Substation Design

Insulation resistance

(determines dryness of insulation and is done after delivery to serve as benchmark for comparison agains future readings.)

Sound

(important for applications in residential and office areas and that can be used as comparison with future sound tests to reveal any core problems)

Impulse

(where lightning and switching surges are prevalent)

LIGHTNING ARRESTER

A device in electrical power system that protects the insulation and conductors of the system from the damaging effects of lightning

GAS CIRCUIT BREAKER

A mechanical switching device capable of making, carrying and breaking currents under normal circuit conditions and also making , carrying for a specified time and breaking current s under specified circuit conditions such as those of short circuit.

DISCONNECT SWITCH

A mechanical switching device which, for reason of safety, provides in the open position an isolating distance in accordance with specified requirements.

SUBSTATION

A part of the total electrical system (generation, transmission, and distribution) that transforms voltage from one level to another.

BASIC INSULATION LEVEL (BIL)

A specific insulation level expressed in kV of the crest value of standard lightning impulse.

STATION-TYPE TRANSFORMER

A transformer designed for installation in a station or substation.

DISTRIBUTION TRANSFORMER

A transformer for transferring electrical energy in a primary distribution circuit to a secondary distribution or consumer's service circuit.

LIQUID IMMERSED TRANSFORMER

A transformer in which the core and coils are immersed in insulation liquid

DRY TYPE TRANSFORMER

A transformer in which the core and coils are in gaseous or dry compound insulation medium.

OUTDOOR TRANSFORMER

A transformer of weather resistant construction suitable for service without additional protection against weather.

UNIT-SUBSTATION TRANSFORMER

A transformer which is mechanically and electrically connected to and coordinated in design with one or more switchgear assemblies.

INDOOR TRANSFORMER

A transformer which because of its construction, must be protected from weather.

POLE TYPE TRANSFORMER

A transformer which is suitable for mounting on a pole or similar structure.

POWER TRANSFORMER

A transformer which transfer electric energy in any part of the circuit between the generator and the distribution primary circuit.

1. Impulse 2. Sound 3. Temperature rise of coils 4. Corona 5. Insulation resistance 6. Insulation power factor

ADDITIONAL TEST that may be applicabe depending upon how and where the transformer will be used:

INSULATION LEVEL

An insulation strenght expressed in terms of withstand voltage.

PAD MOUNTED TRANSFORMER

An outdoor transformer utilized as part of an undergound distribution system with enclosed compartments for high voltage and low voltage cables entering from below, and mounted on a foundation pad.

PERFORMANCE CHARACTERISTICS

Characteristics (such as Impedance, Losses, Di-electric test levels, Temperature rise, Sound level) which descibes the performance of the transformer under specified conditions.

WIRING

Check ______ and control alarm circuits (if provided) to make sure there are no loose connections and no damaged to insulation.

BOLTED CONNECTIONS

Check all ____ for tightness.

TANK FINISH

Check all painted surfaces to make sure that there is no damage or corrosion.

TAP CHANGER SETTING

Check the ______ to ensure it is set to proper position for the required voltage.

GROUNDING

Check to ensure that the transformer tank is permanently and effectively grounded. The transformer tank ground pad is located near the bottom of the tank.

TOOLS

Check to make sure that all equipment are accounted for and have been removed from the transformer.

PRESSURE TEST

Check transformer tank for leaks by pressurizing the tank with the dry air or dry Nitrogen through the pressure test fitting to a pressure of 3 -4 psig.

1. Voltage level (kV) 2. Load capacity (MVA) 3. Environmental considerations 4. Space limitations 5. Right-of-way requirements

DESIGN FACTORS

26

Dielectric Strength of new fluid shall be ___ kV or greater

INSULATING FLUID TEST

Draw a fluid sample and test its Dielectric Strength.

Land Water Vegetation Topography Soil Access Line Entries Pollution

Environmental Considerations

Economical Evaluation Technical Evaluation Community Acceptance

Evaluation Criteria

1. Resistance Test at rated tap setting 2. Turns Ratio 3. Polarity and Phase Relationship 4. Excitation (No Load) Loss at 100% voltage 5. Impedance and Load Loss 6. Applied Potential 7. Induced Potential 8. Leak Test (Positive Pressure) 9. Production line full wave impulse

IEEE ROUTINE TEST

DELTA/WYE and SERIES/MULTIPLE SWITCH SETTING

If these connections are made using an internal board, check to ensure that these connections are made properly according to the chart on the transformer nameplate.

LINE CONNECTION TEST

In preparation for line connections, check to make sure that all mating connector surfaces are clean and smooth.

horn gap arrester

It consists of a horn shaped metal rods A and B separated by a small air gap. The horns are so constructed that distance between them gradually increases towards the top.

multi gap arrester

It consists of a series of metallic (generally alloy of zinc) cylinders insulated from one another and separated by small intervals of air gaps. The first cylinder (i.e. A) in the series is connected to the line and the others to the ground through a series resistance. The series resistance limits the power arc. By the inclusion of series resistance, the degree of protection against traveling waves is reduced.

Rod Gap Arrester

It is a very simple type of diverter and consists of two 1.5 cm rods, which are bent at right angles with a gap in between as shown in Fig.

CONTINUITY TEST AND RESISTANCE TEST

Measure the winding resistance of each winding and compare results to factory test values. An increase of 10% could indicate a loose terminal connection.

less than 5 ohms

NEC Earthing Resistance should be ____

RATED KVA

Output that can be delivered for the time specified at rated secondary voltage and rated frequency without exceeding the specified temperature rise limitations.

INSULATION MEGGER TEST

Perform 1000V megger test and a power factor test to ensure that none of the windings is grounded.

RATIO TEST

Perform ratio test at each tap position to ensure coil ratios and tap changer connections are correct.

Intermediate Class

Protects against surges from lightning and over-voltages but are designed to be used in medium voltage equipment areas such as electrical utility stations, substation transformers or other substation equipment. Designed to protect equipment in the range of 1 - 20MVA

FLUID TEMPERATURE

Read the _______ gauge and make sure the temperature is no lower than 20degC before the unit is energized.

230kV - 1,100kV

Referred to as extra high voltage

69 kV - 230 kV

Referred to as high voltage

600 V and below

Referred to as low voltage

600 V - 69 kV

Referred to as medium voltage

above 1,100 kV

Referred to as ultra high voltage

80%

Rod Gap Arrester. Generally, the gap length is so adjusted that breakdown should occur at ___% of spark-voltage in order to avoid cascading of very steep wave fronts across the insulators.

Power Station Switchyard Main System Transmission Transformation Station

SUBSTATION Generally classified into:

1. Lightning Arrester 2. Disconnect Switch 3. Gas Circuit Breaker (SF6) 4. Current Transformer 5. Potential Transformer 6. Substation Transformer 7. Medium Voltage Switchgear

SUBSTATION COMPONENTS

Secondary Class

Secondary class arresters are designed to protect most homes from lightning strikes Offered the least amount of protection

4.1 Insulation Coordination 4.2 Protection Scheme 4.3 Clearances and safety measures 4.4 Thermal and Mechanical Stresses

Space Limitation The basis for dimensioning the substation and its components is defined in respect of the following factors:

1. Station Class 2. Intermediate Class 3. Distribution Class 4. Secondary Class

TYPES OF LIGHTNING ARRESTER ACCORDING TO CLASS

1. Rod Gap Lightning Arrester 2. Horn Gap Lightning Arrester 3. Multi Gap Lightning Arrester 4. Expulsion Type 5. Valve Type 6. Silicon Carbide 7. Metal Oxide Arrester (MOV)

TYPES OF LIGHTNING ARRESTER FOR OUTDOOR APPLICATION

ROUTINE TEST

Test made for quality control by the manufacturer on every device (or representative sample) to verify during production that the product meets the design specifications.

80 to 90%

The MCOV rating is usually ___ to ___ of the duty cycle rating.

choke coil

The _____ is so designed that it offers small reactance at normal power frequency but a very high reactance at transient frequency.

laminations,

The core, which provides the magnetic path to channel the flux, consists of thin strips of high-grade steel

CORE LOSSES

The power dissipated in a magnetic core subjected to a time varying magnetizing force.

a. maximumreliability, b. maximum flexibility, and c. continuity of service

The substation design objective is to provide

TOTAL LOSSES

The sum of no load and load losses, excluding losses due to accessories.

FLUID LEVEL

The transformer tank is fitted with fluid at the factory to the normal level at a reference temperature of 25degC.

WITHSTAND VOLTAGE

The voltage that an electric equipment is capable of withstanding without failure or disruptive discharge when tested under specified conditions.

0.23 0.36

Thickness ranges from___mm to upwards of ___ mm.

Expulsion Type Arrester

This type of arrester is also called 'protector tube' and is commonly used on system operating at voltages up to 33kV.

LOAD LOSS

Those losses which are incident to the 2 carrying of a specified load. it includes I R loss in the windings due to load and eddy currents; stray loss due to leakage fluxes in windings, core clamps and other parts.

NO-LOAD (EXCITATION) LOSSES

Those losses which are incident to the excitation of transformer. Includes core loss, di-electric loss, conductor loss due to excitation current , conductor loss due to circulation current in parallel windings.

DESIGN TEST

Those tests made to determine the adequacy of the design of a particular type, style or model of equipment to meet its assigned ratings and to operate satisfactorily under normal service conditions (or under special specified conditions) and to demonstrate compliance with appropriate standards of the industry.

Core Form Shell Form

Two basic types of core construction:

Station Class

Typically used in electrical power stations or substations and other high voltage structures & areas Protects against both lightning and over-voltages where the electrical device has more current in the system than is designed to handle Designed to protect equipment above 20MVA

1-2

Upon the completion of this test, reduce the internal pressure to _____ psig.

aluminum

While _____ is lighter and generally less expensive than copper, a larger cross section of aluminum conductor must be used to carry a current with similar performance as copper. Copper has higher mechanical strength and is used almost exclusively in all but the smaller size ranges, where aluminum conductors may be perfectly acceptable.

4th Letter

__ Letter Cooling medium in contact with the external cooling system Type of Circulation

3rd Letter

__ Letter Cooling medium in contact with the external cooling system Type of Medium

2nd Letter

__ Letter Cooling medium in contact with the windings Type of Circulation

1st Letter

__ Letter Cooling medium in contact with the windings Type of Medium

Load break switch

a mechanical switching device capable of making, carrying, and breaking currents under normal circuit conditions which may include specified operating overload conditions.

Helical windings

are also referred to as screw or spiral windings, with each term accurately characterizing the coil's construction. A helical winding consists of a few to more than 100 insulated strands wound in parallel continuously along the length of the cylinder, with spacers inserted between adjacent turns or discs and suitable transpositions i i n n c c l l u u d d e e d d t t o o m m i i n n i i m m i i z ze e circulating currents between parallel strands.

Layer (barrel) windings

are among the simplest of windings in that the insulated conductors are wound directly next to each other around the cylinder and spacers. Several layers can be wound on top of one another, with the layers separated by solid insulation, ducts, or a combination. Several strands can be wound in parallel if the current magnitude so dictates.

non-linear resistor discs

are made of inorganic compound such as thyrite or metrosil. These discs are connected in series. The non-linear resistors have the property of offering a high resistance to current flow when normal system voltage is applied, but a low resistance to the flow of high surge currents. In other words, the resistance of these non-linear elements decreases with the increase in current through them and vice-versa.

metal oxide arresters

are without gaps, unlike the SIC arrester. This "gap-less" design eliminates the high heat associated with the arcing discharges. utilizing zinc- oxide blocks provides the best performance, as surge voltage conduction starts and stops promptly at a precise voltage level, thereby improving system protection. Failure is reduced, as there is no air gap contamination possibility; but there is always a small value of leakage current present at operating frequency.

Pancake Windings

arrangement of conductors into discs. However, the term most often refers to a coil type that is used almost exclusively in shell-form transformers. The conductors are wound around a rectangular form, with the widest face of the conductor oriented either horizontally or vertically. This type of winding lends itself to the interleaved arrangement

Distribution Class

arresters are most commonly found transformers, both dry type and liquid filled. These arresters are found on equipment rated 1000kVA or less

disc winding

can involve a single strand or several strands of insulated conductors wound in a series of parallel discs of horizontal orientation, with the discs connected at either the inside or outside as a crossover point. Each disc comprises multiple turns wound over other turns, with the crossovers alternating between inside and outside. Mostly used for 25kV class and above.

The windings

consist of the current-carrying conductors wound around the sections of the core, and these must be properly insulated, supported and cooled to withstand operational and test conditions.

Insulation resistance (meg-ohmeter testing)

determines dryness of insulation and is done after delivery to serve as benchmark for comparison against future readings.

Corona

for medium voltage(MV) and high voltage(HV) units, which help determine if the insulation system is functioning properly.

silicon carbide arrester

has some unusual electrical characteristics. It has a very high resistance to low voltage, but a very low resistance to high-voltage.

Valve type arresters

incorporate non linear resistors and are extensively used on systems, operating at high voltages. Fig shows the various parts of a valve type arrester. It consists of two assemblies (i) series spark gaps and (ii) non-linear resistor discs in series. The non-linear elements are connected in series with the spark gaps. Both the assemblies are accommodated in tight porcelain container.

Large Power Transformer

is a large, custom-built piece of equipment that is a critical component of the bulk transmission grid. Because LPTs are very expensive and tailored to customers' specifications, they are usually neither interchangeable with each other nor produced for extensive spare inventories.

spark gap

is a multiple assembly consisting of a number of identical spark gaps in series. Each gap consists of two electrodes with fixed gap spacing. The voltage distribution across the gap is line raised by means of additional resistance elements called grading resistors across the gap. The spacing of the series gaps is such that it will withstand the normal circuit voltage. However an over voltage will cause the gap to break down causing the surge current to ground via the non-linear resistors.

Regular Insulation Testing

is one of the most cost effective methods of identifying aging of transformer.

Windings

made of copper conductors wound around the core, providing electrical input and output.

core

made of high- permeability, grain-oriented, silicon electrical steel, layered in pieces; and

Temperature

rise of coils, which helps ensure that design limits will not be exceeded)

Insulation power factor,

which is done at initial installation and every few years therafter to help determine the ageing process of the insulation.