The last few decades have seen very rapid development of renewable energy, especially, distributed photovoltaic (DPV) and wind power. It is estimated that at least 40 per cent of electricity generation by year 2040 would be from renewable energy sources and this would give appreciable reduction of the present level of carbon dioxide emissions of about 10 billion tons per year. The transformer industry is rising to this challenge and has developed special transformers for the DPV generation and wind generation.
Transformers are critical components in solar energy production and distribution. Historically, transformers have ‘stepped-up’ or ‘stepped-down’ energy from non-renewable sources. 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. For example, solar power applications experience steady state loading during inverter operation. When the sun’s out, there’s a dampened reaction process and more constant loading on the transformer.
Also, fault ride through has not been defined for photovoltaic systems. This may be because it is easier to turn solar systems on and off quickly, or because regulatory requirements have not caught up with the young technology. This may change in the future.
As for harmonics, the solar inverter’s typical harmonic content is below 1 per cent, which has almost no impact on the system. The lower harmonic profile is because there are no generators and switching and protective controls such as those found on wind turbines. Solar transformers do require step-up duty. Yet, the solar inverter converts DC input from the PV array to AC voltage for the transformer in a smooth transition with no overvoltage from unloaded circuit. Because solar transformers operate at a steady voltage, with the rated voltage controlled by inverters, voltage and load fluctuations are considerably lower than in wind turbines. PV systems also operate close to their rated loads.
Solar-power systems also have special design issues. Because the largest solar inverter size is about 500 kilovolt Ampère (kVA), designers are building 1,000 kVA solar transformers by placing two inverter connected windings in one box. The transformer must have separate windings to accept completely separate inputs. Design issues also stem from running cables long distances to convert from DC to AC.
Restrictions on inverter size also limit the size of PV systems. Increasing the size by adding more solar inverters into one transformer box is extremely difficult. With the required box size and running cabling to convert DC to AC, things get complex.
The key to solar transformers is to understand the variables in every system. Transformers need to customize to work with each particular system. Inverter technology has been slow to advance, and it remains to be seen whether this comparative disadvantage will be a fatal flaw in the advancement of solar technology to the same level as wind farms.
Transformers for Distributed Photovoltaic (DPV) generation
Electric power is generated by converting solar energy to d.c by using photovoltaic (PV) cells. The DC generated is converted to a.c by inverters and the a.c is connected to the power grid by a step up transformer. The international standard applicable for the transformers for DPV generation is IEEE C57.159 2016 “IEEE Guide on transformers for application in distributed photovoltaic (DPV) power generation system”. At present there are limitations on the power rating and voltage level of the inverter system and therefore one or more inverters are connected to an equal number of secondaries of the step up transformers. Though the most common configuration is 2 to 3 secondaries, at present transformer with 6 secondaries are also manufactured. Inverter manufacturers are now developing inverters with higher power and voltage ratings and this would increase the transformer MVA ratings in future and reduce the number of secondary windings required.
• Non symmetrical load and voltage: The inverter voltage and load current to the 3 phases of the transformer can be unbalanced. If the transformer is fed by more than one inverter, there is a possibility of one of the inverters getting inactive and this can create unbalanced loading of the winding. The unbalanced voltage and current can create excessive leakage flux, stray loss, and overheating of winding and tank.
• Windings: Vertically stacked loosely coupled LV windings with equal number of split HV windings are the preferred design to reduce the effect of unbalances. The impedance characteristics will be defined based on the inverter system and number of inverters connected to a transformer.
• Presence of direct current in winding: There is a possibility that a direct current can come to the inverter fed winding which can increase the core magnetizing current and inrush current peak.
• Wave shapes of inverter output: The wave shapes of 2 or more inverters connected to one transformer may not be synchronized. This can cause change in wave shape and harmonics as well as disturbances of the flux.
• Fast rising pulsed waveform on LV winding: The inverter produces a pulsed output to ground and the pulse can reach a rate of rise (dv/dt) level of 500 V / microsecond. The low voltage winding insulation will have to be designed to withstand the rapid rising voltage for the design life of the transformer. An electrostatic shield is provided between the LV and HV winding to isolate the HV winding from the effect of the fast rising voltage on the LV. The shield acts as an additional dv/ dt filter and filters the voltage gradient of the pulsed inverter output. It also reduces the transfer of the transients from the high voltage winding to the low voltage winding. Copper or aluminium can be used as shielding. Copper shields will produce less eddy current loss when compared to aluminium shield. Accelerated aging test of prototype LV winding insulation can be carried out to check the effect of the fast rising transients on the insulation life. The effect is different for dry type transformer insulation and fluid filled transformer insulation.
• Losses and efficiency of the transformer: The DPV transformer is designed with comparatively low no load loss because the transformer draws exciting power from the system at night. The efficiency at a specified load cycle is optimized to get overall economy of operation. If the power system is designed with a power storage facility like battery system, the transformer will be operating under load continuously and the efficiency level can be fixed on this basis.
• Inrush current considerations: The LV winding of the DPV transformer is normally near the core and therefore the air core reactance of this winding is low. The inrush current when LV is switched on is comparatively high.
• Thermal design: The transformer cooling system is designed to consider the effect of the ambient temperature variation at the site, load curve, effect of harmonics, effect of reactive load.
• Short circuit considerations: The winding configurations and the location of short circuit on the transformers affect the short circuit magnitude and distribution. The effect of the various short circuit conditions of the transformer will have to be considered for the design which include- short circuit on the HV of the transformer, short circuit on any one or more of the LV’s of the transformer and short circuit between any two LV’s of the transformer.
• High frequency switching transients: The HV side of the transformer is controlled by a circuit breaker and in almost all cases vacuum circuit breakers (VCB’s) are used for the circuit interruption. The pre-strikes and re-strikes of VCB’s together with the capacitance of cables and inductance of the transformer generate fast rising transients. These transients can cause insulation failure and detailed analysis is required to ensure adequate insulation design.
• IEEE Standard C57.142-2010:“IEEE Guide to Describe the Occurrence and Mitigation of Switching Transients Induced by Transformers, Switching Device, and System Interaction” deals with this subject. Switching of an unloaded transformer with VCB can cause chopping overvoltage, multiple re-ignitions over voltage and over voltage due to virtual current chopping. Simulation study of transformer model for different frequency intervals up to 2 MHz, using electrical parameters of cable used and transformer design data can be carried out to get the calculated values of over voltages during VCB switching.
• Special installation and operation practices: The inverters are connected to the star connected LV windings of the transformer, and thus neutral is kept floating. The neutral shall not be earthed and/or grounded. It is a safe design practice to keep the neutral isolated inside the transformer. The transformers used are having electrostatic shields shall have single point earthing only.
Power generation from photovoltaic system does not produce carbon emission. However if mineral oil is used for the DPV step up transformer, it is not environmental friendly. The options used now are biodegradable oil filled transformers, dry type cast resin transformer and dry type transformers.
Trends in the development of transformers for PV
The complexity of electrical grid is increasing rapidly due to the use of renewable energy distributed generating systems, usage of large number of nonlinear loads, electric vehicle (EV) charging and so on. Along with this, the need for making the grid versatile and “intelligent” has prompted the development of “smart grid” concept. It is inevitable that the grid will get phenomenal transformation sooner or later and this will require “Smart Intelligent Transformers” in future. The transformers for renewable energy are no exception and the next generation of transformers for renewable energy will have to integrate with the demands of the smart grid. Following are the information regarding the features of these transformers.
• Demands of smart grid and customer: The smart grid demands several unique features and functionalities from the transformer such as,
1)Voltage sag compensation: The present distribution transformers and system cannot correct the voltage levels and ensure constant voltage of the customer terminal. The transformers of the future shall have this feature.
2)Harmonic isolation and filtering: Nonlinear loads produce harmonics and the transformer shall be capable of maintaining clean output waveform
3)DC output: In addition to the stable AC output, the future transformers will have to give DC output for EV charging and other DC loads, reactive power compensation, Advanced Distribution Automation
4)Outage compensation: The transformer shall draw power from the energy system and give outage compensation
5)Fault isolation: The transformer shall isolate the grid from a fault on the load side and also isolate itself from the grid when a fault on the incoming side occurs voltage balancing, protection from single phasing, 3 phase power from single phase supply, reduced weight and size, eliminate oil and/or fluids.
In order to meet the requirements of the smart grid, the transformer will have to be solid state using power electronics. The solid state transformer works on the same principle of the conventional transformers, but at a high frequency to reduce the weight and size. The incoming voltage is converted into high frequency AC by using power electronics converters and fed to the primary of the high frequency (HF) transformer to obtain AC and / or DC output voltage. There are several technical issues to be solved and product development work required for the commercialization of the SST for renewable energy such as,
• Introduction of smart grid by the utilities, which is a slow process now due to investment constraints and legacy issues.
• Commercial availability of high voltage (e.g. 11 kV, 13.2 kV etc.) IGBT or SiC components for converter inverter application. At present cascade connection is employed to get the working voltage level.
• Protection of high voltage power electronic circuit from surges / impulses and system faults
• Availability of low loss magnetic material for the high frequency transformer core
• Overall system efficiency needs improvement. The conventional transformer has high efficiency (more than 99% typically) whereas the overall efficiency of the SST is considerably low
• Special winding material is required for HF application. It is expected that carbon nano tube may offer low weight and low loss solution in future.
Wind power and solar power will dominate the source of electric power in future. This will reduce the carbon dioxide emissions substantially. The transformers required for wind power and solar DPV require special design features to meet the challenging operating conditions. The industry today has developed different types of transformers for the application. The future challenge involves the development of solid state intelligent transformers equipped with the necessary features suitable for integrating these with the emerging smart grid. Time will tell, but the indicators are promising. The transformer industry has to undertake collaborative development work for meeting the future challenges.