, single-phase

or polyphase.2. The type of delivery system: radial, loop, or network. Radial

systems include duplicate and throwover systems.3. The type of construction: overhead or underground.

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Distribution System ConsiderationDESIRED FEATURES

Electrical energy may be distributed over two or more wires. The principal features desired are:

• Safety• Smooth and even flow of power• As far as is practical; and economy.

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Types of Electric System• Direct Current Systems• Alternating Current Single-Phase Systems

– Two-wire Systems– Three-wire Systems

• Alternating Current Three-Phase Systems– Three-wire Systems– Four-wire Systems

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Types of Delivery System• Primary Distribution System

– which carries the load at higher than utilization voltages from the substation (or other source) to the point where the voltage is stepped down to the value at which the energy is utilized by the consumer.

Radial Distribution Ring Distribution

• Secondary distribution– which includes that part of the system operating at

utilization voltages, up to the meter at the consumer ’s premises.

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Types of Delivery System

Radial SystemsThe radial-type system is the simplest and the one most commonly used. It comprises separate feeders or circuits “radiating” out of the substation or source, each feeder usually serving a given area.

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Types of Delivery SystemRing Distribution SystemsAnother means of restricting the duration of interruption employs feeders designed as loops, which essentially provide a two-way primary feed for critical consumers. Here, should the supply from one direction fail, the entire load of the feeder may be carried from the other end, but sufficient spare capacity must be provided in the feeder.

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Load CharacteristicsIn the planning of an electrical distribution system, as in any other enterprise, it is necessary to know three basic things: 1. The quantity of the product or service desired (per unit of time)2. The quality of the product or service desired3. The location of the market and the individual consumers Logically, then, it would be well to begin with the basic building blocks, the individual consumers, and then determine efficient means of supplying their wants, individually and collectively.

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Load CharacteristicsIn general types of load can be divided into the following categories: Residential, Commercial, and Industrial.

• Residential Load: mainly consists of lights, fans, refrigerators, airconditioners, mixer, ovens, small pumping motor, etc.

• Commercial Load; mainly consists of lighting for shops, offices, advertisements etc., fans, heating, air conditioning and many other electrical appliances used in commercial establishments such as market places, restaurants etc.

• Industrial Load: consists of small scale industries , medium scale industries, large scale industries, and heavy industries

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Daily Load CurveDaily load curve of a utility is a composite of demands made by various classes of users

Figure 1.3 Load Curve

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Consumer Factors

It is obvious that an individual consumer is not apt to be using all of the electrical devices that constitute his or her “connected load” at the same time, or to their full capacity.

It would evidently be unnecessary to provide facilities to serve such a total possible load, and much more economical to provide only for a probable load, the load creating the demand on the distribution facilities.

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Consumer Factors• Demand – the load at the receiving terminals averaged

over a specified amount of time• Demand Interval – the time period over which average

load is computed• Maximum Demand – the greatest value of all demands

which occurred in a specified period of time.• Demand Factor – the ratio of maximum demand of the

system to the total connected load of the system

fEQ. 1.1

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Load Curve• Load Factor – the ratio of average load over a designated

period of time to the peak load occurring in that period.

• Utilization Factor – the ratio of the maximum demand of a system to the rated capacity of the system

• Plant Factor – the ratio of the annual energy generation to the plant capacity x Specified time period

EQ. 1.2

EQ. 1.3

EQ. 1.4

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Consumer Factors• Example Demand Factor:

– a consumer may have ten 10-hp motors installed; at any one time, some will not be in use and others will not be fully loaded, so that the actual demand may be only 50 hp; the demand factor is 50 divided by 100, or 50 percent.

• Example Load Factor:– a consumer household may have a maximum demand of 2 kW during the

evening when many of its lights, the TV, the dishwasher, and other appliances are in use. During the 24-h period, the energy consumed may be 12 kWh; thus the average demand or load is 12 kWh divided by 24 h, or 0.5 kW, and the load factor in this case is 0.5 kW divided by 2 kW, or 25 percent.

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Power Demand ProblemsA power station supplies the load as tabulated below:

a) Plot the load curve and find out

the load factor.b) Determine the proper numberand size of generating units to supply this loadc) Find the reserve capacity of the plant and plant factor

d) Find out the operating schedule of generating units selected.

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Power Demand ProblemsSolution:a)Units generated during 24 hours =[(2x1.2)+2+(3x3)+(2x1.5)+(4x2.5)+(2x1.8)+2+(2x1)+6+0.5+0.8] = 37.8 MWhr

b) Maximum Demand = 3 MW. Therefore 4 generating units rating of 1.0 MW each may be selected. During the period of maximum demand 3 units will operate and 1 unit will remain as stand by

c) Plant Capacity = 4 x 1.0 MW = 4.0 MW Reserved Capacity = 4 – 3 = 1.0 MW

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Power Demand ProblemsSolution:c)

d) Operating Schedule will be as follows:One generating unit – 24 hoursSecond generating unit – 6AM – 9PM (15 hours)Third generating unit – 9AM – 12 Noon and 2PM – 6PM (7

hours)

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Supplementary Problems1) The annual load of a substation is given in the following table.

During each month, the power is assumed constant at an average value. Plot the annual load curve. Find the average load and the annual load factor. Find also the proper number, size of generating units to supply this load and the reserve capacity.