A full-bridge non-isolated photovoltaic grid-connected inverter

1. Non-isolated photovoltaic grid-connected inverter

1.1 Photovoltaic grid-connected power generation system

The contribution of photovoltaic power generation to the world's energy is increasing year by year, which is obvious to all.

According to the data of IEA PVPS, in 2009, the project's member countries installed a total of 6.2GW of photovoltaic capacity (about 7GW installed globally), of which more than 95% were grid-connected systems, as shown in Figure 1.

Figure 1 The contribution of photovoltaic power generation to world energy increases year by year

Data source: IEA PVPS, International Energy Photovoltaic Power Systems Programme

1.2 Photovoltaic power generation system

The photovoltaic power generation system consists of a photovoltaic cell array and a grid-connected inverter (as shown in Figure 2). The grid-connected inverter plays an important role in determining the performance and cost of the power generation system.

According to whether they are equipped with transformers, grid-connected inverters can be divided into isolated and non-isolated types, including: power frequency isolated grid-connected inverters, high frequency isolated grid-connected inverters, non-isolated grid-connected inverters (single-stage and multi-stage), etc.

The power frequency isolated grid-connected inverter (as shown in Figure 3) has the advantages of electrical isolation and elimination of the DC component of the current, but it is large in size and weight, high in price, and has a system efficiency of only 94%-96%.

The high-frequency isolated grid-connected inverter (as shown in Figure 4) has advantages such as electrical isolation, volume, weight, and cost reduction, but the system efficiency is only 90%-95%.

Non-isolated grid-connected inverters are divided into single-stage non-isolated grid-connected inverters and two-stage non-isolated grid-connected inverters. Single-stage non-isolated grid-connected inverters are suitable for higher PV voltage and power; while two-stage non-isolated grid-connected inverters are suitable for PV arrays with a wide voltage range. They both have a maximum efficiency of 98.8%, small size, light weight, and low cost, but their disadvantage is that there is an electrical connection between the battery panel and the grid.

Figure 2 Composition and structure of photovoltaic power generation system

Figure 3 Structure diagram of power frequency isolation grid-connected inverter

The electrical connection provides a flow path for leakage current, which is the biggest obstacle to the application of high-efficiency non-isolated photovoltaic grid-connected inverters. The leakage current problem will generate parasitic capacitance of 150nF/kWp, causing a common-mode voltage source at the switching frequency. At present, most circuit structures use SPWM modulation strategies.

Figure 4 High frequency isolated grid-connected inverter

2. Common circuit topologies of non-isolated grid-connected inverters

In the past, we often used bipolar SPWM modulated full-bridge grid-connected inverters (Figure 5 shows its topology) because of its low efficiency, often used in small power applications, and no patent barriers.

Figure 5 Topology of bipolar SPWM modulated full-bridge grid-connected inverter

Here we would like to introduce several patented topologies.

2.1 Sunways patented topology (Figure 6)

Single-phase two-stage series: AT 2700/3000/3600/4500/5000:

Single-phase single-stage series: NT 2500/3700/4200/5000;

Three-phase two-stage series: Three-phase IxIT 10000/11000/12000.

Figure 6 Sunways patent topology

2.2 SMA's patented topology (Figure 7)

Single-phase two-stage series: SB3000TL/4000TL/5000TL;

Single-phase single-stage series: SMC6000TL /7000TL /8000TL

/9000TL /10000TL /11000TL

Figure 7 SMA's patented topology

2.3 Half-bridge topology

The two-level SPWM half-bridge has no patent barriers and is therefore widely used; in addition, there is also a unipolar SPWM three-level half-bridge.

3. Improved full-bridge non-isolated photovoltaic grid-connected inverter

Let’s first look at how the leakage current analysis model of the single-phase grid-connected inverter (as shown in Figure 8) solves the leakage current problem of the single-phase grid-connected inverter.

Filter branch: It is dominated by the grid filter, EMI filter and grid parasitic parameters, and plays a leading role in the common mode current loop impedance;

Parasitic branch: It is composed of parasitic capacitance at the midpoint of the bridge arm and affects the impedance of the common-mode current loop:

We summarize two ways to eliminate leakage current through the leakage current analysis model of the single-phase grid-connected inverter (as shown in Figure 9):

(1) Under the premise of symmetry of circuit and parasitic parameters,

Figure 8 Model for leakage current analysis of single-phase grid-connected inverter

Figure 9 Model for leakage current analysis of single-phase grid-connected inverter

VCM-DM: 0), the VCM voltage generated by the SPWM switching mode is a constant value;

(2) When the VCM voltage generated by the SPWM switching mode is high-frequency time-varying, the circuit parameters are matched to make VCM+VCM-DM=consto.

The leakage current suppression technology of full-bridge single-phase grid-connected inverter includes:

(1) Under the premise of symmetry of circuit and parasitic parameters (i.e. satisfying VCM-DM:0), the V voltage generated by SPWM switching mode is a constant value.

Common circuits are as follows:

Full bridge circuit with AC bypass link;

Full bridge circuit with DC bypass link;

Full bridge circuit with DC side bypass clamp;

Improved full-bridge circuit based on optimized functionality and efficiency.

A controllable switch tube and a voltage-dividing capacitor are added to form a bidirectional clamping branch.

4. Theoretical analysis and experimental research

4.1 Circuit structure and driving timing

The main circuit structure SPWM and drive timing working modes are the positive half cycle of current and the negative half cycle of current.

The voltage clamping operation is to increase or decrease the midpoint voltage during the freewheeling phase as the midpoint voltage fluctuates with the grid voltage.

4.2 Analysis and calculation of power device losses

Taking the photovoltaic voltage of 500V and power of 5kW as an example (as shown in Figure 10), we conducted research under the following experimental conditions.

Input voltage: 340-700VDG

Photovoltaic parasitic capacitance: 2×0.1 u F

Power grid: 220V/50Hz

Network filter: 4mH+6.6uF

Power: 1kW

Switching frequency: 20kHz

The following are four types of circuit experiments:

A: Haric

B:H5

C: H6

D: Optimized H5

Figure 10 Power device loss analysis and calculation

Figure 1 1 Experiment A: Haric

5. Conclusion

Non-isolated photovoltaic grid-connected inverters have the advantages of high efficiency, small size and light weight;

According to the leakage current analysis model of bridge-type non-isolated photovoltaic grid-connected inverter, we can derive two ways to suppress the switching frequency leakage current;