Electrical Questions and Answers Part-4
(1) When Neutral Isolation is required? Or PCC Incomers & Bus Couplers are normally 4-Pole.- 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 bus coupler needs to be 4-pole and not both).
- 3pole or 4pole switches are used in changing over two independent sources, where the neutral of one source and the neutral of another source should not mix.
- 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 cross and create tripping of anyone source breaker.
- 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.
- In India, at low voltage 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 Over current or short circuit protection device to operate in sufficiently quick time before any harm could be done.
- 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. Tertiary winding is may be used for any of the following purposes:
- 1) To limit the fault level on the LV system by subdividing the in feed that is, double secondary transformers.
- 2) The interconnection of several power systems operating at different supply voltages.
- 3) The regulation of system voltage and of reactive power by means of a synchronous capacitor connected to the terminals of one winding.
- To prevent a broken upper line from falling on one or more of the phase lines below.
- The clearance from other phase.
- Mutual inductance minimization.
- Preventing droplet of water/ice to fall on bottom conductor.
- To prevent big birds from bumping their heads against the overhead line when they sit on the wire below.
- 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.
- Neutral is connected to earth at some point, thus it has some value as a return path in the event of say and equipment earth being faulty. It’s a bit like asking ‘why don’t we break the Earth connection’
- It was stupid and dangerous, as it was possible for the neutral fuse to blow; giving the appearance of ‘no power’ when in fact the equipment was still live.
- Motor Insulation Resistance:
- The acceptable meg-ohm value = motor KV rating value + 1 (For LV and MV Motor).
- Example, for a 5 KV motor, the minimum phase to ground (motor body) insulation is 5 + 1 = 6 meg-ohm.
- Panel Bus Insulation Resistance:
- The acceptable meg-ohm value = 2 x KV rating of the panel.
- Example, for a 5 KV panel, the minimum insulation is 2 x 5 = 10 meg-ohm
- IEEE 43 – INSULATION RESISTANCE AND POLARIZATION INDEX (min IR at 400C in MΩ)
| Minimum Insulation Resistance | TEST SPECIMEN |
| R1 min = kV+1 R1 min = 100 | For most windings made before about 1970, all field windings, and others not described below For most dc armature and ac windings built after about 1970 (form wound coils) |
| R1 min = 5 | For most machines with random -wound stator coils and form-wound coils rated below 1kV |
- 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 catenary (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 catenary 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 meters, with the electrode some 2 to 3 meters 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.
- Location
- Country
- Site soil type and resistivity
- Number of Lightning Days
- Expected Voltage + Current of a local lightning strike
- Other – Advise please.
- Service factor is the load that may be applied to a motor without exceeding allowed ratings. For example, if a 10-hp 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.
- 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.
- The earthing must have minimum resistance. yes for small trans. if neutral is directly earthed then the same earthing point can be used for body earthing too, but in case of big size trans. where neutral is grounded through impedance or resistance, the length of body earthing increases that time, so better to use separate earthing pit which will provide minimum resistance path for body,
- You can also ensure that the resistance between two different earthing pits in your company is having almost negligible resistance, so both are redundant.
- In India It is mandatory as per IE rules that u have 2 neutral earth pits exclusively which can also be connected to the earth grid .Body earthing needs separate pits. Earthing does not receive as much attention as one would desire. It may never be in use in its life time. But if and when a fault occurs it should be in condition.
- 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.
- 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 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 require full size neutrals. Indeed, in big UPS installations need to install double neutrals and earths for this reason.
- NEUTRAL is the origin of all current flow. In a poly-phase system, as its 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 utilize 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 (e.g. ELCB, RCD etc) work basically on principle that the phase currents 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 i.e. even minute currents are supposed to trip these.
- One aspect is safety – when someone touches a neutral, you do not want him to be electrocuted.
- Usually if 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.
- 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 circuited, a full load current shall flow in the the short circuited 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.
- 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).
- There won’t 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 phases, 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.
- For high impedance relays (differential or restricted earth fault relays), ‘Class X’ current transformers are recommended to be used.
- Please note that both CTs (neutral & phase) shall have the same characteristics. The following is an example to size the CT:
- Input data:
2. Transformer % impedance (Z): 6% (assumed);
3. Length of cable from neutral CT to the relay located at LVSP: 200 m (given);
4. Cross section of CT cable to be used: 6 mm² -copper (assumed);
5. CT cable resistance: 0.0032 Ω/m (from manufacturer data).
- Step 1: Calculation of CT Rated Primary Current
2. Select the secondary current of the CT 1 or 5 A. selecting 1 A secondary current, as the cross section and length of pilot wires can have a significant effect on the required knee voltage of the CT and therefore the size and cost of the CT. When the relay is located some distance from the CT, the burden is increased by the resistance of the pilot wires.
- Step 2: Calculation of max. Fault Current
2. Ift = 2500/ (0.415×1.732×0.06) = 57968.59 A (rounded to 58000 A)
- Step 3: Calculation of the Knee Voltage of the CT (Vkp)
2. Where:
3. Rct: is the CT resistance (to be given by the manufacturer), for the purpose of illustration, we will use a typical Rct value in this example (1.02 Ω) given by one of the CT manufacturers.
4. Rw: total CT cable resistance= 2x cable length (200 m) x wire resistance= 2x200x0.0032= 1.28 Ω
5. CT transformation ratio = CT Primary Current/CT Secondary Current
6. CT transformation ratio = 4000/5= 800 A, for CT with 5 A secondary current; or,
7. CT transformation ratio = 4000/1= 4000 A, for CT with 1 A secondary current. We will use 1 A in this example.
8. Vkp = (2x58000x (1.02+1.28)/4000)= 66.7 V
- The Vkp of the CT should be higher than the setting of relay stability voltage (Vs), to ensure stability of the protection during max.
- Through fault current. To calculate the stability voltage, you should follow the related formula given by the relay manufacturer, as each relay manufacturer has its own formula.
- You may calculate the Vkp as above using a CT with secondary current of 5 A, and you will notice the difference in the Vkp.
- In section 1.4.2, IEEE Std 142-1991 (Green Book) states: “Numerous advantages are attributed to grounded systems, including greater safety, freedom from excessive system overvoltage that can occur on ungrounded systems during arcing, resonant or near-resonant ground faults, and easier detection and location of ground faults when they do occur.”
- If you effectively ground the neutral, you have just replaced the hazards associated with ungrounded systems with new hazards in the form of arc flash / blast hazards, which are associated with solidly grounded systems.
- In section 7.2.4, IEEE Std 141-1993 (Red Book) states: “A safety hazard exists for solidly grounded systems from the severe flash, arc burning, and blast hazard from any phase-to-ground fault.” For this reason, IEEE Std 142-1991 (Green Book), in section 1.4.3, states the benefits of resistance grounding: “The reasons for limiting the current by resistance grounding may be one or more of the following:
- (1) To reduce burning and melting effects in faulted electric equipment, such as switchgear, transformers, cables, and rotating machines.
- (2) To reduce mechanical stresses in circuits and apparatus carrying fault currents.
- (3) To reduce electric-shock hazards to personnel caused by stray ground-fault currents in the ground return path.
- (4) To reduce the arc blast or flash hazard to personnel who may have accidentally caused or who happen to be in close proximity to the ground fault.
- (5) To reduce the momentary line-voltage dip occasioned by the clearing of a ground fault.
- (6) To secure control of transient overvoltage while at the same time avoiding the shutdown of a faulty circuit on the occurrence of the first ground fault (high-resistance grounding).”
- It’s best to not only ground the neutral, but also to ground through high-resistance (typically 5A) for all systems less than 600V and most systems from 600V to 5kV. For systems more than 5kV, low-resistance grounding (typically 200A or 400A) is used.
- By “not grounding,” let’s assume to an ungrounded system. Ungrounded systems can be extremely unsafe, as per our previously cited excerpts from the IEEE Buff and Red Books. In bonding your neutral to the grounded chassis, you are effectively solidly grounding your generator. The consequence of doing this is that you have now replaced hazards. By limiting the ground fault to 5A, you have avoided the hazards with solidly grounded systems.
- In addition, several generator set manufacturers require resistance grounding, since the generators are not rated for ground faults. In fact, these faults are often significantly higher than 3-phase faults. In section 1.8.1, the IEEE Green Book states: “Unlike a transformer … a generator will usually have higher initial ground-fault current than 3-phase fault current if the generator has a solidly grounded neutral. According to NEMA, the generator is required to withstand only the 3-phase current level unless it is otherwise specified …”
- This is due to very low zero-sequence impedance within the generator, which causes very high earth fault currents. For generators 600V or below, this may not be an issue. However, it is almost always an issue as the voltage class increases.
- The resistor also significantly reduces any circulating currents, which are typically triple harmonics, leading to reduced overheating in the generator windings. Circulating currents are caused by different pitch windings in generators.
- For solidly grounded systems, neutral switching is a viable option. According to the NEC, when the service falls under the requirements of 230.95, you should ground the neutral at each source and switch it where the Code requires ground-fault detection coordination.
- When the service rating equals or exceeds 1,000A (833kVA), 230.95 requires ground-fault protection on the service disconnect. Along with this, if you have an alternate power supply, you must switch the neutral. If you have a service larger than 1,000A, the NEC requires ground-fault protection at the main service disconnect.
- If the generator neutral grounding runs via a solid connection to the main service neutral and the generator experiences a ground fault while feeding the load, the main service disconnect will open. This will not disconnect the arc fault from the generator, and coordination will be lost.
- Furthermore, if the neutrals of the two sources are separately grounded, you must switch the load neutral conductor to the source feeding the load, as per 230.95(C) FPN No. 3. Ground-fault current will return only to the source from which it originates, providing for coordination of the ground-fault protection scheme. It’s not always necessary to separately ground the generator neutral conductor. However, if you do, you may need to switch a load neutral along with its phase conductors when transferring loads between power sources, particularly when you use ground-fault protection. The NEC requires ground-fault protection for 480/277V, 3-phase, 4-wire, wye-connected services rated 1,000A or more, but it’s optional in other configurations that don’t include ground-fault protection. However, where a branch circuit neutral conductor transfers between sources, the switching means should assure the neutral conductor switching contact does not interrupt current.
- 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. On the type of lamp.
- 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 something 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.
- First of all MCB is Miniature Circuit Breaker, since it is miniature it has limitation for Short Circuit Current and Amp Rating Both.
- MCB:
- MCB are available as Singe module and used for :-
- Number of Pole :- 1,2,3,4 – 1+ N , & 3 + N
- Usually Current range for A.C. 50-60 HZ, is from 0.5 Amp – 63 Amp. Also available 80A, 100A, and 125 Amp.
- SC are limited 10 KA
- Applications are as: – Industrial, Commercial and Residential application.
- Tripping Curve: (1) B Resistive and lighting load, (2) C Motor Load, (3) D Highly inductive load.
- MCCB: – Moulded Case Circuit Breaker.
- MCCB:
- MCCB are available as Singe module and used for :-
- Number of Pole :- 3 pole , & 4 Pole
- Usually Current range for A.C., 50-60 HZ, are from 3.2 /6.3/12.5/25/50/100/125/ 160 Amp. – SC 25/35/65 KA.
- 200 250 Amp. – SC 25/35/65 KA
- 400 630/800 Amp – SC 50 KA
- Protection release :-Static Trip :- Continuous adjustable overload protection range 50 to 100 % of the rated current Earth fault protection can be add on with adjustable earth fault pick up setting 15 to 80 % of the current.
- Micro processor Based release:
- Over load rated current 0.4 – 1.0 in steps of o.1 of in trip time at 600 % Ir (sec) 0.2.0.5,1, 1.5 , 2 ,3
- Short Circuit :-2 – 10 in steps of 1 lr , short time delay (sec) 0.02.0.05,0.1, 0.2 ,0.3
- Instantaneous pick up ;- 2 – 10 in steps of 1 in Ground fault pick up Disable, 0.2 – 0.8 in steps of 0.1 of in Ground fault delay (sec) – 0.1 -=to 0.4 in steps of 0.1
- MCB (Miniature Circuit Breaker) Trip characteristics normally not adjustable, factory set but in case of MCCB (Moulded Case Circuit Breaker)—Trip current field adjustable.
- Most troubles with Capacitors — either open or short.
- A multi meter is good enough. A shorted C will clearly show very low resistance. An 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.
- If it is a single speed motor then you have to identify 6 leads.
- Use IR tester to identify 3 windings and their 6 leads. Then connect any two leads of two winding and apply small voltage across it and measure the current.
- Then again connect alternate windings of same two windings and apply small amount of voltage (same as before) and measure current.
- Check in which mode you get the max current and then mark it as a1-a2 & b1-b2. You get max current when a2-b1 will be connected and voltage applied between a1-b2.
- Follow the same process to identify a1-a2, b1-b2, c1-c2.now you will be able to connect it in delta or star.
- Connecting a capacitor across a generator always improves power factor, 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, do not connect a capacitor across an alternator while it is picking up or without any other load.
- 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. Tertiary
winding is may be used for any of the following purposes:
- (1) To limit the fault level on the LV system by subdividing the in feed that is double secondary T.C.
- (2) The interconnection of several power systems operating at different supply voltages.
- (3) 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 magnetizing current must thus flow through the neutral of a star-connected winding, where the neutral of the supply and the star-connected 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.
- Each bulb when independently working will have currents (W/V= I)
- 200/230=0.8696 A
- 100/230=0.4348 A
- 60/230=0.2609 A respectively
- Resistance of each bulb filament is (V/I = R)
- 230/0.8696= 264.5 ohms
- 230/0.4348 = 528.98 ohms and
- 881.6 ohms respectively
- Now, when in series, current flowing in all bulbs will be same. The energy released will be I2R
- 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 minimum current through it. So minimum power dissipated across it as min current X equal Voltage across =s min power dissipation.
- 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 neutral.
- 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.
- 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.
- The first parameter is voltage. By choosing HRG on systems 600V or below, you reduce ground-fault currents to less than 25A. For systems between 600V and 5kV, we can use either HRG or LRG arrangements.
- In either case, the resistance chosen must be such that the desired let-through ground-fault current is above the system capacitive charging current. On systems above 5kV, LRG is the typical choice in that it reduces ground-fault currents to between 25A and 1,200A. (Most people use a value of 200A or less.)
- The next parameter is system capacitive charging current. Every system has a capacitance value, mostly due to the system’s cables and surge arresters/capacitors. The general rule of thumb for estimating this charging current is to use 1A per 1,000kVA. If necessary, you can perform a more detailed calculation based on system components.
- During commissioning, you should determine this value by taking an actual measurement. Once you know the system capacitive charging current, you can select the resistor let-through current or desired current. As previously stated, the desired resistor current (IR) must be greater than the system capacitive charging current to avoid transient overvoltage. You determine the neutral grounding resistance by using the following equation: R = VL-N ÷ IR
- Where R is neutral grounding resistance, VL-N is line-to-neutral voltage, and IR is ground-fault current
- For systems less than 600V, the system capacitive charging current is typically 1A to 3A. Therefore, most people use 5A neutral current as a standard. Because this is less than 10A, all 600V systems are high-resistance grounded.
- For systems between 600V and 5kV, you can use either HRG or LRG. The decision is typically based on system capacitive charging current, which can vary from 1A to 10A. This value then determines whether to use HRG or LRG. For systems above 5kV, the system capacitive charging current may be greater than 25A, so LRG is almost always used.
- Another factor is system continuity. An HRG system allows a distribution system to continue to operate with one fault, without the faulted feeder being tripped. When the possible total earth fault current is such that an earth fault can be sustained continuously without risk of further fault damage, then the system is considered HRG. Electrical installation codes in various jurisdictions have rules governing this.
- On systems with grounding resistor let-through currents higher than 10A — or for systems of voltage ratings greater than 5kV — the faulted feeder should be tripped and the fault isolated. These are generally called LRG systems.
- You can broadly classify medium-voltage (MV) grounding systems into four categories: solidly grounded, low-resistance grounded (LRG), high-resistance grounded (HRG), and insulated neutral (ungrounded) systems. A good reference is ANSI/IEEE Std. 242 (Buff Book), “Protection and Coordination of Industrial and Commercial Power Systems.”
- 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.
- Direct or solidly grounded system showing phase-to-ground fault and ground-fault current path. The NEC requires ground-fault protection on services of more than 150V to ground, but not exceeding 600V phase-to-phase, for each service disconnect rated 1,000A or higher.
- With the solidly grounded system, there is no intentional impedance in the neutral-to-earth path. Instead, the neutral is solidly connected to earth. This is why the term “earthing” is sometimes used in place of “grounding.”
- The phase-to-ground voltage remains constant during a ground fault, and there are very high fault current flows, which can result in extensive damage. The protective device closest to the fault must trip and isolate the circuit as fast as possible.
- If the fault is in a rotating machine, then there is a high possibility of core damage and replacement costs. The cost associated with the downtime also can be significant.
- 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
- Low-resistance grounding system showing phase-to-ground fault and ground-fault current path. Ground resistor limits the magnitude of the ground-fault current.
- With LRG systems, the ground-fault current is controlled and normally limited to between 25A and 1,000A. The voltage to ground on the un-faulted phases can increase up to the phase-to-phase voltage level, so you must use adequately rated insulation systems and surge suppression devices.
- You also must detect and isolate the ground fault. Since the ground-fault current is smaller and controlled, ground-fault relaying still has the requirement of fast tripping. However, you can achieve better time current coordination with this type of grounding system. Damage at the fault point is also reduced; therefore, maintenance and repair costs are reduced.
- The neutral grounding resistor needs to be short time rated (usually 10 seconds), as the fault will be cleared by the protective relay closest to the fault.
- For high-resistance grounding, there are two options:
- (1) Install resistance grounding on each source; or
- (2) Derive a neutral on the paralleling bus via a zigzag transformer, and then add resistance grounding on the derived neutral.
- 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.
- (1) Install resistance grounding on each source :
- High-resistance grounding system showing phase-to-ground fault and ground-fault current path. Ground resistor limits the magnitude of the ground-fault current to very low levels and allows continuity of power while locating the fault.
- With an HRG system, the ground-fault current is in the 10A range. The intention here is to allow the system to operate without tripping, even with a phase-to-ground fault on one phase.
- When a ground fault does occur, only an alarm is raised. This permits time to locate the fault while power continuity is maintained. This also allows repairs to be done at a scheduled shutdown of the faulty equipment. Maintenance costs should be less than that for a low-resistance grounding system. Damage at the fault location also should be small.
- If the fault is in a rotating machine, there usually is no iron damage in the stator. The system, with one phase faulted to earth, operates with the un-faulted phases now raised from earth to the full phase-to-phase voltage for an extended period.
- As such, the insulation system needs to be rated for phase-to-phase voltage. For phase-to-ground voltages to remain at the phase-to-phase level — and not increased beyond that level — the net capacitive charging current at the fault must be less than that from the controlled resistive ground-fault current fed from the neutral grounding. The grounding resistor also needs to be continuously rated because it will carry the let-through current in the event of a fault for an extended period of time.
- (2) Derive a neutral on the paralleling bus via a zigzag transformer, and then add resistance grounding on the derived neutral.
- Zigzag transformer used to derive a neutral. Resistance grounding can then be added on the derived neutral.
- Be careful here in that you cannot use this neutral for any loads or connect it to anything except the resistor. There are advantages/disadvantages for each option. By having a resistor on each source, the total ground-fault current is dependent upon the total number of sources in operation. However, the system is always grounded. If the resistor is on the paralleling bus, the ground-fault current is always the same value however, the system is only grounded if the paralleling bus in operation. Most people choose option 2 and have a resistor on the paralleling bus
- Ungrounded system showing phase-to-ground fault and ground-fault current path. When a ground fault occurs, the fault current is contributed by the system capacitance to earth on the un-faulted phases. This is usually small, and the system can be operated without tripping.
- With the insulated neutral (ungrounded) system, there is no intentional connection of the system to ground. In effect, the three phases of the system float.
- When a ground fault occurs, the fault current is contributed by the system capacitance to earth on the un-faulted phases. This is usually small, and the system can be operated without tripping.
- Because the system is floating, if the ground fault is of the arcing or intermittent type, then there is the possibility of substantial transient overvoltage, which can be six to eight times the phase voltage.
- These transients often cause a subsequent failure elsewhere, thus raising the possibility of a phase-to-earth-to-phase fault, and leading to high fault current and extensive damage. Coordinated tripping is often difficult, and extensive damage is seen at the two faulted locations. Maintenance costs are typically the highest among the four types of grounding systems — now at least two pieces of equipment need repair.
- Ungrounded system showing phase-to-ground-to-phase fault and ground-fault current path.
- If the ground fault is an arcing or intermittent type, there is possibility of substantial transient overvoltage, which can be six to eight times the phase voltage.
- These transients often cause a subsequent failure elsewhere, thus raising the possibility of a phase-to-earth-to-phase fault.
- The standards and best practices in various countries generally follow ANSI or IEC standards. The technical literature supports these practices. In power plant applications, MV systems occur in two places: generation and station service. In practice, both station service and generators are low- or high-resistance grounded.
- For station service at distribution voltages of less than 15kV, power continuity is very important. Here, we would size the neutral grounding resistor so that the let-through ground-fault current is higher than the net current from the distributed capacitance. If the let-through ground-fault current is less than 10A, then this would be high-resistance grounding. If this current were more than 10A, then it would be low-resistance grounding.
- Although it’s rare to have station service voltage that is higher than 15kV, if the voltage is higher than this, then the same rule as noted above would apply, except the fault should be detected and isolated by tripping the faulted feeder at the closest protective device.
- For generators, the ground-fault current is almost always controlled, and we can employ resistance grounding. The resistor let-through current will be dependent on the size of the generator and generation voltage. Typically, 5A to 400A let-through current grounding is used.
- More recently, hybrid grounding has been proposed. Here, two resistors in parallel are used: one of low resistance; the other of high resistance (5A).
- In the event of an internal earth fault in the stator winding of the generator, a fast-acting generator ground differential relay opens the low-resistance grounding path, thus allowing the high resistance (5A let-through resistor) to control and lower the fault current and reducing the stator damage caused by the internal ground fault after the generator has been isolated (while it is slowing down).
- Without this reduction of current, the generator would continue to feed energy into the fault while it is coming to a stop. The result would be extensive stator iron damage at the ground-fault location.
- For CT’s either we use for 3 phases or 2 phase or even if you use only 1 CT’s for the Over current 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 throuw the CT’s.
- In generally, tripping of Earth faults and Over current Protection has nothing to do with the earthing the neutral of the CT’s. Even these CT’s are not Grounded or Earthed, these over current and the Earth Faults Protection Relay still can operated.
- Operating of the Over current Protection and the Earth Faults Relays are by the Kirchhoff 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 differences of the leakage current, the Earth Faults Relays will operated.
- The following points need to check before going for Neutral Grounding Resistance.
- (1)Fault current passing through ground, step and touch potential.
- (2) Capacity of transformer to sustain ground fault current, w.r.t winding, core burning. Manufacturer shall be able to give this data.
- (3) Relay co-ordination and fault clearing time.
- (4) 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 senility of relay.
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