Tuesday, 17 October 2017

Breakthrough -- Natural Cooling Design of String Inverter

Photovoltaic (PV) inverters are the core equipment of the solar power generation system, therefore the reliability of the inverters determines the reliability of the PV system. The heat dissipation performance is the key factor influencing the reliability of PV inverters. The power component, a core component of inverters, is sensitive to temperature, which means change in temperature could impact connection/disconnection process. In the case of high temperature, the power switch may suffer poor performance or even break down. Therefore, inverter heat dissipation is a key factor when it comes to inverter performance and quality.

PV power plants are generally situated in deserts and plains in MENA, where the sunlight is abundant. However, these areas also have extreme high temperatures and strong sand-wind weather. For those that are close to the sea, the corrosion is severe. In these scenarios, inverters are directly exposed to extreme environmental conditions, leading to deteriorating exterior components. That is why the prime concern of inverters becomes how to strike the right balance between environmental adaption and heat dissipation.

In recent years, utility-scale power plants around the world have widely adopted string inverters over central inverters due to obvious advantages: higher yields, smaller footprint, no equipment room, reliable operation, and simpler O&M (operation and maintenance).

1. Common Heat Dissipation Mode and Relevant Problems in Inverters
In general, inverters dissipate heat through natural or air cooling. Normally, central inverters are installed in containers or equipment rooms that often adopt direct ventilation. The heat dissipation capability of string inverters is improved through external fans, but string inverters are normally used in harsh environments, and this poses higher requirements on the protection level of external fans. Fans wear out quickly and cannot be totally protected from dust. Harsh environment could accelerate the deterioration of fans, resulting in frequent replacement. Fans need to be replaced whenever they are faulty, which brings endless troubles to PV plants that are supposed to run for 25 years.

Popular container-type equipment rooms are said to be protected to IP54. However, as such equipment rooms usually use air-cooled heat dissipation method, the actual Ingress Protection Rating can only reach IP44 if not lower, and therefore the Ingress Protection Rating of the entire system fails to reach IP65. What’s more, dust and corrosive gas cannot be blocked out of the inverter. The accumulation of dust on the circuit board and terminal block inside the inverter could shorten the creepage distance, and pose safety risks such as discharge and fire. In addition, to protect fans from rain, the air channel has to be designed in a complex manner while the air channel form is limited. Once the fan fails to work, the heat dissipation capability is badly impaired, causing the inverter output power to derate and energy yields to decline, greatly affecting the benefits of customers. What’s worse, the lifespan of fans could be tremendously shortened due to years of exposure to rain, dust and other corrosive objects. Causing fans needing multiple replacement within the lifespan of the inverter, resulting in high O&M cost.

On the other hand, high-power fan rotates at high speed when the inverter works in the daytime and brings dust into the equipment room from outside. Some particles accumulate on the equipment room floor, some block the air filter, and some fine dust particles enter the cabinet. In the desert region of MENA, the air filter needs to be replaced or cleaned every two to three months. If the air filter is not maintained or cleaned in time, the blockage will decrease the supplied air volume to the equipment room, thus exaggerating heat dissipation and raising the temperature in the equipment room. The inverter will then deliver derated output power or even shut down due to poor heat dissipation. If the derating and protection mechanisms are not effective, the inverter may be burnt or on fire.

Below figure shows a fan cooling inverter used in a typical power plant. We can see that after a year of operation, the fan is dusty and badly corroded.

A fan is damaged mostly due to sandstorm, corrosion, and wear and tear, rather than temperature. A large number of statistics from PV plants show that the external fans of central inverters in a PV plant need to be all replaced after five to ten years. The logistics cost, labor cost, and energy yield loss resulting from fan replacement are very considerable. What is more important is that fan replacement severely affects the O&M efficiency as O&M personnel have to do a large quantity of valueless maintenance work.

2. Solutions to Inverter Heat Dissipation Problems
At present, natural cooling design is the advanced solution. Natural cooling technology allows the protection level of inverters to reach IP65 with fully-sealed design while retaining a high reliability in hot & moist environment. In addition, noise is reduced, reliability is improved, power deration is eliminated, maintenance is simplified, and cost is lowered.

As the output power of the inverter increases and its size decreases, heat dissipation becomes a major bottleneck. To address this issue, including copper and aluminum composite, heat pipes, temperature equalization plate, may be employed to reduce thermal resistance and improve the dissipation capability of radiators.

By adopting the mentioned heat dissipation concept and employing the heat equalization design, heat isolation and heat shield technologies, Huawei SUN2000 string inverter is sealed completely and designed with natural cooling. By taking the thermal isolation and thermal shielding technologies, heating components and thermosensitive components are deployed in different compartments to ensure no partial hot spots, improving the reliability of heat dissipation, and ensuring that the inverter can operate properly outdoor under extreme high temperature, without power derated. Whether it is in the desert of the Middle East or in the coastal regions of Japan, Huawei string inverters withstand the tests of sandstorms and corrosion, bringing high reliability, effective protection, sound operation, and quality performance.