What are the different single-phase transformerless PV inverter configurations?

A transformerless PV inverter has no galvanic isolation between the input and the output, leading to current leakage problems. Parasitic capacitance plays a crucial role in the circulation of leakage current. Several types of transformerless PV inverter configurations can address this issue. This FAQ discusses why parasitic capacitance matters and the four commonly used configurations to overcome leakage current circulation.

Why does parasitic capacitance matter in single-phase transformerless PV inverters?

Parasitic capacitance is commonly present in all transformerless PV inverter configurations; hence, it is good to know about them before we discuss the inverter configurations further. Parasitic capacitance in photovoltaic (PV) panels is an inherent electrical characteristic that arises from the design and installation of the modules. This capacitance is not required for the primary function of the PV array but is a byproduct of its mechanical structure and installation. Figure 1 shows parasitic capacitance created by PV panel and their surrounding environment.

The following are four ways parasitic capacitance influences the transformerless PV inverter:

1. Parasitic capacitance can lead to leakage currents, which may cause grid current distortion and electromagnetic interference.
2. The parasitic capacitance can influence the operating behavior of inverters, especially transformerless types.
3. While parasitic capacitance does not affect the insulation of PV modules, the resulting leakage current may pose a safety risk if not properly managed.
4. In dry conditions, the parasitic capacitance is typically small enough that its effect on PV system operation is negligible.

Now, as illustrated in Figure 2, let us examine four commonly categorized single-phase transformerless PV inverters. We will briefly discuss how each type is different and how they benefit the electrical grid.

Common-ground type single-phase transformerless PV inverter

The common ground-type single-phase transformerless PV inverter shown in Figure 3 is a configuration in which the negative terminal of the PV panel is directly connected to the grid’s ground line. This connection offers the advantages of constant common mode voltage (CMV) and eliminates leakage current.

By connecting the PV panel’s negative terminal directly to the grid ground, the parasitic capacitors (Cpv1 and Cpv2), the primary source of leakage current, are clamped to zero potential. This clamping prevents the formation of a resonant circuit that would otherwise be excited by varying CMV, thereby eliminating leakage current.

While common ground configurations excel at suppressing leakage current, they can present challenges in controlling flying capacitors or inductors. However, the benefits of constant CMV and the elimination of leakage current make them highly desirable for transformerless PV inverters, particularly in size and efficiency.

DC-decoupled type single-phase transformerless PV inverter

DC-decoupled transformerless inverters are a full-bridge single-phase PV inverter that utilize an extra circuit at the dc bus to decouple the PV panels from the ac side during the freewheeling period. This decoupling aims to suppress leakage current. Figure 4 shows a dc-decoupled type inverter, commonly known as the H5 configuration, which has a switch at the input side of the inverter to help eliminate leakage current.

Switches S1 and S3 operate at grid frequency but are phase-shifted by 180 degrees. Like S1 and S3, switches S2 and S4 operate at switching frequencies separated by 180-degree phase angles. Therefore, the switch combination of S1, S2, S3, and S4 acts as an H-bridge inverter.

However, during the operation of any H-bridge inverter, there is a zero-voltage state during the transition between positive and negative cycles, which causes leakage of current circulation. This is the precise moment when switch S5 disconnects the grid and the PV modules to prevent leakage of current circulation.

AC-decoupled type single-phase transformerless PV inverter

Instead of acting on the DC side, AC-decoupled configurations introduce a decoupling circuit on the AC side, specifically at the AC output port. This approach offers alternative paths during freewheeling, reducing leakage current. Another advantage of this approach is that these configurations achieve low total harmonic distortion (THD) in the output voltage and current. Figure 5 shows the AC-decoupled type inverter with a pair of switches at the AC output port.

During the freewheeling period, when the inverter output voltage is zero, the inductive current from the grid needs a path to flow. The ac-decoupling circuit provides this alternative path, allowing the inductive current to circulate without flowing back through the PV panel’s parasitic capacitances (CPV1 and CPV2). By diverting the current away from the parasitic capacitances, ac decoupling minimizes the potential difference across them, thereby suppressing the generation of leakage current.

Neutral point clamped (NPC) Type single-phase transformerless PV inverter

As discussed in our previous conversations, leakage current arises in transformerless inverters due to the absence of galvanic isolation and parasitic capacitances (Cpv1 and Cpv2) between the PV panel and ground.

NPC configurations achieve CMV clamping by introducing additional components, typically capacitors, and diodes, to create a virtual neutral point within the inverter circuit, as shown in Figure 6. This virtual neutral point is a reference for voltage levels, limiting CMV fluctuations and reducing leakage current.

NPC configurations can achieve lower THD in the output current than other transformerless inverter types, leading to better power quality. The values of clamping capacitors in NPC circuits influence the effectiveness of CMV clamping and the ripple in the clamped voltage. Careful selection is essential to optimize performance.

Summary

We discussed four types of transformerless PV inverter configurations. The common ground type configuration is the simplest, using fewer semiconductors and a small filter with very high efficiency. However, using a flying capacitor or inductor could present control difficulties.

The dc-decoupled and ac-decoupled configurations isolate the dc and ac part of the inverter to prevent leakage current circulation. However, both the inverter configurations are bigger than the common-ground configuration. The NPC configuration has a special feature called constant CMV. Still, the inverter size is larger than the dc-coupled and ac-coupled configurations, and the efficiency is lower than that of the common ground configuration.

References

A Comparative Review on Single Phase Transformerless Inverter Topologies for Grid-connected Photovoltaic Systems, MDPI
Energy efficiency enhancement in full-bridge PV inverters with advanced modulations, ScienceDirect
Transformerless Inverter Topologies for Single-Phase Photovoltaic Systems, Aalborg University
Capacitive Leakage Currents, SMA Solar Technology
Leading Leakage Currents, SMA Solar Technology

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