FAQs - Inverter

DC-Side

What DC to AC, or stacking ratio, can I use?

The ratio of PV module STC power rating to inverter output power rating is often referred to as “Stacking Ratio.”  Most PV installations are designed for a stacking ratio greater than 1, and less than 1.5.  However, the only real limitation on stacking ratio for the HiQ inverter is the voltage and current limitations of the string inputs.  The Vmp range for the string inputs is 425 to 850VDC.  The specified maximum short circuit current of the DC source is 30A. Two strings with a Vmp of 850VDC and a short circuit current capability of 30A would yield a stacking ratio of almost 6.375 (8 kW) and 8.9 (5.75 kW), which are obviously excessive.

 

A more practical limitation is due to the power and current limits imposed by the inverter firmware.  Input power is limited to 5kW and input current is limited to 10A per string by the firmware.  The total of these two strings exceeds the output power limit of 8kW, so in reality something less than 5kW would be harvested from each string.

 

Ideal stacking ratio varies by installation. For the vast majority of HiQ installations, a stacking ratio between 1.25 and 1.5 makes perfect sense.  Under some conditions (constant high irradiance) a lower stacking ratio may be preferred.  A stacking ratio of 1.1 to 1.25 typically allows for energy harvest loss factors such sub-optimum module orientation, inverter efficiency losses, module soiling, module aging, etc.  Nevertheless in some cases a stacking ratio of 1.5 or greater can be advantageous.  For example in the northern hemisphere a North facing array, or an East or West-facing array may justify a higher stacking ratio.

 

This string calculator may be helpful.

 

Can I use modules with an Isc of more than 10A?

The maximum input current on each string is 10A for maximum output power. However, the

‘DC maximum input short circuit current’ for each input, i.e. the maximum current the unit is rated for under fault conditions, is 30A. Therefore there is no issue with use of modules with Isc’s up to this value.

 

Array grounding

The TrueString product is a non-isolated inverter for use with ungrounded DC systems

 

 

Are DC disconnects required for HiQ inverters?

As stated in the manufacturers specifications for the 480V TrueString inverter

and the 208V TrueString inverter, the connectors are the certified means of

disconnect. More background on this topic may be found in the

application note shown here.

 

 

 

 

 

 

 

 

Where can I purchase the MC4-style connectors used on TrueString inverters?

These are manufactured by Renhe Solar (www.renhesolar.com).

For more details, please contact them directly: ian@renhesolar.com

Connectors as Disconnects

AC-Side

Which grid-types are supported?

480V and 208V wye only – neutral, ground and 3 phases.  Also known as a 5-wire system. 480V example follows.

 

Line diagram of 480V wye with 5 wires, 3 live, neutral and ground

Do you have an example line diagram?

Line diagram 480V HiQ Solar TrueString PV inverter, OCPDs, Gateway, AC Splice

Thanks to Brad Carlson of Solar Carports for the following customer examples.

Is grounding required?

The metal enclosure of the inverter must be grounded per NEC 690.43.  Equipment ground is provided through the AC connection. Optionally, grounding may be achieved also through attaching an Equipment Ground Conductor (EGC) to a lay-in lug (not provided) attached to the chassis.

 

Grounding may also be achieved using the mounting bolts to any properly grounded metallic structure provided a paint-cutting washer is employed, such as a stainless steel star washer.

 

Is a neutral required?

All 5 circuits (L1, L2, L3, Neutral, Ground) must be connected and Neutral must be bonded to earth ground.  Failure to do so can damage the unit and void the warranty.

 

Note that particular care is required to ensure that an effective neutral is maintained when a transformer forms part of the installation.

 

Neutral wire sizing guidance

The HiQ inverter exports power only on the Line conductors.  The neutral conductor is

used solely for voltage sensing and PowerLine communications.

 

 

Are AC disconnects required for HiQ inverters?

As stated in the manufacturers specifications for the 480V TrueString inverter

and the 208V TrueString inverter, the connectors are the certified means of

disconnect. More background on this topic may be found in the

application note shown here.

 

 

 

 

Do HiQ inverters meet the requirements for NEC 2014 Rapid Shutdown?

Our inverters easily comply with this requirement. More details may be found here.

 

 

 

 

How does connecting up to 3 HiQ inverters on a single OCPD comply with the NEC?

This is answered comprehensively in the document here.

Connectors as Disconnects

Wiring

Tech. Brief

Can I use TrueString Inverters with a Transformer?

  • The inverter must have wye grid connection (3 hot phases, neutral & equipment ground)
  • Neutral MUST be bonded to equipment ground or damage to the inverter may result
  • Transformers are often isolating – so the neutral may lose any path to equipment ground that was provided elsewhere, once the transformer is inserted into the circuit
  • Example is shown below where neutral is bonded to equipment ground in the load center, but many other system configurations and methods are possible
  • Utility side of transformer - configuration to be determined by the design engineer to suit site requirements

Mounting

How should I mount the inverter?

The TrueString inverter is NEMA6 certified, and easy to mount in a wide variety of ways. Unlike many string inverters, the unit can be mounted on it’s back, upside down, under modules etc. It can also use the ballast-mounting solution from HiQ Solar described here.

 

Ideally the unit would be at least 1” above the roof, and with 4-5” clearance above the cooling fins to allow air to circulate to minimize the likelihood of thermal power limiting.

How do I replace a TrueString inverter?

The following video shows how easy it is to replace a TrueString inverter.

Inverter Communications

What methods are available to communicate with TrueString inverters?

These are explained here.