T & D - 42) The Structure Of Power System

An interconnected power system is a complex enterprise that may be subdivided into the following major subsystems:

• Generation Subsystem
• Transmission and Subtransmission Subsystem
• Distribution Subsystem
• Utilization Subsystem

Generation Subsystem

Generation subsystem includes generators and transformers.

Generators

Three-phase ac generator from around 1895

An essential component of power systems is the three-phase ac generator known as synchronous generator or alternator. Synchronous generators have two synchronously rotating fields: One field is produced by the rotor driven at synchronous speed and excited by dc current. The other field is produced in the stator windings by the three-phase armature currents.

The dc current for the rotor windings is provided by excitation systems. In the older units, the exciters are dc generators mounted on the same shaft, providing excitation through slip rings. Current systems use ac generators with rotating rectifiers, known as brushless excitation systems. The excitation system maintains generator voltage and controls the reactive power flow. Because they lack the commutator, ac generators can generate high power at high voltage, typically 30 kV.

The source of the mechanical power, commonly known as the prime mover, may be hydraulic turbines, steam turbines whose energy comes from the burning of coal, gas and nuclear fuel, gas turbines, or occasionally internal combustion engines burning oil.

A steam turbine used to provide electric power

Steam turbines operate at relatively high speeds of3600 or 1800 rpm. The generators to which they are coupled are cylindrical rotor, two-pole for 3600 rpm, or four-pole for 1800 rpm operation. Hydraulic turbines, particularly those operating with a low pressure, operate at low speed. Their generators are usually a salient type rotor with many poles. In a power station, several generators are operated in parallel in the power grid to provide the total power needed. They are connected at a common point called a bus.

With concerns for the environment and conservation of fossil fuels, many alternate sources are considered for employing the untapped energy sources of the sun and the earth for generation of power. Some alternate sources used are solar power, geothermal power, wind power, tidal power, and biomass.

The motivation for bulk generation of power in the future is the nuclear fusion. If nuclear fusion is harnessed economically, it would provide clean energy from an abundant source of fuel, namely water.

Transformers

High voltage transformer 40MVA

The transformer transfers power with very high efficiency from one level of voltage to another level. The power transferred to the secondary is almost the same as the primary, except for losses in the transformer.

Using a step-up transformer will reduce losses in the line, which makes the transmission of power over long distances possible.

Insulation requirements and other practical design problems limit the generated voltage to low values, usually 30 kV. Thus, step-up transformers are used for transmission of power. At the receiving end of the transmission lines step-down transformers are used to reduce the voltage to suitable values for distribution or utilization.

The electricity in an electric power system may undergo four or five transformations between generator and consumers.

Top

Transmission and Subtransmission Subsystem

An overhead transmission network transfers electric power from generating units to the distribution system which ultimately supplies the load.

Transmission lines also interconnect neighboring utilities which allow the economic dispatch of power within regions during normal conditions, and the transfer of power between regions during emergencies.

Standard transmission voltages are established in the United States by theAmerican National Standards Institute (ANSI). Transmission voltage lines operating at more than 60 kV are standardized at 69 kV, 115 kV, 138 kV, 161 kV, 230 kV, 345 kV, 500 kV, and 765 kV line-to-line.

Transmission voltages above 230 kV are usually referred to as extra-high voltage (EHV).

High voltage transmission lines are terminated in substations, which are called high-voltage substations, receiving substations, or primary substations.

The function of some substations is switching circuits in and out of service; they are referred to as switching stations. At the primary substations, the voltage is stepped down to a value more suitable for the next part of the trip toward the load. Very large industrial customers may be served from the transmission system.

The portion of the transmission system that connects the high-voltage substations through step-down transformers to the distribution substations is called the subtransmission network. There is no clear distinction between transmission and subtransmission voltage levels.

Typically, the subtransmission voltage level ranges from 69 to 138 kV. Some large industrial customers may be served from the subtransmission system. Capacitor banks and reactor banks are usually installed in the substations for maintaining the transmission line voltage.

Top

Distribution Subsystem

The distribution system connects the distribution substations to the consumers’ service-entrance equipment. The primary distribution lines from 4 to 34.5 kV and supply the load in a well-defined geographical area.

Some small industrial customers are served directly by the primary feeders. The secondary distribution network reduces the voltage for utilization by commercial and residential consumers. Lines and cables not exceeding a few hundred feet in length then deliver power to the individual consumers.

The secondary distribution serves most of the customers at levels of 240/120 V, single-phase, three-wire; 208Y/120 V, three-phase, four-wire; or 480Y/277 V, three-phase, four-wire. The power for a typical home is derived from a transformer that reduces the primary feeder voltage to 240/120 V using a three-wire line.

Distribution systems are both overhead and underground. The growth of underground distribution has been extremely rapid and as much as 70 percent of new residential construction is via underground systems.

Top

Load Subsystems

Power systems loads are divided into industrial, commercial, and residential.

Heavy-Duty Single-Phase Capacitor Start And Run Induction Motor

Industrial loads are composite loads, and induction motorsform a high proportion of these loads. These composite loads are functions of voltage and frequency and form a major part of the system load.

Commercial and residential loads consist largely of lighting,heating, and cooking. These loads are independent of frequency and consume negligibly small reactive power. The load varies throughout the day, and power must be available to consumers on demand.

The daily-load curve of a utility is a composite of demands made by various classes of users.

The greatest value of load during a 24-hr period is called the peak or maximum demand. To assess the usefulness of the generating plant the load factoris defined. The load factor is the ratio of average load over a designated period of time to the peak load occurring in that period. Load factors may be given for a day, a month, or a year.

The yearly, or annual load factor is the most useful since a year represents a full cycle of time.

The daily load factor is:

Daily L.F. = average load / peak load            (1.1)

Multiplying the numerator and denominator of (1.1) by a time period of 24 hr, we obtain:

Daily L.F. = average load x 24 hr / peak load

Daily L.F. = energy consumed during 24 hr / peak load x 24 hr          (1.2)

The annual load factor is:

Annual L. F. =  total annual energy / peak load x 8760 hr          (1.3)

Generally there is diversity in the peak load between different classes of loads, which improves the overall system load factor.

In order for a power plant to operate economically, it must have a high system load factor. Today’s typical system load factors are in the range of 55 to 70 percent. Load-forecasting at all levels is an important function in the operation, operational planning, and planning of an electric power system. Other devices and systems are required for the satisfactory operation and protection of a power system.

Some of the protective devices directly connected to the circuits are called switchgear. They include instrument transformers, circuit breakers, disconnect switches, fuses and lightning arresters. These devices are necessary to deenergize either for normal operation or on the occurrence of faults.

The associated control equipment and protective relays are placed on switchboards in control houses.

Reference: Electrical Energy Systems by Mohamed E. El-Hawary (Dalhousie University)

T & D - 21) Defining Size and Location of Capacitor in Electrical System (1)

Defining Size and Location of Capacitor in Electrical System (1)

Content

• Type of Capacitor Bank as per Its Application:
1. Fixed type capacitor banks
2. Automatic type capacitor banks
3. Types of APFC – Automatic Power Factor Correction
• Type of Capacitor as per Construction
• Selecting Size of Capacitor Bank
• Selection of Capacitor as per Non Liner Load
• Configuration of Capacitor:
1. Star-Solidly Grounded
2. Star-Ungrounded
3. Delta-connected Banks
• Effect of series and Parallel Connection of capacitor:
1. Parallel Connection
2. Series Connection

Type of Capacitor Bank as per Its Application

1. Fixed type capacitor banks

The reactive power supplied by the fixed capacitor bank is constant irrespective of any variations in the power factor and the load of the receivers. These capacitor banks are switched on either manually (circuit breaker / switch) or semi automatically by a remote-controlled contactor.

This arrangement uses one or more capacitor to provide a constant level of compensation.

These capacitors are applied at the terminals of inductive loads (mainly motors), at bus bars.

Disadvantages:

• Manual ON/OFF operation.
• Not meet the require kvar under varying loads.
• Penalty by electricity authority.
• Power factor also varies as a function of the load requirements so it is difficult to maintain a consistent power factor by use of Fixed Compensation i.e. fixed capacitors.
• Fixed Capacitor may provide leading power factor under light load conditions, Due to this result in overvoltages, saturation of transformers, mal-operation of diesel generating sets, penalties by electric supply authorities.

Application:

• Where the load factor is reasonably constant.
• Electrical installations with constant load operating 24 hours a day
• Reactive compensation of transformers.
• Individual compensation of motors.
• Where the kvar rating of the capacitors is less than, or equal to 15% of the supply transformer rating, a fixed value of compensation is appropriate.
• Size of Fixed Capacitor bank Qc ≤ 15% kVA transformer

2. Automatic type capacitor banks

The reactive power supplied by the capacitor bank can be adjusted according to variations in the power factor and the load of the receivers.

These capacitor banks are made up of a combination of capacitor steps (step = capacitor + contactor) connected in parallel. Switching on and off of all or part of the capacitor bank is controlled by an integrated power factor controller.

The equipment is applied at points in an installation where the active-power or reactive power variations are relatively large, for example:

• At the bus bars of a main distribution switch-board,
• At the terminals of a heavily-loaded feeder cable.

Where the kvar rating of the capacitors is less than, or equal to 15% of the supply transformer rating, a fixed value of compensation is appropriate.

Above the 15% level, it is advisable to install an automatically-controlled bank of capacitors.

Control is usually provided by contactors. For compensation of highly fluctuating loads, fast and highly repetitive connection of capacitors is necessary, and static switches must be used.

Types of APFC – Automatic Power Factor Correction

Automatic Power Factor correction equipment is divided into three major categories:

1. Standard = Capacitor + Fuse + Contactor + Controller
2. De tuned = Capacitor + De tuning Reactor + Fuse + Contactor + Controller
3. Filtered = Capacitor + Filter Reactor + Fuse + Contactor + Controller.

Advantages:

• Consistently high power factor under fluctuating loads.
• Prevention of leading power factor.
• Eliminate power factor penalty.
• Lower energy consumption by reducing losses.
• Continuously sense and monitor load.
• Automatically switch on/off relevant capacitors steps for consistent power factor.
• Ensures easy user interface.
• Automatically variation, without manual intervention, the compensation to suit the load requirements.

Application:

• Variable load electrical installations.
• Compensation of main LV distribution boards or major outgoing lines.
• Above the 15% level, it is advisable to install an automatically-controlled bank of capacitors.
• Size of Automatic Capacitor bank Qc > 15% kVA transformer.

Method	Advantages	Disadvantages
Individual capacitors	Most technically efficient, most flexible	Higher installation & maintenance cost
Fixed bank	Most economical, fewer installations	Less flexible, requires switches and/or circuit breakers
Automatic bank	Best for variable loads, prevents over voltages, low installation cost	Higher equipment cost
Combination	Most practical for larger numbers of motors	Least flexible

Type of Capacitor as per Construction

1. Standard duty Capacitor

Construction: Rectangular and Cylindrical (Resin filled / Resin coated-Dry)

Application:

1. Steady inductive load.
2. Non linear up to 10%.
3. For Agriculture duty.

2. Heavy-duty

Construction: Rectangular and Cylindrical (Resin filled / Resin coated-Dry/oil/gas)

Application:

1. Suitable for fluctuating load.
2. Non linear up to 20%.
3. Suitable for APFC Panel.
4. Harmonic filtering

3. LT Capacitor

Application:

• Suitable for fluctuating load.
• Non linear up to 20%.
• Suitable for APFC Panel & Harmonic filter application.

Selecting Size of Capacitor Bank

The size of the inductive load is large enough to select the minimum size of capacitors that is practical.

For HT capacitors the minimum ratings that are practical are as follows:

System Voltage	Minimum rating of capacitor bank
3.3 KV , 6.6KV	75 Kvar
11 KV	200 Kvar
22 KV	400 Kvar
33 KV	600 Kvar

Unit sizes lower than above is not practical and economical to manufacture.

When capacitors are connected directly across motors it must be ensured that the rated current of the capacitor bank should not exceed 90% of the no-load current of the motor to avoid self-excitation of the motor and also over compensation.

Precaution must be taken to ensure the live parts of the equipment to be compensated should not be handled for 10 minutes (in case of HT equipment) after disconnection of supply.

Crane motors or like, where the motors can be rotated by mechanical load and motors with electrical braking systems, should never be compensated by capacitors directly across motor terminals.

For direct compensation across transformers the capacitor rating should not exceed 90 % of the no-load KVA of the motor.

Selection of Capacitor as per Non Liner Load

For power Factor correction it is need to first decide which type of capacitor is used.

Selection of Capacitor is depending upon many factor i.e. operating life, Number of Operation, Peak Inrush current withstand capacity.

For selection of Capacitor we have to calculate Total Non-Liner Load like: UPS, Rectifier, Arc/Induction Furnace, AC/DC Drives, Computer, CFL Blubs, and CNC Machines.

• Calculation of Non liner Load, Example: Transformer Rating 1MVA,Non Liner Load 100KVA
• % of non Liner Load = (Non Liner Load/Transformer Capacity) x100 = (100/1000) x100=10%.
• According to Non Linear Load Select Capacitor as per Following Table.

% Non Liner Load	Type of Capacitor
<=10%	Standard Duty
Up to 15%	Heavy Duty
Up to 20%	Super Heavy Duty
Up to 25%	Capacitor +Reactor (Detuned)
Above 30%

Configuration of Capacitor

Power factor correction capacitor banks can be configured in the following ways:

1. Delta connected Bank.
2. Star-Solidly Grounded Bank.
3. Star-Ungrounded Bank.

1. Star-Solidly Grounded

• Initial cost of the bank may be lower since the neutral does not have to be insulated from ground.
• Capacitor switch recovery voltages are reduced
• High inrush currents may occur in the station ground system.
• The grounded-Star arrangement provides a low-impedance fault path which may require revision to the existing system ground protection scheme.
• Typically not applied to ungrounded systems. When applied to resistance-grounded systems, difficulty in coordination between capacitor fuses and upstream ground protection relays (consider coordination of 40 A fuses with a 400 A grounded system).
• Application: Typical for smaller installations (since auxiliary equipment is not required)

2. Star-Ungrounded

Industrial and commercial capacitor banks are normally connected ungrounded Star, with paralleled units to make up the total kvar.

It is recommended that a minimum of 4 paralleled units to be applied to limit the over voltage on the remaining units when one is removed from the circuit.

If only one unit is needed to make the total kvar, the units in the other phases will not be overloaded if it fails.

In industrial or commercial power systems the capacitors are not grounded for a variety of reasons. Industrial systems are often resistance grounded. A grounded Star connection on the capacitor bank would provide a path for zero sequence currents and the possibility of a false operation of ground fault relays.

Also, the protective relay scheme would be sensitive to system line-to-ground voltage Unbalance, which could also result in false relay tripping.

Application: In Industrial and Commercial.

3. Delta-connected Banks

Delta-connected banks are generally used only at distributions voltages and are configured with a Single series group of capacitors rated at line-to-line voltage. With only one series group of units no overvoltage occurs across the remaining capacitor units from the isolation of a faulted capacitor unit.

Therefore, unbalance detection is not required for protection and they are not treated further in this paper.

Application: In Distribution System.

Effect of series and Parallel Connection of capacitor

Parallel Connection

This is the most popular method of connection. The capacitor is connected in parallel to the unit. The voltage rating of the capacitor is usually the same as or a little higher than the system voltage.

Series Connection

This method of connection is not much common. Even though the voltage regulation is much high in this method,

It has many disadvantages.

One is that because of the series connection, in a short circuit condition the capacitor should be able to withstand the high current. The other is that due to the series connection due to the inductivity of the line there can be a resonance occurring at a certain capacitive value.

This will lead to very low impedance and may cause very high currents to flow through the lines.

T & D - 15) Reducing Distribution Line Losses

Reducing Distribution Line Losses

One of the main benefits of applying capacitors is that they can reduce distribution line losses. Losses come from current through the resistance of conductors. Some of that current transmits real power, but some flows to supply reactive power. Reactive power provides magnetizing for motors and other inductive loads. Reactive power does not spin kWh meters and performs no useful work, but it must be supplied.

Using capacitors to supply reactive power reduces the amount of current in the line.

Since line losses are a function of the current squared,I²R, reducing reactive power flow on lines significantly reduces losses.

Engineers widely use the “2/3 rule” for sizing and placing capacitors to optimally reduce losses. Neagle and Samson (1956) developed a capacitor placement approach for uniformly distributed lines and showed that the optimal capacitor location is the point on the circuit where the reactive power flow equals half of the capacitor var rating. From this, they developed the 2/3 rule for selecting and placing capacitors. For a uniformly distributed load, the optimal size capacitor is 2/3 of the var requirements of the circuit.

The optimal placement of this capacitor is 2/3 of the distance from the substation to the end of the line. For this optimal placement for a uniformly distributed load, the substation source provides vars for the first 1/3 of the circuit, and the capacitor provides vars for the last 2/3 of the circuit (see Figure 1).

A generalization of the 2/3 rule for applying n capacitors to a circuit is to size each one to 2/(2n+1) of the circuit var requirements. Apply them equally spaced, starting at a distance of 2/(2n+1) of the total line length from the substation and adding the rest of the units at intervals of 2/(2n+1) of the total line length. The total vars supplied by the capacitors is 2n/(2n+1) of the circuit’s var requirements.

So to apply three capacitors, size each to 2/7 of the total vars needed, and locate them at per unit distances of 2/7, 4/7 and 6/7 of the line length from the substation.

Figure 1 - Optimal capacitor loss reduction using the two-thirds rule

Grainger and Lee (1981) provide an optimal yet simple method for placing fixed capacitors on a circuit with any load profile, not just a uniformly distributed load. With the Grainger/Lee method, we use the reactive load profile of a circuit to place capacitors.

The basic idea is again to locate banks at points on the circuit where the reactive power equals one half of the capacitor var rating.

With this 1/2-kvar rule, the capacitor supplies half of its vars downstream, and half are sent upstream. The basic steps of this approach are:1. Pick a size
Choose a standard size capacitor. Common sizes range from 300 to 1200 kvar, with some sized up to 2400 kvar. If the bank size is 2/3 of the feeder requirement, we only need one bank. If the size is 1/6 of the feeder requirement, we need five capacitor banks.
2. Locate the first bank
Start from the end of the circuit. Locate the first bank at the point on the circuit where var flows on the line are equal to half of the capacitor var rating.
3. Locate subsequent banks
After a bank is placed, reevaluate the var profile. Move upstream until the next point where the var flow equals half of the capacitor rating. Continue placing banks in this manner until no more locations meet the criteria.

There is no reason we have to stick with the same size of banks. We could place a 300-kvar bank where the var flow equals 150 kvar, then apply a 600-kvar bank where the var flow equals 300 kvar, and finally apply a 450-kvar bank where the var flow equals 225 kvar. Normally, it is more efficient to use standardized bank sizes, but different size banks at different portions of the feeder might help with voltage profiles.

The 1/2-kvar method works for any section of line. If a line has major branches, we can apply capacitors along the branches using the same method. Start at the end, move upstream, and apply capacitors at points where the line’s kvar flow equals half of the kvar rating of the capacitor. It also works for lines that already have capacitors (it does not optimize the placement of all of the banks, but it optimizes placement of new banks).

For large industrial loads, the best location is often going to be right at the load.

Figure 2 - Placement of 1200-kvar banks using the 1/2-kvar method

Figure 2 shows the optimal placement of 1200-kvar banks on an example circuit. Since the end of the circuit has reactive load above the 600-kvar threshold for sizing 1200-kvar banks, we apply the first capacitor at the end of the circuit. (The circuit at the end of the line could be one large customer or branches off the main line.) The second bank goes near the middle. The circuit has an express feeder near the start.

Another 1200-kvar bank could go in just after the express feeder, but that does not buy us anything. The two capacitors total 2400 kvar, and the feeder load is 3000 kvar. We really need another 600-kvar capacitor to zero out the var flow before it gets to the express feeder.

Figure 3 -Sensitivity to losses of sizing and placing one capacitor on a circuit with a uniform load. (The circles mark the optimum location for each of the sizes shown.)

Fortunately, capacitor placement and sizing does not have to be exact. Quite good loss reduction occurs even if sizing and placement are not exactly
optimum. Figure 3 shows the loss reduction for one fixed capacitor on a circuit with a uniform load. The 2/3 rule specifies that the optimum distance is 2/3 of the distance from the substation and 2/3 of the circuit’s var requirement.

As long as the size and location are somewhat close (within 10%), the not-quite-optimal capacitor placement provides almost as much loss reduction as the optimal placement.

Consider the voltage impacts of capacitors. Under light load, check that the capacitors have not raised the voltages above allowable standards. If voltage limits are exceeded, reduce the size of the capacitor banks or the number of capacitor banks until voltage limits are not exceeded. If additional loss reduction is desired, consider switched banks as discussed below.

Energy Losses

Use the average reactive loading profile to optimally size and place capacitors for energy losses. If we use the peak-load case, the 1/2-kvar method optimizes losses during the peak load. If we have a load-flow case with the average reactive load, the 1/2-kvar method or the 2/3 rule optimizes energy losses. This leads to more separation between banks and less kvars applied than if we optimize for peak losses.

If an average system case is not available, then we can estimate it by scaling the peak load case by the reactive load factor, RLF:

RLF = Average kvar Demand / Peak kvar Demand

The reactive load factor is similar to the traditional load factor except that it only considers the reactive portion of the load. If we have no information on the reactive load factor, use the total load factor. Normally, the reactive load factor is higher than the total load factor.

Figure 4 - Example of real and reactive power profiles on a residential feeder on a peak summer day with 95% air conditioning

Figure 4 shows an example of power profiles; the real power (kW) fluctuates significantly more than the reactive power (kvar).