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Posted on 01 August 2019

Optimising Modern Solar Installations

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The inverter uses the current transducers

In addition to the incentives and pressures brought by political initiatives such as the Kyoto protocol, the increasing costs of many forms of energy and the search for ‘cleaner’ sources of power are boosting interest in alternative energy such as solar.

By Stéphane Rollier, LEM

 

Many new designs are emerging to make use of these sources of energy at the most profitable and efficient level possible. These designs are supported by today’s electronic technologies, including current transducers.

When electricity generated by solar panels is fed back into the grid (a "grid connected” system), the connection can be made in two ways:

1) The solar array is linked to an inverter, which is connected to the grid via a transformer (Figure 1).

The solar array is linked to an inverter, which is connected to the grid via a transformer

2) The inverter is directly connected to the grid, avoiding the transformer (transformerless system) (Figure 2).

The inverter is directly connected to the grid, avoiding the transformer

Another solution is not to feed the electricity to the grid, but to charge batteries to power autonomous installations. This is the “offgrid“.

For use in applications such as remote buildings: mining settlements, remote settlements in Australia, Canada, or villages in third world countries,  and for road signs, underground lighting, etc.

Today, solar inverters handling power from 500W to 10kW are available on the market, and installations with a capacity up to 500kW are possible, allowing, for example, the continuous lighting of an underground parking floor for a large stadium. System lifetimes of up to 20 years are possible. Both types of system (transformer or transformerless) can supply a single-phase output (for lower power systems) or three-phase output (for high power), depending on the targeted grid and power installation.

Two or three different kinds of inverters are currently in common use, depending on the design goals for the system, which include size, weight, robustness, electrical separation from the grid, price, efficiency and losses. It is important to measure current in all types of solar inverter – to help to improve efficiency and to protect the system.

The transformerless design is the most efficient type, as there are no transformer losses. Sometimes, in this configuration, a stepup converter (DC/DC converter) is used between the photovoltaic (PV) arrays and the inverter (DC/AC), to adapt the voltage from the arrays to the inverter input voltage.

Often, a Maximum Power Point Tracking (MPPT) module is used just after the PV arrays in order to ensure that they work at their maximum power operating levels. A special software algorithm is used with dedicated electronics to control the operating points of the panels (cells), using current and voltage transducers for that function. Generally speaking, one current transducer can then be used to measure the singlephase output (current supplied to the grid) and one to measure the input DC current (from 10 to 25A). In the case of a threephase output, two transducers are then used to measure the AC current of the threephase output. The DC/AC inverter connected to the grid is a full bridge inverter converting the DC signal into a sine wave.

The inverter output current (15 to 50ARMS) flowing to the grid is measured by a transducer for feedback to the controller for pulse width modulation (PWM) sine wave control. Controllers are mainly based on microprocessors or DSPs supplied with +5V and working with voltage references shared with other active components of the electronic control system. LEM’s HMS current transducers operate from a +5V power supply. Their internal reference voltage (2.5 V) is provided on a separate pin, allowing them to be used easily with DSPs or microcontrollers. But, they can also accept an external reference (between 2 and 2.8 V) from these same DSPs, from which they then derive their own reference. This symbiosis between all the electronic elements of the control system makes the overall application more efficient (reference drift cancellation in the error calculation). The HMS current transducers are very well suited for all the current measurements required in solar inverters.

The current transducer can be used for peak current detection, for a comparison of real values versus the setup points. The inverter also uses the current transducers in the system that controls the output frequency. Indeed, the inverter shuts down in a short time (less than 2 seconds) whenever the frequency moves outside a pre-selected range.

Offset and temperature drifts have to be the best possible, because on the grid (AC side), low DC values are necessary with a level that must not be exceeded. Another requirement for the grid connection is that DC current must not be supplied into the grid. The DC current generated by the transducer offset or IGBT commutation would cause difficulties in the network. This current could generate a saturation in the transformer, which would generate more losses and more harmonics in the network. With a transformerless configuration, this is less of an issue.

The accepted level differs from country to country, but common requirements are 0.5 percent or 1 percent of the nominal output current or, in some countries, it is a defined limit (20mA in the UK, 1A in Germany and Benelux, 100mA in Japan, or 50mA in China and the USA). If the DC current is above the limit, the system must be disconnected from the grid. Today it is still not clear if it is a requirement to measure the DC current or just to detect the threshold.

In future solar designs, this current could be compensated. The DC component would be calculated by measuring the mean value of the AC current; which represents the DC component. Therefore, the DC offset of the current transducer used in the control loop of the inverter should be as low as possible. Also, DC offset as a result of the IGBT’s switching delay in the inverter should be avoided or kept as small as possible. The consequence of this DC offset can be a saturation of the network distribution transformers. To decrease this DC offset, new topologies of inverters are being developed.

HMS current transducers measuring only 16 (L) x 13.5 (W) x 12 (H) mm and integrating the primary conductor are ideal when the space for current measurement on the printed circuit board is tight. The models are directly surface-mounted onto a printed circuit board, reducing manufacturing costs, and avoiding mixing different soldering processes. Despite these small dimensions, 8 mm creepage and clearance distances are achieved in the HMS design. Cumulated with a CTI of 600 for their plastic case, this allows HMS to provide high isolation performance (test isolation voltage : 4.3 kVRMS/50 Hz/1 min).

Image 1

Four standard models are available to cover nominal AC, DC, pulsed and mixed isolated current measurement of 5, 10, 15 or 20ARMS, up to 50kHz, with a measuring span of up to ± 3 x IPN. The same mechanical design is used for all four models so that they can be used to measure current across a complete range of end products. Gain and offset are fixed and set so that, at Ipn, the output voltage is equal to Ref in or Ref out ± 0.625 V.

A unique LEM ASIC designed for use with open-loop Hall-effect technology has been used to provide performance improvements. These include better offset and gain drifts and linearity, in addition to an extended operating temperature range (-40 to +85°C) compared to traditional discrete technology.

The transducers are CE marked and conform to the EN 50178 standard. They can be used in industrial applications such as power inverters (solar, wind, etc) as well as in home appliances, variable speed drives, UPS, SMPS, and air-conditioners to make them more efficient.

 

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