What is Earthing Transformer?

Have you heard about Earthing Transformer? Some says YES, but most will say NO. It is not surprise since most of electrical engineers are being exposed with normal types of transformer such as power transformer (132/33kV and etc), distribution transformer (33/11kV or 11/0.433kV) and instrument transformer (VT and CT).

Earthing transformer is usually being used in electrical substation. The main purpose is to create a neutral point in areas where earth point is not available. Earthing transformer having  the zig-zag (interstar) winding. This winding arrangement is to achieve the required zero phase impendence stage which provides the possibility of neutral earthing condition. In addition auxiliary windings can also be provided to meet the requirement of an auxiliary power supply.

As for connection, the earthing can be connected directly, through an arc-suppression reactor or through a Neutral Earthing Resistor (NER). The excess current from power transformer is being fed to the NER then to the HV part of earthing transformer. This earthing transformer will convert the incoming power to 415V of power supply. This power supply eventually will be supplied to the LV Board which is being used for the power consumption of the substation itself. In the nutshell, the leakage current from power transformer is being used as a source of power for the whole substation rather than it’s going directly to the ground as wastage.

Generally, there are 4 inspection/tests need to be done to ensure good functionality of the earthing transformer:

i)              Physical Check

ii)             Electrical Test

iii)            Insulating Oil Test and

iv)           Functionality check.

Physical Check

Below is the itemized that need to be check;

i)              Check for any corrosion, damage or defect

ii)             Coordinating gaps (alignment and gap where applicable)

iii)            Name plate

iv)           Earth connection

v)            Pressure Relief Device (PRD)

vi)           Bucholz relay

vii)          Breather (leaks inspection, silica gel and etc)

viii)         Valves in operational position

ix)           Oil level in conservator tank

x)            Missing components

xi)           Ensure all terminal blocks, gauges and other accessories are fully sealed and moisture proof.

Electrical Test
Below is the testing that needs to be carried out:

No.	Test	Description	Remarks
1	Insulation Resistance	It shall be perfomed using a 5kV insulation tester. The following measurement must be recorded: a) Between Primary and Earth b) Between Secondary and Earth c) Between Primary and Secondary	All value must acceed 200MΩ
2	Polirization Index	It shall be perfomed using a 5kV insulation tester. The following measurement must be recorded: a) Between Primary and Earth b) Between Secondary and Earth c) Between Primary and Secondary	value shall be > 1.2 (R10/R1min)
3	Turn Ratio	Ration of HV over the LV winding	Within 0.5% of the nameplate ration of the rated voltage.
4	Impedance Voltage	Percentage measurement of zero and positive sequence impedance	As declared on nameplate
5	Winding Power Factor	Measurement of power factor of all windings	shall be < 0.5% at 20˚C
6	Vector Group	Checking the vector group of the transformer	measured vector diagram shall be equal with manufacturer's vector diagram

Insulating Oil Test

Below is the testing that needs to be carried out:

No.	Test	Description Highest Voltage for Equipment (≤72.5kV)	Remarks
1	Moisture Content (ppm)	≤ 15	IEC 814
2	Neutralization Value (ppm)	≤ 0.03	IEC 296
3	Interfacial Tension (mN/m)	> 35	IEC 6295
4	Colour	≤ 2.0	IEC 296
5	Dielectric Breakdown Voltage (kV) Test Gap (2.5mm)	> 50	IEC 156
6	Oil Power Factor	< 0.1% at 20°C
No.	Test	Test Method	Remarks
7	Dissolved Gas Analysis (DGA)	Using Full Gas Extraction and Headspace Methods Standard ASTM Standard D 3612-01 Analysis od Gasses Dissolved in Electrical Insulationg Oil by Gas Chromotophraphy Method A & C IEC567 Standard Section 7.3 Vacuum Extraction by Partian Degassing Method.	Hydrogen gas levels should not exceed 20ppm No detactable amount of Methane, Ethylene and Acetylene gases. This test dhall be carried out one to two eweek after continuous energisation of transformer.

Functional Check

To ensure that the transformer is functionally well, the itemized below need to be inspected:

i)               Transformer guard check: alarms, indications, flags, facia and relay operation at transformer, relay panel and control panel shall be recorded clearly. Actual operation for each signal of all earthing transformer guards shall also be check and mentioned except for PRD.

ii)             Oil level indicator shall be functional.

iii)            All temperature indicators shall be operational.