What is a centralized inverter?

What is a centralized inverter?

Centralized inverters are mainly used in large-capacity photovoltaic power generation systems such as ground power stations and large workshops. The total system power is large, generally above the megawatt level. Inverter power is usually greater than 100kW. There are many photovoltaic modules connected to a single inverter. The power devices use high-current IGBTs. The system topology uses DC-AC first-level power electronic devices to convert full-bridge inverters. Most of the early centralized inverter products only had one MPPT controller. Current manufacturers have mass-produced centralized inverters with multiple MPPT controllers.

The topology of the centralized inverter is shown in Figure 1. The DC power output from the DC power distribution cabinet is sent to the inverter through the DC circuit breaker. The input DC power enters the MPPT module after DC EMC filtering. The DC power modulated by the MPPT module enters the inverter module. The output of the inverter module passes through the AC filter, AC EMC filter, AC contactor and AC circuit breaker inside the inverter. The converter is connected to the step-up transformer unit.

What is a centralized inverter?
Figure 1 Centralized inverter topology

The photovoltaic power generation system of a centralized inverter is shown in Figure 2, which generally includes photovoltaic modules, DC cables (first-level bus cables), combiner boxes, DC cables (secondary bus cables), DC power distribution cabinets, DC cables or copper Rows, inverters, step-up transformers, AC power distribution. Several photovoltaic modules are connected in series to generate a DC voltage that meets the input requirements of the inverter. They are connected to the DC combiner box in parallel through the first-level combiner cable, and several DC combiner boxes are connected in parallel to a DC distribution cabinet through the second-level combiner cable. Connect the inverter through a DC cable or copper bar to convert DC power into AC power. The two inverters are connected in parallel to a step-up transformer to convert low-voltage AC power into high-voltage AC power, which is connected through the high-voltage ring network cabinet. Into the grid. Because the photovoltaic array containing hundreds of kilowatts of photovoltaic modules only uses one grid-connected inverter, the entire system has a simple structure and the inverter has a high efficiency.

What is a centralized inverter?
Figure 2 Centralized inverter scheme

The main advantages of centralized inverters are: ①the number of inverters is small, which is easy to manage; ②the number of inverter components is small, and the reliability is high; ③the harmonic content is small, the DC component is small, and the power quality is high; ④the inverse The converter has high integration, high power density and low cost; ⑤The inverter has complete protection functions and high power station safety; ⑥With power factor adjustment function and low voltage ride-through function, the power grid has good adjustability.

The main disadvantages are: ①The failure rate of the DC combiner box is high, which affects the entire system; ②The MPPT voltage range of the centralized inverter is narrow, generally 450~820V, and the component configuration is not flexible. In rainy and foggy areas, power generation The time is short; ③The inverter itself consumes electricity and the ventilation and heat dissipation of the computer room, and system maintenance is relatively complicated; ④There is no redundancy in the centralized grid-connected inverter system. If a failure occurs, the entire system will stop generating electricity; ⑤Blocking and Bypass diodes increase the loss of the system; ⑥The ability to resist hot spots and shadows is poor, and the system power mismatch phenomenon is serious; ⑦The characteristic curve of the photovoltaic array appears complex and multi-peak, and a single centralized structure is difficult to achieve good MPPT; ③This The structure requires a relatively high-voltage DC bus to connect the grid-connected inverter to the photovoltaic array, which not only reduces safety, but also increases system cost.

Although there are shortcomings, with the increasing power of photovoltaic power stations, the centralized structure is still widely used because its output power can reach the megawatt level and the unit power generation cost is low.

At present, the maximum input voltage of most centralized inverters is 1000V (DC), and the MPPT range is 460~850V. Therefore, the number of photovoltaic modules in each photovoltaic module string must ensure that the no-load voltage of the photovoltaic module string does not exceed the maximum input voltage , The operating voltage during operation is within the MPPT range.

Early centralized photovoltaic inverters were all single-channel MPPT tracking, and the number of photovoltaic modules connected to the inverter was huge, which caused many photovoltaic modules to actually not work near the MPPT working point for a long time, causing the loss of photovoltaic power generation capacity . In the case of single-channel MPPT tracking, all photovoltaic modules are connected in series and connected to the inverter. If there is a photovoltaic module failure in the string, the overall MPPT efficiency of the circuit will be affected, which may greatly reduce the efficiency of the system. No matter how high the conversion efficiency and tracking efficiency of the inverter are, it will not be able to play a role. In order to reduce the risk of system efficiency reduction caused by such failures, centralized photovoltaic inverters introduced the technology of small photovoltaic inverters, and also produced centralized photovoltaic inverters with multiple independent MPPT tracking, multiple MPPT It can reduce the loss caused by the failure of a single MPPT, and bring greater flexibility to the configuration of the system.

Because each string in the photovoltaic array has its own operating voltage, the voltages are very likely to be inconsistent. If the traditional centralized solution is adopted, a parallel mismatch will occur when the strings are collected in the combiner box, and another parallel mismatch will occur when the combiner box is connected to the inverter, which will seriously affect the power generation efficiency of photovoltaic modules. As shown in Figure 3, in the case of single-channel MPPT, photovoltaic modules are affected by various factors, resulting in two or more crests. Tracking any crest will cause loss of power generation. In the case of multi-channel MPPT, the characteristics of each string can be accurately tracked to form multiple MPPT curves, so that each string of photovoltaic strings can exert its maximum efficiency as much as possible.

What is a centralized inverter?
Figure 3 Comparison of single-channel MPPT curves

There are many factors that cause the mismatch of PV modules in series and parallel, such as shadow shading, inconsistency of module factory products, inconsistent attenuation, inconsistent inclination angle of solar cell modules due to terrain limitations, inconsistent temperature and light in large areas of photovoltaic power stations, etc. . If there is only one MPPT tracking for a MW square matrix, the impact of parallel mismatch will be very large. If shadow occlusion, temperature/light inconsistency, inconsistent module performance, etc. are all probabilistic factors, then in complex terrain such as mountains, the parallel loss caused by the inconsistent installation of photovoltaic modules is basically a deterministic factor.

The multi-channel MPPT technology design is so precise that every few photovoltaic strings corresponds to 1 MPPT tracking, which solves the problem of parallel mismatch. As shown in Figure 4 below, multi-channel MPPT can form multiple MPPT curve tracking, and every several strings form an MPPT curve. This refinement makes every several strings work at maximum efficiency and solves the parallel mismatch of photovoltaic power plants. The problem is to greatly increase the power generation of photovoltaic power stations, thereby greatly increasing the revenue of the power station.

What is a centralized inverter?
Figure 4 Multi-channel MPPT curve