Low Frequency Common Mode Research Articles

Nonisolated Three-Phase Current DC-Link Buck–Boost EV Charger With Virtual Output Midpoint Grounding and Ground Current Control

Non-isolated three-phase AC/DC converter concepts facilitate more compact and more efficient realizations of future EV chargers. However, without the galvanic isolation and/or high common-mode (CM) impedance provided by an isolation transformer, non-isolated chargers must employ other means to suppress CM leakage currents to ground sufficiently and to prevent nuisance tripping of mandatory residual current devices (RCDs). Typically, the required EMI filters reduce high-frequency (HF) CM leakage currents to uncritical values. However, low-frequency CM voltages, e.g., generated by third-harmonic injection, may drive significant LF CM currents through the parasitic capacitances of the DC output (including the battery pack) to protective earth (PE). Therefore, considering a non-isolated three-phase buck-boost current DC-link PFC rectifier system that consists of a buck-type current-source rectifier (CSR) stage and a three-level boost-type DC/DC-stage, this paper first proposes a virtual grounding control (VGC) of the DC output voltage midpoint. VGC employs the DC/DC-stage to compensate the LF (third-harmonic) CM voltage inherently generated by the CSR-stage, and thus controls the LF CM voltage between the DC output midpoint and PE to zero. This enables further a direct connection of the DC output midpoint to PE, where an additionally proposed ground current control (GCC) ensures near-zero LF CM leakage current. The proposed concepts are verified with a 10kW hardware demonstrator (power density of 6.4 kW/dm3 or 107.5 W/in3, full-load peak efficiency of 98.5%) considering TT (Terra-Terra) and TN (Terra-Neutral) grounding systems. With a direct connection of the DC output midpoint to PE, GCC limits the LF CM leakage current to < 6mA RMS, i.e., significantly below typical RCD trip levels, and, using the human-body impedance model according to UL 2202, achieves a test voltage of 110mV that is clearly below the most stringent limit (250 mV) of the standard.

A Transformerless Converter with Common-Mode Decoupling in Low-Voltage Hybrid Grids

Compared with isolated converters, transformerless converters are a preferred choice in low-voltage grids due to their efficiency and lower cost. However, leakage current and common mode (CM) voltage appear through the converter and ground in hybrid grids, which consist of AC and DC subgrids. The leakage current and CM voltage seriously influence operation and power quality in low-voltage distribution systems. This paper proposes a common-ground-type (CGT) converter equipped with a CM decoupling control strategy to eliminate the leakage current and CM voltage. A CM model is derived, and the leakage current and CM voltage are analyzed in detail. A CGT four-leg converter is constructed to eliminate the high frequency CM voltage. A dual DQ current control loop is developed to suppress the DC double-frequency ripple. Additionally, an active damping method is proposed, based on the neutral current feed-forward plus inductor current feedback, to attenuate the low frequency CM voltage. The proposed converter and control strategy guarantees excellent performance in suppressing leakage current and CM voltage. The DC voltage of the converter connected to the DC grid maintains stability and symmetry. The leakage current is significantly reduced, and the leakage current suppression performance is improved by 83%. The high frequency CM voltage is attenuated from 50%udc to 2%udc, and the low frequency CM voltage is suppressed from approximately 32%udc to 3%udc, which is a significant improvement compared with the traditional method. In addition, the proposed control strategy has good transient performance when the load changes abruptly. Finally, an experimental platform is established to validate the feasibility and performance. The experiment results showed that the proposed control strategy improves the system performance and power quality.

A Study on Various Causes of Low Frequency Components in Common Mode Voltage of a Space Vector Pulse Width Modulated Three– Phase Quasi–Z–Source H–Bridge Inverter Fed from a Three–Phase Diode Bridge Rectifier

Abstract The presence of low frequency components in the common mode voltage can cause harmful electromagnetic interference. A critical study on various causes of low frequency components in the common mode voltage of a space vector pulse width modulated Quasi–Z– source three–phase H–bridge voltage source inverter fed from a three–phase diode bridge rectifier is presented in this paper. The Quasi–Z–source network is utilized in boosting the rectified dc voltage which increases the overall voltage gain. The study considers the effect of boosting on the low frequency components. The input three-phase diode bridge rectifier has its influence in modulating the instantaneous common mode voltage and contributes low frequency components. The unbalanced three-phase supply can contribute additionally ac supply frequency component in the common mode voltage. The major contribution of this paper is the analytical, simulated and experiment-based study on various causes of the low frequency common mode voltages due to the combined action of the input non-ideal three phase grid, the front-end diode bridge rectifier as well as the load-end Z-source H-bridge three-phase inverter feeding a three-phase inductive load, while operated through a space vector pulse width modulation strategy.

Design of the Number of Transformer Shielding Winding Turns for Minimizing Low-Frequency Common-Mode Noise in Flyback Converters

Transformer shielding techniques are widely applied to suppress common-mode (CM) noise. As an important shielding method, winding shielding gains lots of attention because of its convenience and cost effectiveness. For suppressing CM noise effectively, the number of shielding winding turns should be carefully decided. This paper proposes a design method of the number of transformer shielding winding turns for minimizing low-frequency CM noise in flyback converters. This is realized through exploring the relation between low-frequency CM current and the number of shielding winding turns. With this method, the optimal number of shielding winding turns can be obtained without calculation and comparison of CM currents in converters with different shielding winding turns. By comparing CM noise measurement results, the proposed method is validated. The proposed method can be a guideline on the design of transformer shielding winding.

A Proposed Terminal-Ground EMI Filter for Reduction of Conducted Emissions Considering Cable Radiation and Safety

This article proposes an optimal design of a terminal-ground electromagnetic interference filter (TGEF) for reduction of common-mode (CM) conducted emissions (CEs) with considering cable radiations and safety. The TGEF is free from risks of magnetic saturation and heat problem. In experimental tests with a 3.3-kW air conditioner system, a prototype TGEF well reduced low-frequency CM CE noises, but increased disturbance power (DP) due to cable radiations. To investigate the principle of cable radiations, the impedance and parameters in the DP measurement setup are extracted by measurements using a vector network analyzer and validated with full-wave simulations. The effects of the TGEF on total CM DP currents at power cables are also experimentally demonstrated and modeled in simulations. Based on the analysis of DP measurement environments, a quantitative guideline for a TGEF design was proposed without increasing cable radiations while maintaining CE noise attenuation performances. The performances of the optimized TGEF are then validated by CE and DP measurements in the real product system. Moreover, safety of the optimized TGEF in a ground-fault condition was experimentally confirmed.

Elimination of Common-Mode Choke Saturation Caused by Self-Resonance of the EMI Filter in a Variable-Frequency Drive System

The saturation of a common-mode (CM) choke is a practical problem in the electromagnetic interference filter application. As an important factor of the CM choke saturation, the self-resonance of the CM path is analyzed based on the low-frequency CM model of the variable-frequency drive system. Combining the analysis and the conducted emission test results of the system, the main cause of the CM choke saturation in our experiment is studied. Meanwhile, a novel online impedance measurement method is proposed to observe the saturation effect of the CM choke. To eliminate the saturation effect, a new method is proposed in this paper. A damping resistor is connected in parallel with Y capacitor of an electromagnetic interference filter to suppress the self-resonance. Through comparing the experimental results of the original filter and the improved filter, the effectiveness of the resonance suppression method is validated and the saturation of the CM choke is prevented.

A Bidirectional High-Efficiency Transformerless Converter With Common-Mode Decoupling for the Interconnection of AC and DC Grids

This paper presents an interface ac–dc converter to connect the 380 V bipolar dc microgrid with the 240 V split-phase single-phase ac utility. The design targets are high efficiency, leakage current attenuation, and symmetric bipolar dc bus generation. A transformerless two-stage topology is proposed to decouple the common-mode (CM) voltage between the connected ac and dc systems. The two-stage structure also reduces the required capacitance in single-phase power conversion by allowing a large voltage ripple on the intermediate dc link. Adaptive dc-link voltage control and interleaving of state-of-the-art silicon carbide (SiC) mosfet s are adopted to achieve high efficiency. Interleaving angles are optimized to minimize the total volume of both differential-mode and CM magnetics. An active CM control method is incorporated to regulate the dc and low-frequency CM voltages on the dc buses with respect to the ground. The resultant dc bus voltages are rendered symmetric to the ground, and the leakage current is suppressed. The generated ±190 V dc bus is suitable for bipolar 380 V dc power distribution systems. Pluggable phase-leg modules with embedded driving and protection circuit are designed and tested, based on which a 10 kW all-SiC converter prototype is built achieving an efficiency >97%.

Third Harmonic Injection SPWM Method Based on Alternating Carrier Polarity to Suppress the Common Mode Voltage

In this paper, a third harmonic injection sinusoidal pulsewidth modulation (THISPWM) method based on alternating carrier polarity (ACP) is proposed for three-phase PWM inverter. Compared with the space vector pulsewidth modulation (SVPWM), the THISPWM has the advantage of lower low-frequency common mode voltage (CMV). However, the phase of the third harmonic injected must be strictly synchronous with the phase of the reference voltage. The synchronization will cause the difficulty of the injected third harmonic generation. First, the relationship between the injected third harmonic and the reference voltage of the PWM inverter output is analyzed to obtain a real-time conversion method, thereby solving the synchronization problem. Then, a modulation method based on ACP is proposed to eliminate zero switch states of the PWM inverter. The middle amplitude-modulated voltage is used to select the negative polarity carrier. In addition, the low-frequency and high-frequency CMV, which are separately defined according to the modulated fundamental voltage frequency and the carrier frequency, are taken into account. Therefore, the analysis results manifest that the proposed method could suppress both the low-frequency and high-frequency CMVs and has the same voltage utilization as the SVPWM. Finally, a three-phase PWM inverter circuit based on an FPGA was built, and the experimental results verified the proposed method.

Low-Frequency Common-Mode Voltage Control for Systems Interconnected With Power Converters

In this paper, an active common-mode (CM) duty cycle injection method is proposed to control the dc and low-frequency CM voltage in grounded systems interconnected by power converters. Specifically, a 380-V dc nanogrid and a single-phase ac distribution system interfaced with a transformerless ac–dc converter is considered. In such architecture, the ac and dc CM quantities are coupled through the ground; while the high-frequency noise is filtered by passive components, the dc and low-frequency CM voltage need be controlled to generate symmetric dc-bus voltages and effectively mitigate the ground leakage current. Using a two-stage bidirectional ac–dc converter as an example, the operating range of the proposed method is identified under different ac and dc voltages and alternative grounding schemes. Further, a CM equivalent circuit is derived and experimentally validated, and used to design the controller of the proposed control system. Experiments with resistive and constant power loads in rectification and regeneration modes are used to validate the performance and stability of the control method. As a result, the dc-bus voltages are rendered symmetric with respect to ground, and the leakage current is suppressed. Finally, the control method is generalized to three-phase ac–dc converters for larger power systems.

A Generalized Common-Mode Current Cancelation Approach for Power Converters

The interwinding parasitic capacitance of transformers and the parasitic capacitance between semiconductor switches and the ground are two major contributors to the common-mode (CM) noise currents in switched mode power converters. In this paper, a generalized CM current cancelation approach is proposed for the reduction of CM noise in isolated power converters. The approach is demonstrated in a forward converter. In this approach, the total effect of the two parasitic capacitances on CM noise is represented with an equivalent parasitic capacitance (EPC) at low frequencies. With this EPC, different CM current cancelation techniques can be efficiently organized to simultaneously cancel the low-frequency CM noise caused by these two parasitic capacitances. Furthermore, the EPC can be used to evaluate and quantify the performance of CM noise reduction techniques. Both theoretical analysis and experimental results show that the proposed approach is easy to implement and can significantly attenuate low frequency CM noise and therefore greatly reduce CM filter size and cost.

Common mode noise rejection properties of amplitude and phase noise in a heterodyne interferometer

High precision metrology systems based on heterodyne interferometry can measure the position and attitude of objects to accuracies of picometer and nanorad, respectively. A frequently found feature of the general system design is the subtraction of a reference phase from the phase of the position interferometer, which suppresses low frequency common mode amplitude and phase fluctuations occurring in volatile optical path sections shared by both the position and reference interferometer. Spectral components of the noise at frequencies around or higher than the heterodyne frequency, however, are generally transmitted into the measurement band and may limit the measurement accuracy. Detailed analytical calculations complemented with Monte Carlo simulations show that high frequency noise components may also be entirely suppressed, depending on the relative difference of measurement and reference phase, which may be exploited by corresponding design provisions. While these results are applicable to any heterodyne interferometer with certain design characteristics, specific calculations and related discussions are given for the example of the optical metrology system of the LISA Pathfinder mission to space.

Common Mode Voltage Cancellation in a Buck-Type Active Front-End Rectifier Topology

AC/AC power conversion is widely used to feed AC loads with a variable voltage and/or a variable frequency from a constant voltage constant frequency power grid or to connect critical loads to an unreliable power supply while delivering a very balanced and accurate sinusoidal voltage system of constant amplitude and frequency. The load specifications will clearly impose the requirements for the inverter stage of the power converter, while wider ranges of choices are available for the rectifier. This paper investigates the utilization of a buck-type current source rectifier as the active front-end stage of an AC/AC converter for applications that require an adjustable DC-link voltage as well as elimination of the low-frequency common mode voltage. The proposed solution is to utilize a combination of two or more zero current vectors in the Space Vector Modulation (SVM) technique for Current Sources Rectifiers (CSR).

AC coupling technique for Josephson waveform synthesis

We demonstrate a new bias technique that uses low-pass and high-pass filters to separate the current paths of the sampling and signal frequencies in a Josephson waveform synthesizer. This technique enables the output voltage of the array to be directly grounded by removing the low-frequency common mode signal that previously prevented direct measurement of the array voltage with low-impedance instruments. We directly measure the harmonic spectra of 1 kHz and 50 kHz synthesized sine waves. We also use a thermal transfer standard to compare the rms voltages of synthesized sine waves at frequencies from 1 kHz to 50 kHz. Finally, we describe a new circuit that should enable a significant increase in output voltage by allowing several distributed arrays to be biased in parallel at high frequency, while combining their low frequency output voltages in series.