Application of Current Sensors in Solar Installations

In addition to the momentum and pressure from political strategies such as the Kyoto Protocol, the increasing cost of many forms of energy and the search for "cleaner" sources of power are driving interest in alternative energy sources such as solar energy. Many new designs are emerging to make the most effective and efficient use of these energy sources. These designs are supported by today's electronic technology, including current sensors.

When the electricity generated by solar panels is fed back into the grid (a "grid-connected" system), two connection options are possible:

Connect the solar panels to the inverter and connect them to the grid via a transformer (Figure 1), or connect the inverter directly to the grid to avoid using a transformer (transformer-less system) (Figure 2).

Figure 1

Figure 2

Another solution is not to feed the power into the grid, but to charge batteries that are used to power automation. This is "off-grid". For remote building applications such as mining subsidence, remote subsidence in villages in Australia or Canada or third world countries, as well as road signs and underground lights.

Today, solar inverters are available on the market that handle power from 500W to 10KW, and installations up to 500KW capacity are possible, such as continuous lighting in underground parking lots of large stadiums. System service life may be up to 20 years. Both types of systems (with and without transformers) can provide a single-phase output (for smaller power systems) or a three-phase output (for large power systems), depending on the target grid and power installation.

Two or three different inverters are used today, depending on the system design objectives, including size, weight, robustness, electrical separation from the grid, price, efficiency, and losses. To help improve efficiency and protect the system, it is important to measure the current in all types of solar inverters.

Transformerless design is the most efficient type because it does not incur transformer losses. In this configuration, a boost converter is sometimes used between the photovoltaic (PV) array and the inverter (DC/AC) to convert the module voltage to the inverter input voltage.

Maximum Power Point Tracking (MPPT) components are usually used just after the PV array to ensure that the array operates at its maximum power operating level. A special software algorithm is applied together with dedicated electronics to control the panel (battery) operating point using current and voltage sensors for the tracking function. Generally, one current sensor can be used to measure the single-phase output (current supplied to the grid) and another sensor can be used to measure the input DC current (10-25A). In the case of a three-phase output, two sensors can be used to measure the AC current of the three-phase output. The DC/AC inverter connected to the grid is a full-bridge inverter that converts the DC signal into a sine wave.

The inverter output current (15-50ARMS) flowing into the grid is measured by a sensor in order to be fed back to the controller for pulse width modulated (PWM) sine wave control. The controller is mainly based on a microprocessor or DSP (digital signal processor) that is supplied with +5V and shares the reference voltage with other active components of the electronic control system. The HMS current sensor from LEM operates from a +5V supply. Its internal reference voltage (2.5V) is provided by a separate terminal, allowing the sensor to be easily used with a DSP or microprocessor. However, the sensor can also accept an external reference voltage (between 2V and 2.8V) from these same DSPs, from which the sensor obtains its own reference voltage. This symbiosis between all electronic components of the control system makes the entire application more efficient (reference drift in error calculations is eliminated). The HMS current sensor is very suitable for all current measurements required by solar inverters.

Current sensors can be used for peak current detection, for comparison of the actual value with the set point. Inverters also use current sensors in systems that control the output frequency. In fact, whenever the frequency moves outside the preselected range, the inverter stops operating for a short while (less than two seconds).

Since low DC values ​​that must not be exceeded are required on the grid (AC side), offset and temperature drift must be as good as possible. Another requirement for the grid connection is that no DC currents can be fed into the grid. DC currents generated by sensor offsets or IGBT communication can cause network problems. This current can saturate the transformer, which in turn generates more losses and more harmonics in the network. For transformerless configurations, this is not a big problem.

Although each country has its own different acceptance values, the common requirement is 0.5% or 1% of the nominal output current, or in some countries a limit value (20mA in the UK, 1A in Germany and the Benelux, 100mA in Japan, 50mA in China and the United States). If the DC current is greater than this limit, the system must be disconnected from the grid. There is no clear definition as to whether the DC current needs to be measured or just the critical value is detected.

In future solar designs, this current may be compensated. The DC component is calculated by measuring the average value of the AC current; this represents the DC component. Therefore, the DC offset of the current sensor used in the inverter control loop should be as low as possible. Also, the DC offset due to inverter IGBT switching delays should be avoided or minimized. This DC offset can lead to saturation of the network distribution transformer. New inverter topologies are being developed to reduce this DC offset.

The dimensions of the HMS current sensor are only 16 (length) x 13.5 (width) x 12 (height) mm.

Furthermore, when space on the PCB for current measurement is tight, it is ideal to integrate the primary conductor. Surface mounting these modules directly onto the PCB reduces manufacturing costs while also avoiding the confusion of various soldering processes. In addition to its small form factor, the HMS design also achieves 8mm creepage and clearance distances. Accumulated with a 600 CTI in its plastic case, the HMS has high isolation performance (test isolation voltage: 4.3 kVRMS/50 Hz/1 minute).

Four standard modules are available covering nominal AC, DC, pulsed and mixed isolated current measurements, which can measure 5, 10, 15 or 20 ARMS currents up to 50kHz over a wide measurement range of ± 3 x IPN. The mechanical design of the four modules is exactly the same, so the modules can be used to measure currents across the entire end product range. Gain and offset are fixed and set so that at lpn, the output voltage is equal to the input or output reference voltage ± 0.625 V.

A unique LEM ASIC designed for use with open-loop Hall-effect technology has been used to improve performance. These performance improvements include better offset and gain drift and linearity, in addition to a wider operating temperature range (-40 to +85°C) compared to conventional discrete technologies.

The sensor is CE marked and complies with EN 50178.

These sensors can be used in industrial applications such as power inverters (solar, wind, etc.) as well as in home appliances, variable speed drives, UPS, switch mode power supplies (SMPS), and air conditioners to make these devices more efficient.