Solar Sales� Guide to Buying a Grid Connected Solar Power System






Handbook for Positive Grounded
Batteryless Grid Connected
PV Systems








Handbook for Positive Grounded Batteryless Grid Connected PV Systems





Contents Page


1. Introduction
2. Surface Polarisation
3. Positive Grounding Systems without Batteries
4. Inverters
I. KACO Blue Planet
II. Fronius IG
III. SMA SunnyBoy
IV. Other Inverters

















2 Handbook for Positive Grounded Batteryless Grid Connected PV Systems

1.

INTRODUCTION

SunPower and other high-efficiency back-contact
solar modules can suffer from a temporary decrease in
performance if they are installed according to
conventional methods without system earthing.

Ironically this is due to the modules unique back-contact
high-efficiency design. When installed according to the
manufacturers recommendations (positive system
earthing) this design increases the efficiency of the solar
cells and thus increases the power per unit area
compared to other solar panels which makes
SunPower panels some of the best modules on the
market.

The purpose of this handbook is to explain the module performance problem
and provide an explanation of how systems using these high-efficiency
modules should be installed using positive system earthing/grounding.



3 Handbook for Positive Grounded Batteryless Grid Connected PV Systems
2.

SURFACE POLARISATION

The temporary decrease in performance experienced by back-contact solar
cells is due to a surface polarisation effect which is caused by a small amount
of leakage current from the module. All modules experience some amount
of leakage current; however the surface polarisation effect seems to be
unique to thin-film and back-contact solar cells.

If the leakage current is not channeled in the right direction, a small static
charge builds up on the surface of the solar cell. The effect of this static
charge is to encourage the recombination of electron-hole pairs formed in the
semiconductor when exposed to solar radiation (sunlight). Thus fewer
electrons are available to the external circuit and module performance is
temporarily reduced as long as the surface charge is present. However if the
leakage current is channeled correctly - as in a positive-grounded system - the
surface charge has no opportunity to form and module performance is not
affected (in fact, module performance can improve under these conditions.)

Figure 1: Back-contact solar cell subject to surface polarisation.

4 Handbook for Positive Grounded Batteryless Grid Connected PV Systems

SunPower recommends that all systems using their modules be positive
earthed:

Systems using SunPower modules require a transformer-
based, grid isolated inverter bonded to ground on the DC
positive side to receive optimal performance and to provide the
highest possible energy yield.

[SunPower Corporation Technical Note]

Fortunately the surface polarisation effect is 100% reversible and does not
cause any damage to the module. This means that a system that has been
installed according to conventional methods (without system earthing) and is
experiencing decreased performance can be restored to full operational
performance simply by wiring it to be a positive-grounded system.

All new installations involving SunPower solar panels should be positive-
grounded systems in order to avoid complications and ensure the best
performance from the solar array.

A more detailed explanation of the surface polarisation phenomenon is
available for download from the SunPower website:

http://www.sunpowercorp.com/techpapers/polarization.pdf


5 Handbook for Positive Grounded Batteryless Grid Connected PV Systems
3.

POSITIVE GROUNDING SYSTEMS WITHOUT BATTERIES


Most PV systems only require grounding once - on the AC side of the inverter
(Power Conditioning Unit.) On the DC side it is often only the module frames
that are grounded if anything. That is, most PV systems are floating on the
DC side. Grounding and bonding of PV systems and components is dealt
with in Section 5 of AS/NZS 5033 and AS/NZS 3000.

In the case of SunPower back-contact modules however, the PV system
itself must be grounded in order to avoid the effects of surface polarisation.
Furthermore the system ground must be done to positive side. This involves
grounding the main positive current carrying conductor of the PV array. The
question then is:

At what point in the circuit should the
positive pole of the PV array be grounded?
And how should it be done?

PV system grounding is not necessarily required by the Australian Standards,
but must comply with them when done in order to deal with the problem of
surface polarisation. According to the Standard:

When the PV array is earthed, the connection to
earth shall be made at a single point and this point
shall be connected to the installation earth. In
systems without batteries, this connection point shall
be between the PV array disconnection device and
the power conditioning unit and as close as possible
to the power conditioning unit.

[AS/NZS 5033:2005 Section 5.1.2]


Figure 2: PV system ground location from AS/NZS 5033 Section 5.

6 Handbook for Positive Grounded Batteryless Grid Connected PV Systems

The preceding diagram (Figure 2Figure 2: PV system ground location from
AS/NZS 5033 Section 5.) does not identify the polarity of the PV conductors
since the Australian Standard does not specify whether the positive or
negative should be grounded. However if SunPower modules are being
used it must be the positive conductor that is grounded.

Positive grounding can be accomplished by different methods depending on
which inverter is being used in the system. Solar Sales has developed a
simple system for positive grounding of Batteryless Grid Connected PV
Systems with non-SMA inverters. SMA has developed its own method for
dealing with the positive grounding issue that involves installing an additional
Positive Grounding Set. The SMA method is described in more detail further
along in this document.

For Fronius and KACO inverters Solar Sales recommends the use of one of
our pre-wired Grid Connect AC/DC Isolation Enclosures. These
enclosures incorporate the following features:
Compliance with Australian Standards, PV Disconnect (DC), AC
Disconnect, PV Combiner (configuration for one or multiple PV arrays as needed), Fused overcurrent protection for PV strings (if more than one), Incorporation of positive system grounding.

The Grid Connect AC/DC Isolation Enclosure combines the PV Disconnect
and the AC Disconnect into one pre-wired box. The installer will only need to
bring in and connect the following conductors:
DC positive and negative from each string of PV, DC positive and negative to the inverter, AC cable (Active, Neutral, Earth) from the inverter, AC cable (Active, Neutral, Earth) out to the Switchboard for the grid
connection point.

The enclosure is bonded to the main system earth in the Switchboard.
Positive system earthing is accomplished by pre-wiring the PV+ conductor to
the ground bus within the enclosure.


7 Handbook for Positive Grounded Batteryless Grid Connected PV Systems
Figure 3 below shows the basic schematic for the Grid Connect AC/DC
Isolation Enclosure for one string of PV. As can be seen in the schematic
the positive pole is grounded on the load side of the main PV Isolation Switch.
Thus PV+ is connected directly to ground and positive grounding is achieved.


Figure 3: Grid Connect AC/DC Isolation Enclosure.

Using such a setup keeps the issue of positive grounding simple and
manageable. The installer simply has to bring in the PV+ and PV- leads from
the solar array to the DC circuit breaker. The PV+ and PV- must then be taken
from the DC circuit breaker (PV I