Specification for Earthing Transformers
1.0 SCOPE
1.1 This section covering specification for earthing transformers, does not purport to include all the necessary provisions of a contract. For general requirements and tests, reference shall be made to other sections of the Transformer Manual.
2.0 GENERAL
2.1 Unless otherwise modified in this section the earthling transformers shall comply with latest versions of IS 5553 (Part 6) and IS 2026.
2.2 Three phase earthing transformers provide an artificial neutral and are used for the following purposes :
(a) to earth an otherwise unearthed system
(b) to connect single phase loads between lines and neutral (c) to connect an arc suppression coil
(d) to limit fault current during a line to earth fault determined by the zero sequence impedance of earthing transformers and also by the possible addition of resistors and thereby permitting selective protection.
*Note : The provision of the earthing transformer does not necessarily make the system effectively earthed.
(e) Earthing transformers with zigzag (inter-star) connected winding can have a star connected secondary winding to provide an auxiliary supply.
2.3 Construction of earthing transformer is similar to conventional oil filled transformer.
Usually cooling specified is ONAN type.
3.0 WINDING CONNECTIONS
Earthing transformers are usually connected either in zigzag (inter-star) or delta. For star-delta transformer the secondary star-delta winding shall always be connected in closed star-delta. The neutral of star connected main winding is earthered. Earthing transformer which consists of a single winding connected in inter-star may also provided with, an auxiliary (secondary) winding. This secondary winding when provided shall be connected in star.
(a) Primary - Zigzag (interstar), Secondary - Star
(b) Primary - Star Secondary - Delta
(c) Primary - Zigzag (interstar)
3.1 For the purpose of fault current limitation resistors/reactors can be inserted either between primary neutral point and earth or in series with primary terminals of interster or star connected primary windings to adjust the zero sequence impedance (Figs. 1 and 2).
For star-delta connected earthing transformer the delta connected winding may be of the open type in order to permit the insertion of a resistor or reactor to adjust the zero sequence impedance.
3.2 Also connecting the resistor/reactors at the neutral end would be preferable.
4.0 TAPPINGS AND TAP CHANGING
4.1 For zigzag connected earthing transformer having auxiliary winding if tappings are required for voltage variation, it shall be provided on zigzag connected main winding. Equal and uniform number of tappings shall be provided on both zig and zag windings of main windings.
(a) Current-limiting resistor in neutral (b) Current limiting resistor in line Fig. l Interconnected star (zigzag) neutral earthing transformer
(a) Current limiting resistor in neutral (b) Current limiting resistors in line Fig. 2 3 phase star-delta neutral earthing transformer
Range of variation: +5 to -5% in steps of 2.5%
4.2 Tap changing shall be carried out by means of an off circuit externally operated self-positioning switch (when the transformer is in de-energised condition. Position No. 1 shall correspond to maximum plus tappings. Provisions shall be made for locking the tap changing switch handle in position.
4.3 However, tappings are not preferred for earthing transformer.
5.0 INSULATION LEVEL
The insulation level for the line terminals of an earthing transformer shall correspond to those specified for transformers as per IS: 2026 (Part 3).
6.0 LOSSES AND IMPEDANCE 6.1 Losses
6.1.1 Only no-load losses should be specified for earthing transformer not provided with additional auxiliary windings. The tolerance on specified no. load losses will be subject to limits specified in IS: 2026.
6.1.2 Both no-load and load losses will be specified for earthing transformers provided with windings' suitable for supplying auxiliary loads. The load losses specified should be based on the rating of the auxiliary winding. These losses are also subject to tolerance in accordance with IS : 2026.
6.2 Impedance
6.2.1 Zero sequence impedance of each earthing transformer shall be specified in ohms per phase and this impedance will be subject to a tolerance of +20%. -0%.
6.2.2 When earthing transformers are provided with auxiliary winding impedance between the auxiliary winding and the main interstar (zigzag) winding must be specified and this impedance shall be subject to tolerance as per IS : 2026.
However, if any difficulty arises to achieve both the specified zero sequence impedance of main winding and the percentage impedance between the main winding and auxiliary winding, in such cases either external resistors/reactors may be provided on main windings to adjust the zero sequence impedance or current limiting resistors/reactors may be provided on auxiliary side to limit the fault current on auxiliary side to the specified value.
7.0 CONTINUOUS AND SHORT TIME CURRENT RATING 7.1 Continuous Current
7.1.1 Rated Neutral Continuous Current
Continuous neutral current is specified either in the case where phase unbalance of the system exists or when the earthing transformer is to be designed for connection of single phase loads between line and the neutral.
7.1.2 Rated Continuous Current
The current flowing through the line terminals continuously when a rated power of a secondary winding is specified.
Note : The earthing transformer shall carry the specified neutral or rated continuous current and comply as regards the temperature rise with appropriate requirements of IS : 2026 when it is energised at rated voltage and frequency
7.2 Rated Short Time Current in the Neutral
The earthing transformer shall carry the specified neutral fault current for the specified duration without exceeding the winding temperature of 250oC for copper and a temperature of 200oC for aluminium.
When an earthing transformer is designed for the neutral point to be connected to a current limiting impedance in the connection to earth, it should also be capable of withstanding, for a period of 5 seconds, the maximum earth fault current that can flow without the additional impedance in circuit. This safe guard is necessary should, for instance, the bushing of an earth resistor flash over.
When earthing transformer are operated without external resistor, the rated short time current and zero sequence impedance shall have the following relationship :
Ish =
Vph is the maximum permissible operating phase voltage
zo is the zero sequence impedance per limb of earthing transformer Ish is the short time neutral current of the transformer
7.3 Ability to Withstand Rated Short Time Current
7.3.1 The earthing transformers shall be capable of withstanding the mechanical and thermal stresses caused by the rated short time current flowing for the specified duration.
The thermal ability can be demonstrated by calculation using the following formula as per clause 9.1 of IS 2026 (Part 1) - 1977
θ1 = θ0 + a J2 t x 10-3oC where
θ1 is the highest average temperature attained by the winding due to short time current maintained over the specified duration and shall not exceed 250oC for copper winding and 200oC for aluminium winding.
θo is the initial temperature in degree celsius 3. Vph
zo
J is the short time current density in ampere per square millimetre t is the duration in seconds
a is a function of 1/2 (θ2 + θ0), in accordance with Table 1.
θ2 is the maximum permissible average winding temperature, 250oC for copper and 200oC for aluminium.
7.3.1.1 Where earthing transformers are used with external resistor/reactors to limit the earth fault current, the earthing transformer should also be able to withstand dynamically and thermally the maximum earth fault current without external resister/reactors for a period of 5 seconds.
7.3.1.2 For earthing transformers without secondary winding θ0 shall be taken as the sum of the maximum ambient temperature and manufacturers guaranteed average oil temperature rise of the earthing transformer under normal operating conditions.
7.3.2 For earthing transformer with loaded secondary windings θ0 shall be the sum of the appropriate maximum ambient temperature and the relevant temperature is specified in IS : 2026 measures by change in resistance.
Table 1
a - function of ½ (θ1 + θ2) oC
½ (θ0 + θ2) Copper windings Aluminium windings
140 7.41 16.5
160 7.80 17.4
180 8.20 18.3
200 8.59 19.1
220 8.99
-240 9.38
-7.3.3 Ability of earthing transformer to withstand mechanical stresses due to the rated short time current flowing in the windings under fault conditions shall be determined by tests described as per clause 8.6 of IS 5553 (part 6).
8.0 TESTS 8.1 Type Test
l Impulse Voltage Withstand Test (IS : 2026 (Part III).
l Heat run test (clause 4 of IS : 2026 (Part 2).
Applicable only in the case of earthing transformers having auxiliary winding.
8.2 Special test
l Short circuit withstand test.
Clause 8.6 of IS : 5553 (Part 6).
Note : After short circuit test, success of test shall be verified by usual inspection, by repetition of dielectric test at 75 per cent of its original value and by a check measurement of zero sequence impedance.
8.3 Routine Test
l Measurement of winding resistance clause 16.2 of IS : 2026 (Part 1).
l Measurement of insulation resistance (clause 16.0 of IS : 2026 (Part I).
l Measurement of zero sequence impedance (clause 16.10 of IS : 2026 (Part I).
Note : Zero sequence impedance may be measured at any current between 25 per cent to 100 per cent rated short time neutral current and is expressed in ohms per phase. It shall be ensured that the applied current shall not exceed the current carrying capability of the winding or metallic constructional parts.
l Measurement of no load loss and no load current (clause 16.5 of IS : 2026 (Part I).
l Dielectric tests (IS: 2026 (Part 3).
l Separate source voltage withstand Test (Clause 10 of IS : 2026 (Part 3).
l Induced over voltage test (clause 11 of IS : 2026 (Part 3). Applicable only in the case of earthing transformer with a secondary winding.
l Check of voltage vector relationship and polarity (clause 16.3 of IS : 2026 (Part 1).
l Measurement of voltage ratio (clause 16.3 of IS : 2026 (Part 1). Applicable only in the case of earthing transformer with a secondary (auxiliary) winding.
l Ratio measurement of zigzag connected earthing transformer with star connected auxiliary winding.
For a zigzag connected earthing transformer the zig and zag windings constituting one phase are physically wound on two different limbs of the core. Hence if a single phase supply is applied between line and neutral of interstar (zigzag) connected winding, the voltage induced in zig and zag winding, will be different. Due to this, voltage induced on secondary winding of same phase will not be the same as that defined by the per phase voltage ratio of the transformer. Thus voltage ratio measurement with single phase application will give misleading results if application and measurement is made on per phase basis. (ie between line and neutral).
A vector diagram of a 33/0.435 kV ZNynl connected earthing transformer is given as Fig. 3.
The zig and zag windings per limb are designed for 11 kV and LV is designed for 0.435/3kV.
Here 3 phase voltage ratio is defined as the ratio of the HV line to neutral voltage to LV line to line voltage (IR-IN/2R-2Y) ie 33/3/0.435 = 43.799
Fig. 3 Vector and voltage relationship or a 33/0.435 kV ZNynl connected earthing transformer For single phase application, the ratio IR-IN/2R-2Y will be :
11 kV x x 2 = 33 /3 x 2 /0.435 / 3 x 2
= 33/ 3 / 0.435 kV
= 43.799
i.e., for a zig zag connected earthing transformer to get actual design ratio with single phase application, the ratio measurement shall be made by applying line to neutral voltage (per phase) on interstar connected main winding and measuring the induced line to line voltage on corresponding star connected secondary windings or vice versa.
Note : For ratio measurement with 3 phase application equal and balanced supply (w.r. to voltage and phase difference) shall be applied, otherwise ratio error will be high.
8.3.1 Measurement of impedance voltage short circuit impedance and loss (see clause 16.4 of IS : 2026 (Part I) 1977.
9.0 REFERENCES
l IS : 5553 (Part 6)
l IS : 2026 (Part 1,2,3,4 & 5)
l ISC 289 section 6
l IEC 76
l CBIP Manual on Transformers, Section H - 1987 “Specification for Earthing Transformer”
l The J & P Transformer Book, Tenth edition chapters 11, 20 and 21.
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