The Second Age of Solar - USA Electrical Codes, Practices & Standards for 1000 Volt Solar Farms

The word “bankability” is being tossed around again during the ongoing evolution of the utility scale solar market.  Sometimes it is presented that the sales revenue size of the company defines bankability for solar farm financiers.  I doubt it.  At one conference, a $175+ Billion USA utility described their financial terms, conditions and consequences their supplier would incur from non-performance.  And then they said they were “a small fish in a big pond” when including all of the other USA utilities combined.  Therefore, solar supplier’s sales revenue is not a good measure of bankable in the utility scale solar farm development market - supplier size does not matter all that much to the utilities - but performance, reliabilty and safety do.

So when defining company bankability in the context of utility scale solar farms, the true measure may be how well a diversified-market company has supplied to significantly larger customers over several decades without incurring the fiscal consequences of any one customer.  After supplying multi-megawatt power conversion, distribution & control systems to the oil & gas industry for over 65 years to over 150+ customers with significantly larger sales revenues (eg, more than $1 trillion USD combined), demanding turnaround times (eg., “we need it now”), and certainly litigious when it comes to lack of performance (e.g., the aftermath of any oil spell) – our long term company financial success and hence company bankability is based upon the use of global codes, practices & standards to deliver high quality products on time and within budget.

So for solar, product bankability may be defined as “use of the best in class design & manufacturing codes, practices & standards with performance, reliability and safety built in throughout”.  One engineering firm’s study concluded that USA solar O&M costs are now 8% to 10% of the initial cost annually, and most of that is due to inverters systems – so this definition of product bankability is directly related to solar farm ROI over the project lifecycle – whereas supplier sales revenue metrics indicate nothing about the supplier’s product performance, reliability and safety that directly impacts project ROI.

USA utilities pursue the building of anything first time right, and have their own codes, practice and standards for operating, extending, upgrading and maintaining their power generation systems and distribution networks for up to 50 years.  However when privately funded power generation systems are built, such as utility scale 1000 Volt solar farms, AHJs – Authorities Having Jurisdiction – are the ones applying their knowledge of, at minimum, NEMA, NEC, UL, ANSI, IEEE, NFPA and OSHA electrical codes, practices and standards, to the inspection of these projects.  While in the past 1000 Volt solar farms were not as scrutinized because of “being behind the fence and above NEC/UL 600 Volt codes”, that notion is fading as AHJs are hiring medium voltage specialists to develop an improved set of inspection criteria – at the encouragement of the utilities who have to interconnect these privately operated systems to their regional grid.

So in this series of “Second Age of Solar” articles, we will review the more relevant USA Electrical Codes, Practices & Standards for Utility Scale 1000 Volt Solar Farms that encourage best in class design & manufacturing codes, practices & standards with performance, reliability and safety built in throughout.  The code, practice or standard itself will not be debated from any financial, engineering, construction, operation or maintenance perspective, as it is assumed it provides a merit or benefit towards safety, health and welfare, human and financial (as there is plenty of documentation on that outstanding).  So, let’s start this article off with discussion about this trend…

AHJs extend existing USA Electrical Codes, Practices & Standards to 1000 Volts for Solar Farm Inspections & Audits

Some believe that when designing and deploying 1000 Volt utility scale solar farms, there are no USA electrical codes or standards that need to be followed because 1000 Volts is above NEC’s codebook of 600 Volt practices and below ANSI’s 15 or 38 kVolt practices.  Others may believe that USA electrical codes, practices & standards do not apply to solar farms, because they are behind the fence as a utility power generation system and thus outside the domain of NEMA, NEC, UL, ANSI, IEEE, NFPA and OSHA, to name a few. 

While there may be fewer codes, practices & standards called out specifically for 1000 Volt solar farms – there are however significant, tested and proven bodies of knowledge called out in general for 600 to 1500 Volt power conversion, distribution and control systems, that are applicable and usable across different industries with respect to occupational safety, health and welfare. In today’s litigious world, those italicized phrases are often heard in our court system, with the end result usually being that it was less costly to abide by available codes, practices and standards, than to avoid them.

Self-certifying IEC proponents often point to paragraphs that cite “utilities are exempt from USA codes, practices and standards (U-CPS)”.  Yes, that is initially true – and then no, that is not true over time.  All USA utilities adopt and incorporate U-CPS into their operating body of knowledge as quickly as the U-CPS is proven – it just takes them time to do it.  U-CPS adoption is often done more quickly when local USA municipalities – the AHJs – request NRTL test reports regarding equipment under inspection that does not have a USA agency certification.  And because 1000 Volt solar farm designs have few certifying agencies, USA municipalities have stepped up their investigations into 1000 Volt solar farm developments by using medium voltage testing specialists to translate existing U–CPS up to 1000 Volts as part of their inspection authority.

With respect to electrical systems, the AHJs start with U-CPS from NEMA, NEC, UL, ANSI, IEEE, NFPA and OSHA, among others.  NEMA, NEC and UL already have written and addressed wiring & cabling practices beyond 600 Volts, and as a body of knowledge, it is applicable to 1000 Volt PV wiring, cabling and junction boxes – or combiners.  ANSI and IEEE have already written and addressed “motor control center” power conversion practices applicable to 1000 Volt adjustable frequency power drive systems – or inverters.  NFPA and OSHA have already addressed electrical arc flash hazard conditions regardless of voltage.  So within these bodies of knowledge, 1000 Volt U-CPS can be easily derived by the AHJs – and as the utilities know – the AHJs have the final say regarding electrical commissioning against any U-CPS they apply.

So for 1000 Volt PV wiring & cabling practices, NEC 210.6 (E) “Over 600 Volts between Conductors” states “Circuits exceeding 600 volts between conductors shall be permitted to supply utilization equipment in installations where conditions of maintenance and supervision ensure that only qualified persons service the installation.” This is where the “behind the fence” ideology originally was born – over 600 volts, one definitely does have to keep non-qualified persons out of the site. 

But NEC 300.37 “Aboveground Wiring Methods” dictates a more specific practice, “Aboveground conductors shall be installed in rigid metal conduit, in intermediate metal conduit, in electrical metallic tubing, in rigid nonmetallic conduit, in cable trays, as busways, as cablebus, in other identified raceways, or as exposed runs of metal-clad cable suitable for the use and purpose. In locations accessible to qualified persons only, exposed runs of Type MV cables, bare conductors, and bare busbars shall also be permitted. Busbars shall be permitted to be either copper or aluminum.”

So PV panel wires that are not NEC-armored should be contained within raceways, and this suggests that PV panel junction boxes should accommodate conduit or raceway entrances to ensure that non-armored wires are fully contained according to NEC 300.37.  If not, then even 600 Volt solar farms must be behind a fence as well, because tie-wrapping non-armored 600 Volt PV wire to the PV panel racks does not address NEC 300.37 – which is why AHJs push solar farm developers back to NEC 210.6 (E) to exclude non-qualified persons from the site, and up goes a fence around the 600 Volt solar farm project.

Again, we’re not going to debate the NEC code – this is just an example of how AHJs use U-CPS to decide whether a solar farm is commissionable.  In this example, it was illustrated that if one code provision is not met, then the AHJ falls back to another code provision, and enforces code compliance using that directive.  That is the way it is.

When it comes to 1000 Volt power conversion, ANSI via NEMA ICS 61800 provides a series of design & construction practices regarding Motor Control Centers for Low Voltage Adjustable Frequency Power Drive Systems up to 1000 Volts – so there is a body of knowledge for 1000 volt power conversions as well.  UL 1741 goes further regarding grid interconnection, which is a key U-CPS for AHJs because it comprehensively covers four different U-CPS in one: material specification (the equipment has to be proven to be designed with fire preventative materials in accordance with NFPA), electrical creepage & clearance (the equipment has to be proven to be designed to NEC dielectric and insulation requirements for that operating voltage), ground fault detection & interruption (in this case, the equipment must be proven to detect and interrupt a ground fault in either the PV+ or PV- leg of the solar array using only UL-certified components), and utility grid interconnection as specified by IEEE 1547.1.  In absence of a USA agency certification, AHJs accept OSHA approved NRTL witnessed test reports – the AHJs are looking for third-party validation that the equipment has been built to U-CPS – and will not consider the OEM’s self-certified IEC report.  This is another example of how AHJs – and utilities – use U-CPS to decide whether inverter equipment can be commissioned on the farm and connected to the grid.

Thus far, it seems that U-CPS are AC voltage oriented bodies of knowledge – DC voltages apparently do not seem to have any U-CPS.  Their lack of existence sometimes gets translated into the false notion that “DC is safe”.  However, there is one major body of DC knowledge that exists and has been written for rail transportation motion systems using 750 to 1500 Volts DC motor drive systems – power conversion voltages similar to 1000 Volt solar farm designs.  The European CPS reveal that since the current does not go through zero like an AC sine wave, a DC arc once started will not extinguish itself in any short amount of time unless the DC arc is stretched apart.  To understand the dangers of a 1000 Volt DC arc flash more completely – think of striking an arc with an arc welder rod, and then continue on with the process of welding.  Arc welding wire is rated at 2000 volts – not all that far removed from 1000 Volt PV wire.  And once a solar panel is exposed to the sun – it is a live DC generator with a high voltage always across it when the sun is shining.  So, EPCs face DC voltage dangers every day of PV farm construction.

The rail transportation industry understands these dangers and has written many codes and practices for DC systems, which then were then converted into British EN standards for 750 to 1500 Volt DC rail transportation systems.  This represents a significant body of knowledge regarding best practices for design & construction with 750 to 1500 Volt DC systems, along with DC shock hazard protection and arc flash prevention.  The solar industry may want to utilize EN CPS to derive their own U-CPS regarding DC arc flash equipment safety with respect to DC cable & wiring, DC fusing and DC disconnect, to provide AHJs with a body of knowledge for their inspection.

Unlike DC, AC shock hazard protection and arc flash prevention U-CPS are prevalent at many worksites, including utility power generation sites.  USA utilities have translated these U-CPS into facility safety programs with defined responsibilities, calculating arc flash hazards for relevant equipment, providing appropriate PPE clothing for live work, training workers on arc flash hazards and safe work practices, providing appropriate tools for working with energized equipment, and placing warning labels on equipment that poses an arc flash risk.  Every utility transcribes NEMA, NEC, UL, ANSI, IEEE, NFPA and OSHA into their own operating procedures for approval by their public utility commission, their independent system operator regionally and FERC & NERC at a federal level. 

EPCs also conform to the U-CPS regarding AC shock hazard protection and arc flash prevention at the site – but what makes their lives more complicated is equipment not built to U-CPS, so the EPC has to expend additional time & cost to bring equipment into compliance with U-CPS, and that impacts 1000 Volt solar farm project ROI.

Legacy solar inverters provide EPCs with the most U-CPS non-compliance issues. Many of these inverters have exposed power circuit busbars alongside control circuit-boards without any compartmenting or deadfronting – which presents some of the greatest arc flash hazard conditions on the farm.  In these cases, the drop of a metallic tool adjusting a control circuit nearby a power circuit busbar can result in a serious AC or DC arc flash.  NFPA 70E cites one should at least four feet from an arc flash incident to remain safe – and since most of these inverter systems have almost no compartmenting or dead-fronting – such an arc flash would result in an injury to the employee. 

For equipment built in this manner, NFPA 70E requires the appropriate level of protective clothing – safety hat & visor, flash proof suit and thick rubber gloves – which then makes adjusting a control circuit near impossible in such attire.  This is another reason why bankability has been defined using “best in class design & manufacturing codes, practices & standards with performance, reliability and safety built in throughout” – because power conversion suppliers that have used arc flash mitigating dead-fronting doors and enclosed compartments to stop arc flash incidents from happening, have not been put out of business by their customers for these kinds of equipment design and manufacturing defects. OSHA views a preventable arc flash incident as a product defect.

That statement alone will not stop 1000 volt solar inverter companies from proclaiming NFPA 70E is an expensive design and construction practice, because it impacts initial lower cost per watt.  But again, when an AHJ interprets the U-CPS for an incident that results in bringing in OSHA, who then determines that an arc flash incident, AC or DC, was found to be preventable by the inverter OEM, the EPC site builder, the O&M operator and/or the site Owner, the fine is $70K for each person having responsibility of preventing the incident, along with overall workplace fines of up to $1M or more.  And the legal and insurance professions have been extracting additional payments up to $10M in civil lawsuits to recover fiscal, medical and human value costs in the aftermath of an arc flash incident.  So, unsafe lower cost per watt equipment design & manufacturing can translate to elimination of the solar farm project’s ROI with just one arc flash incident.

As a result, it only makes sense that utility scale 1000 Volt solar farm design, operations & maintenance should be performed against relevant U-CPS proven in applications across several industries and over time.  OSHA’s polices, in effect since March 2010, now applies to equipment manufacturers contributing to any U-CPS neglect or avoidance.  AHJs, as commissioners with the final say, have already begun applying 1000 Volt related U-CPS bodies of knowledge during their investigations to determine whether a utility scale solar farm is safe to operate & maintain or not.  While NEMA, NEC, UL, ANSI, IEEE, NFPA and OSHA CPS, among others, may not be at the top of any developer’s list regarding project elements that improve cost per watt – U-CPS do provide significant guidance to avoid losses per watt regarding regulatory design non-conformance, operating site non-conformance, and at worst case, electrical incidents and accidents.  So when coming back to product bankability, 1000 Volt solar farm equipment suppliers need to provide U-CPS compliance along with competitive cost per watt.

Now that we have covered the landscape of U-CPS probably more than you may have ever wanted to know, let’s examine how they impact 1000 Volt solar farm inversion system equipment regarding their input, inversion and output sections.

Utility Scale 1000 Volt Solar Inverter PV Input Design Codes, Practices & Standards

Almost every EPC is interested in DC grounding practices – it is not spelled out in U-CPS bodies of knowledge for DC as well as it is for AC.  Whether to ground or float the PV array into the inverter is still a debate between the PV Panel and Inverter OEMs.  PV Panel OEMs offer improvements in the lifetime performance of their solar cell technology by grounding one leg of the PV array to earth ground.  Inverter OEMs avoid DC common mode voltage and AC phase imbalance issues when floating the PV array into the inverter without an earth grounding of the array.  And then the California Public Utility Commission recently issued a mandate that any DC ground fault detected in either the PV+ or PV- pole of the array must trip off the inverter – thereby negating the PV Panel OEMs reason to earth ground one pole of the PV array. 

These debates will continue on a project by project basis, but there is growing trend among the EPCs to float the PV array into the inverter with a wye-ungrounded transformer secondary output.  The evidence suggest this configuration performs best for reducing the DC common mode voltage that can be impressed upon a grounded PV pole inverter during cloud fades, which can result in AC phase imbalance nuisance tripping.  How the ungrounded array practice affects the projected performance of PV Panels over their life is open for discussion and probably is covered in their body of knowledge – but inverters can accommodate any grounding practice, but some nuisance trip off because of cloud fade induced DC common-mode AC phase imbalances.

Three other DC related issues have become increasingly problematic for 1000 Volt solar farm development – DC field fusing rating, DC field fusing configuration, DC field fusing coordination.  Much of this has not been addressed by NEC 490 and 690 clearly enough to offer U-CPS guidance – yet such guidance is available from U-CPS in the AC world of fuse selection, configuration and coordination.

With respect to fuses, regardless of AC or DC, the maximum continuous fuse voltage allowed is 80% of their rated voltage.  Thus, a 1000 Volt fuse can operate only up to 800 volts continuously.  If operated continuously above that voltage within a humid and dusty environment, eventually small leakage currents from end cap to end cap will eventually flash over which may start a fire, perhaps destroying the equipment as well.  So when 1000 Volt solar farms are designed with Vmpp of 800 Volts or more, the DC fuse ratings should be increased to 1200 Volts.  All DC fuse manufacturers have been fairly straightforward on this – some have claimed the solar industry is not listening to them.

Another issue is DC fusing & disconnect configuration.  For floating (ungrounded) PV arrays, the use of one fuse with a one pole disconnect switch, instead of two pole fusing and a two pole disconnect switch, is dangerous and basically is a site design defect.  Since every PV Panel is in reality a DC generator always producing power when the sun shines, using one pole fusing on one leg of a floating PV array leaves the potential for severe shock hazard, just as much as fusing only one phase of an AC generator creates the same shock hazard condition – there is too great a chance someone will touch the circuit at the wrong point to complete the electrical circuit to ground, and thus get injured.  The other reason to fuse both poles in a floating PV array is to have a means to completely isolate the PV string so that it can be safely worked upon for the reason discussed.  The practice of one pole fusing and one pole disconnect is an area where developers tout saving some cost per watt upfront, but it also results in more O&M costs later because of the difficulty of completely isolating circuits to troubleshoot the PV array – not to mention the lack of protection, especially in ungrounded bi-polar PV array configurations that use this practice.

Finally, many DC fields are designed and implemented without appropriate fuse coordination.  While NEC provides short circuit fusing guidance (1.56 times the Isc) – it stops short of applying coordination to the entire DC field fusing system, just like that would be done for AC system fusing systems.  Most PV field combiners use slow blow fuses on the PV strings, which is an acceptable practice because of DC transients and surges in the PV array.  However, inverters are often supplied with slow blow fuses.  This results in the field combiner and inverter master re-combiner fusing systems becoming uncoordinated with respect to the fuse I2T curves.  In this situation, if the field combiner PV string fusing fails to open before the PV array’s short circuit current pours down into that PV string fault, then there is a high probability that since the master re-combiner slow blow fuse is also too slow to open, the path of least resistance for the PV array’s short circuit current will be right on through unprotected inverter IGBTs – which is about 10 cents per watt of investment destroyed for the lack of a 0.006 cents per watt high-speed fuse that should have been used in the master re-combiner.  DC fuse coordination typically is the responsibility of EPC performing the site design, because the drawings require a Professional Engineer stamp of approval for submittal to AHJs.

Fuse coordination problems becomes even more apparent when stringing the PV array above 1.2 DC input times the AC output, where the short circuit current in the larger PV array is now very high and very capable of bursting through the unprotected IGBTs very quickly regardless of the I2T of the slow blow fuse.  Above that 1.2 hot run ratio, the inverter IGBTs should be protected by their own very high speed fuse against the higher PV array short circuit current.  This master re-combiner and inverter IGBT DC fusing scheme can be referred to as a “two tier” fusing scheme, because the fusing to protect the PV array is assigned to the fuses of the master re-combiner for DC side faults, and the fusing to protect the inverter IGBTs is assigned to the fuse in front of the IGBTs for AC side faults.  This is standard practice in when paralleling inverters to create multi-megawatt power conversion systems up to several megawatts.

When inverter OEMs use only one level of fusing to protect against the two different “fault current directions”, typically the IGBTs are the devices that are destroyed as part of improper DC fuse coordination.  Uncoordinated fusing is never discovered until years later in the field when the first PV array faults start to happen.  This is not inverter wear & tear – it is an inverter design & manufacturing defect because there are U-CPS governing the coordination of AC fusing systems that relates to DC fusing systems.

Next Generation 1000 Volt Solar Inverter Protection Practices

The practice of all DC poles & all AC phases disconnect switching the inverter off both the PV and the AC grid simultaneously provides several benefits.  First, it provides complete power circuit load break to reduce the arc flash through the DC disconnect switch, which destroys the switch after several uses when performing direct DC load break – these DC switches can only withstand 5 to 8 full load break switching cycles before the DC arc flash destroys them (again, not inverter wear & tear – it is an inverter design & manufacturing defect because DC disconnect switch OEMs tell this to the inverter OEMs).  When DC load breaking is done with contactors instead of the disconnect switch behind dead-fronted doors, this results in NPFA 70E PPE Level 1 clothing requiring no special suit needed to take the inverter off the grid. 

This offline condition of the inverter then reduces medium voltage arc flash hazard conditions when using the MV AC disconnect switch on the transformer to completely isolate the inverter and transformer off the grid.  In addition, complete galvanic cut-off of the inverter from both DC and AC grids reduces power circuitry offline tare loss to zero.  Another benefit derived from taking the inverter off the grid is the reduction in destruction potential from night-time lightning strikes onto the utility grid, which are far more prevalent and more damaging at night than during the day. 

And if the power modules do not need to be on the grid, they should be taken offline anyway – transformers, being magnetic, can handle utility grid power surges fairly well – inverter power modules, which are electronic, do not handle surges and transients as well as the transformers.  Leaving them online with the grid when not necessary just opens the door for a grid transient to take out the power modules.  Given the power quality of most utility grids, theses kind of surges and transients happen more often than one would think.  A survey of solar system integrators indicated that inverters were more often destroyed on the AC side than the DC side regarding surges & transients, so at minimum, lightning surge arrestors should be standard offering in any inverter.

The purported life for inverters in the field ranges from three to six years – how long would the inverters have lasted, if for one-half of each day each was powered down, isolated and protected from surge & transient damage from the DC and the AC sides?  When is comes to this subject, this body of knowledge is just emerging for the solar industry, but the oil & gas industry has been using multi-megawatt power conversion systems on drilling rigs in the worst climates for over 30 years – that is one big body of knowledge to bank on – so 1000 Volt solar farm financiers and developers should be looking there as to how multi-megawatt power conversion has been done for decades.

1000 Volt Solar Inverter AC Output Design Codes, Practices & Standards

Although there is a significant body of knowledge regarding U-CPS for this subject, it continues to expand and re-define itself.   Let’s start with the interconnection standard itself, the core of which is IEEE 1547.1, but in the USA the wrapper is the new UL 1741 Year 2010 standard.  For many years it was assumed that since the UL certifying agency mark was for inverters up to 600 Volts, UL 1741 was not required for 1000 Volt inverters, just IEEE 1547.1.

That did not sit well with AHJs – IEEE is not a third party or a witness testing agency and carries no authority as a judge regarding equipment design & construction, so there was substantial inspection & commissioning work EPCs had to do with the AHJs.  But with 15% more watts per string driving higher project ROIs available with 1000 Volt solar farm designs, both utilities and EPCs pushed for a way to do unipolar 1000 Volt farms without resorting to the more expensive 600 Volt bipolar solar farm designs – EPCs have determined at least 5 cents per watt savings in just cabling costs alone when switching to 1000 Volt unipolar farm designs using the more flexible 2000 volt flexible stranded “arc welding wire”.

Over the past few years, grid interconnection of solar inverters has become more problematic, as the utilities began pushing back upon inverters that implement IEEE 1547.1 using their own software instead of an approved utility interconnection relay.  Many utilities were becoming concerned about inverters not disconnecting from the grid when the grid goes down, creating a significant shock hazard to their maintenance workers.  So the utilities began insisting upon inverters with UL listed IEEE 1547 interconnect relays in inverters along with UL 1741 witness test results of the overall system, to be certain that utility interconnection rules are followed.

As a result, new entrants to the industry brought UL 1741 Year 2010 third party witness-tested at 1000 Volts to the market.  In order to gain significant utility interconnect and AHJ acceptance to U-CPS, along with significant bankability regarding reliability and safety throughout, these inverter suppliers implemented a three tier UL approval strategy for their 1000 Volt inverters: UL 508C listed power conversion modules to demonstrate proven power conversion reliability of the IGBT system on the grid, UL listed IEEE 1547.1 interconnect relay to demonstrate proven interconnection safety when connecting to the grid, and NRTL witness testing of the 1000 Volt inverter system to UL 1741 to demonstration performance, reliability and safety of the inverter system at 1000 Volts. 

So with the three U-CPS bodies of knowledge brought back to the utilities and the AHJs in the form of a third-party test report, 1000 Volt solar farm development has become much easier to inspect and commission.  Even still, utilities are shifting towards having a UL listed IEEE 1547.1 interconnection relay in the medium voltage central substation looking at the utility grid to shutdown the substation if the utility grid goes down, AND, a UL listed IEEE 1547.1 interconnection relay in the inverter looking at the collection grid to shutdown the inverter if the collection grid goes down, as part of their “two layer interconnection trip” strategy.  This U-CPS body of knowledge has not yet been completely written when it comes to distributed generation of power on a regional grid.

The next challenge regarding inverter output systems came about with increased low voltage ride through requirements of inverters and converters from the utilities seeking to shore up their grids while wind and solar farms began to propagate across their territories.  As a result, active grid voltage compensation moved to the forefront when the American Wind Energy Association and FERC agreed upon changing the FERC 661A requirement for low voltage ride through from 5 cycles / 15% of the grid volts to 15 cycles / 0% of the grid volts.  For wind turbines, this can be accomplished by transferring the power of the wind turbine blades spinning down while pumping reactive power into a utility grid fault for 15 cycles.  This was the result of utility grid power analysis studies concluding this shoring up the grid needed to be done when interconnecting renewable energy projects.

As a result, many utilities are in one way or another moving towards adoption of this new requirement.  This in essence is transferring local voltage regulation control from the utility network to the utility scale solar farm developer, as part of the interconnection process cost sharing for resolving voltage droop issues caused by the hard turn on & off of the solar farm as a major cloud passes over the PV array.  On some grids and especially at the end of their distribution line, this is becoming a serious problem.

One approach for controlling short-term droop is to inject VARs to shore up the grid voltage at that point.  Soft start-up of solar power is possible with some inverters, but some inverters cannot provide soft shut-down of power if designed as two-quadrant power supplies, which many legacy solar inverters are.  Four quadrant inverters – which are in essence motor/generator drives – can provide voltage regulation capabilities by injecting VARs directly on demand into the grid.  This is the basis behind dynamic VAR compensation, where a four-quadrant converter is able to provide a combination of real solar power plus reflected reactive power into the grid. 

The bottom line is that if the utility chooses a significant level of dynamic VAR compensation to improve the grid’s low voltage ride through – a solar farm developer using the legacy lower cost per watt two quadrant inverter may not be allowed to connect to the grid.  This is already happening on the northeast coast, and will likely appear as a requirement on the southwest coast soon, where in both cases end of line utility grid power instability conditions are appearing as the result of interconnection of more multi-megawatt solar farms.  Here, solar farm developers will need to supply utility grade solar inversion equipment that meets the needs of the local utility grid interconnection requirements, if they are to expect grid interconnection of their project at all. 

Currently, obtaining grid power quality specification from the utilities in order to design and construct equipment that resolve these issues more effectively has become a challenging process.  Unless that information is provided by the Electrical Power Research Institute on behalf of their utilities, it will be a slow going process to resolve these newer issues created by renewable energies, and as a result, some 1000 Volt solar farm development projects are unlikely to be interconnected, leaving portions of the grid more and more exposed to grid instability issues rather than being addressed and resolved with solar power perhaps in combination with energy storage systems.  Here, the utilities have to take leadership of this U-CPS, possibly via EPRI, NIST, FERC and NERC.

In Summary

In this article, we’ve examined bankability using best in class design & manufacturing codes, practices & standards as based upon successful performance, reliability and safety found in other energy markets with significantly longer histories than solar.  We reviewed the most relevant USA codes, practices and standards (U-CPS) from NEMA, NEC, UL, ANSI, IEEE, NFPA and OSHA, along with specific DC practices from EN, to observe how they are applied by AHJs and utilities for witness-testing acceptance of the site and witness-testing acceptance of the equipment.  And then we reviewed how U-CPS acceptance by AHJs and utilities can be applied to the design & manufacturing of the inverter input, conversion and output sections to present the most accepted 1000 Volt solar inverter system for the USA 1000 Volt solar farm development market. 

Although the approach seems to increase initial cost per watt, a solar project’s ROI is measured over a 20 to 30 year life.  As a result, solar farm developers should be assessing the total lifecycle cost per watt, especially since solar farm O&M costs on average have soared to 8 to 10% of the farm’s initial cost, not to mention the costs of electrical fires and arc flash incidents occurring on solar farms.

While 1000 Volt solar farm developers have somewhat avoided U-CPS for the reason that “the projects are behind the fence”, they then require warranties of 10 years or more on equipment because the absence of best in class design & manufacturing U-CPS is behind many premature equipment failures.  In contrast, traditional energy power conversion equipment utilizes design & manufacturing U-CPS very thoroughly, purchasing only 1 year warranties on equipment, which lasts 10 to 15 years before failure, in the several of the world’s worst climates. So for solar, bankability really should be based on a more thorough set of CPS with equipment that is witness-tested against the CPS.  Having been in the electrical industry for more than 30 years, witness-tested U-CPS product once costed 5% to 10% more but always lasted 2 to 3 times longer than non U-CPS products used on the USA's electrical grid - with today's weak USA dollar, the costs between U-CPS and non U-CPS product has been wiped out by the exchange rate. 

So when assessing bankability, one needs to investigate if they should be building a better house that costs 5% to 10% more upfront with better market value in terms of complete code compliance that is amortized over 30 years and has far fewer maintenance problems, or should they build a house at the lowest cost per foot amortized over 30 years that now requires 8% to 10% per year of the initial cost for the next 30 years to keep the house usable, along with requiring additional capital investment later to bring the house up to code in order to sell it.  There are plenty of home builders everywhere that can sell at a low cost per foot, just like there are plenty of solar farm developers that can sell at a low cost per watt.  The question for the buyer is, is that what the buyer wants for their project ROI? USA’s codes, practices & standards exist to protect uninformed Buyers from below-board Sellers – not the other way around – that is the purpose of U-CPS.