While proceeding towards the reticulated use of electricity to supply the human needs attention on utilisation of natural forces has always been locked up in fossil fuels. The magnitude of the fossil fuels which is on the verge of depletion swung our concentration towards the renewable sources of energy. In recent decades, harnessing energy from solar has been enhanced due to the efficient photo voltaic technologies.
Due to concern of environmental quality, today photo voltaic power plants (PV) are rapidly spreading all over the countries. In a traditional PV plant, a large number of PV modules are series connected in long strings whose output is provided as input to single centralised inverter for the voltage inversion. Basic architecture of solar power plant is shown in figure 1. Step-up transformers are required to boost the 350 to 690 V inverters output voltage to the 11 or 33 kV of the medium voltage utility network. More sophisticated architectures have been developed where PV modules are arranged in strings, or even substrings, each one connected to the step-up transformer through a dedicated inverter, or a dedicated DC/DC converter and a centralised inverter.
Inverter Duty solar Transformers
For pumping of electrical power to the utility network inverter duty type solar transformers are widely used, either singly or paralleled. There are different types of solar transformers including distribution, station, sub-station, pad mounted and grounding. All solar transformers have specialised needs that impact costs. The inverter-duty solar transformer which is a key element of a PV system, are generally manufactured with multiple LV windings, that enables to connect several PV panel strings to the grid with less number of transformers in total.
While designing inverter-duty transformers having three-windings or five-windings for grid connected photovoltaic systems, special care is taken in design and manufacturing to address the harmonics that are usually appear in the transformer windings.
Inverter windings are specifically designed to withstand voltages excursions that arise due to pulsed mode inverter operation. Inverter windings are capable of withstanding voltages with high rate of rise (dV/dt).
The special feature of Inverter duty transformers is offering galvanic isolation between the input power circuit (PV array) and the grid so as to prevent dangerous DC faults to be transmitted to the AC side.
An earthed screen is normally installed between the primary (inverter winding) and secondary (MV) windings of Inverter duty solar transformers. Electrostatic shield between inverter winding and MV winding decouples capacitive nature of primary and secondary winding so as to eliminate the effects of high frequency transients on the other winding.
During normal lifetime, these transformers are subjected to a variety of electrical, mechanical and thermal stresses. One of the most critical situations is that caused by external short circuits, which produces high currents in the transformer windings and hence high internal forces in the windings. Thus, before installing these multi secondary winding transformers, one of the transformers should undergo short circuit withstand test for design verification.
Role of CPRI
CPRI is a pioneer testing organisation in India with six decades expertise in the short circuit and dielectric testing, short circuit design data reviews, quality control checks and stage inspection of various power system equipment. Presently, CPRI is expanding its testing activities globally with international institutions such as ASTA Intertek UK and also as a member of STL for testing and certification of various LV and MV Switchgears and Power & Distribution Transformers as per International Standards. CPRI is continuously engaged in testing of various types of switchgear equipment from last six decades and issuing test certificates and test reports as per national and international standards.
To prove the satisfactory performance of these inverter-duty solar transformers, various tests have been carried out as specified in the international and national standards. In last four years, different rating of inverter duty transformers of various manufacturers for solar application came for short circuit dynamic withstand test at CPRI.
Ability to withstand short circuit test
The short circuit test is carried out to verify the integrity for stresses, primarily mechanical, developed when short circuit current flows through the transformer. Due to the speed at which short circuit faults appear and are cleared, the current exert electro-dynamic forces onto the windings that causes mechanical stresses in the radial direction (tending the inner turns to compressive stress and outer turns to tensile stress) and in axial direction (with pulsating compressive forces). Radial forces regularly leads to bucking of the winding, axial and radial forces have been observed to result in spiralling and/or tilting of the turns. Permanent deformation of the winding may lead to immediate damage or long-term issues like deterioration of insulation, obstruction of oil flow, material weakness or loosening of mechanical structures, etc.
Short circuit test procedure
The standards which are used for short circuit testing and evaluation of transformers are as follows:
- IS 2026-part 5: 2011
- IEC 60076-part 5: 2006
- IEEE/ANSI C57.12.90
- IEEE/ANSI C57.12.00
Preparation of test Transformer prior to short-circuit tests
The short circuit test must be carried out on a new transformer ready for service, protection accessories such as Buchholz relay and pressure relief device must be mounted. Test Set-up of the inverter duty solar transformer undergoing short circuit withstand test is shown in figure 3. Routine Tests that must be carried out prior to short circuit test according to the standards are as follows:
- Measurement of winding Resistance
- Measurement of voltage Ratio & check of phase displacement
- Measurement of short -circuit impedance and load loss
- Measurement of no-load loss and current
- Dielectric Routine tests such as Separate Source AC withstand voltage test and Induced AC voltage tests
If windings have tappings, the reactance and resistance must be measured for the tapping positions at which the short circuit test will be carried out.
Short-Circuit Current calculation
Fault currents flowing through transformers are significantly higher than the rated currents of the transformers. In worst case, the current would be as high as the current that would flow if system voltage was applied to the primary terminals while the secondary terminals are shorted – limited by the transformer impedance only. These currents produce both mechanical and thermal stresses in the transformers. Thus the r.m.s. value of the symmetrical current If that is intended to flow through the transformer during short circuit test shall be calculated as follows
Where U is the rated voltage of the winding under consideration,
Zt is the short-circuit impedance of the transformer referred to the winding under consideration,
Zs is the short-circuit impedance of the system
Test current peak value
Forces resulting from the currents passing through the transformer act on the conductors as a function of the peak asymmetrical current (the highest peak value of any cycle of the current), which is usually at its highest during the first half cycle of the fault. The transformer manufacturer needs to ensure these forces do not damage the transformer. So, the test shall be performed with current holding maximum asymmetry as regards the phase under test. The amplitude î of the first peak of the asymmetrical test current is calculated as follows:
where factor k accounts for the initial offset of the test current
Duration and number of tests
The thermal stress is caused by the high current causing heating in the transformer. Both the RMS symmetrical current magnitude and duration of the fault contribute to the heating of the transformer. The transformer manufacturer needs to ensure the components of the transformer do not become hot enough to be damaged.
Thus, the duration of each test shall be:
- -0.5 s for transformers rated up to 2500kVA,
- -0.25 s for transformers rated more than 2500kVA.
The recommended number of tests for short circuit withstand ability test on three-phase transformers are nine (With regard to tap-changer position, test sequence shall be three tests in the position corresponding to the highest voltage ratio, three tests on the principal tapping and three tests in the position corresponding to the lowest voltage ratio).
Detection of faults and evaluation of test results
During each test, recordings shall be taken of the applied voltages, the phase currents, and Tank current. After each test, the oscillograms taken during the test shall be checked, the gas-and oil-actuated relay need to be inspected and the short-circuit reactance has to be measured.
The figure 4 shows a recording of applied voltages, LV side phase currents, Tank current of Inverter duty Solar Transformer which has undergone short circuit withstand test.
Criteria to pass the test
The results of the short-circuit reactance measurements and the oscillograms shall not indicate any anomalies. The dielectric tests and other routine tests when applicable have been successfully repeated.
The out-of-tank inspection does not reveal any defects such as displacements, shift of laminations, deformation of windings, connections or supporting structures, so significant that they might endanger the safe operation of the transformer. No traces of internal electrical discharge to be found.
For transformers rated up to 100 MVA, reactance variation should not be more than 2 per cent for transformers with circular concentric coils and sandwich non-circular coils and 7.5 per cent for transformers with non-circular concentric coils.
Conclusion
CPRI is always inclined towards meeting our nation’s strategic energy objectives such as secure and environmentally responsible energy system by certifying qualitative LV and MV switchgears, distribution and power transformers as per National and International standards.
