Currently, photovoltaic (PV) power generation systems mainly operate in two modes: stand-alone and grid-connected. This section will focus on the grid-connected mode. A grid-connected PV power generation system refers to a system that converts the direct current (DC) output from the PV array into alternating current (AC) with the same amplitude, frequency, and phase as the grid voltage, and then connects it to the grid.
1.1 Scheduled and Unschedulable Systems
Currently, common grid-connected photovoltaic (PV) power generation systems can be divided into two categories based on their system functions: one is the "unschedulable PV grid-connected power generation system" without batteries; the other is the "schedulable PV grid-connected power generation system" that includes battery banks as energy storage. The system configuration diagrams for both are shown in Figures 1 and 2. The dispatchable grid-connected PV system incorporates energy storage devices, serving as both an uninterruptible power supply and an active filter, and is beneficial for grid peak shaving. However, its energy storage component typically suffers from short lifespan, high cost, bulky size, and low integration, therefore, this type is currently less commonly used. The biggest difference between the dispatchable and unschedulable PV grid-connected power generation systems is the inclusion of energy storage, typically using lead-acid battery banks, the capacity of which can be configured according to actual needs. Functionally, the dispatchable system has certain expansions and improvements, mainly including:
(1). In addition to the grid-connected inverter, the system controller also includes a battery charge and discharge controller, which manages the energy of the battery pack according to the system functional requirements;
(2). When the AC power grid fails, the dispatchable system can function as an uninterruptible power supply (UPS) to power important local AC loads;
(3) Larger capacity dispatchable photovoltaic grid-connected power generation systems can also control the grid-connected output power according to operational needs, and realize a certain grid peak-shaving function.
While functionally superior to non-dispatchable grid-connected photovoltaic (PV) systems, dispatchable PV grid-connected systems have significant drawbacks due to the addition of energy storage. These drawbacks are the main reasons currently limiting the widespread application of dispatchable PV grid-connected systems, including:
(1) Adding a battery pack increases system costs;
(2). The lifespan of the battery is relatively short, much shorter than that of other components in the system: Currently, the lifespan of maintenance-free lead-acid batteries under reasonable use is usually 3 to 5 years, while photovoltaic arrays can generally work stably for more than 20 years;
(3) Discarded lead-acid batteries must be recycled, otherwise they will cause serious environmental pollution.
1.2 Structure of Photovoltaic Grid-Connected Power Generation System
Most electrical devices primarily use AC power, and the DC power generated by photovoltaic arrays needs to be converted into AC power by inverters to supply the loads. Therefore, inverters play a crucial role in photovoltaic grid-connected power generation systems. The structure of a photovoltaic grid-connected power generation system is closely related to its power rating. Currently, the commonly used structures for photovoltaic grid-connected power generation systems mainly include four types: centralized inverters, integrated inverters, string inverters, and multiple string inverters. Among them, centralized inverters are mainly used in photovoltaic power plants, while the latter three types are widely used in distributed photovoltaic grid-connected power generation systems.
1.2.1 Central inverters
A centralized inverter mainly consists of a photovoltaic array, an inverter, and a DC bus. It was the earliest inverter type used in photovoltaic power generation systems. In this system, all photovoltaic devices are connected in series and parallel to form a photovoltaic array. The energy of this array is centrally converted into alternating current by an inverter, hence the name centralized inverter. Its structure is shown in Figure 3.
Advantages of centralized inverters: Output power can reach megawatt levels, unit power generation cost is low, and they are mainly used in high-power applications such as photovoltaic power plants. To obtain sufficient power and voltage, its photovoltaic array consists of photovoltaic modules connected in series and parallel.
However, this series and parallel connection method of photovoltaic devices easily leads to the following disadvantages:
① In the same array, photovoltaic devices are affected not only by the mutual influence of the characteristics of series modules, but also by the mutual influence of the characteristics of parallel modules. Therefore, the output power of photovoltaic devices will be affected, and the utilization rate of photovoltaic devices in this inverter is lower than that of other methods.
② When a component in a photovoltaic array is covered by shadow, that component not only cannot output power, but also becomes a load on the system, causing the component to heat up.
Experiments show that at an ambient temperature of 12 degrees Celsius, the temperature of normally functioning photovoltaic modules is 22 degrees Celsius, while the temperature of modules affected by shading can reach 70 degrees Celsius. This not only reduces the system's output power but also shortens the lifespan of the modules. Furthermore, this structure requires a high-voltage DC bus to connect the inverter and the photovoltaic array, increasing costs and reducing safety.
Companies like Hefei Sungrow Power and SMA have set 100kW as the lower limit for the rated power of centralized inverters. Taking Sungrow Power's SG100K3 photovoltaic grid-connected inverter as an example, it provides 6 DC input terminals.
Furthermore, an external combiner box is required at the DC input front end to meet the requirements, which is related to the limitations of the inverter's own technical parameters. This mainly involves the limitation of the photovoltaic module series connection conditions. For the SG100K3 photovoltaic grid-connected inverter, the important parameters related to the photovoltaic modules are Udc range (450-650), maximum tracking voltage range MPPT (550-620), and maximum open-circuit voltage range Uoc (700-780).
Taking a company's 170Wp polycrystalline silicon inverter as an example (under STC conditions, 25 degrees Celsius, Vmp = 35V, Voc = 44.5V, Pm = 270W), the recommended Voc and Vmp configurations for the SG100K3 model inverter and the method for determining the number of photovoltaic modules connected in series are as follows:
The minimum number of cascades, n1 = V1/Vmp, is rounded up using the rounding method. V1 is the lower limit of the recommended MPPT range.
The maximum number of series connections, n2 = V2/Voc, is rounded down using the rounding method. V2 is the upper limit of the recommended range of Uoc.
Based on the above calculations, the number of photovoltaic modules in series should be 16-18. Even if 18 are chosen, the rated power of one string is only 3.06KW. If a combiner box is not used, the maximum power will not exceed 20KW, which is less than 20% of the rated power. Therefore, a combiner box is essential.
1.2.2 Module Integrated Inverter
An integrated inverter (also called an AC photovoltaic module system) refers to a photovoltaic power generation system that integrates an inverter and photovoltaic modules together. The disadvantages of integrated inverter systems are: lower power output, typically between 50W and 400W. Each integrated inverter has its own MPPT circuit, maximizing the efficiency of the photovoltaic devices. The advantages of integrating this type of inverter with photovoltaic modules are: high efficiency, no need for a DC bus, direct connection of the output to the power grid, and improved system safety.
As shown in Figure 4, the integrated inverter and the photovoltaic modules it connects to constitute a complete photovoltaic system that can operate independently. This feature provides great flexibility for system expansion: users can select the appropriate number of integrated inverters based on the required power.
Furthermore, this connection method improves the overall system reliability; even if one inverter fails, the rest of the system can continue to operate normally. Integrated inverters can also operate in applications that are far from the grid but require AC power. However, under the same power level, integrated inverters are more expensive than other topologies and are less efficient compared to high-power inverters. Due to the structural limitations of integrated inverters, failures in the inverter or photovoltaic devices can cause inconvenience in maintenance and replacement.
1.2.3 String Inverter
A string inverter refers to a topology in which photovoltaic devices are connected in series to form a photovoltaic array to provide energy to a photovoltaic power generation system. Its advantage is that it avoids the disadvantage of parallel modules causing the system to malfunction due to voltage drops.
Figure 5 Schematic diagram of string photovoltaic inverter
Figure 5 shows the schematic diagram of a string inverter. String inverters are based on a modular concept, with each photovoltaic string (1kW-5kW) connected to an inverter. They feature maximum power point tracking (MPPT) on the DC side and are connected in parallel to the grid on the AC side. Many large photovoltaic power plants use string inverters because they are unaffected by differences in modules between strings and shading, while also reducing the mismatch between the optimal operating point of the photovoltaic modules and the inverter, thus increasing power generation. String inverters introduce a "master-slave" concept between strings, allowing the system to connect several photovoltaic strings in parallel when a single string of power is insufficient to operate a single inverter, enabling one or more of these strings to operate and thus producing more electricity.
1.3 Overview of Topology: Selection of Single-Level and Two-Level Topologies:
Taking Hefei Sunshine as an example:
1.5/2.5/4 kW topology diagram
5k/6kw Topology Diagram
