Why does a solar inverter need a reference current from a grid
Why does a solar inverter need a reference current from a grid while in operation? Inverter fed by solar panel needs a reference current because solar panel can inject power equal to generated by it which is determined by the current as solar array voltage is controlled by MPPT (Maximum Power Point Tracking) controller). MPPT output of Solar array voltage is further controlled by either DC-DC converter or inverter voltage control match the inverter output to load or grid voltage.
Reference current is obtained by comparison of MPPT output and DC link capacitor voltage and feeding this comparison outcome to the PI (Position-integral) or any other controller. This reference current is compared with output current and that gives the switching sequence of inverter. Figure below shows the block diagram of a inverter control,
Why does a solar inverter need a reference current from a grid while in operation?
Solar Inverters, not all types require the grid connectivity.
A grid connected or grid tied Solar Power plant requires a connection to grid while in operation for various reasons to serve it’s purpose of net- metering & bill savings, by injecting excess plwer generated by the SPV plant to the grid so as to spin the billing in reverse direction, thus net metering.
Now, we’ll understand why it needs a grid connection to do this.
All grid tie Solar Inverters have to comply with an international norm for SPV called the IEC 61683, aka Anti islanding operation.
where this connection to grid is utilized for islanding detection (unavailability of grid is referred as islanding) based on few factors like
- Voltage
- frequency
- reactive power infusion
- power factor
- surge
- load mismatch
These factors help the SPV system to generate power & utilize it via captive loads & feed in the excess power into the grid by mimicking the factors in a range pre-programmed in factory settings for each country.
Solar Inverters get disconnected from the grid in matter of milliseconds if any of the above stated is out of the working range for the inverter so as to protect the SPV system from interfering with grid failures & surges.
Anti islanding operation – disconnecting of the inverter from grid, on uneven grid operation or unavailability of grid, so as to not inject any power into the transmission lines, which might be under maintaining work,
thus protecting the line man operating on repairs from. being electrocuted by SPV powe injection into the grid.
This IEC compliance is not applied on offgrid/hybrid inverters,
because in offgrid as the name suggests, in hybrid the inverters are programmed to utilize power from various sources.
Hope this gives you an insight of why grid connection is important for a Grid tied Solar inverter.
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