Environmental Engineering Reference
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More importantly, it adds a mechanism to limit the current harmonics caused by the harmonic
components in the grid voltage.
For applications with a reasonably high switching frequency, the LCL filter can be designed
in two steps, i.e. to design the LC filter first and then add the grid inductor. For applications
with a very low switching frequency, e.g. at MW-level, extra care should be taken (Rockhill
et al . 2011; Teodorescu et al . 2011). Some other guidelines about the design of LCL filters
can be found in (Araujo et al . 2007; Bolsens et al . 2006; Liserre et al . 2005).
When designing the output filter, it is also important to check the reactive power of the filter
capacitor. It should not considerably affect the rating of the inverter.
1.3.4 Voltage and Current Sensing
Due to the requirement on the galvanic isolation between the power part and the electronic
part of an inverter, voltage and current sensors with galvanic isolation are often adopted to
measure relevant voltages and currents needed by the controller. Moreover, these signals can
be applied to achieve over-current and/or over-voltage protection.
It is possible to use conventional current and voltage transformers to measure currents and
voltages but integrated voltage and current transducers based on the Hall effect are very popular
due to the compact package, accuracy, high linearity, low temperature drift, wide frequency
bandwidth, zero insertion loss, high immunity to external interference, etc. For example, LAH
25-NP from LEM is a compensated (closed-loop) multi-range current transducer (rated up to
25A) using the Hall effect (LEM n.d.a.). It can be mounted directly on a printed circuit board
to measure AC, DC and pulsed currents. A sketch view of LAH 25-NP is shown in Figure
1.34(a), where R M is the measuring resistor in the secondary circuit. The output from the
sensor is a current that is in proportion to the current measured. The current flows through
the measuring resistor and the resulting voltage can be processed further. The range of the
measuring resistance depends on the type of the detected signal (DC or AC), the operating
temperature and the supply voltage, but the range is quite wide and a suitable value can
be easily chosen. The measurement range of the current can be changed via changing the
connections of the input terminals, as shown in Table 1.3.
LV 25-P is a voltage transducer using the Hall effect (LEM n.d.b). Its sketch view is shown
in Figure 1.34(b). In principle, this is a current transducer that works within a narrow current
range around the rated primary current 10 mA (RMS). A resistor R 1 with an appropriate value
(a) Current sensor LAH 25-NP
(b) Voltage sensor LV 25-P
Figure 1.34 Sketch view of voltage and current sensors
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