How to Protect Agains Edge of Cloud Effect

By Ray Saka, Sales Manager, TMEIC

In 2013, the U.S. solar market is projected to make the 5-GW marker with new installations. Utility-scale projects are expected to account for most 2.5 GW of that total. As more systems are installed, system reliability and performance becomes an increasing concern for owners, utilities and contained system operators (ISOs). When viewing reliability and operation of the PV power plant, in that location are 3 levels to consider including the component, system and institute levels.

Component Level
Inverter failures can significantly impact the overall energy harvest of the establish. System owners have modeled this touch on through advanced calculation based on uptime by using failures rates such as mean time between failures (MTBF), and hateful time to recovery (MTTR). Catastrophic failures on insulated-gate bipolar transistors (IGBTs) or the cooling system would result in unabridged inverter shutdown, decreasing the uptime of the system. Inverter manufacturers are challenged with high temperature and power stress of the IGBTs.

New engineering such as multi-level topology and advanced cooling systems assistance mitigate these risks. Multi-level topology distributes stress among a higher number of IGBTs to reduce the voltage stress on private IGBTs. Cooling technology simplifies and reduces the number of components to lower failure rate and then increase uptime. There are several types of cooling technology including liquid cooled, air cooled and convection cooled. Both liquid-cooled and air-cooled systems are challenged with high failure-rate moving mechanical parts (fans, pumps, etc.) On the flipside, convection cooled systems are simple cooling systems with lower failure rates; withal, they are not every bit efficient at cooling compared to liquid-cooled. An case of hybrid cooling by using air-cooled and convection cooled technology leads to mitigation of reducing mechanical moving part count, and at the aforementioned time providing an efficient cooling system. It'due south the best of both worlds, so to speak. Back-up of cooling system components is also a mitigation factor to reduce downtime; however, this needs to be carefully evaluated due to the increase in cost.

Organization Level
It is also important to consider the performance of the inverter as a system. This means considering inverter performance and reliability when the inverter is connected to a DC source (the PV panels) and AC (typically a medium-voltage step-up transformer). Most developers utilize performance-modeling software such as PVSYST, which models efficiency and the DC voltage MPPT window. However, blueprint risks associated with interactions between the DC array and the inverter are not modeled in PVSYST.
With the recent panel price degradation, many PV designers are starting to load more DC power higher up the inverter's Air conditioning nameplate to capture as much energy harvest as possible. Since inverters are current-limiting in nature, every bit more DC power is loaded, DC voltage slides up. This becomes more critical every bit the system encounters situations such as the cloud-edge effect and when discussing low voltage ride through (LVRT) functions.

The deject-edge upshot is when a thin distributed cloud passes over the DC assortment and the edges of the cloud intensify the irradiance moving instantaneous transient ability to the inverter. This tin can potentially cause an over-voltage issue in the inverter reaching above its nominal MPPT range thereby shutting downwards the inverter. Well-nigh all inverters use an IGBT switching device to convert DC (PV Console) to Ac (grid) ability. These devices have limitation based on blueprint gene. Some designs would not let you to safely track MPPT beyond 800V for a 100-V organisation for instance. When deject-edge effect happens in the field, the inverters typically limit the current, thereby raising the DC voltage above nominal condition reaching above MPPT range. This will cause the inverters to shut-down; thus, accumulating downtime.

The MPPT range besides comes into play when discussing LVRT office in the inverter. LVRT is a new requirement in the industry, so information technology has not been studied every bit much. While riding through a fault upshot using the LVRT characteristic, the voltage could exist as low as 10% or 20% of nominal condition. This would hateful, restriction of Air conditioning power downward to 10% to 20% of nominal condition, and it would hateful the same for the DC side. While limiting and riding through of the overall output of the inverter due to grid fault event, the DC side voltage will ascent. Depending on the DC array pattern and DC loading gene, the voltage may shoot beyond the allowed MPPT window of the inverter, causing inverter shut-down. Inverter and arrangement design should exist carefully discussed and assessed in terms of existent-life scenarios, IGBT design and tolerance to these types of weather. Having a wider MPPT window is quite of import to withstanding cloud-edge effects and LVRT situation.

Plant Level
A PV ability plant consists of two or more ability blocks paralleled together and interconnected to the filigree at the substation. As many inverters are connected, intelligent power optimization using i cardinal power controller becomes increasingly important. For case, inverters that are capable of generating above their nominal kW typically have reserved capability for reactive ability output, which tin can be used equally agile power when needed. While cloud coverage is causing energy loss in some of the plant's blocks, the other healthy part of the plant would exist able to compensate for the lost energy past using this characteristic and therefore optimizing institute energy harvest.

Intelligent ability optimization is also helpful during inverter failures and maintenance. While some inverters are shutdown during maintenance, other inverters are online and able to compensate for those losses. The reactive power and voltage can also be dynamically controlled with one centralized controller to match what is required at the point of common coupling (POCC). As the larger bulk of PV power avails are being installed in the grid system, being able to interact and support the grid arrangement becomes increasingly of import.

By Ray Saka, Sales Manager, TMEIC

Desire more? Try these articles:

Do We Need Solar Inverter Reliability Standards? Here'south What Manufacturers Think

2014 Trends: Fundamental Solar Inverters

How To Calculate Solar Reliability

woolfbeheriseed83.blogspot.com

Source: https://www.solarpowerworldonline.com/2014/04/three-levels-solar-reliability/