What causes negative sequencing?

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I'm working on the 13.8kv main switchgear of a large hospital. There are 13.8kv to 120v control transformer's with 1 amp primary fuses (for the 120v controls on the board) that have failed multiple times over the past year.

I've gathered information off of the power quality meter on each of the 13.8kv branch breakers within the switchgear and they are consistently reporting "negative sequencing". I'm not an engineer, so this is new to me. 2 Questions:

1. Is it safe to assume that because all of the 13.8 branch breakers are reading similar voltage imbalances, that this is a utility issue? Or is possible something downstream of the main switchgear could cause this?

2. What are some potential causes of consistent utility voltage imbalances?
 

Phil Corso

Senior Member
Larry,

you are probaly aware that negative-sequence is related to mostly line-current imbalance! What is important is the degree of imbalance. \\

What are your current readings for: Ph-Ph voltages; Ph-Grnd voltages; and Line-Currents?

Regards, Phil Corso
 

junkhound

Senior Member
Location
Renton, WA
Occupation
EE, power electronics specialty
In addition to line current imbalance, differential v0ltage drop, power factor, and phase shifts also generate negative sequence.

A brief explanation since you indicate you are unfamiliar with negative sequence.

Negative sequence is only a mathematical tool to quantify power quality.

If you take the 3 phase vectors of the system, each phase will have a voltage and phase. Ideally, that is 13.800000V per phase and 120.0000 degrees between phases.

In actuality, these values vary. Line currents and power factors may produce 12.8, 13.1, and 12.6 kV at of zero degrees, 122 degrees, and 236 degrees.

If you take the vectors of these voltages, you can break them down into positive, negative and zero sequence. The positive sequence is all voltages identical and at exactly 120 degrees spacing, the NEGATIVE sequence is all voltages identical and 120 degrees spacing BUT in the opposite phase rotation, hence the negative terminology. The zero sequence is a single vector that adds onto the negative and positive sequences to produce the actual line voltages and phase relationships.

One way to look at positive and negative is that the positive sequence will run a motor in the forward direction, while the negative sequence produces a rotation torque in the opposite direction.

SO:
1. It is a combination of both utility load factors and your downstream of switchgear. Best way to know is put a scope on the open circuit utility power and measure open circuit voltages and phase relationship and calculate the positive and negative sequence voltages. Anything worse than that when your load is connected is due to your loads.

2. Sources of neg seq: Uneven poco loading of phases, another nearby industry on same substation with atrocious power factor loads, bad PF an unequal loading of your own loads.


PS: 1amp pot xfmr fuses blowing. Hospital loads - does the hospital exercise it's standby generators monthly? Are the fuse openings at all related? Have seen cases where when the standby gen voltage regulators stay active during shutdown, the slower speed of the gen during shutdown leaves the voltage high as the frequency drops, causing the potential xfmr primary to saturate and blow small fuses. Solution is to make sure the volt second capability of the pot xfmr is not exceeded during shutdown.
 
The PQ meters on the main breakers generated multiple events the night that the fuse blew. In what appears to be the most extreme event the voltages were reported at 6.72kv (AB), 8.72kv (BC), and 14.10 (CA). The phase displacement numbers at that same time reported as AB=0 degrees, BC=312 degrees, and CA=152 degrees.

Unfortunately we haven't been able to pull amp readings from the meter. I'm an amateur, but this appears to be a very unbalanced system. However, I still don't know the potential cause of such a utility event (the utility is telling me that they didn't have any issues that night) and why this would cause the VT fuse (a time-delay fuse) to blow. :?
 

LMAO

Senior Member
Location
Texas
I would also check for harmonics. Hospitals are notorious for having arcing furnaces and other large switching devices.

Beat me to it. Some harmonics are also negative sequenced. In case of a 6 pulse VFD, 5th, 11th and 17th are negative sequence.
 

mbrooke

Batteries Included
Location
United States
Occupation
Technician
The PQ meters on the main breakers generated multiple events the night that the fuse blew. In what appears to be the most extreme event the voltages were reported at 6.72kv (AB), 8.72kv (BC), and 14.10 (CA). The phase displacement numbers at that same time reported as AB=0 degrees, BC=312 degrees, and CA=152 degrees.

Unfortunately we haven't been able to pull amp readings from the meter. I'm an amateur, but this appears to be a very unbalanced system. However, I still don't know the potential cause of such a utility event (the utility is telling me that they didn't have any issues that night) and why this would cause the VT fuse (a time-delay fuse) to blow. :?

That is very poor power quality IMO.
 

junkhound

Senior Member
Location
Renton, WA
Occupation
EE, power electronics specialty
That is very poor power quality IMO.

Do not think anyone would disagree with your opinion on that!

Was trying to visualize how a 3 phase system would be so poor - possibly fed by an overloaded rotary converter and not a 'real' 3 phase system, or an open phase somewhere with low load motors on the line acting as rotary phase converters?
 

mbrooke

Batteries Included
Location
United States
Occupation
Technician
Do not think anyone would disagree with your opinion on that!

Was trying to visualize how a 3 phase system would be so poor - possibly fed by an overloaded rotary converter and not a 'real' 3 phase system, or an open phase somewhere with low load motors on the line acting as rotary phase converters?

I can only guess but that could do it also. I am willing to be its all POCO, which I would defiantly call them out on it.
 

coop3339

Senior Member
Location
NJ
The PQ meters on the main breakers generated multiple events the night that the fuse blew. In what appears to be the most extreme event the voltages were reported at 6.72kv (AB), 8.72kv (BC), and 14.10 (CA). The phase displacement numbers at that same time reported as AB=0 degrees, BC=312 degrees, and CA=152 degrees.

Unfortunately we haven't been able to pull amp readings from the meter. I'm an amateur, but this appears to be a very unbalanced system. However, I still don't know the potential cause of such a utility event (the utility is telling me that they didn't have any issues that night) and why this would cause the VT fuse (a time-delay fuse) to blow. :?

The voltage numbers could possibly indicate phase lost on delta primary somewhere up stream. With phase loss on a delta primary you get one leg about 100% voltage and 2 at about 50% voltage. I don't know about the phase angles, but maybe someone here does.
 

Phil Corso

Senior Member
Coop,

Your observation that post-fault (possibly during-fault) ratio-magnitudes being about 1:1/2:1/2, being indicative of phase-loss, is correct!

Unfortunately, however, nothing is known about pre-fault measurement magnitudes! Nor is there information on the event-duration!

Regards, Phil Corso
 

mbrooke

Batteries Included
Location
United States
Occupation
Technician
The voltage numbers could possibly indicate phase lost on delta primary somewhere up stream. With phase loss on a delta primary you get one leg about 100% voltage and 2 at about 50% voltage. I don't know about the phase angles, but maybe someone here does.


Id imagine phase angles near identical across the two phases still in service ?
 
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