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Compliance and Code Challenge for Central Inverter Replacement

The purpose of this Article is to discuss the code compliance and challenges customer faces when comes to replacing older central inverter of their existing PV project with new inverters.
Older solar protects, almost all of them are grounded PV system where one of the DC currents carrying conductors is, as per then NEC requirement, needed to be grounded through Ground Fault Detection and Interrupter circuit. As a result, older inverters used in NA were designed for grounded DC system to be in compliance with NEC requirement.
When it comes of electrical system, grounded system is generally used in North America while in Europe, ungrounded is generally used. In NA almost all solar projects up until recently have used only grounded DC systems whereas Europe and most of the Asian countries that follow European standard have used ungrounded systems. The Europeans have demonstrated that ungrounded systems result in higher system efficiencies and lower cost. Now NA is following European lead and started to use ungrounded systems to take advantage of lower cost and the higher efficiencies.
Most of the inverter manufacturers have evolved to the manufacturing of transformerless ungrounded
inverters considering higher equipment efficiency and pricing competitiveness. Newer string inverters are transformer-less and designed for higher dc voltage, 1000Vdc or 1500Vdc. But, having no internal transformer inside the inverter means there is no galvanic isolation between DC and AC circuit and as a result DC input connected to solar inverter cannot be connected to ground as a result newer grounded inverter can’t be replaced with dc grounded system. In order to use ungrounded inverter, PV array need to converter to ungrounded system. NEC has revised code and has laid out the requirement for ungrounded system to facilitate ungrounded system. So, this article discuss NEC code compliance related to ungrounded system and what modification to be done to replace older grounded 600Vdc central inverter with ungrounded string inverters.
To clarify this issue, it is necessary to have a closer look at Article 690 in NEC code book. Based on Article 690.41, Photovoltaic system can either be grounded or ungrounded. String inverters are transformerless and ungrounded. For ungrounded systems, Article 690.35 should be executed and applied. The requirements include:
(A) Disconnects. All PV source and output circuits conductors shall have disconnects complying with 690, Part III . NEC690.35(A) requires means of disconnecting both (+Ve & -Ve) ungrounded conductors. This  means if existing project is grounded system then it will have disconnecting means only on ungrounded conductor. To convert grounded system to ungrounded system, NEC690.35(A) requires adding disconnecting means to both ungrounded conductors which means external or internal disconnects will have switch pole for both conductors coming from PV array.
(B) Overcurrent Protection. All PV source and output circuits conductors shall have overcurrent protection complying with 690.9. DC overcurrent protection is required for ungrounded current carrying conductor. So NEC690.35(B) calls for overcurrent protection for both +Ve and -Ve conductor of PV source circuit which means multiple strings of modules will have a overcurrent protection on both +Ve and -Ve DC inputs from each string of modules.

(C) Ground-Fault Protection. All PV source and output circuits shall be provided with a ground-faultprotection device or system that complies with 690.35(1) through (4):
(1) Detects ground fault(s) in the PV array dc current-carrying conductors and components.
(2) Indicates that a ground fault has occurred
(3) Automatically disconnects all conductors or causes the inverter or charge controller connected to the faulted circuit to automatically cease supplying power to output circuits.
(4) Is listed for providing PV ground-fault protection. What this means is inverter must be listed and marked clearly for use with ungrounded PV Array. Inverter must be equipped with ground fault detection circuit to detect the ground fault on PV array dc current carrying conductor and components. If ground fault is detected, inverter will be required to shutdown and indicate that a ground fault has occurred. Khehua string inverters model SPI50K BHV & SPI60K BHV are designed and satisfy this requirement?? (Confirm with Connor)
(D) Conductors. The PV source conductors shall consist of the following:
(1) Metallic or nonmetallic jacketed multiconductor cables.
(2) Conductors installed in raceways.
(3) Conductors listed and identified as PV wire installed as exposed, single conductors or,
(4) Conductors that are direct buried and identified for direct burial use.
(E) Battery Systems. The PV power system direct direct-current circuits shall be permitted to be used with ungrounded battery systems complying with 690.71(G)
(F) Marking. The PV power source shall be labeled with the following warning at each junction box, combiner box, disconnect, and device where energized, ungrounded circuits may exposed during
service:

(G) Equipment. The inverters or charge controllers used in systems with ungrounded PV source and output circuits shall be listed for the purpose.

Summary:

Older central inverter that are designed for grounded DC system can be replaced with newer more efficient, cheaper and compact string inverter. To do that system need to converted from grounded to ungrounded system and NEC Article 690.35 laid out the standard to achieve that. However, if grounding a transformerless system is your goal, you can still achieve it by series a resistor with high resistance (22k+ ohm) between the negative pole and ground. By doing this, even though the
short circuit (grid to ground) happens, the current will be massively reduced by the resistor and won’t damage the inverter. This is so called “High Impedance Grounding”. We understand that meeting all codes and compliance can be intimidating and overwhelming. We hope this article with help clarify some of your questions. Of course, it is always recommended that you communicate your installation needs with certified installers as well as local AHJ (Authority Having Jurisdiction).