Coupled-Inductor-Based DC Current Measurement Technique for Transformerless Grid-Tied Inverters

Grid-tied photovoltaic inverters must fulfill several requirements, including high efficiency and reduced cost and complexity of the overall system. Hence, transformerless operation is advantageous in order to achieve the prior requirements. Meanwhile, such operation results in several demerits, such as the dc current component injection into the grid. This component should be effectively mitigated in order to avoid some impacts, such as the saturation of the transformers in the distribution network. On the other hand, limiting this component up to few milliamperes is a challenging issue due to the various measurement errors. Accordingly, different blocking and measurement techniques have been proposed and studied to overcome this issue, where some demerits are seen behind each technique such as the implementation complexity, the common-mode voltage problems, and the high filter requirements. Moreover, none of them measures the dc component directly, but predicts its value using different approaches. Hence, this letter proposes a new technique to measure this dc current component with high accuracy using a coupled inductor combined with a small-range Hall effect current sensor in order to achieve the lowest possible cost with the highest possible accuracy. The proposed technique is introduced, analyzed, and tested experimentally to verify its principle of operation. Also experimental measurement of the dc current component using a 5-kVA transformerless grid-tied voltage-source inverter is introduced with and without the proposed technique in order to validate its operation.


I. INTRODUCTION
The use of grid-tied photovoltaic (PV) inverters is continuously increasing and several requirements have to be fulfilled in order to achieve the highest possible performance of the employed power conditioning stage (PCS).Recently, transformerless operation, i.e. eliminating the low frequency power transformer from the grid-tied PV inverter, is followed in order to improve the PCS efficiency and reduce its cost and volume [1]- [5].Even though the gained merits from the transformerless operation, it results in several problems, where the dc current component injection into the grid is a major one.This dc current component might exist due to one or more of the following reasons [5]: • an asymmetry in the switching scheme; • some problems in the gate drive circuits; • non-identical turning ON and OFF times or voltage drops of the employed switches; • measurement errors in the employed sensors.Furthermore, it is of importance to mitigate this dc current component in order to avoid the following impacts [5]- [8]: • affecting the operating point of the power transformers along the distribution network; • increasing the system losses due to the circulation between the inverter phase legs or between the paralleled inverters; • affecting the normal operation of the connected ac motors; • corrosion problems in the grounding wires.
Several standards have been established in order to consider this issue and limit its effect on the power network elements.For example, in the IEEE standard 1547-2003 [9] and the Italian standard CEI 0-21 [10], the injected dc current component shall not exceed 0.5% of the nominal output current, while the Australian standard AS4777.2[8] has established a limit of 0.5% of the nominal output current or maximum 5 mA.Accordingly, it is a challenging issue to measure such few milliamperes within several tens of amperes due to the measurement accuracy, i.e. very high accuracy current sensor is mandatory, which is usually expensive, especially if the target is to limit the component below the 5 mA limit.Hence, several research activities have been conducted in order to address this issue and fulfill the prior standards requirements.These different research activities have proposed several methods to limit this dc current component by considering a blocking method or a measurement approach combined with a dedicated control scheme.It is worth to note that there are some current sensors with very high accuracy that can be used to detect the dc current component with higher accuracy, but these sensors are usually more suitable for measurement equipment (i.e. it is not usually suitable for the PV application due to the cost constrains).
In literature, two blocking approaches have been introduced in [3], [11] using blocking capacitors.The authors in [3] discussed the effect of using a half-bridge single-phase voltage source inverter (VSI) to block the dc component utilizing the dc link capacitors.Meanwhile, the authors in [11] proposed to use an output series dc capacitor instead of an ac capacitor to achieve the same goal, in which a certain control scheme is utilized in order to control the average voltage across the output capacitor, in addition to using protective diodes for over and reverse voltages protections.On the other hand, several measurement techniques combined with control algorithms have been introduced as well.In [12], the authors demonstrated how to detect a small level of dc current in the single-phase full-bridge grid-tied inverters.This measurement technique uses a small 1:1 voltage transformer and an RC circuit to detect the dc voltage.It is quite difficult to measure the dc voltage with high accuracy due to the existing ac voltage, which increases in magnitude with the increase of the employed phase shift between the grid and the inverter voltages.On the other hand, the authors in [13], [14] are discussing a measurement technique to detect the dc offset between the inverter output terminals using RC filters and voltage transformers, in which the common mode voltage problems should be effectively considered.Similarly, the authors in [15], [16] are discussing the dc current component measurement and mitigation using output low-pass filters and voltage sensors for single-phase and three-phase systems.Also in [17], the authors are using a reactor with a specific design combined with a current transformer and an LC filter to mitigate the dc component.This reactor is working at the knee point of the magnetizing curve or higher, where this measurement technique is seen as a bulky and complicated one.The authors in [18], [19] are discussing the effect of using means of dc offset calibration on the dc current injection by measuring the dc link current, in which the gain and the linearity errors are neglected and they might affect the measurement in different scenarios.
Among the different measurement techniques discussed before, none of them measures the dc current component directly ::::: except ::: the ::::::::: technique ::::::::: introduced ::: in :::: [24].These prior art techniques physically block the dc current component or predict its value from the measured voltage.Hence, this paper is proposing a new measurement technique to mitigate this dc current component.The proposed technique utilizes a 1:1 coupled-inductor combined with a small range current sensor in order to extract this component with the highest possible accuracy and lowest cost and complexity.
The rest of this paper is organized as follows: Section II discusses the proposed technique, showing its principle of operation and its design challenges.The control scheme of a grid-tied voltage source inverter (VSI), including the proposed measurement technique, is introduced in Section III.Finally, experimental results are shown in Section IV, where the results include the obtained dc current component measurement with and without the proposed measurement technique using a 5 kV A transformerless grid-tied VSI.

II. PROPOSED DC CURRENT MEASUREMENT TECHNIQUE
It has been seen that several demerits exist behind the prior mentioned dc current injection blocking and measurement techniques.Furthermore, none of them measures the dc component directly, but predicts its value using different approaches.
Hence, this letter proposes a different technique by measuring the injected dc current itself, i.e. it does not predict for its value from the equivalent dc voltage.This proposed technique uses a 1:1 coupled-inductor combined with a small range hall effect current sensor in order to extract the dc current as shown in Fig. 1(a).

III. PV INVERTER CONTROL SCHEME IN GRID-TIED MODE WITH THE PROPOSED TECHNIQUE
Grid-tied PV inverters are commonly controlled using two control schemes.The first one is based on a proportional-integral (PI) current controller in the synchronous reference frame [25], [26], while the second one is based on a proportional-resonant (PR) or a proportional-integral-resonant (PIR) current controller in the stationary reference frame [27]- [29].In this paper, a single loop PIR current controller is used, whose block diagram is shown in Fig. 2 [29].

IV. EXPERIMENTAL VERIFICATION
In order to verify and evaluate the proposed technique, a dc current component measurement board has been designed and implemented as shown in Fig. 3.This board comprises two sets of the proposed dc current measuring unit (A and B), where each set has a 1:1 coupled-inductor combined with a hall effect current sensor in order to measure the dc component in a 5 kVA three-phase three-wire grid-tied system.Each coupled-inductor utilizes an RM 14 ungapped ferrite core, whose material is N41, and a CT 0.4-P hall effect LEM current sensor, where the nominal measuring range is equal to ±400 mA.:::::   The implemented dc current component measurement board, shown in Fig. 3, has been tested separately in order to validate the introduced concept.At first an ac current is passed through the primary winding, and the current in the secondary winding and the sensor output are measured as shown in Fig. 4(a).Then, a small dc current has been passed through the primary winding, and the current in the secondary winding and the sensor output are measured again as shown in Fig. 4(b).These figures confirm the proposed concept and shows the possibility of detecting a few milliamperes of dc current easily and with high accuracy.Note that there is an RC filter after the sensor output which is increasing the phase angle of the residual ac component and reducing its amplitude.to ::: the :::::::::::: measurements ::::: noise :: in ::: the ::::::: current ::::: probe.
Finally, this board has been used in a 5 kVA three-phase three-wire grid-tied VSI.The control scheme of this inverter is as shown in Fig. 2.This prototype is utilized in order to examine the functionality of the proposed measurement technique.The dc current component is measured twice in the different phases using a KinetiQ PPA5530 power analyzer with and without the proposed measurement technique, where in the latter case a PIR current controller is utilized (i.e. an integral term has been added to limit the dc component).Fig. 5 shows the obtained measurement, in which the dc component has been effectively mitigated using the proposed measurement technique.:::: Note :::: that ::: the ::::: added :::::: losses ::::: using ::: the :::::::: proposed ::::: board :::: can :: be ::::::::: calculated

Fig. 2 .
Fig.2.Control scheme of a grid-tied three-phase three-wire voltage source inverter (VSI) using a proportional-integral-resonant (PIR) current controller, in which the proposed dc current component measurement technique is utilized considering two-phases.

Fig. 3 .
Fig. 3. Implementation of the proposed dc current component measurement technique for a 5 kVA three-phase three-wire grid-tied system.

i s ( 3 AFig. 4 .Fig. 5 .
Fig. 4. Experimental results of testing the implemented dc current component measurement board, where the current through the primary and the secondary windings, and the sensor output are shown.(a) considering an ac current component in the primary winding; (b) considering a dc current component in the primary winding.