New technologies are allowing photovoltaic (PV) inverters to switch at ever higher frequencies. Consequently, these inverters are becoming much smaller and lighter. Meanwhile, international competition and reduced available subsidies for new installations brings strong pressure on cost. The current transducers used in PV inverters must follow the trends: they must have a reduced footprint while maintaining or improving performance at a lower cost, as compared to the transducers they replace. Typically, PV installations use current transducers in three places. On the DC side, they are needed for the maximum power point tracking (MPPT) system. On the AC side, they are used to define the parameters of the output current waveform, and in the safety system as an important component of ground fault or residual current monitors (RCM).
Figure 1: Main components around an inverter in a PV system typically used in residential installations of up to approximately 20kW. Several such inverters may be combined to make the complete installation, which is connected to the grid via metering apparatus.
During the last decade, new silicon MOSFETs have been introduced in inverters, and in the future MOSFETs based on SiC and GaN will begin to replace those using silicon. This allows higher frequency switching, which in turn means that reactive components (inductors, capacitors) of lower value and with smaller physical dimensions can be used. A 2kW inverter available in 2010 and weighing over 20 kg, according to the manufacturer´s datasheet, has been replaced in 2016 by a model weighing less than 10 kg. In order for the current transducers used as measurement devices in a PV system to continue to use a negligible part of the overall space and weight budget, the transducer size must also reduce without any performance degradation. Similarly, their cost must reduce to follow the downwards cost trend of the complete inverter system.
The three LEM current transducers in Figure 1 all contain custom proprietary CMOS ASICs with fully integrated Hall cells. On the DC side of the inverter, there is an open-loop current transducer; on the AC side, a closed loop current transducer is used for the inverter control system and at the output a differential closed-loop transducer is used for residual current monitoring (RCM).
Figure 2: The voltage waveforms on the DC and AC sides of the inverter. Note that in a transformer-less system, the DC side does indeed have a DC voltage corresponding to the output of the photovoltaic cells between the PV+ and PV- nodes (this may be increased by a DC-DC converter) but each of the PV nodes also has an AC voltage whose peak value is similar to the peak output voltage of the AC side. If not considered at the system level, this represents a serious safety hazard.
DC side current transducers
Depending on the illumination intensity of the PV cells, the load which maximizes the power transferred from them varies, and so the control system uses a real-time MPPT algorithm to load the cells for maximum power transfer. In the case of motorized PV panels, the MPPT algorithm can also be used to obtain the optimum orientation. Since the target of the algorithm is simply to find the peak in the power transfer, the accuracy requirement on the current transducer used is not demanding, and a simple open-loop transducer is ideal for this purpose.
AC side current transducers
The transducer shown after the inverter in Figure 1 is a key element of the control loop which drives the inverter switches, and so governs the accuracy of the current output waveform. It must have a fast response time, low noise and good linearity, and in particular the offset and its drift with temperature must be low so that the DC component of the current injected into the grid meets regulatory requirements.
Closed-loop transducers have an architecture which, due to the transformer effect, provide good speed, noise and linearity performance. Historically, the low offset requirements have been met using a fluxgate as the magnetically-sensitive element. However, many of LEM´s current transducers now achieve low offset (and low offset drift) thanks to design innovations in the CMOS ASIC used. The ASIC includes Hall cells and low offset amplifiers merged in a new patented architecture, resulting in sensors whose construction is simpler than that of the fluxgate families with similar performance.
Residual current measurement for safety
The nodes PV+ and PV- of figure 1 are physically large in a typical PV system. The average voltage on each node, relative to ground, is half of the voltage from the PV cells, but upon this is added an AC voltage whose peak-peak value is similar to that of the cells. In the event of a person touching the PV+ or PV- nodes (or, in general, any node on the DC side of the inverter), a leakage current will flow out of the system through the person to ground.
Because there is only one node in the system whose potential is maintained at ground level, the N node at the output, this leakage must flow back into the system through the N node, and this will cause a DC current imbalance, or residual current, between the L and N outputs. This residual current must be detected, permitting the system to take very fast action to protect the person who has caused the residual current to flow. Among the challenges in RCM are:
- The absolute value of the current to be detected is low, some 10´s of mA, and so the transducer offsets must be low enough for this level of current to be detected;
- The AC current at the output is between zero and 10´s of A, and the residual current must be detected in the presence of this;
- Capacitance between the PV panels and ground mean that there is always some current flowing to ground, and the system objective is to distinguish these from additional residual current caused by dangerous human contact.
Figure 2 shows the leakage current path in a simplified inverter system using a LEM closed-loop differential current sensor for RCM. Of the three challenges listed, the sensor achieves the first and second challenges through a special differential measurement design dedicated to RCM; the third is achieved by applying a signal processing algorithm to the transducer output.
Figure 3: The RCM principal: a Hall cell ASIC is the heart of a closed-loop transducer. The AC currents I1 and I2 cancel, and the low residual current is detected by the Hall cell ASIC and compensated by a secondary winding.
Detailed analyses of the effect of the position of the primary conductors in figure 5 shows that the cancellation of I1 and I2 is not perfect and the residual magnetic field in the air gap depends on their position. Therefore, it was decided to fix the stationary primary positions by placing them on a multi-layer PCB inside the transducer. Furthermore, only a few dozen turns are required for the secondary coil for RCM, which means they can also be designed as traces on the PCB. In this way, an innovative sensor has been designed whose construction is far simpler than that of earlier sensors. Having the primary conductors on a PCB limits the maximum primary current, but the allowed value of 35 A in each conductor is more than enough for domestic installations.
With primary currents of this magnitude, the design of the PCB on which the transducer is mounted is important. Simulations have shown that with an optimized design the temperature rise in the transducer due to a 35 A primary current is limited to a 13o C increase.
Figure 6 shows a simplified drawing of a transducer with its package removed. For test purposes, an additional coil is wound on the ASIC PCB concentrically with the secondary circuit. This is useful for a system test: a current passed through it will give a transducer output in the same turns ratio as the measured current difference between the primaries.
Figure 4: Transducer with planar primary conductors and magnetic core
The ASIC is designed for minimum offset, and the offset referred back to the input current is reduced by placing a hole in the PCB under the ASIC to provide the smallest possible air gap in the magnetic circuit. Because of the transducer´s high sensitivity, a magnetic shield (not shown in figure 4, for clarity) is placed around the ASIC and air gap within the device.
In general, the leakage currents detected by the transducer will have an AC and a DC component and each user application will implement a specific algorithm on the transducer output to determine when a leakage is ‘excessive´ and then take appropriate action. A particularly challenging case occurs when there is a large natural and variable AC leakage component (depending on ambient humidity, for example) through parasitic capacitances, and the extra leakage caused by a person touching the DC side must still be detected. The impedance presented by a person is largely resistive, and so, as shown in Figure 8, the extra current flowing makes almost no difference to the RMS value of the leakage current; the main effect is a change of phase.
Figure 5: The effect of adding a resistive path to the leakage
Generally, there is also noise adding to the real and reactive currents of figure 5. In a case where only one known frequency must be analysed in a sampled waveform, the Goertzel algorithm is particularly efficient. In figure 6, a 30 mA rms ‘person leakage´ current is added to a 300 mA rms ‘capacitive leakage´ current with 7.5 mA rms of noise at time = 0.1 s. The visible effect on the total leakage current is quite burried, but after treatment with the Goertzel algorithm the 30mA current step is easily recovered. If this leakage exceeds a predefined threshold value, appropriate action can be taken at the system level.
Figure 6: Simulation of residual current during fault and output of the Goertzel algorithm
Recent advances in current transducers have reduced size and cost while maintaining or improving performance. By moving the complexity of transducer design into the custom Hall effect ASICs, transducers can now be designed without the magnetic circuit or fluxgate component previously needed.
Written by David Jobling, Thomas Harge Stephane Rollier, LEM SA