Grid-Tie Inverters

In Singapore, most solar power installations are developed as Grid-Tie systems so that each installation can contribute to the overall Power Grid.  All installations will need one or more inverters to transform the DC output of solar panels into the AC power used in the Power Grid.  This post focuses on Grid-Tie Inverters.

Referencing the 2012 edition of the Solar Electricity Handbook by Michael Boxwell (the chapter "Components for Grid-Tie systems"):

Methods of connecting the Inverter to the Solar Array

There are 3 ways to connect solar panels to the inverter.  The first way is the high voltage in series method, where solar panels are connected in series before reaching the inverter.  This design is cost effective, simple, and minimise losses in the solar array due to the high voltage, but it is also dangerous due to the high voltage.  There can also be problems if there is damage to the solar panel wiring, which can become a fire hazard, or if there are reduced output from one or two solar panels, which leads to an overall reduction in solar output due to those few solar panels.

The second way to connect is the low voltage method, where solar panels are connected in a shorter series with multiple series or strings connected in parallel to the inverter.  This method is safer than the high voltage in series method, losing little in power efficiency and isn't as problematic, but it is more costly as the inverter has to be one that accepts strings of solar arrays connections in parallel.  Alternatively, we may also need to get separate inverters for each string in parallel.

The third way is the micro-inverter method and is the most costly, but prices are falling.  Each solar panel is individually connected to an inverter, and all inverters are then connected into the main electrical system.  The advantages are that reductions in output from one or two panels will not affect output from other panels, that there is no high voltage safety issues, that installation, fault finding, maintenance and system upgrading becomes easier, and that solar panel installations can be more flexible.

Grid-Tie Inverters (GTI) considerations

Grid-Tie Inverters (GTI) are special inverters that must synchronise with the Power Grid through generating a pure AC sine waveform.  It must cut off power from the solar array if the Power Grid shuts down.  It must also deal with the widely fluctuating voltage outputs of the solar panels, especially if they are connected in a long series.  Hence, GTI must be certified for use with the Power Grid.

The fluctuating voltage outputs from the solar array must be properly planned for when getting a GTI.  It is important to ensure that the peak voltage of the solar array can be managed by the GTI, not just the nominal voltage of the array.  If the peak voltage of the GTI is exceeded, the GTI will shut down to avoid damage, but this may not be enough to prevent permanent damage to the GTI.  The GTI also has a maximum voltage rating, which must be larger than the maximum open circuit voltage of the solar array.

Next, we need to consider all the power ratings of the GTI: the input power rating - consisting of the nominal power rating, the min and max power range, and the start-up power rating - and the output power rating. 

The input power rating shows the min and max wattage range of the GTI - the bigger the range, the more efficient the GTI is.  The nominal power rating indicates the max DC power that the GTI can transform into AC.  If this rating is exceeded, the additional power will be lost as heat.  If exceeded for too long, the GTI may shut down to avoid overheating.  The min power rating indicates the min amount of power that the solar array must generate for the GTI to start producing output power.  If the max power rating is exceeded, the GTI may be damaged.  If the solar array produces less DC power than the start-up power rating, the GTI will switch off. 

The output power rating shows the max continuous AC power that the GTI can produce.  The output power information includes the output voltage, the nominal output power, the max output current, and the AC frequency.  It also shows the max efficiency rating of the GTI in percentages.  A cheap GTI may be significantly less efficient than the usual 90% to 94%. 

A GTI should also ideally incorporate max power point tracking (MPPT), which adjusts the output voltage of the solar array into the correct voltage of the GTI to remove the inefficiencies of a fluctuating voltage.  This can produce more energy compared to a non-MPPT GTI.

In order to connect solar arrays in the 2nd low voltage method, the GTI must be able to connect to multiple strings, which are connected independently of each other.  This is beneficial when solar panels are of different sizes or face different directions.  Problems with tree shade falling on solar arrays are usually not applicable to Singapore, where most buildings are high rise.

Other considerations include: diagnostics and reporting information (using a self display on the GTI, a separate monitoring unit, or direct or wireless connection to the PC or internet), built in and additional safety protection, and physical installation and operating environmental concerns.