Fuseless is not useless
This post addresses the VFD protection concept and particularly the concept eliminating medium voltage fuses – so called fuseless design. We explain the advantages of such approach and why it is definitely not a useless feature.
Concept of fuseless design
Solid protection concept is key point for safe and reliable operation of a drive system.
The VFD as the “brain” of such system has a specific role. It can (fully or partly) protect other system components but it must seamlessly protect itself. In medium voltage drives the fuseless concept is very common and used by several manufacturers across multiple VFD platforms. What is the motivation to get rid of the fuses and why is fuseless design so popular?
Limitation of power fuses
To answer that question we can look at the alternative design using fuses. First, we need to find an appropriate fuse. Power semiconductors have very low thermal capacity and the junction temperature in a fault case rises rapidly. Therefore, the fuse must act very fast to avoid semiconductor damage. Fuses with special characteristics suitable for protection of power electronis are required.
Fuses are also subject of aging. They need to be maintained and replaced after several years. Otherwise, their proper reaction cannot be guaranteed.
Basic question is also where to place the fuse? The location is crucial to determine characteristic and rating of the fuse. Just when looking at converter grid side there are several options:
- fuses on transformer primary (HV) side → rough protection, fuses must be able to withstand transformer inrush current (unless VFD can eliminate the inrush e.g. by pre-magnetization), limited choice depending on grid voltage
- fuses on transformer secondary side → they need to be rated for quite high nominal current
- fuses in series with power semiconductors → good protection, need for special “fast fuses”, high quantity
In high power range there is a limited availability of suitable fuses. During commissioning phase there is typically an increased probability of unusual behavior which may lead to blowing of the fuses. That is extra cost as those fuses are not cheap. Replacing the fuse takes some time (although not that much) thus increasing the time to repair and reducing availability.
What if there is a fuseless design that can mitigate above issues? Good news is that such solution exists and is used since many years. Thus, it is well proven and mature concept.
Fuseless protection concept
Due to above limitations most manufacturers of medium voltage drives implemented so called fuseless protection concept. It may sound little exotic but in fact it is quite straight forward, robust, proven and powerful. The concept includes the incoming circuit breaker located upstream of the input isolation transformer. After detecting an overcurrent the VFD issues a trip signal to the breaker. According to the specification the breaker must open within some 60…100 ms. The VFD is designed in such way that it can withstand the short circuit current until the circuit breaker opens.
Principle
How to achieve it? The semiconductors can be overloaded for a very short period of time. Information about such overloadability is characterized by the overload integral i²t stated in the datasheet. Thyristor based devices (conventional thyristor or IGCT) are particularly powerful for short time overloading. The VFD is designed to not exceed the limits of the semiconductor. There are basically three sources of short circuit current:
- Short circuit current from the grid side (Ik”_grid)
- Short circuit current from the dc link (Ik”_dc link)
- Short circuit current from the machine side (Ik”_machine)
The maximum level of each current contribution can be limited by system design. Minimum required short circuit impedance of the input transformer limits the fault level coming from the grid side. Sizing of the dc link capacitors potentially combined with small dc reactors limits the discharge current of the capacitor cans. And finally the fault current from the machine side is limited by minimum required machine stray reactance (for machines outside of the required range an output reactor may help). By that the maximum total fault current is clearly defined.
Next step is the distribution of the fault current. Fuseless drives may do something called protection firing. It basically means to turn on all active devices and to create multiple paths for the short circuit. It means that the initial fault will not see the entire fault current but just a portion of it.
If that level is still too high then a crowbar can be added into the design. Crowbar typically consists of thyristors. The device is normally turned off. When an overcurrent is detected, the crowbar is turned on simultaneously with all other active semiconductors. Latest after implementing a crowbar the fault level is such that the VFD components can handle it until the main circuit breaker opens.
Strong points of fuseless design
The fuseless protection concept has many advantages. First is, of course, the elimination of power fuses. It saves cost and maintenance effort. Fuseless concept has very fast reaction time. The fuseless solution also opens the way for ultra-fast arc elimination which is obviously a big advantage for safety and availability.
Summary
Power fuses in medium voltage power electronic systems present several limitations and disadvantages:
- Special “fast fuses” required to adequately protect power electronics
- Limited availability in high power range (and need for paralleling them)
- Protection against light overload is not possible with fuses
- Large quantity for certain VFD topologies
- Transient overvoltages during arcing of fuse
- Ageing of fuses → periodic replacement needed
- Fuses must be replaced after a fault → longer downtime, increased lifecycle cost
An alternative is a fuseless concept. The solution is based on defined maximum short circuit current (limited by design), opening of main circuit breaker, short time overloadability of power semiconductors and smart distribution of fault current into multiple paths. Fuseless is definitely not useless.
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References
[1] Short circuit currents in variable speed drive systems, https://mb-drive-services.com/short-circuit-currents-in-variable-speed-drive-systems/
[2] How to calculate the short circuit current in a correct way? https://mb-drive-services.com/how-to-calculate-the-short-circuit-current/
[3] What makes protection firing so brilliant? https://mb-drive-services.com/what-makes-protection-firing-so-brilliant/
