Fuse Protection of Power Transformers 4

[Mbrooke]

What are the cons of protecting 5-60MVA power transformers with primary fuses? Why is the practice frowned upon?

 

[cuky2000]

Al thought fuses are susceptible to aging degradation, fuses used in the power industry are reliable with infrequent failure rate. Moisture it is recognized of one of the multiple stressor that can lead to failure. However, the predominant failure mode (80%)is attributed to the fatigue of the fuse link over long operation time due to exposure to elevated temperature, voltage transients, or short duration over current condition, follow by a failure of holder clips (~9%), link connection to end caps (~7%), fuse holder wiring lugs (2%) & blade/ferrule ware (~2%) in accordance with an study prepare by the NRC.

The typical minimum clearing time of a circuit breaker (including relay & CB interrupting time) is 2-3 cycles or larger. In contrast, fuses could trip within 1/2 cycles.

The decision selecting simple fuse or better protective system has a profound impact on the economics and to be able to achieve a require reliability and maintainability of the electrical system. As larger is the transformer size, the cost ratio of the protection system became marginal.
On the other hand, if a higher reliability is required. It not unusual to see relay system for transformer in the range of 2-5 MVA. Relay system it is require to facilitate remote control, monitoring, detection of incipient failures, obtain historic data, eliminate single phasing, reduce outage time, ability to provide differentI al protection, among others protective features. Consider the above, protective system base in microprocessor relay is a superior system than fuses limited to provide overcurrent protection only.

 

[stevenal]

The typical minimum clearing time of a circuit breaker (including relay & CB interrupting time) is 2-3 cycles or larger. In contrast, fuses could trip within 1/2 cycles.

But we're talking about a transformer application. For through faults limited by transformer impedance you won't be seeing a 1/2 cycle clearing time with a fuse sized for load. A through fault within a differential or restricted earth fault relay/breaker zone will clear much faster.

Of course you could always go old school and use your relay to trigger a high speed grounding switch on the primary side to blow the fuses much faster.

 

[bacon4life]

Fuses have a much lower fault current rating. Perhaps 10 kA at 115 kV? This was the driver for retrofits at my utility.

Fuses cannot provide overload protection, though typically utility owned transformer relays are not set for overload protection either.

Lack of event reporting. If a fuse blows, you probably have to completely redo Doble/oil testing before re-energizing the transformer. Prior to getting rid of the 115 kV fuses, we had several fuses blow and no transformer failures.

The simple self powered nature seems like the biggest advantage.

 

[Mbrooke]

Why did those fuses blow where no trafo failure was discovered?

 

[bacon4life]

Some were unexplained, some were overly fast melts for distribution faults.

 

[Mbrooke]

When did these unexplained blows happen ie after a trip and reclose? Load pickup? Lightning strike?

Those that melted from distribution faults, how were they sized relative to the transformer's full load current?

 

[cuky2000]

Below Is an impartial attempt to summarize the pros and cons of use of power fuses vs. relays for distribution transformer protection based on this post recommendations, accepted industry practice and some degree by personal experience;

PRO
a) Fuses used in the power industry are reliable with infrequent failure rate.
b) Fuses are self powered. No need of DC, CTs, or low voltage wiring; all possible failure points.
c) Fuses are great to protect VT’s.
d) Fuses are simple thermal devices and very economical to purchase and maintain.
e) There is high probability that fuses has unlimited life if loaded under 60% of the rated current. The worst performance is estimated for 90% loading factor with frequent loading cycles.
f) If fault current is extremely high, a fuse can be faster than a breaker [sub]Fuses can clear faults within 0.5-2 cycles after inception. The fastest type are “current limiting fuse” (CLF) but careful selection is advised to clear the inrush current and avoid nuisance tripping.
[/sub].
g) For lower fault current a ‘fast grounding switch” is an alternative to improve the clearing time. However, this approach will increase cost and still create O&M and reliability challenges.

CONS
1. Potential for single phasing. Single phasing causes very high negative sequence voltage and current and low voltages (Line-to-Neutral & Line-to-Line).[sub]The resultant voltage may be worse than no voltage due to the overheating that it can cause to certain types of equipment, such as three phase motors.[/sub]
2. Fuse link require replacement after fault This could increase outage and maintenance time. CLF fuse can be damaged by inrush current if not properly selected.
3. Poorer protection when compared with breaker + relays. a) Fuses are slow to operate at low & moderate current. [sub] i) Time for fuses to blow is much slower than proper breaker. ii) Fuses will not sense low level faults, such as near the neutral of the transformer, and hence trip only after the fault has evolved into a high current event. iii)This put the transformer at higher risk for being irreparable after an internal fault and at higher risk of failing in some catastrophic manner, such as a fire. iv) The low sensitivity of fuses means they are poor at backing up secondary overcurrent protection devices, especially for faults remote from the transformer secondary and especially for ground faults on delta/wye transformer banks.[/sub] .
. b) Fuses cannot provide overload.
[sub] i) To allow short overloads, a transformer fuse is typically selected to carry per the NEC 150-300% of the transformer rated current. ii) Most fuses can carry over 125% of rated current for very long times. iii) and just begin to reliably trip for faults in the range of 150-200% of the fuse rating, and at this level generally take tens of seconds to trip. The effect is that a fuse might carry current in the range of 3 to 5 times transformer rated current for an extended period. [/sub]
4. Fuses are susceptible to aging degradation [sub] i) Fuses are subjected to gradual damage from heavy through faults, leading to an eventual fast trip for a low magnitude fault. ii) Fuses are not as precise in operating characteristics. Characteristics change slightly with temperature, pre-fault and loading cycles. iii) Moisture is recognized of one of the multiple stressor that can lead to failure. However, the predominant failure mode (80%)is attributed to the fatigue of the fuse link over long operation time due to exposure to elevated temperature, voltage transients, or short duration over current condition.[/sub]

5. Fuses have a much lower fault current rating This could limit the application for smaller units than circuit breakers.

6. Lack of event reporting & monitoring features. [sub]i) If a fuse blows, probably the transformer have to completely redo Doble/oil testing before re-energizing the transformer resulting in increasing outage time. quote: ii)Prior to getting rid of the 115 kV fuses, we had several fuses blow and no transformer failures.[/sub]
c) Fuses can not sense current unbalance as relays does with small flow of current for relays differential protection. [sub]In the absence of a fault in the protected zone, this unbalance tends to be small and the flows into the zone are closely matched to the flows leaving. Accordingly, such relays can be more sensitive than phase overcurrent relays and need not be delayed to coordinate with other relays during external faults.[/sub]

7) Transformer MVA Size & primary voltage kV Those parameters alone are not a definite guide for transformer protection and should consider reliability, maintainability and life cycle cost. [sub] i) Fuses are the predominant choice for transformers below 10 MVA and 15 kV class probably influence by general recommendation developed years ago. ii) Under 3MVA, breakers on the high side are seen only in special applications (e.g., some small generation sites may use a high side breaker). iii) The cost of protection and ancillaries equipments is marginal for large transformers. iv) Important units requires relay system to protect the asset and achieve high reliability and maintainability.[/sub]

 

[stevenal]

Cuky2000,

Did you mean fuses cannot provide overload protection above? I think the statement following indicates they provide overload capability just fine. I don't believe this is a con, since overloading of transformers this size should be monitored by operators rather than tripped by protection. The operator can shift or shed load, avoiding a complete outage.

 

[cuky2000]

Hi Stevenal: For single fuse it is unfrequent (cheap approach) to monitor the transformer overloading through a remote SCADA and most of the time there is only occasional maintenance personnel. In this case appear that most unanticipated overload conditions will happen undetected and depending upon the rating of the fuse selected there is a risk to trip or overload the trafo. This is not an ideal situation if there is need to maintain a decent reliability index and a good maintainability practice.

 

[stevenal]

How does poorer protection present a disadvantage?

So here's a real world example: 69-12.47kV 10 MVA delta wye, formerly protected by an S&C SMD1A 125E fuse. A line to ground fault on the secondary side cleared in 9.3s on the primary side.

With a transformer relay and circuit switcher, and the l-g fault within the differential and restricted earth fault zones: relay time (0.5 cycle), lockout relay time (0.5 cycle) plus switcher time (6 cycle) = 0.117 s.

 

[bacon4life]

It has been more than 10 years since we got rid of the substation transformer fuses, so I do not have the exact details anymore. With just high side fuses and low side electromechanical relays, there was not a lot of information available at time.

I remember hearing some debate as to whether all three fuses need to be replaced anytime one of the fuses blows.

 

[Mbrooke]

9.3 seconds isn't bad

4. Fuses are susceptible to aging degradation i) Fuses are subjected to gradual damage from heavy through faults, leading to an eventual fast trip for a low magnitude fault. ii) Fuses are not as precise in operating characteristics. Characteristics change slightly with temperature, pre-fault and loading cycles. iii) Moisture is recognized of one of the multiple stressor that can lead to failure. However, the predominant failure mode (80%)is attributed to the fatigue of the fuse link over long operation time due to exposure to elevated temperature, voltage transients, or short duration over current condition.

I remember hearing some debate as to whether all three fuses need to be replaced anytime one of the fuses blows.

I have to find it, but S&C specifically mentions there fuses are none damaging and that un-blown units do not have to be replaced. Also that fuses can take overloads without weakening if the fuse link.

S&C appears very confident the melting time will remain constant regardless of what ever is thrown at the fuse (within reason of course).

voltage transients

I'm confused- how do voltage transients effect fuses? They are a series device, isolated from ground.

Single phasing causes very high negative sequence voltage and current and low voltages (Line-to-Neutral & Line-to-Line).

What do you mean by high negative sequence voltage? Will a blown fuse on a delta wye cause over voltages?

 

[Mbrooke]

I take it a delta-delta unit doesn't have the same problems with slow side secondary clearing?

 

[3DDave]

Where is there confusion? A fuse cannot detect voltage transients and can never respond to them. Breakers can be tripped by voltage transient sensors.

 

[Mbrooke]

I know a fuse can't detect them, but a previous member mentions that transients effect fuses. I want to think not... but Cucky2000 knows more than I do.

 

[davidbeach]

Voltage transients can cause the arrestor to conduct, the current through the arrestor can blow the fuse.

 

[Mbrooke]

Point well made- but- I haven't seen to many arrestors trip a trafo's differential.

 

[davidbeach]

Generally an event of short enough duration that the differential hasn't decided whether it's looking at inrush or a fault. Also lots of transformer differentials use CTs on the bushings and the arrestor is outside the zone.

 

[stevenal]

Mbrooke, You take it wrong, your delta-delta still has impedance that will slow down secondary fault clearing.

 

[Mbrooke]

Any idea to what extent? I'd imagine L-L is less current demanding that L-L-L.