Safety considerations for assessing module-level shutdown solutions

module level shutdown safety

Now that California – the largest solar market in the U.S. – has adopted the 2017 National Electrical Code (NEC), many solar installers wonder what a good solution for the Module Level Shutdown (Art. 690.12) requirement looks like. With safety being paramount, this article identifies the major criteria for assessing solutions and potential issues with current solutions.

The new requirement for Module Level Shutdown (Art. 690.12) is intended to improve first responder safety with solar systems. The code requires that rapid shutdown results in conductors in the PV array boundary “shall be limited to not more than 80 volts within 30 seconds”, reducing the shock risk for first responders. Therefore, safety should be the main criteria for assessing solutions and is broken down into three goals:

Safe and reliable switching in an emergency

Since Module Level Shutdown is for fire fighter safety, solutions have to be reliable when needed. When Rapid Shutdown is activated, the module-level shutdown switches must reliably switch and de-energize the array. If this doesn’t occur, the risk could actually be higher than without having the shutdown function at all, because fire fighters would assume that the array is in fact de-energized.

So how can safe switching be ensured? Mechanical DC disconnects (e.g., on an inverter) are subject to testing to the UL98B standard. This standard requires testing in typical environmental conditions in which the disconnect is supposed to function, with thousands of iterations along with other safety relevant tests. This ensures the DC disconnect functions properly since because people’s health and life depends on it.

In stark contrast, today’s solid-state DC switches used in Module Level Shutdown compliance are only tested to UL 1741. This is not a DC disconnect or safety switch standard. In addition, the environment that they are supposed to work in is actually more severe than the environment of a regular DC disconnect at or on the inverter. This becomes especially precarious in case of fire that can lead to extremely high temperatures. This scenario is exactly when Module Level Shutdown is needed for a fire fighter to safely work around the PV array.

Accurate arc fault detection and proper shutdown response

Electronic components can create electrical noise. On the DC side of a solar system, this is typically not a problem. However, with today’s MLSD devices, the noise levels can increase significantly and interfere with proper arc fault detection. AFCI algorithms monitor DC current and voltage of the solar system and detect certain characteristics (such as sudden changes), which look like an arc, based on a set of arc signatures indicating what a typical arc looks like. If the measured current and voltage patterns look similar to an arc signature, the AFCI is triggered to extinguish an arc or prevent a potential arc before any damage.

If the electrical noise created by MLSD devices (e.g., from optimizers switching frequencies) creates signals that look like real arcs, the AFCI algorithm has difficulty discerning a real arc. This noise increases the likelihood of nuisance tripping or even worse, of not detecting a real arc. Furthermore, this additional noise could ironically interfere with the shutdown signal of some systems, making it difficult to rely on shutting down when needed – even in normal operating conditions, e.g. when the noise is mistaken for the stay-alive signal.

Fewer connection points mean less risk of arcing

Studies conducted by Fraunhofer Institute in Germany and BRE National Solar Centre in the UK concluded that DC connectors are a main cause for serial arcs in a PV-array. There are two causes for DC Connectors to be the Achilles’ heel of solar systems:

1. Installation errors. The most common installation faults are connectors that are not fully inserted and poor on-site crimping, both resulting in bad connections that can lead to arcing. Typical mistakes include use of incorrect crimping tools, mounting of connectors without enough precision, or simply insufficient installation training.

2. Lack of a DC connector standard. A mismatched connection is one between a male and female DC connectors made by different manufacturers. Because different materials are used by different manufacturers, incompatibility leads to corrosion, ingress of water, different thermal expansion behavior, or simply loose connections. Despite being against an industry best practice, it often happens when wanting to mate different manufacturer’s devices that have different connectors; installers fear voiding a warranty by cutting off the unwanted connector type to place their own connector.

Therefore, any Module Level Shutdown solution that significantly increases the number of connection points adds risk to the system. Most of today’s solutions increase the number of connection points by 300-400% compared to a system without MLSD.

On this last point, cost and operational reliability should also be considered when assessing Module Level Shutdown solutions. Cost-effectiveness drives the adoption of solar energy – the more affordable solar is for customers, the quicker we will be able to transition to a renewable energy supply system. In addition, solar systems are 20+ years investments and must operation reliably over the lifetime. Any system that has fewer components and less exposure of power electronics to harsh roof environments will show higher reliability.

Conclusion

Safety is a top industry priority, so when assessing MLSD solutions for safety, it is crucial that the switching mechanism is safe and reliable, so it works when needed the most. This means making sure it can perform reliably in all situations and extreme temperature conditions. Furthermore, the shutdown devices must not create noise that could interfere with AFCI algorithms and thus risking nuisance tripping, not detecting real arcs, or interfering with and causing the shutdown function to fail. Last but not least, additional connection points should be avoided to reduce arc risk. Once safety criteria are met, cost and operational reliability can be taken into account as well.

This article was submitted by Fronius USA.

— Solar Builder magazine

[source: https://solarbuildermag.com/featured/safety-concerns-for-solar-pv-module-level-shutdown-solutions/]

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