Electrical Q&A Part-1
Electrical Q&A Part-1 Download in PDF (1) Why ELCB can’t work if Neutral input of ELCB do not connect to ground? ELCB is used to detect earth leakage fault. Once the phase and neutral are connected in an ELCB, the current will flow through phase and that much current will have to return neutral so resultant current is zero. Once there is a ground fault in the load side, current from phase will directly pass through earth and it will not return through neutral through ELCB. That means once side current is going and not returning and hence because of this difference in current ELCB will trip and it will safe guard the other circuits from faulty loads. If the neutral is not grounded, fault current will definitely high and that full fault current will come back through ELCB, and there will be no difference in current. (2) Why in a three pin plug the earth pin is thicker and longer than the other pins? It depends upon R=rl/a where area(a) is inversely proportional to resistance (R), so if (a) increases, R decreases & if R is less the leakage current will take low resistance path so the earth pin should be thicker. It is longer because the The First to make the connection and Last to disconnect should be earth Pin. This assures Safety for the person who uses the electrical instrument. (3)Why Delta Star Transformers are used for Lighting Loads? For lighting loads, neutral conductor is must and hence the secondary must be star winding. and this lighting load is always unbalanced in all three phases. To minimize the current unbalance in the primary we use delta winding in the primary. So delta / star transformer is used for lighting loads. (4)What are the advantages of star-delta starter with induction motor? (1)The main advantage of using the star delta starter is reduction of current during the starting of the motor. Starting current is reduced to 3-4 times Of current of Direct online starting. (2) Hence the starting current is reduced , the voltage drops during the starting of motor in systems are reduced. (5)What is meant by regenerative braking? When the supply is cut off for a running motor, it still continue running due to inertia. In order to stop it quickly we place a load (resistor) across the armature winding and the motor should have maintained continuous field supply. so that back e.m.f voltage is made to apply across the resistor and due to load the motor stops quickly. This type of breaking is called as “Regenerative Breaking”. (6)When voltage increases then current also increases then what is the need of over voltage relay and over current relay? Can we measure over voltage and over current by measuring current only? No. We can’t sense the over voltage by just measuring the current only because the current increases not only for over voltages but also for under voltage(As most of the loads are non-linear in nature).So, the over voltage protection & over current protection are completely different. Over voltage relay meant for sensing over voltages & protect the system from insulation break down and firing. Over current relay meant for sensing any internal short circuit, over load condition ,earth fault thereby reducing the system failure & risk of fire. So, for a better protection of the system. It should have both over voltage & over current relay. (7)If one lamp connects between two phases it will glow or not? If the voltage between the two phases is equal to the lamp voltage then the lamp will glow. When the voltage difference is big it will damage the lamp and when the difference is smaller the lamp will glow depending on the type of lamp.
(8) What are HRC fuses and where it is used? HRC stand for “high rupturing capacity” fuse and it is used in distribution system for electrical transformers (9)Mention the methods for starting an induction motor? The different methods of starting an induction motor DOL:direct online starter Star delta starter Auto transformer starter Resistance starter Series reactor starter (10)What is the difference between earth resistance and earth electrode resistance? Only one of the terminals is evident in the earth resistance. In order to find the second terminal we should recourse to its definition: Earth Resistance is the resistance existing between the electrically accessible part of a buried electrode and another point of the earth, which is far away. The resistance of the electrode has the following components: (A) the resistance of the metal and that of the connection to it. (B) the contact resistance of the surrounding earth to the electrode. (11)What is the power factor of an alternator at no load? At no load Synchronous Impedance of the alternator is responsible for creating angle difference. So it should be zero lagging like inductor (12)Why most of analog o/p devices having o/p range 4 to 20 mA and not 0 to 20 mA ? 4-20 mA is a standard range used to indicate measured values for any process. The reason that 4ma is chosen instead of 0 mA is for fail safe operation . For example- a pressure instrument gives output 4mA to indicate 0 psi, up to 20 mA to indicate 100 psi, or full scale. Due to any problem in instrument (i.e) broken wire, its output reduces to 0 mA. So if range is 0-20 mA then we can differentiate whether it is due to broken wire or due to 0 psi. (13)Two bulbs of 100w and 40w respectively connected in series across a 230v supply which bulb will glow bright and why? Since two bulbs are in series they will get equal amount of electrical current but as the supply voltage is constant across the bulb(P=V^2/R).So the resistance of 40W bulb is greater and voltage across 40W is more (V=IR) so 40W bulb will glow brighter. (14)What is meant by knee point voltage? Knee point voltage is calculated for electrical Current transformers and is very important factor to choose a CT. It is the voltage at which a CT gets saturated.(CT-current transformer). (15)What is reverse power relay? Reverse Power flow relay are used in generating stations’ protection. A generating stations is supposed to fed power to the grid and in case generating units are off, there is no generation in the plant then plant may take power from grid. To stop the flow of power from grid to generator we use reverse power relay. (16)What will happen if DC supply is given on the primary of a transformer?
Mainly transformer has high inductance and low resistance. In case of DC supply there is no inductance ,only resistance will act in the electrical circuit. So high electrical current will flow through primary side of the transformer. So for this reason coil and insulation will burn out When AC current flow to primary winding it induced alternating flux which also link to secondary winding so secondary current flow in secondary winding according to primary current. Secondary current also induced emf (Back emf) in secondary winding which oppose induced emf of primary winding and thus control primary current also. If DC current apply to Primary winding than alternating flux is not produced so no secondary emf induced in secondary winding so primary current may goes high and burn transformer winding. (17)What are the advantage of free wheeling diode in a Full Wave rectifier? It reduces the harmonics and it also reduces sparking and arching across the mechanical switch so that it reduces the voltage spike seen in a inductive load. (18)Different between megger and contact resistance meter? Megger used to measure cable resistance, conductor continuity, phase identification where as contact resistance meter used to measure low resistance like relays ,contactors. (19)When we connect the capacitor bank in series ? we connect capacitor bank in series to improve the voltage profile at the load end in transmission line there is considerable voltage drop along the transmission line due to impedance of the line. so in order to bring the voltage at the load terminals within its limits i.e (+ or – %6 )of the rated terminal voltage the capacitor bank is used in series (20)What is Diversity factor in electrical installations? Diversity factor is the ratio of the sum of the individual maximum demands of the various subdivisions of a system, or part of a system, to the maximum demand of the whole system, or part of the system, under consideration. Diversity factor is usually more than one. (21)Why humming sound occurred in HT transmission line? This sound is coming due to ionization (breakdown of air into charged particles) of air around transmission conductor. This effect is called as Corona effect, and it is considered as power loss. (22)What is different between grounding and earthings? Grounding means connecting the neutral point of the load to the ground to carry the residual current in case of unbalanced conditions through the neutral to the ground whereas earthing is done in an electric equipment in order to protect he equipment in occurrence of fault in the system. (23)Why should be the frequency is 50 hz only why not others like anything, why should we maintain the frequency constant if so why it is only 50? We can have the frequency at any frequency you like, but than you must also make your own motors, transformers or any other equipment you want to use. We maintain the frequency at 50hz or 60hz cos the world maintains a standard at 50 /60hz and the equipments are made to operate at these frequency. (24)If we give 2334 A, 540V on Primary side of 1.125 MVA step up transformer, then what will be the Secondary Current, If Secondary Voltage=11 KV? As we know the Voltage & current relation for transformer-V1/V2 = I2/I1 We Know, VI= 540 V; V2=11KV or 11000 V; I1= 2334 Amps. By putting these value on Relation-
540/11000= I2/2334 So,I2 = 114.5 Amps (25)what are the points to be consider for MCB(miniature circuit breaker selection? I(L)*1.25=I(MAX) maximum current. Mcb specification are done on maximum current flow in circuit. (26)what is the full form of KVAR? We know there are three types of power in Electricals as Active, apparent & reactive. So KVAR is stand for “Kilo Volt Amps with Reactive component. (27)In three pin plug 6 Amp. 220v AC rating. why earth pin diameter is higher than ohter two pin? what its purpose ? Because Current flow in the conductor is inversely proportional to the conductor diameter. So if any short circuits occur in the system first high currents bypassed in the Earthling terminal.( R=Pl/a area of the conductor increases resistance value decreases) (28)How can you start-up the 40w tube lite with 230v AC/DC without using any choke/Coil? It’s possible by means of Electronic choke. Otherwise it’s not possible to ionize the particles in tube. light, with normal voltage. (29)What is “pu” in electrical engineering? Pu stands for per unit and this will be used in power system single line diagram there it is like a huge electrical circuit with no of components (generators, transformers, loads) with different ratings (in MVA and KV). To bring all the ratings into common platform we use pu concept in which, in general largest MVA and KV ratings of the component is considered as base values, then all other component ratings will get back into this basis. Those values are called as pu values. (p.u=actual value/base value). (30)Why link is provided in neutral of an ac circuit and fuse in phase of ac circuit? Link is provided at a Neutral common point in the circuit from which various connection are taken for the individual control circuit and so it is given in a link form to withstand high Amps. But in the case of Fuse in the Phase of AC circuit it is designed such that the fuse rating is calculated for the particular circuit (i.e load) only.So if any malfunction happen the fuse connected in the particular control circuit alone will blow off (31) What is the diff. between Electronic regulator and ordinary rheostat regulator for fans? The difference between the electronic and ordinary regulator is that in electronic reg. power losses are less because as we decrease the speed the electronic reg. give the power needed for that particular speed but in case of ordinary rheostat type reg. the power wastage is same for every speed and no power is saved.In electronic regulator triac is employed for speed control.by varying the firing angle speed is controlled but in rheostatic control resistance is decreased by steps to achieve speed control. (32) What happen if we give 220 volts dc supply to d bulb r tube light? Bulbs [devices] for AC are designed to operate such that it offers high impedance to AC supply. Normally they have low resistance. When DC supply is applied, due to low resistance, the current through lamp would be so high that it may damage the bulb element (33)What will happen when power factor is leading in distribution of power? If there is high power factor, i.e if the power factor is close to one: 1.losses in form of heat will be reduced, 2.cable becomes less bulky and easy to carry, and very
cheap to afford, & 3. it also reduces over heating of tranformers. (34) what the one main difference between UPS & inverter ? And electrical engineering & electronics engineering? uninterrupted power supply is mainly use for short time . means according to ups VA it gives backup. ups is also two types : on line and offline . online ups having high volt and amp for long time backup with with high dc voltage.but ups start with 12v dc with 7 amp. but inverter is startwith 12v,24,dc to 36v dc and 120amp to 180amp battery with long time backup (35)Which type of A.C motor is used in the fan (ceiling fan, exhaust fan, padestal fan, bracket fan etc) which are find in the houses ? It is Single Phase induction motor which mostly squirrel cage rotor and are capacitor start capacitor run. (36)what is the difference between synchronous generator & asynchronous generator? In simple, synchronous generator supply’s both active and reactive power but asynchronous generator(induction generator) supply’s only active power and observe reactive power for magnetizing. This type of generators are used in windmills. (37)What is the Polarization index value ? (pi value)and simple definition of polarization index ? Its ratio between insulation resistance(IR)i.e meager value for 10min to insulation resistance for 1 min. It ranges from 5-7 for new motors & normally for motor to be in good condition it should be Greater than 2.5 . (38)What is Automatic Voltage regulator(AVR)? AVR is an abbreviation for Automatic Voltage Regulator. It is important part in Synchronous Generators, it controls the output voltage of the generator by controlling its excitation current. Thus it can control the output Reactive Power of the Generator. (39)Difference between a four point starter and three point starter? The shunt connection in four point starter is provided separately form the line where as in three point starter it is connected with line which is the drawback in three point stater (40)What is the difference between surge arrestor and lightning arrestor? LA is installed outside and the effect of lightning is grounded, where as surge arrestor installed inside panels comprising of resistors which consumes the energy and nullify the effect of surge. (41)What happens if i connect a capacitor to a generator load? Connecting a capacitor across a generator always improves powerfactor,but it will help depends up on the engine capacity of the alternator, otherwise the alternator will be over loaded due to the extra watts consumed due to the improvement on pf. Secondly, don’t connect a capacitor across an alternator while it is picking up or without any other load (42)Why the capacitors works on ac only? Generally capacitor gives infinite resistance to dc components(i.e., block the dc components). it allows the ac components to pass through. (43)How many types of colling system it transformers? ONAN (oil natural,air natural) 2. ONAF (oil natural,air forced) 3. OFAF (oil forced,air forced) 4. ODWF (oil direct,water forced) 5. OFAN (oil forced,air forced)
(44)What is the function of anti-pumping in circuit breaker? when breaker is close at one time by close push button, the anti-pumping contactor prevent re close the breaker by close push button after if it already close. (45)There are a Transformer and an induction machine. Those two have the same supply. For which device the load current will be maximum? And why? The motor has max load current compare to that of transformer because the motor consumes real power.. and the transformer is only producing the working flux and its not consuming. Hence the load current in the transformer is because of core loss so it is minimum. (46)what is boucholz relay and the significance of it in to the transformer? Boucholz relay is a device which is used for the protection of transformer from its internal faults, it is a gas based relay. whenever any internal fault occurs in a transformer, the boucholz relay at once gives a horn for some time, if the transformer is isolated from the circuit then it stop its sound itself other wise it trips the circuit by its own tripping mechanism. (47)Why we do 2 types of earthing on transformer (ie:)body earthing & neutral earthing , what is function. i am going to install a 5oo kva transformer & 380 kva DG set what should the earthing value? The two types of earthing are Familiar as Equipment earthing and system earthing. In Equipment earthing: body ( non conducting part)of the equipment should be earthed to safeguard the human beings. System Earthing : In this neutral of the supply source ( Transformer or Generator) should be grounded. With this, in case of unbalanced loading neutral will not be shifted. So that unbalanced voltages will not arise. We can protect the equipment also. With size of the equipment( transformer or alternator)and selection of relying system earthing will be further classified into directly earthed, Impedance earthing, resistive (NGRs) earthing. (48)What is the difference between MCB & MCCB, Where it can be used? MCB is miniature circuit breaker which is thermal operated and use for short circuit protection in small current rating circuit. MCCB moulded case circuit breaker and is thermal operated for over load current and magnetic operation for instant trip in short circuit condition. Under voltage and under frequency may be inbuilt. Normally it is used where normal current is more than 100A. (49)Where should the lighting arrestor be placed in distribution lines? Near distribution transformers and out going feeders of 11kv and incoming feeder of 33kv and near power transformers in sub-stations. (50)Why Delta Star Transformers are used for Lighting Loads? For lighting loads, neutral conductor is must and hence the secondary must be star winding. and this lighting load is always unbalanced in all three phases. To minimize the current unbalance in the primary we use delta winding in the primary. So delta / star transformer is used for lighting loads. (51) If 200watt,100 watt and 60 watt lamps connected in series with a source of 230V AC supply, which lamp glow brighter??Each lamp voltage rating is 230V. Each bulb when independently working will have currents (W/V= I) For 200 Watt Bulb current (I200) =200/230=0.8696 A For 100 Watt Bulb current (I100) =100/230=0.4348 A For 60 Watt Bulb current (I60) =60/230=0.2609 A Resistance of each bulb filament is (V/I = R) For 200 Watt Bulb R200= 230/0.8696= 264.5 ohms
For 100 Watt Bulb R100= 230/0.4348 = 528.98 ohms and For 60 Watt Bulb R60= 230/0.2609=881.6 ohms respectively Now, when in series, current flowing in all bulbs will be same. The energy released will be I 2R Thus, light output will be highest where resistance is highest. Thus, 60 watt bulb will be brightest. The 60W lamp as it has highest resistance & minimum current requirement. Highest voltage drop across it X I [which is common for all lamps] =s highest power. Note to remember: Lowest power-lamp has highest element resistance. And highest resistance will drop highest voltage drop across it in a Series circuit And highest resistance in a parallel circuit will pass minmum current through it. So minimum power dissipated across it as min current X equal Voltage across =s min power dissipation (52) How to check Capacitor with use of Multi meter. Most troubles with Capacitors — either open or short. A ohmmeter (multi meter) is good enough. A shorted C will clearly show very low resistance. A open C will not show any movement on ohmmeter. A good capacitor will show low resistance initially, and resistance gradually increases. This shows that C is not bad. By shorting the two ends of C (charged by ohmmeter) momentarily can give a weak spark. To know the value and other parameters, you need better instruments
(53)What do AC meters show, is it the RMS or peak voltage? AC voltmeters and ammeters show the RMS value of the voltage or current. DC meters also show the RMS value when connected to varying DC providing the DC is varying quickly, if the frequency is less than about 10Hz you will see the meter reading fluctuating instead. (54)In the transmission tower construction Middle arm is longer than the upper and lower arm wires will stay apart. To prevent big birds (Ostriches etc) from bumping their heads against the cable above when they sit on the wire below. Designed to maintain the mechanical requirement to prevent arching between conductors while maintaining a tower height that is manageable, and of course preventing head injuries to birds the arms are of different links to prevent a broken upper line from falling on one or more of the phase lines below. 1.The clearance from other phase is a criteria. 2. Mutual inductance minimization is another criteria. 3. Preventing droplet of water/ice to fall on bottom conductor is another criteria. (55)What is the difference between Surge Arrester & Lightning Mast Transmission Line Lightning Protection – General: The transmission line towers would normally be higher than a substation structure, unless you have a multi-storey structure at your substation. Earth Mats are essential in all substation areas, along with driven earth electrodes (unless in a dry sandy desert site). It is likewise normal to run catenaries’ (aerial earth conductors) for at least 1kM out from all substation structures. Those earth wires to be properly electrically to each supporting transmission tower, and bonded back to the substation earth system.
It is important to have the catenaries’ earth conductors above the power conductor lines, at a sufficient distance and position that a lightning strike will not hit the power conductors. In some cases it is thus an advantage to have two catenary earth conductors, one each side of the transmission tower as they protect the power lines below in a better manner. In lightning-prone areas it is often necessary to have catenary earthing along the full distance of the transmission line. Without specifics, (and you could not presently give tower pictures in a Post because of a CR4 Server graphics upload problem), specifics would include: Structure Lightning Protection – General: At the Substation, it is normal to have vertical electrodes bonded to the structure, and projecting up from the highest points of the structure, with the location and number of those electrodes to be sufficient that if a lightning strike arrived, it would always be a vertical earthed electrode which would be struck, rather than any electrical equipment. In some older outdoor substation structures, air-break isolator switches are often at a very high point in the structure, and in those cases small structure extension towers are installed, with electrodes at the tapered peak of those extension towers. The extension towers are normally 600mm square approximately until the extension tower changes shape at the tapered peak, and in some cases project upwards from the general structure 2 to 6 metres, with the electrode some 2 to 3 metres projecting upwards from the top of the extension tower. The substation normally has a Lightning Counter – which registers a strike on the structure or connected catenary earth conductors, and the gathering of that information (Lightning Days, number per Day/Month/Year, Amperage of each strike) gives the Engineering Staff good statistics for future substation design. 1.
Site soil type and resistivity
Number of Lightning Days
Expected Voltage + Current of a local lightning strike
Other – Advise please.
(56)Use of Stones/Gravel in electrical Switch Yard helps in Reducing Step and Touch potentials during Short Circuit Faults Eliminates the growth of weeds and small plants in the yard Improves yard working condition Protects from fire which cause due to oil spillage from transformer and also protects from wild habitat. What is service factor? Service factor is the load that may be applied to a motor without exceeding allowed ratings. For example, if a 10hp motor has a 1.25 service factor, it will successfully deliver 12.5 hp (10 x 1.25) without exceeding specified temperature rise. Note that when being driven above its rated load in this manner, the motor must be supplied with rated voltage and frequency. Keep in mind, however, that a 10-hp motor with a 1.25 service factor is not a 12.5-hp motor. If the 10-hp motor is operated continuously at 12.5 hp, its insulation life could be decreased by as much as two-thirds of normal. If you need a 12.5-hp motor, buy one; service factor should only be used for short-term overload conditions (57)why transmission line 11Kv OR 33KV,66Kv not in 10kv 20kv? The form factor of an alternating current waveform (signal)
is the ratio of the RMS (Root Mean Square) value to the average value (mathematical mean of absolute values of all points on the waveform). In case of a sinusoidal wave, the form factor is approximately 1.11. The reason is some thing historical. In olden days when the electricity becomes popular, the people had a misconception that in the transmission line there would be a voltage loss of around 10%. So in order to get 100 at the load point they started sending 110 from supply side. This is the reason. It has nothing to do with form factor (1.11). Nowadays that thought has changed and we are using 400 V instead of 440 V, or 230 V instead of 220 V. Also alternators are now available with terminal voltages from 10.5 kV to 15.5 kV so generation in multiples of 11 does not arise. The Basic Idea behind a desired transmission voltage was still the form factor. In ancient times when we needed to use 10 kV at destination, simply multiplied the form factor to it Say 1.11X10=1.11 =11KV aprox.(we had taken 10% losses as standard thumb rule) similarily for 30 & 60 KV. Form Factor = RMS voltage/Average Voltage For AC sine wave Form Factor is 1.11. Now a days when, we have voltage correction systems, powerfactor improving capacitors, which can boost/correct volatge to desired level, we are using the exact voltages like 400KV inspite of 444KV (58) How Corona Discharge Effect Occur: In a power system transmission lines are used to carry the power. These transmission lines are separated by certain spacing which is large in comparison to their diameters. In Extra High Voltage system (EHV system ) when potential difference is applied across the power conductors in transmission lines then air medium present between the phases of the power conductors acts as insulator medium however the air surrounding the conductor subjects to electro static stresses. When the potential increases still further then the atoms present around the conductor starts ionize. Then the ions produced in this process repel with each other and attracts towards the conductor at high velocity which intern produces other ions by collision. The ionized air surrounding the conductor acts as a virtual conductor and increases the effective diameter of the power conductor. Further increase in the potential difference in the transmission lines then a faint luminous glow of violet colour appears together along with hissing noise. This phenomenon is called virtual corona and followed by production of ozone gas which can be detected by the odor. Still further increase in the potential between the power conductors makes the insulating medium present between the power conductors to start conducting and reaches a voltage (Critical Breakdown Voltage) where the insulating air medium acts as conducting medium results in breakdown of the insulating medium and flash over is observed. All this above said phenomenon constitutes CORONA DISCHARGE EFFECT in electrical Transmission lines. (59) Factors Affecting Corona Discharge Effect: Corona Discharge Effect occurs because of ionization if the atmospheric air surrounding the voltage conductors, so Corona Discharge Effect is affected by the physical state of the atmosphere as well as by the condition of the lines. (1) Conductor: Corona Discharge Effect is considerably affected by the shape, size and surface conditions of the conductor Corona Discharge Effect decreases with increases in the size (diameter) of the conductor, this effect is less for the conductors having round conductors compared to flat conductors and Corona Discharge Effect is concentrated on that places more where the conductor surface is not smooth. (2) Line Voltage:
Corona Discharge effect is not present when the applied line voltages are less. When the Voltage of the system increases (In EHV system) corona Effect will be more. (3) Atmosphere: Breakdown voltage directly proportional to the density of the atmosphere present in between the power conductors. In a stormy weather the ions present around the conductor is higher than normal weather condition. So Corona Breakdown voltage occurs at low voltages in the stormy weather condition compared to normal conditions (4) Spacing between the Conductors: Electro static stresses are reduced with increase in the spacing between the conductors. Corona Discharge Effect takes place at much higher voltage when the distance between the power conductors increases. (60) Methods to reduce Corona Discharge Effect: Critical Breakdown voltage can be increased by following factors By increasing the spacing between the conductors: Corona Discharge Effect can be reduced by increasing the clearance spacing between the phases of the transmission lines. However increase in the phases results in heavier metal supports. Cost and Space requirement increases. By increasing the diameter of the conductor: Diameter of the conductor can be increased to reduce the corona discharge effect. By using hollow conductors corona discharge effect can be improved. By using Bundled Conductors: By using Bundled Conductors also corona effect can be reduced this is because bundled conductors will have much higher effective diameter compared to the normal conductors. By Using Corona Rings or Grading Rings: This is of having no greater significance but i presented here to understand the Corona Ring in the Power system. Corona Rings or Grading RIngs are present on the surge arresters to equally distribute the potential along the Surge Arresters or Lightning Arresters which are present near the Substation and in the Transmission lines. (61) What is electrical corona? Corona is the ionization of the nitrogen in the air, caused by an intense electrical field. Electrical corona can be distinguished from arcing in that corona starts and stops at essentially the same voltage and is invisible during the day and requires darkness to see at night. Arcing starts at a voltage and stops at a voltage about 50% lower and is visible to the naked eye day or night if the gap is large enough (about 5/8 at 3500 volts). (61). What are the indications of electrical corona? A sizzling audible sound, ozone, nitric acid (in the presence of moisture in the air) that accumulates as a white or dirty powder, light (strongest emission in ultraviolet and weaker into visible and near infrared) that can be seen with the naked eye in darkness, ultraviolet cameras, and daylight corona cameras using the solar-blind wavelengths on earth created by the shielding ozone layer surrounding the earth. (62) What damage does corona do? The accumulation of the nitric acid and micro-arcing within it create carbon tracks across insulating materials. Corona can also contribute to the chemical soup destruction of insulating cements on insulators resulting in internal flash-over.
The corona is the only indication. Defects in insulating materials that create an intense electrical field can over time result in corona that creates punctures, carbon tracks and obvious discoloration of NCI insulators. (63) How long does corona require creating visible damage? In a specific substation the corona ring was mistakenly installed backwards on a temporary 500kV NCI insulator, at the end of two years the NCI insulator was replaced because 1/3 of the insulator was white and the remaining 2/3 was grey. (64) What voltage are corona rings typically installed at? It varies depending upon the configuration of the insulators and the type of insulator, NCI normally start at 160kV, pin and cap can vary starting at 220kV or 345kV depending upon your engineering tolerances and insulators in the strings. (65) What causes flash-over? Flash-over causes are not always easily explained, can be cumulative or stepping stone like, and usually result in an outage and destruction. The first flash-over components are available voltage and the configuration of the energized parts, corona may be present in many areas where the flash-over occurs, flash-over can be excited by stepping stone defects in the insulating path. (66) How to test insulators? Always remember to practice safety procedures for the flash-over voltage distance and use a sturdy enclosure to contain an insulator that may shatter, due to steam build-up from moisture in a cavity, arcing produces intense heat, an AM radio is a good RFI/arcing detection device, a bucket truck AC dielectric test set (130KV) is a good test set for most pin and cap type insulators. A recent article said the DC voltage required to “search out defects can be 1.9 times the AC voltage. Insulators have a normal operating voltage and a flash-over voltage. Insulators can have internal flash-over that are/are not present at normal operating voltage. If the RFI is present, de-energize the insulator (line) and if the RFI goes away, suspect the insulator (line). Then there can be insulators that have arcing start when capacitor or other transients happen, stop when the line is de-energized or dropped below 50% of arc ignition voltage. Using a meg-ohm-meter can eliminate defective insulators that will immediately arc-over tripping the test set current overload. (67) Conductor corona is caused by? Corona on a conductor can be due to conductor configuration (design) such as diameter too small for the applied voltage will have corona year-around and extreme losses during wet weather, the opposite occurs during dry weather as the corona produces nitric acid which accumulates and destroys the steel reinforcing cable (ACSR) resulting in the line dropping. Road salts and contaminants can also contribute to starting this deterioration. (68) What is flash-over and arcing? Flash-over is an instantaneous event where the voltage exceeds the breakdown potential of the air but does not have the current available to sustain an arc, an arc can have the grid fault current behind it and sustain until the voltage decreases below 50% or until a protective device opens. Flash-over can also occur due to induced voltages in unbounded (loose bolts, washers, etc) power pole or substation hardware, this can create RFI/TVI or radio/TV interference. Arcing can begin at 5 volts on a printed circuit board or as the insulation increases it may require 80kVAC to create flash-over on a good cap and pin insulator. (69) What are causes of insulator failure? Electrical field intensity producing corona on contaminated areas, water droplets, icicles, corona rings, … This corona activity then contributes nitric acid to form a chemical soup to change the bonding cements and to create
carbon tracks, along with ozone and ultraviolet light to change the properties of NCI insulator coverings. Other detrimental effects include water on the surface or sub-surface freezing and expanding when thawing, as a liquid penetrating into a material and then a sudden temperature change causes change of state to a gas and rapid expansion causing fracture or rupture of the material. (70) Causes of Corona Corona is causes by the following reasons: The natural electric field caused by the flow of electrons in the conductor. Interaction with surrounding air. Poor or no insulation is not a major cause but increases corona. The use of D.C (Direct Current) for transmission.(Reason why most transmission is done in form of AC) (71) Effects of Corona 1.
Line Loss – Loss of energy because some energy is used up to cause vibration of the air particles.
Long term exposure to these radiations may not be good to health (yet to be proven).
Electromagnetic Interference to telecommunication systems
Ozone Gas production
Damage to insulation of conductor.
(72) Minimizing Corona Effects Installing corona rings at the end of transmission lines. A corona ring, also called anti-corona ring, is a toroid of (typically) conductive material located in the vicinity of a terminal of a high voltage device. It is electrically insulated. Stacks of more spaced rings are often used. The role of the corona ring is to distribute the electric field gradient and lower its maximum values below the corona threshold, preventing the corona discharge. (73) Types of grounding system which are: There are primarily three types of grounding system which are: (1)Solid grounding – The neutral point of the system is grounded without any resistance. If the ground fault occurs, high ground current passes through the fault. Its use is very common in low voltage system, where line to neutral voltage is used for single phase loads. (2) Low Resistance grounding (LRG) – This is used for limiting the ground fault current to minimize the impact of the fault current to the system. In this case, the system trips for the ground fault. In this system, the use of line to neutral (single phase) is prohibited. The ground fault current is limited to in the rage from 25A to 600A. (3) High Resistance Grounding (HRG) – It is used where service continuity is vital, such as process plant motors. With HRG, the neutral is grounded through a high resistance so that very small current flows to the ground if ground fault occurs. In the case of ground fault of one phase, the faulty phase goes to the ground potential but the system doesn’t trip. This system must have a ground fault monitoring system. The use of line to neutral (single phase) is prohibited (NEC, 250.36(3)) in HRG system, however, phase to neutral is used with using the additional transformer having its neutral grounded. When ground fault occurs in HRG system, the monitoring systems gives alarm and the plant operators start the standby motor and stop the faulty one for the maintenance. This way, the process plant is not interrupted. The ground fault current is limited to 10A or less. There are other two types such as Corner Grounding (for Delta system) and Ungrounded system but they are not commonly used. (74) Why the up to dia 70mm² live conductor,the earth cable must be same size ?Above dia 70mm² live conductor the earth conductor need to be only dia 70mm² ? The current carrying capacity of a cable refers to it carrying a continuous load.
An earth cable normally carries no load, and under fault conditions will carry a significant instantaneous current but only for a short time – most Regulations define 0.1 to 5 sec – before the fuse or breaker trips. Its size therefore is defined by different calculating parameters. Broadly speaking however the magnitude of earth fault current depends on: (a) the external earth loop impedance of the installation (i.e. beyond the supply terminals) (b) the impedance of the active conductor in fault (c) the impedance of the earth cable. i.e. Fault current = voltage / a + b + c Now when the active conductor (b) is small, its impedance is much more than (a), so the earth (c) cable is sized to match. As the active conductor gets bigger, its impedance drops significantly below that of the external earth loop impedance (a); when quite large (here in NZ above 120mm2, but in your region apparently 70mm2) its impedance can be ignored. At this point there is no merit in increasing the earth cable size i.e. Fault current = voltage / a + c (c) is also very small so the fault current peaks out. The neutral conductor is a separate issue. It is defined as an active conductor and therefore must be sized for continuous full load. In a 3-phase system, if balanced, no neutral current flows. It used to be common practice to install reduced neutral supplies, and cables are available with say half-size neutrals (remember a neutral is always necessary to provide single phase voltages). However the increasing use of non-linear loads which produce harmonics has made this practice dangerous, so for example the current NZ Regs require full size neutrals. Indeed, in big UPS installations I install double neutrals and earths for this reason. (75) The difference between Ground and Neutral? NEUTRAL is the origin of all current flow. In a poly-phase system, as it’s phase relationship with all the three phases is the same, (i.e.) as it is not biased towards any one phase, thus remaining neutral, that’s why it is called neutral. Whereas, GROUND is the EARTH on which we stand. It was perceived to utilise this vast, omnipresent conductor of electricity, in case of fault, so that the fault current returns to the source neutral through this conductor given by nature which is available free of cost. If earth is not used for this purpose, then one has to lay a long. long metallic conductor for the purpose, thus increasing the cost. Ground should never be used as neutral. The protection devices (eg ELCB, RCD etc) work basically on principle that the phase currects are balanced with neutral current. In case you use ground wire as the neutral, these are bound to trip if they are there – and they must be there. at least at substations. And these are kept very sensitive ie even minute currents are supposed to trip these. One aspect is safety – when some one touches a neutral, you don’t want him to be electrocuted – do you? Usually if you see the switches at home are on the phase and not neutral (except at the MCB stage). Any one assumes the once the switch is off, it is safe (the safety is taken care of in 3 wire system, but again most of the fixtures are on 2 wire) – he will be shocked at the accidental touching of wire in case the floating neutral is floating too much. (76) What is impedance of a transformer? If you mean the percentage impedance of the transformed it means the ratio of the voltage( that if you applied it to one side of the transformer while the other side of the transformer is short cuitcuted, a full load current shall flow in the the short ciruted side), to the full load current.
More the %Z of transformer, more Copper used for winding, increasing cost of the unit. But short circuit levels will reduce, mechanical damages to windings during short circuit shall also reduce. However, cost increases significantly with increase in %Z. Lower %Z means economical designs. But short circuit fault levels shall increase tremendously, damaging the winding & core. The high value of %Z helps to reduce short circuit current but it causes more voltage dip for motor starting and more voltage regulation (% change of voltage variation) from no load to full load. (77)How to measure Transformer Impedance? Follow the steps below: (1) Short the secondary side of the transformer with current measuring devices (Ammeter) (2) Apply low voltage in primary side and increase the voltage so that the secondary current is the rated secondary current of the transformer. Measure the primary voltage (V1). (3) Divide the V1 by the rated primary voltage of the transformer and multiply by 100. This value is the percentage impedance of the transformer. When we divide the primary voltage V1 with the full load voltage we will get the short circuit impedance of the transformer with refereed to primary or Z01. For getting the percentage impedance we need to use the formula = Z01*Transformer MVA /(Square of Primary line voltage). (78) PCC Incomers & Bus Couplers are normally 4-Pole./ When is Neutral Isolation Required? Neutral Isolation is mandatory when you have a Mains Supply Source and a Stand-by Power Supply Source. This is necessary because if you do not have neutral isolation and the neutrals of both the sources are linked, then when only one source is feeding and the other source is OFF, during an earth fault, the potential of the OFF Source’s Neutral with respect to earth will increase, which might harm any maintenance personnel working on the OFF source. It is for this reason that PCC Incomers & Bus Couplers are normally 4-Pole. (Note that only either the incomer or the buscoupler need to be 4-pole and not both). 3pole or 4pole switches are used in changing over two independant sources ,where the neutral of one ssource and the neutral of another source should notmix,.the examples are electricity board power supply and standalone generator supply etc. the neutral return current from one source should not mix with or return to another source. as a mandatory point the neutral of any transformer etc are to be earthed, similarly the neutral of a generator also has to be earthed. While paralling (under uncontrolled condition) the neutral current between the 2 sources will criss cross and create tripping of anyone source breakers. also as per IEC standard the neutral of a distribution system shall not be earthed more than once.means earthing the neutral further downstream is not correct, (79) why 3No of Current transformer in 3 phase Star point is grounded. For CT’s either you use for 3 phase or 2 phase or even if you use only 1 CT’s for the Overcurrent Protection or for the Earth Faults Protection, their neutral point is always shorted to earth. This is NOT as what you explain as above but actually it is for the safety of the CT’s when the current is passing thru the CT’s. In generally, tripping of Earth faults and Overcurrent Protection has nothing to do with the earthing the neutral of the CT’s. Even these CT’s are not Grounded or Earthed, these Overcurrent and the Earth Faults Protection Relay still can operated. Operating of the Overcurrent Protection and the Earth Fauts Relays are by the Kichoff Law Principle where the total current flowing into the points is equal to the total of current flowing out from the point.
Therefore, for the earth faults protection relays operating, it is that, if the total current flowing in to the CT’s is NOT equal total current flowing back out of the CT’s then with the differeces of the leakage current, the Earth Faults Relays will operated. (80) Power Transformer Neurtal Earthing The following points need to check before goint for Neutral Grounding Resistance. Fault current passing through groung, step and touch potential. Capacity of transformer to sustain ground fault current, w.r.t winding, core burning. Manufacturer shall be able to give this data. Relay co-ordination and fault clearing time. Standard practice of limiting earth fault current. In case no data or calculation is possible, go for limiting E/F current to 300A or 500A, depending on sensivity of relay. (81) why a neutral grounding contactor is needed in diseal generator? There would not be any current flow in neutral if DG is loaded equally in 3 phases , if there any fault(earth fault or over load) in any one of the phase ,then there will be un balanced load in DG . at that time heavy current flow through the neutral ,it is sensed by CT and trips the DG. so neutral in grounded to give low resistance path to fault current. An electrical system consisting of more than two low voltage Diesel Generator sets intended for parallel operation shall meet the following conditions: (i) neutral of only one generator needs to be earthed to avoid the flow of zero sequence current. (ii) during independent operation, neutrals of both generators are required in low voltage switchboard to obtain three phase, 4 wire system including phase to neutral voltage. (iii) required to achieve restricted earth fault protection (REF) for both the generators whilst in operation. Solution: Considering the requirement of earthing neutral of only one generator, a contactor of suitable rating shall be provided in neutral to earth circuit of each generator. This contactor can be termed as “neutral contactor”. Neutral contactors shall be interlocked in such a way that only one contactor shall remain closed during parallel operation of generators. During independent operation of any generator its neutral contactor shall be closed. Operation of neutral contactors shall be preferably made automatic using breaker auxiliary contacts. (82) Neutral grounded system vs solidly grounded system In India, at low volatge level (433V) you MUST do only Solid Earthing of the system neutral. This is by IE Rules 1956, Rule No. 61 (1) (a). Because, if you opt for impedance earthing, during an earth fault, there will be appreciable voltage present between the faulted body & the neutral, the magnitude of this voltage being determined by the fault current magnitude and the impedance value. This voltage might circulate enough current in a person accidentally coming in contact with the faulted equipment, as to harm his even causing death. Note that, LV systems can be handled by non-technical persons too. In solid earthing, you do not have this problem, as at the instant of an earth fault, the faulted phase goes to neutral potential and the high fault current would invariably cause the Overcurrent or short circuit protection device to opearte in sufficiently quick time before any harm could be done. (83) Difference between Restricted Earth Fault & Unrestricted Earth Fault protections? Restricted earth fault is normally given to on star connected end of power equipment like generators, transformers etc.mostly on low voltage side. for REF protection 4 no’s CTs are using one each on phase and one in neutral. It is working on the principle of balanced currents between phases and nuetral.Unrestricted E/F protection working
on the principle of comparing the unbalance on the phases only. For REF protection PX class cts are using but for UREF 5P20 Cts using. For Differential Protection CTs using on both side HT & LV side each phase, and comparing the unbalance current for this protection also PX class CTs are using. (84) Transformer tertiary winding Providing a tertiary winding for a transformer may be a costly affair. However, there are certain constraints in a system which calls for a tertiary transformer winding especially in the case of considerable harmonic levels in the distribution system. Following is an excerpt from the book “The J&P Transformer Book”. Tertiary winding is may be used for any of the following purposes: (A)To limit the fault level on the LV system by subdividing the infeed that is, double secondary transformers. (B)The interconnection of several power systems operating at different supply voltages. (C) The regulation of system voltage and of reactive power by means of a synchronous capacitor connected to the terminals of one winding. It is desirable that a three-phase transformer should have one set of three-phase windings connected in delta thus providing a low-impedance path for third-harmonic currents. The presence of a delta connected winding also allows current to circulate around the delta in the event of unbalance in the loading between phases, so that this unbalance is reduced and not so greatly fed back through the system. Since the third-order harmonic components in each phase of a three-phase system are in phase, there can be no third-order harmonic voltages between lines. The third-order harmonic component of the magnetising current must thus flow through the neutral of a star-connected winding, where the neutral of the supply and the starconnected winding are both earthed, or around any delta-connected winding. If there is no delta winding on a star/star transformer, or the neutral of the transformer and the supply are not both connected to earth, then line to earth capacitance currents in the supply system lines can supply the necessary harmonic component. If the harmonics cannot flow in any of these paths then the output voltage will contain the harmonic distortion. Even if the neutral of the supply and the star-connected winding are both earthed, then although the transformer output waveform will be undistorted, the circulating third-order harmonic currents flowing in the neutral can cause interference with telecommunications circuits and other electronic equipment as well as unacceptable heating in any liquid neutral earthing resistors, so this provides an added reason for the use of a delta connected tertiary winding. If the neutral of the star-connected winding is unearthed then, without the use of a delta tertiary, this neutral point can oscillate above and below earth at a voltage equal in magnitude to the third-order harmonic component. Because the use of a delta tertiary prevents this it is sometimes referred to as a stabilizing winding. When specifying a transformer which is to have a tertiary the intending purchaser should ideally provide sufficient information to enable the transformer designer to determine the worst possible external fault currents that may flow in service. This information (which should include the system characteristics and details of the earthing arrangements) together with a knowledge of the impedance values between the various windings, will permit an accurate assessment to be made of the fault currents and of the magnitude of currents that will flow in the tertiary winding. This is far preferable to the purchaser arbitrarily specifying a rating of, say, 33.3%, of that of the main windings. (85) What is a transformer and how does it work? A transformer is an electrical apparatus designed to convert alternating current from one voltage to another. It can be designed to “step up” or “step down” voltages and works on the magnetic induction principle. A transformer has no moving parts and is a completely static solid state device, which insures under normal conditions, a long
and trouble-free life. It consists, in it’s simplest form, of two or more coils of insulated wire wound on a laminated steel core. When voltage is introduced to one coil, called the primary, it magnetizes the iron core. A voltage is then induced in the other coil, called the secondary or output coil. The change of voltage (or voltage ratio) between the primary and secondary depends on the turns ratio of the two coils. (86) Why do transformers hum? Transformer noise is caused by a phenomenon which causes a piece of magnetic sheet steel to extend itself when magnetized. When the magnetization is taken away, it goes back to its original condition. This phenomenon is scientifically referred to as magnetostriction. A transformer is magnetically excited by an alternating voltage and current so that it becomes extended and contracted twice during a full cycle of magnetization. The magnetization of any given point on the sheet varies, so the extension and contraction is not uniform. A transformer core is made from many sheets of special steel to reduce losses and moderate the ensuing heating effect. The extensions and contractions are taking place erratically all over a sheet and each sheet is behaving erratically with respect to its neighbor, so you can see what a moving, writhing construction it is when excited. These extensions are miniscule proportionally and therefore not normally visible to the naked eye. However, they are sufficient to cause a vibration, and consequently noise. Applying voltage to a transformer produces a magnetic flux, or magnetic lines of force in the core. The degree of flux determines the amount of magnetostriction and hence, the noise level. Why not reduce the noise in the core by reducing the amount of flux? Transformer voltages are fixed by system requirements. The ratio of these voltages to the number of turns in the winding determines the amount of magnetization. This ratio of voltage to turns is determined mainly for economical soundness. Therefore the amount of flux at the normal voltage is fixed. This also fixes the level of noise and vibration. Also, increasing (or decreasing) magnetization does not affect the magnetostriction equivalently. In technical terms the relationship is not linear. (87) How can I reduce airborne noise? Below is a list of your most effective options: (1)Put the transformer in a room in which the walls and floor are massive enough to reduce the noise to a person listening on the other side. Noise is usually reduced (attenuated) as it tries to pass through a massive wall. Walls can be of brick, steel, concrete, lead, or most other dense building materials. (2)Put the object inside an enclosure which uses a limp wall technique. This is a method which uses two thin plates separated by viscous (rubbery) material. As the noise hits the inner sheet some of its energy is used up inside the viscous material. The outer sheet should not vibrate. (3)Build a screen wall around the unit. This is cheaper than a full room. It will reduce the noise to those near the wall, but the noise will get over the screen and fall elsewhere (at a lower level). Screens have been made from wood, concrete, brick and with dense bushes (although the latter becomes psychological) (4)Do not make any reflecting surface coincident with half the wave length of the frequency. What does this mean? Well, every frequency has a wave length. To find the wave length in air, for instance, you divide the speed of sound, in air (generally understood as 1130 feet per second) by the frequency. If a noise hits a reflecting surface at these dimensions it will produce what is called a standing wave. Standing waves will cause reverberations (echoes) and an increase in the sound level. If you hit these dimensions and get echoes you should apply absorbent materials to the offending walls (fiberglass, wool, etc.) (88) What are taps and when are they used?
Taps are provided on some transformers on the high voltage winding to correct for high or low voltage conditions, and still deliver full rated output voltages at the secondary terminals. Taps are generally set at two and a half and five percent above and below the rated primary voltage. (89) What is the difference between “Insulating”, “Isolating”, and “Shielded Winding” transformers? Insulating and isolating transformers are identical. These terms are used to describe the separation of the primary and secondary windings. A shielded transformer includes a metallic shield between the primary and secondary windings to attenuate (lessen) transient noise. (90) Can transformers be operated at voltages other than nameplate voltages? In some cases, transformers can be operated at voltages below the nameplate rated voltage. In NO case should a transformer be operated in excess of its nameplate rating unless taps are provided for this purpose. When operating below the rated voltage the KVA capacity is reduced correspondingly. (91) Can 60 Hz transformers be operated at 50 Hz? Transformers 1 KVA and larger, rated at 60 Hz, should not be used on 50 Hz service due to higher losses and resultant heat rise. However, any 50 Hz transformer will operate on 60 Hz service. (92) Can transformers be used in parallel? Single phase transformers can be used in parallel only when their voltages are equal. If unequal voltages are used, a circulating current exists in the closed network between the two transformers which will cause excess heating and result in a shorter life of the transformer. In addition impedance values of each transformer must be within 7.5% of each other. (93) Can MGM Transformers be reverse connected? MGM dry type distribution transformers can be reverse connected without a loss of KVA rating, but there are certain limitations. Transformers rated 1 KVA and larger single phase, 3 KVA and larger three phase can be reverse connected without any adverse effects or loss in KVA capacity. (94) Why do I need a bigger breaker when reverse feeding a transformer? Typically the output winding is wound first and is therefore closest to the core. When used as exciting winding a higher inrush current results. In most cases the inrush current is 10 to 12 times the full load current for 1/10 of a second. When the transformer is reverse fed the inrush current can be up to 16 times greater. In this case a bigger breaker with a higher AIC rating must be used to keep the transformer online. (95) Do taps work the same when a transformer is reverse fed? Taps are normally in the primary winding to adjust for varying incoming voltage. If the transformer is reverse fed, the taps are on the output side and can be used to adjust the output voltage. (96) Why may I get the wrong output voltage when installing a step up transformer? Transformer terminals are marked according to high and low voltage connections. An H terminal signifies a high voltage connection while an X terminal signifies a lower voltage connection. A common misconception is that H terminals are primary and X terminals secondary. This is true for step down transformers, but in a step up transformer the connections should be reversed. Low voltage primary would connect to X terminals while high voltage secondary would connect on the H terminals. (97) Can a single phase transformer be used on a three phase source? Yes. Any single phase transformer can be used on a three phase source by connecting the primary leads to any two wires of a three phase system, regardless of whether the source is three phase 3-wire or three phase 4-wire. The transformer output will be single phase. (98) Can transformers develop three phase power from a single phase source?
No. Phase converters or phase shifting devices such as reactors and capacitors are required to convert single phase power to three phase. (99) What is meant by regulation in a transformer? Voltage regulation in transformers is the difference between the no load voltage and the full load voltage. This is usually expressed in terms of percentage. (100) what is temperature rise in a transformer? Temperature rise in a transformer is the average temperature of the windings and insulation above the existing ambient temperature. (101) what is “Class” in insulation? Insulation class was the original method used to distinguish insulating materials operating at different temperature levels. Letters were used for different designations. Letter classifications have been replaced by insulation system temperatures in degrees celsius. The system temperature is the maximum temperature at the hottest spot in the winding. (102) Are temperature rise and actual surface temperature related? No. This can be compared with an ordinary light bulb. The filament temperature of a light bulb can exceed 2000 degrees yet the surface temperature of the bulb is low enough to permit touching with bare hands. (103) What is meant by impedance in transformers? Impedance is the current limiting characteristic of a transformer and is expressed in percentage. (104) Why is impedance important? It is used for determining the interrupting capacity of a circuit breaker or fuse employed to protect the primary of a transformer. (105) Can single phase transformers be used for three phase applications? Yes. Three phase transformers are sometimes not readily available whereas single phase transformers can generally be found in stock. Three single phase transformers can be used in delta connected primary and wye or delta connected secondary. They should never be connected wye primary to wye secondary, since this will result in unstable secondary voltage. The equivalent three phase capacity when properly connected of three single phase transformers is three times the nameplate rating of each single phase transformer. (106) What is BIL and how does it apply to transformers? BIL is an abbreviation for Basic Impulse Level. Impulse tests are dielectric tests that consist of the application of a high frequency steep wave front voltage between windings, and between windings and ground. The BIL of a transformer is a method of expressing the voltage surge that a transformer will tolerate without breakdown. (107) What is polarity, when associated with a transformer? Polarity is the instantaneous voltage obtained from the primary winding in relation to the secondary winding. Transformers 600 volts and below are normally connected in additive polarity. This leaves one high voltage and one low voltage terminal unconnected. When the transformer is excited, the resultant voltage appearing across a voltmeter will be the sum of the high and low voltage windings. This is useful when connecting single phase transformers in parallel for three phase operations. Polarity is a term used only with single phase transformers. (108) What is exciting current? Exciting current is the current or amperes required for excitation. The exciting current on most lighting and power transformers varies from approximately 10% on small sizes of about 1 KVA and less to approximately 2% on larger sizes of 750 KVA. (109) Can air cooled transformers be applied to motor loads?
This is an excellent application for air cooled transformers. Even though the inrush or starting current is about 5 to 7 times normal running current, the resultant lower voltage caused by this momentary overloading is actually beneficial in that a cushioning effect on motor starting is the result. (110) Can a three phase transformer be loaded as a single phase transformer? Yes, but the load can not exceed the rating per phase and the load must be balanced. (KVA/3 per phase) For example: A 75 kVA 3 phase transformer can be loaded up to 25 kVA on each secondary. If you need a 30 kVA load, 10 kVA of load should be supplied from each secondary. (111) How many BTU’s of heat does a transformer generate? The heat a transformer generates is dependent upon the transformer losses. To determine air conditioning requirements multiply the sum of the full load losses (obtained from factory or test report) of all transformers in the room by 3.41 to obtain the BTUs/hour. For example: A transformer with losses of 2000 watts will generate 6820 BTUs/hour.