A 63-Transformers for Photovoltaic Solar Projects-Part 2
In the first part of this series, we have seen the types of transformers required for a typical Solar Photovoltaic Power Park or Farm. We covered the main transformer in the farm- inverter transformer that steps up inverter output AC voltage to MV voltage levels. In this part we will see details of balance solar farm transformers viz collector transformers, auxiliary transformers, earthing transformers and voltage regulators.
First Part of the series: A 62-Transformers for Photovoltaic Solar Projects - Part 1
2.0?Collector Transformer
2.1?Power from individual inverter transformers at 11-33 kV is collected by the Collector Transformer and stepped up to HV voltage of 66-400 kV.?Maximum MVA of collector transformer is limited by the rating of MV breaker used on LV side. Maximum breaking capacity of 36 kV breaker is limited at about 160 MVA of transformer capacity with normal percentage impedances. Currently collector transformers are used up to a rating of 315 MVA 33-33/ 400 kV with LV side split in to two circuits to limit fault level within the MV breaker capacity. These transformers are with OLTC on HV neutral end for HV variation by +-10 %. In India ratings and specifications of collector transformers were standardized in 2021 by Central Electricity Authority, India. ( www.cea.nic.in.) The specifications “Standard Technical Specifications of Power Transformers for Solar Park Pooling Stations” (260 pages) can be downloaded from the above website.
2.2?Specifications:
?????????????? ? ?80-160 MVA - ONAN/ONAF (ONAN 70 %)
250 -315 MVA- ONAN/ONAF1/ONAF-2 or ONAN/ONAF/ODAF?(60 % -80 % -100 %)
Comments – It is better to limit the variety by selecting only 40-100-160 -250 -315 MVA ratings; Limit maximum specific loadings to 1.6 T and 3.5 A/mm2 and revise losses accordingly. These transformers will be fully loaded only 5-6 hours a day and are almost at no-load at night. In case of double LV transformers, LV1-LV2 shall be decoupled by adopting preferably, an axial split arrangement of LV and HV windings.
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3.0 Auxiliary Transformer
Auxiliary Transformer is a low kVA 3 phase transformer to supply power to inverter. It can be a standalone unit or integrated with the inverter enclosure. Primary may be connected to (a) power grid or to (b) the pulsed output voltage of the inverter. In case of (a) primary voltage will be the grid voltage and in case of (b) it will be the pulsed output voltage of inverter. LV will be suitable for inverter supply voltage. Auxiliary transformer shall be suitable for asymmetric loading (up to 80 %) and the windings shall be of isolated ones and not to be auto connected.??
4.0 Earthing Transformer
Sometimes an earthing (grounding) transformer (zig- zag connected) will be required at the ungrounded MV circuit to get a grounding neutral. Earthing Transformers should comply with standards IEC 60076-6 ed1.0-2007 Section 10 / IEEE C57.32-2015 / IS 5553-6 (R2001)/Section L of CBIP Pub No.317 -Transformer Manual. This is a single winding zig-zag connected, short time (10 second) rated transformer, with the neutral solidly grounded or grounded through a resistor. A star/delta connected transformer also can meet this purpose.
5.0?Voltage Regulator
A booster (voltage regulator) with On-Load Tap-Changer can be provided on LV side or HV side of inverter transformer to take care of the voltage fluctuation of grid while feeding power to grid. These are small three phase auto-transformers with buck-boost OLTC for regulating the output voltage by +-10% in 16 or 32 steps. When used on LV side, ratings up to 250 kVA and for MV side transformers up to 8 MVA are commonly used.
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Senior Testing Engineer at Sudhir Power Limited
2 年P. Ramchandra sir I want to know that Why we used mostly 3.6 KV LV bushing in Solar transformer ?
Transformer Core Specialist
2 年Please contact me for your requirements of CRGO lamination, CRNGO Laminations, CRGO and CRNO slit coils and Toroidal Round and Rectangular cores Rasquinha Transcore Electricals Pvt Ltd Navi Mumbai www.rasquinhatranscore.in [email protected] WhatsApp - 7021764479
Specialist, Power Transformers
2 年One of my friends asked a clarification on my statement of double LV is provided due to limitation of MV breaker capacity. Please note this statement is only for collector transformer where MV breaker may see the full short circuit current passing through collector transformer . Double or quadruple LV is provided in inverter transformer not due to that reason ( ie LV breaker capacity) . Inverter will give only 1.5 to 2 times rated current of inverter during any short circuit. Each inverter requires a separate galvanically insulated low voltage winding. Hence parallel connection of several inverters to one LV winding is not possible.
Project Manager at Millennium Challenge Account Nepal (MCA-Nepal)
2 年Sir, great knowledge product once again. Sir I have few queries regarding the conductor material generally used in inverter transformer proper. In general still Copper is probably ruling as the prime conductor material for say in all ranges of Transformer starting from distribution to large size power transformer except DT in small range where probably Aluminium also is increasing being used. But then why Double winding, Three winding and even Five Winding Inverter transformers upto even 3-5MVA are found to be using Aluminium in either both HV and LV windings or HV(Al) and LV(Copper) etc? Despite Trafo being bigger in size and other manufacturing and repair issues with Aluminium and the problem of tracking, what could be the main reason of increasing use of Aluminium specially for inverter(solar) trafo application? Sorry sir that i missed to post my this query earlier.