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101 renewable - small wind inverters

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Last Updated
1st of October, 2018

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wind inveter connection diagram.jpg

When the wind gusts,  the wind generator blades are rotating.   At this moment the wind turbine generator it is characterized by an RPM value and a torque value at its own ax. This is the components of the Mechanical Power (kinetic power) harvested by the wind generator. How Mechanical Power it is translated into Electrical Power,  that is the next legitimate question.  As a thumb rule, the wind turbine translates RPMs in Voltage and Torque in Amps.

The wind is not a constant kinetic power source. It is vary over  very short time intervals and the wind generator more or less behaves like an alternative power source. It is far from a pure or modified alternative electrical power source. Therefore we need to rectify the wind turbine generator output value into a DC power output using a rectifier, respective condition the DC rectified obtained voltage up to a value where it becomes high enough to be used by an inverter, thus we will generate a pure alternative power and sync that one with the external grid voltage and frequency, or in case of an off-grid installation we will feed the inverter output power into the critical load distribution panel. Everything must be done as per standard safety and electromagnetic compatibility specified by UL 1741 and CSA C22.2 No. 107.1-01. and  Article 694 from NEC 2011

The inverters  used in wind industry can generally be classified as: 

Synchronous Inverter: wind inverters connected to grid;

Standalone Inverter: wind inverters independent of the grid.

To understand the wind inverter role in power generation we have to go through  the elements which characterize the functions of a wind inverter.

After it is connected to the grid,  the inverter will first initialize and sync  in a process which lasts some minutes from the time the grid connection has been established. Then it will sit in a low power state waiting for wind ready turbine to start exporting raw electric power  almost instantly as soon as there is enough wind input. When working as a grid tie generator, the turbine asynchronous machine connection to the grid it is established through a DC - AC galvanic isolated inverter and it is working based on the wind kinetic power only. When the value received from the wind generator translates to a certain voltage value called 'cut in'  voltage, the inverter will start the wind generator as a motor revving it up to the steady state speed.

From there the turbine asynchronous machine it is speeded up by the wind rotor up to the synchronous speed level and the current it is pulled into the rectifier respective into the inverter , the power it is delivered to the grid. At this moment the electrical connection between the wind turbine generator and the grid it is  open through an inverter. When the output voltage generated on the AC side falls to a certain set value the inverter it is no more pulling in current from the wind generator. At this point we reach the 'cut out' status and the wind generator it is electrically removed  from the grid connection by the inverter. From here the inverter will  initiate a new cut in and cut out cycle. The wind turbine motor mode start-up voltage it is called 'operating turbine mode voltage' and it is characterized by two distinctive values: the cut in and cut off voltage values.

Usually in the case of small wind turbines the programmable cut-in voltage it is set to 60VAC and the cut-out voltage it is set to 30VAC . This will allow some hysteresis which translates in a delay for the wind turbine. When the inverter starts pulling current from the turbine generator and the input voltage drops, it does not instantly go below the cut-out voltage and the inverter starts generating a voltage oscillations resonated in the turbine coils, that in turn can damage the turbine. In this case, the hysteresis translates into a delay enough to allow the wind turbine to catch up with the inverter and avoid voltage oscillations between turbine and inverter.

Rectification and DC booster. Thus, inverters build as dedicated wind inverters have built-in rectification  circuitry. For the inverters which are hybrid type of inverters (working as PV and wind inverter at the same time) or inverters which are reprogrammed to work as wind inverters originally being a PV inverter,  the rectification and the DC boost must  be provided separate and  in many of the cases supported by a dedicated rectifier and controller, " Wind Boy Protection Box " by SMA or " Wind Interface " by Power-One in " Aurora Wind " series.

The Role of Rectifier: The Rectifier it is the first in line to receive the wind generator output voltage and to deliver a constant DC voltage to the DC booster. The Rectifier as an external controller to the main inverter provides over-voltage protection and drives a dump-load braking system (electrical breaking) being connected to an external resistive diversion load.

Operating in Turbine Mode the input voltage range has to avoid electrical breaking dump-load  and the need for an external resistive diversion load. The benefits of having  such a high operational input  voltage allow the inverter to continue to produce the maximum amount of power even during wind gusts that would otherwise cause over voltage on the grid. This is useful in rural locations during lighting local grid loading conditions. Operating Turbine Mode input voltage it is in a range from  30VAC input up to 400VAC and in some cases up to 470VAC input. High input voltage means that the electrical braking or external dump loads used to dissipate excess input power aren't needed.

The role of DC Booster. The input voltage received at the rectifier  it is conditioned in a boost circuit to increases the DC voltage value high enough in order to be shaped into a smooth sine wave and to match the grid voltage. This allows the inverter to export power from a very low input voltage. The DC Booster  allows  the inverter to start working at very low wind turbine output voltage.

Thermal throttling. Wind inverters are rated for continuous full power operation up to 45°C (113°F), they are capable of surviving and producing reduced power up to 60°C (140°F).  They include built-in thermal sensing, and will reduce output power or shutdown automatically if temperatures exceed safe operating conditions. The turbine will not be able to function over such temperatures at high values. That will expose the wind turbine generator to over currents and permanent damage the internal coil system. The wind turbines have own thermal protection but in any case, the inverter must be turned off otherwise the inverter will continue to put an electrical load on the turbine trying to restart  the wind electric generator incorporated into a wind turbine.  Each time the process goes through, until the turbine thermal protection shuts down, the increase in coils  temperature will eventually burn the wind  electric generator.

Sleep mode: The wind inverters are programmed to power off into a sleep mode when for a long period of time it is no power demand on AC side. It is called the idle state. If you leave your inverter on, the battery bank which largely supports the inverter core of logical functions will deplete and put the inverter out of work. Being in the sleep mode the inverter will sense the power demand on the AC side and turns itself on.

Invertor service battery recharge function: Since the inverter is powered by its own battery (bank), the wind inverter has a service charging feature. Thus, when the inverter is in use, it will direct a large fraction of its energy to recharge the service battery. The battery must be a deep cycle battery to cover long periods when the demand is idle and the wind generator is off. That will be able to keep the inverter alive despite a long time of service power lost.

Surge Capacity: The inverter is capable of powering up to provide the short surge of energy required. As industry standard, the surge capacity is between 5% to10% of the nominal output power at the grid.

 

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