Earthing Transformer PRESENTED BY PROF VG PATEL Wednesday

Earthing Transformer PRESENTED BY PROF. VG PATEL Wednesday, June 9, 2021 VG PATEL 1

TRANSFORMER ENCYCLOPAEDIA EARTHING TRANSFORMER INTRODUCTION Equipments connected to power systems and networks are operating under variable complex stresses in service. In power systems the faults are not avoidable even after taking utmost care at every stage of planning, designing, installing, commissioning and proper maintenance. It is a well-known fact that, the grounding of the circuit reduces potential stresses under conditions of faults. For a switchgear and protection engineer neutral grounding is very important because: • The earth protection is based on the method of neutral earthing; and • The system voltage during earth fault depends on neutral earthing. Wednesday, June 9, 2021 2 VG PATEL

TRANSFORMER ENCYCLOPAEDIA EARTHING TRANSFORMER The neutral point (star-point) is usually available at every voltage level from generator or transformer. In the absence of power transformer of suitable capacity, connection and design separate grounding transformer can be used. They are inductive devices intended primarily to provide a neutral point for grounding purpose (32. 1. 15 in AIEE Standard No. 32). They are interconnected star (Zigzag), star-delta and `T’ connected. Factors like single winding, smaller capacity and economy factors preferred zigzag transformers compared to other two types. This article is restricted to zigzag type with oil filled transformers. Some of the major benefits of operating with grounded neutral systems are: Wednesday, June 9, 2021 VG PATEL 3

TRANSFORMER ENCYCLOPAEDIA EARTHING TRANSFORMER • improved reliability; • improved fault protection; • fewer faults; • simplification of locating the faults; • greater safety; • limitation of transient over voltages; • lower cost and less maintenance. Wednesday, June 9, 2021 VG PATEL 4

TRANSFORMER ENCYCLOPAEDIA EARTHING TRANSFORMER Earthing Transformer –A Brief Account: A transformer is a device which electro magnetically transforms ac voltage from one level to another. An earthing transformer is a transformer primarily to provide a neutral point for grounding purpose. The sole duty of the grounding transformer is to pass ground current during earth fault and it carries no useful load. Desirable qualities of earthing transformer are low zero sequence impedance and low losses (no load losses). It is having a 5 limb construction. Wednesday, June 9, 2021 VG PATEL 5

TRANSFORMER ENCYCLOPAEDIA EARTHING TRANSFORMER Function of Earthing Transformers: Apart from providing easy path to ground current during earth fault, the following additional functions are also to be achieved: vholding the neutral shift within limits; vpermitting the circulation of unbalanced load; vto meet the current during line to earth fault; vto earth the system; vto provide single phase line to neutral load; and v can be used with resistance / reactance / arc suppression coil. Wednesday, June 9, 2021 VG PATEL 6

TRANSFORMER ENCYCLOPAEDIA EARTHING TRANSFORMER Comparison between Earthing and Power Transformers: The selection of any transformer mainly by two factors namely primary voltage and load. For an earthing transformer, load is short duty or short time (only at the time of fault), normally from a few seconds to one minute. The features, which distinguish earthing transformers from power transformers, are to be clearly understood before discussing on earthing transformer. The comparison of 2. 29 MVA (short time) 2 MVA continuous duty power transformer details both in general as well as design parameters are given in the table below. Though in principle and operation both are same, yet they differ in so many respects. Salient differences and similarities enumerated for the benefits of power engineers. Wednesday, June 9, 2021 VG PATEL 7

TRANSFORMER ENCYCLOPAEDIA EARTHING TRANSFORMER Comparison Characteristics TRANSFORMER GENERAL PARAMETERS TRANSFORMER DESIGN PARAMETERS Earthing Power To Raising or provide a Purpo lowering Core Frame neutral se the supply (cm) point for voltages earthing Wednesday, June 9, 2021 VG PATEL D* L* A* Earthi Power ng (2 (2. 29 MVA) 135 240 530 725 340 450 8

TRANSFORMER ENCYCLOPAEDIA EARTHING TRANSFORMER Specif IS: 3151 ication Rating in k. VA Short Time Voltag e (in Upto 33 gener k. V al) IS: 2026 240 686 320 1360 Flux Density (T) 1. 65 1. 66 Maximum Current Density (A/mm 2) 16 4. 06 Continuou Weight of Core (kg) s High voltages (220 k. V and above) Z or Y – d Vector DY 11, Yd, (Not Group Yy 0, etc, . loaded) Wednesday, June 9, 2021 Weight of Copper (kg) VG PATEL 9

TRANSFORMER ENCYCLOPAEDIA EARTHING TRANSFORMER No Short Circuit 155 65 (Exists in Essenti End (2 (10 Secondary Yd without al Temperature Seconds) loading) (ºC) ) One / Number of No Load Loss One / Two / 0. 52 2. 2 Windings (k. W) Three Bi. Direction of Unidirectio Load Loss directio 28. 6 Power Flow nal (k. W) nal Short Circuit 10 2 Impedance withstand seconds to second 7. 5 (%) duration 1 minute s Wednesday, June 9, 2021 VG PATEL 10

TRANSFORMER ENCYCLOPAEDIA EARTHING TRANSFORMER Losses Magnitisin g Current No load losses and load losses Very less Priority of Zero sequence Impedance impedance Short circuit impedance Zero Sequence Impedance (Ω / Phase) 5 - Weight of Core and Winding (ton) 0. 8 2. 25 Weight of Oil (ton) 0. 38 1. 17 1. 55 6. 1 Depends on system from Short Up to 25 times few hundred Circuit the normal Total Weight (ton) amperes to few Current current thousand amperes Wednesday, June 9, 2021 VG PATEL 11

TRANSFORMER ENCYCLOPAEDIA EARTHING TRANSFORMER Leakage Resistance Little More Dimensio Length 1. 5 2. 3 ns Breadth 0. 7 3. 05 (Meter) Height 1. 7 2. 7 Voltage Not Off circuit Cost Factor Regulation applicabl / No load (Present Cost) (by means of) e Constant / Type of Voltage (in Variable / Regulation general) and Mixed Required to Not Operate in applicabl Frequent Parallel e Wednesday, June 9, 2021 VG PATEL 1 5 12

TRANSFORMER ENCYCLOPAEDIA EARTHING TRANSFORMER Transportation Road Short Circuit 250 Temperature (ºC) Harmonic Residuals Free Load Factor Low Insulation Uniform Efficiency More Cost Less Wednesday, June 9, 2021 VG PATEL Road / Rail 250 Exist High Uniform / Graded Less More 13

TRANSFORMER ENCYCLOPAEDIA EARTHING TRANSFORMER *D = Diameter; L = Limb Length and A = Centre Distance With the above comparison we can infer that for an earthing transformer: Ø weights, losses, dimensions and cost factors are much lower; Ø rise of oil / winding temperature is very less; Ø higher specific loading and efficiency; and Ø more significant factor is zero sequence impedance. Wednesday, June 9, 2021 VG PATEL 14

TRANSFORMER ENCYCLOPAEDIA EARTHING TRANSFORMER Significance of Zero Sequence Parameters: In symmetrical components, the positive sequence is one having normal phase sequence, the second is of negative sequence with reversed phase sequence and the third has no sequence called as a zero sequence are the important parameters. An earth fault is one that part of the current in the faulty phase return to the supply through the earth. This is due to a symmetrical component consisting of three single-phase currents one in each line and all being in phase. In general, the transformer impedance of the transformer is the main factor which influences application. The zero sequence impedance of standard earthing transformer can be calculated from the formula: Wednesday, June 9, 2021 VG PATEL 15

TRANSFORMER ENCYCLOPAEDIA EARTHING TRANSFORMER Where Zs = Impedance in Ω / Phase; E = Line–to-line voltage in k. V; and In = Neutral current in amperes. In certain system it is necessary to insert resistance or reactance to restrict the fault current. The zero sequence component of three line currents are compounded they will add together to produce a resultant called residue, whilst the positive and negative sequence components get cancelled in transformer. The magnetic field produced by zero sequence set of currents is radically different from those produced by negative or positive sequence currents and therefore zero sequence impedance is generally very different from positive and negative impedances. Zero sequence reactance is equal to or less than positive sequence and it depends on the form of core construction and disposition of the windings. Wednesday, June 9, 2021 VG PATEL 16

TRANSFORMER ENCYCLOPAEDIA EARTHING TRANSFORMER Why Interconnected Star Winding is used? Purpose to establish a suitable ground path can be achieved with the help of three pairs of concentric coils connected to oppose ampere-turns. As the fluxes oppose, the transformer takes very small magnetizing current during normal condition. Lines of force enclosing both coils on one limb are therefore impossible. The earth fault current finds little impedance. This connection has unique characteristics and preferred to other types of neutral deriving transformers. Wednesday, June 9, 2021 VG PATEL 17

TRANSFORMER ENCYCLOPAEDIA EARTHING TRANSFORMER The main features are: vwinding has much lower impedance to zero sequence currents; vfree from harmonic residuals; vstable neutral can be obtained; vmost economical and practical; vcan be used with three phase system without secondary winding; vpermitting single phase line to neutral load; vcan be economically used with resistance/reactance/are suppression coil; vequivalent k. VA is approximately 57. 5% of star-delta transformer; vavoidance of undesirable stresses in the insulation; v can be used with either delta or star connected windings to feed desired load; v it keeps zero sequence impedance constant even auxiliary winding under load; and vfault current is not reflected on to the secondary side (auxiliary winding). v From the above, it is very clear that, the inter connected star winding can be utilized as earthing transformer or power transformer or in combination depending upon the requirement. Wednesday, June 9, 2021 VG PATEL 18

TRANSFORMER ENCYCLOPAEDIA EARTHING TRANSFORMER Rating and Inter Related Parameters of Earthing Transformer: The earthing transformers are of short time rating (Ten seconds to one minute). The rating of earthing transformer is entirely different from power transformers. Power transformers are designed to carry total load continuously, whilst the earthing transformers carries no load and supplies current only if one of the lines becomes grounded. It is usual to specify the single-phase earth fault current that the earthing transformer must carry for sufficient time. Since it is almost working on no-load, dictates to have low iron losses. Because of short time device, its size and cost are less than that of continuous duty transformer of equal k. VA rating. The k. VA rating of a three phase earthing transformer or a bank is the product of normal line to neutral voltage (k. V) and the neutral or ground amperes that the transformer is designed to carry under fault conditions for a specified time. In figure 1, if 3 I is the total earth fault current and V is the line voltage, the earthing transformer short time rating is equal to root 3 VI. Wednesday, June 9, 2021 VG PATEL 19

TRANSFORMER ENCYCLOPAEDIA EARTHING TRANSFORMER When specifying the rating of an earthing transformer , the important parameters are: Voltage: The line-to-line voltage of the system; Current: The maximum neutral current to carry for a specified duration. In grounded system it is based on type of grounding. Depending on their duration, several rates short time currents may be specified when a load is to be connected to secondary winding; Wednesday, June 9, 2021 VG PATEL 20

TRANSFORMER ENCYCLOPAEDIA EARTHING TRANSFORMER Wednesday, June 9, 2021 VG PATEL 21

TRANSFORMER ENCYCLOPAEDIA EARTHING TRANSFORMER Time: Designed to carry rated for short time duration of 10 seconds to 60 seconds. Depending upon the time setting of the protective gear on the system and the location of the transformer. Earthing transformer time is 10 seconds for protection and for feeder it is 60 seconds. Reactance: This quality is a function of the initial symmetrical three-phase short circuit KVA. It also based on the type of grounding and type of application of lightning arrester and transient over voltages. Wednesday, June 9, 2021 VG PATEL 22

TRANSFORMER ENCYCLOPAEDIA EARTHING TRANSFORMER Major Design, Constructional and Manufacturing Features: Desired electrical parameters of the earthing transformer are to be achieved by verifying the calculations based on electrical, mechanical and thermal computations. Such as the required electrical strength, mechanical ruggedness, dynamic and thermal resistance of the windings in the event of short circuit are to be solved carefully at the design stage. When designed as an earthing transformer it is usually manufactured as an autotransformer (inter star). Wednesday, June 9, 2021 VG PATEL 23

TRANSFORMER ENCYCLOPAEDIA EARTHING TRANSFORMER Wednesday, June 9, 2021 VG PATEL 24

TRANSFORMER ENCYCLOPAEDIA EARTHING TRANSFORMER Wednesday, June 9, 2021 VG PATEL 25

TRANSFORMER ENCYCLOPAEDIA EARTHING TRANSFORMER The following brief details refers to core type oil filled transformer with cylindrical windings for zigzag with or without auxiliary winding which are popular all over the world. The core of such transformer is built in the same manner as that of transformer with the only difference that instead of two windings per limb will have single winding per limb which is divided into two equal portions and connected as shown in figure 2. The current flowing are in opposition, thereby undesirable harmonics are avoided. The choice of the winding configurations is dictated by zero sequence impedance in single winding. With on auxiliary by both zero sequence and short-circuit impedance. Auxiliary winding may be used continuously to supply station auxiliaries. Wednesday, June 9, 2021 VG PATEL 26

TRANSFORMER ENCYCLOPAEDIA EARTHING TRANSFORMER The type of windings may be either multilayer, helical or disc windings mainly based on current and voltages. Generally, voltages of these transformers are upto 33 k. V and currents up to several kilo-amperes based on system requirements. Figure shows the different configurations and is required to meet both zero sequence impedance and short circuit impedance. Magnetic and electric loadings are similar to power transformer. The basis of ultimate temperature rise calculation is that all the heat stored in the copper without dissipation to insulating media or surroundings. According to standards, end temperature rise depends on the duration of the fault, ultimate being 250º C for copper winding in oil. For continuous rating, calculation is similar to normal power transformers. Insulation design is mainly based on dielectric constants of paper and oil. Wednesday, June 9, 2021 VG PATEL 27

TRANSFORMER ENCYCLOPAEDIA EARTHING TRANSFORMER The ideal stress distribution can be obtained through field plotting by finite element method (FEM). Because of frequent short circuit capability, remedial measures like strengthening of the coils, proper materials, and avoidance of magnetic asymmetry are to be observed. Since, only iron losses are to be dissipated, the tank dissipation is adequate in majority of the cases. With auxiliary loading proper care is necessary in respect of heat dissipation for the reliability of the transformers. In case of earthing transformers for industrial applications and for heavily polluted atmosphere, demands bushings of higher creepage distances to avoid frequent flashovers. Further, external surfaces are to be painted with epoxy base paint. Testing of these transformers is in accordance with National and International standards to verify various parameters. Transportation of these transformers is similar to small power transformers on road. Wednesday, June 9, 2021 VG PATEL 28

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