5 Converter Topologies for Integrated Solar and Storage Systems

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5 Converter Topologies for Integrated Solar and Storage Systems [Copy link]

Energy storage systems are becoming more affordable and electricity prices are rising, so the demand for renewable energy is increasing. Many homes now use a combination of solar power generation and battery storage to ensure energy is available when solar power cannot meet demand. Figure 1 shows a residential use case, and Figure 2 shows how a typical PV inverter system can be integrated with an energy storage system.

Figure 1: A residential solar power generation and energy storage system installation scheme

Figure 2: Typical PV inverter system with energy storage system

Ideally, this type of system has efficient power management components that enable AC/DC and DC/DC conversion and high power density (with the smallest possible solution size), which are highly reliable (ultra-low losses) and help bring products to market quickly. However, these requirements are not always possible at the same time, and trade-offs need to be made regarding the ideal power conversion topology for these sub-blocks.

Common to existing power topologies for AC/DC and DC/DC buck and boost power converters is the interleaved operation of half-bridges or converter branches, designed to increase power levels in DC/DC converters or to achieve three-phase operation in AC/DC inverters or power factor correction stages by placing three branches operating at 120 degrees phase shift. Figure 3 shows a simplified schematic of five power topologies.

Figure 3: Half-bridge power supply topology and equivalent branch power supply topology

Topology 1: In a two-stage converter topology, pulse width modulation (PWM) signals are applied to power devices Q1 and Q2 as complementary (with time delay to avoid shoot-through due to overlapping switching signals). For a positive sine wave at the output, Q1 is applied with a duty cycle of >50%. For a negative sine wave at the output, Q2 is applied with a duty cycle of >50%. Controlling the output power is a simple concept, but the output signal before the line filter has a full bus voltage swing, which requires a larger filter to reduce EMI. The ripple frequency entering the filter is the PWM frequency, which affects the filter size.

Compared to a two-stage converter, a three-level topology allows for the use of smaller passive components and has lower EMI. There are four three-level topologies:

Topology 2: The T-type topology gets its name from the way the transistors are arranged around the neutral point (VN). Q1 and Q2 are connected to the DC link, while Q3 and Q4 are in series with VN. The ripple frequency seen by the filter is equal to the PWM frequency applied to switches Q1 to Q4. This determines how large the filter components need to be to achieve the desired low total harmonic distortion at the AC line frequency. Q1 and Q2 see the full bus voltage, which needs to be rated at 1,200V when the DC link voltage in the system is 800V. Since Q3 and Q4 are connected to VN, they only see half of the full bus voltage, and in an 800-V DC link voltage system, they are rated at 600V, which can save on converter type costs. Learn about the 10kW bidirectional three-phase three-level (T-type) inverter and PFC reference design.

Topology 3: In the active neutral point clamped (ANPC) converter topology, VN is connected to active switches Q5 and Q6 and is set at the middle of the DC link voltage. As with the T-type converter, the ripple frequency seen by the filter is equal to the PWM frequency used to size the AC line filter. The advantage of this architecture is that all switches are rated for half the maximum DC link voltage; in an 800V system, switches rated for 600V can be used, thus helping to save cost. When shutting down this converter, it is important to limit all voltages on each switch to half the DC link voltage. In other words, the controlling microcontroller (MCU) needs to handle the shutdown timing. The TMS320F280049C and other devices in the C2000TM product family have configurable logic that allows the shutdown logic to be implemented in hardware, offloading software tasks from the MCU.

Topology 4: The Neutral Point Clamped (NPC) converter topology is derived from the ANPC topology. Here VN is connected through diodes D5 and D6, setting VN at the middle of the DC link voltage. The output ripple frequency seen by the filter is equal to the PWM frequency used to size the AC line filter. As with the ANPC topology, all switches are rated for half the maximum DC link voltage, but two additional switches are changed to two fast diodes. The NPC topology is slightly less expensive than the ANPC topology at the expense of slightly reduced efficiency. The turn-off timing requirements are also the same as the ANPC topology. The NPC topology is easily derived from the ANPC reference design mentioned above.

Topology 5: Flying Capacitor Topology You already know what happens in this converter; a capacitor is connected to the switch nodes of the stacked half-bridge implemented by Q1 and Q2, and Q3 and Q4. The voltage across the capacitor is limited to half the DC link voltage and drifts periodically between V+/V–; power delivery occurs during this drift. This topology uses all switches during both the positive and negative sine waves. In this topology, the output ripple frequency seen by the filter is twice the PWM frequency provided by the flying capacitor drifting each cycle, so the size of the AC line filter is smaller. Also, all switches are rated for half the maximum DC link voltage, which helps save cost

Table 1 lists the advantages and challenges of different topologies.

Table 1: Advantages and challenges of different converter topologies

All four three-level topologies offer clear advantages over conventional two-level converters in terms of power density (with the smallest possible solution size), highly reliable operation, and fast time to market. These advantages are further enhanced by the use of wide bandgap devices and high-performance MCUs at reasonable costs.

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