String type VS centralized type: Which one should you choose for PV inverter?

At the 2014 Inter Solar Forum in Munich, senior photovoltaic practitioner Manfred Bachler (formerly the chief technology officer of Phoenix Solar, the world's largest EPC manufacturer) proposed a solution to transform existing centralized inverters with string inverters. He concluded that the transformation costs could be recovered in 5 to 6 years. This is mainly because centralized inverters are difficult to maintain and have poor availability. In terms of availability alone, they are 6% worse than string inverters.

Recently, there have been many different opinions in the industry regarding the failure rate and reliability of string and centralized inverters. This article will analyze them in detail from the following perspectives to stimulate discussion.

1. Differences in the basic principles of system reliability

In the string-type solution, the components and inverter are directly connected, and the inverter output is connected to the grid through a step-up transformer. There are fewer transmission and transformation link devices, the DC cables are short, and the transmission is mainly based on AC cables. The main equipment of the centralized solution is the DC combiner box, DC distribution cabinet, inverter and step-up transformer. There are more transmission and transformation link devices and more DC cables in the transmission line. This article will analyze the differences in the reliability principles of the system solutions from the following aspects.

1.1 Impact of DC and AC lines on system safety performance

The characteristics of direct current are that it is easy to produce arc faults and is difficult to extinguish, and there is a risk that it cannot be extinguished, because as long as there is light, current will be generated, which is very harmful. Alternating current has a zero crossing point, so even if an arc fault occurs, the arc will be extinguished at the zero crossing point, and the harm is small.

1.2 System Failure Response Time

When a short circuit occurs on the AC side, since the energy comes from the power grid and is large enough, the electrical protection device can trip in time, cut off the short circuit path, and protect the electrical equipment. When a short circuit occurs on the DC side, since the fault current is small and the circuit breaker often has a derating design, the circuit breaker cannot quickly protect and cut off the short circuit path, during which time the insulation may age and soften, causing a fire.

1.3. Maturity of key equipment

Since AC technology has been developed for more than 100 years, power generation technology is stable, mature, and has a wide range of applications, and related electrical components have also developed and matured. However, there is little accumulation of photovoltaic DC protection technology, and there are many technical problems that need to be solved urgently; and the DC voltage range is wide, the energy difference is large, and the development of related application components is still immature. For example, only a very small number of manufacturers can provide components for high-voltage DC protection.

1.4. Selection of key components of the system

At present, when selecting inverter components, some manufacturers pursue low costs, and it is very common to use AC circuit breakers on the DC side of centralized inverters, which will bring great safety hazards to the system. First, due to the different voltage levels of AC and DC, AC circuit breakers are used in DC scenarios, and the working voltage exceeds the rated voltage of the device. Long-term use will cause the circuit breaker to fail, posing a great safety hazard; secondly, due to the high DC voltage level and large working current, the circuit breaker is prone to arcing during the cutting process. The characteristics of DC and AC are different, and the design of the circuit breaker arc extinguishing device is bound to be different. When AC circuit breakers are used in DC scenarios, the DC arc cannot be effectively extinguished. If the arc lasts too long (tens of ms), an explosion accident will occur.

1.5 Analysis and comparison

From the above system analysis, we can see that string inverters are more reliable than centralized inverters, and string solutions are safer and more reliable than centralized solutions. According to statistics, there are major fire accidents of centralized inverters almost every month; while string DC cables are very short, and the safety of the AC part has been verified for more than 100 years. So far, no serious fire accidents have been heard in string power stations above 10G worldwide.

2. Difference in inverter failure rate

Centralized and string inverters have different power levels, so their structural characteristics, heat dissipation methods, and protection levels will be different, resulting in differences in the failure rate of the entire machine.

2.1. Failure rate of string inverters

Some well-known string inverter manufacturers in the industry design their inverters to ensure reliable operation for 25 years, using natural heat dissipation without external fans to achieve internal and external environmental isolation, with a protection level of IP65, which allows internal components to operate in a dust-free and stable environment, greatly reducing the impact of external environments such as temperature, sand, moisture, and salt spray on component life, and greatly enhancing reliability. At the same time, the system has no vulnerable parts, no fuses or other components that need to be replaced regularly, and the overall failure rate is less than 1%, making the system maintenance-free.

2.2 Centralized inverter failure rate

Centralized inverter solutions generally use conventional civil buildings or container-type machine rooms with a protection level of IP54. However, because machine rooms generally use direct ventilation cooling solutions, they can only reach IP44 or lower in practice, and cannot prevent sand, dust and corrosive gases from entering the inverter. Therefore, the internal circuit components of centralized inverters are easily exposed to harsh working environments. For example, the accumulation of dust on the circuit boards and terminal blocks inside the inverter will reduce the creepage distance, and ultimately cause safety risks such as discharge and fire.

Wet dust can easily cause leakage and corrosion effects between PCBs or components, causing signal abnormalities or high-voltage arcing and sparking. It may also cause a short circuit between the power grid and PE, causing the inverter to shut down abnormally or explode. At the same time, the inverter fan is a vulnerable part with an average service life of about 5 years. Many power stations in the north are built on the Gobi Desert, which is actually a typical saline-alkali land, and the dust contains a large amount of salt ions. In the day-and-night intermittent working mode, the probability of corrosion and leakage is much greater than that of conventional electrical equipment; according to statistics, the failure rate of centralized inverters is greater than 3%.

2.3 Analysis and comparison

Through the comparative analysis of the above aspects, it can be seen that the failure rate of centralized inverters is more than 3 times that of string inverters. Compared with centralized inverters, string inverters have higher reliability and better system stability.

3. Differences in inverter heat dissipation principles

Due to different power levels, the heat generation levels of string inverters and centralized inverters will be very different. The inverter heat dissipation method determines the reliability of the entire product.

3.1 Differences in heat generation mechanisms

The power consumption of a single string inverter is several hundred watts, which can be designed for natural heat dissipation and can meet the requirements of reliable operation at an ambient temperature of 60°C. For centralized inverters, especially the 10-foot container solution, the loss of 1MW is as high as 30kW, which is equivalent to 30 1kW electric furnace wires baking inside the 10-foot container. It is difficult to achieve reliable heat dissipation with just a few fans, which greatly increases the risk of burning.

3.2 Impact of application environment on inverter heat dissipation

In the western ground power station, the string inverter is installed on the components, the environment is open and well ventilated, and the inverter has good heat dissipation; the centralized inverter is installed in the machine room. Due to the severe wind and sand in the northwest region, the dust accumulation in the centralized inverter machine room causes the dust net to be blocked, the heat dissipation performance of the whole machine deteriorates, and the temperature of the high-power consumption device rises sharply, causing temperature alarms. In more serious cases, it will cause damage to the internal IGBT device, the equipment will restart continuously, and even burn out. Table 1 shows the temperature test results of the northwest ground power station inverter. It can be seen that since the centralized inverter is installed in the machine room, the temperature rise is higher than the string type. In the hot summer, it is common for the temperature in the machine room to exceed 50℃. The temperature inside the centralized inverter cabinet is higher and worse, and the high temperature brings about a reduction in the life of the device, a reduction in the operation of the whole machine, and even the risk of explosion. Since the first half of this year, there have been serious accidents such as explosion of centralized inverters due to heat dissipation problems almost every month.

4. Inverter availability and maintainability

As the core component of the photovoltaic power generation system, the availability and maintainability of the inverter play a decisive role in the reliability, power generation and power generation income of the photovoltaic system. The following article compares and analyzes the impact of centralized inverters and string inverters on system reliability from two aspects.