Open Phase Detection

[jghrist]

One of our municipal clients had a situation where someone shot one phase on their wholesale power supplier's 100 kV transmission line serving the client's 100-25 kV substation. The phase opened but no trip occurred, leaving a phase open on the primary of the delta-wye transformer. In this situation, two of the secondary phases are 50% of nominal phase-to-neutral and 180 degrees out of phase from the third phase.

This caused considerable damage to our client's customers' motors.

This situation can be detected using some undervoltage logic on the 25 kV side. It can also be distinguished from a VT fuse blowing situation. This protection was not provided, however.

Have any utility people out there had similar occurrances? Do you normally (ever?) provide protection against this at distribution substations? If so, do you trip the station or just send a SCADA alarm?

 

[busbar]

Most likely seems to be as high on up the oneline at LV facility {~service entrance}—exampli gratia 3200A airframe tripped by dedicated utility-grade 47 device—but not at MV level or other group loads. Seems partial service continuity outweighs motor thermal damage. Group/feeder tripping seems rare even in most industrial/petrochem applications. In western US, feeder trip seems fairly rare.

Typically in the western US, it is most often avoided at the distribution level:

“…voltage balance between phases will be maintained… as close as practicable to 2½ percent…” BUT,

“…responsibility to equip his three?phase motor installations {with}…open-phase protection to prevent damage due to overheating in the event of loss of voltage on one phase.”
General thought seems that protective devices furnished closest to motor are most effective. Voltage sensing [versus current-balance or negative-sequence overcurrent] is seen as marginally reliable and of limited security.

On distribution circuits, where negative-sequence overvoltage sensing is applied, tendency is for annunciation but no trip. Not usually dedicated 47 devices, but synthesized from transducer phase-voltage readings.

IEC ‘block-relay’ individual-motor overcurrent protection is claimed at being more sensitive to phase unbalance than eutectic-alloy devices, but seem to be marginal at best…with nebulous sales hype far more prevalent than published numbers.

 

[jghrist]

Thanks, busbar. I'm not sure what the legal liabilities are here, but our client will probably end up paying for some equipment even if not strictly liable. Damage may not have been only to three-phase equipment seeing a voltage imbalance, but also to single phase equipment on the phases that had 50% voltage. I'm investigating more tomorrow.

There are no voltage relays at the substation now, but there are voltage transducers. How would you synthesize negative-sequence voltage from phase voltage transducer outputs? There is no phase angle information in the transducer output. I'm thinking more in line with alarming when any phase voltage is greater than 40% but less than 80% of nominal. This should catch any open phase primary situation but exclude blown VT fuses.

Now, can you tell me why the power supply on an RTU at a 100 kV switching station burnt up? The switching station is between the open phase point and the distribution substation. I am suspecting ferroresonance on the 100 kV system because of the delta windings being in series with the overhead transmission line capacitance.

 

[busbar]

I should have said, phase-to-phase and phase-to-neutral voltages could provide information to figure phase angles.

Depending on the ‘flavor’ of 47, it can react only to imbalance and not assert for voltage changes that remain symmetrical. I’d have refer to ILs to elaborate. Also, 47 devices seem to be available in definite-time, inverse-time and high-speed versions. The inverse-time version is apparently desirable to model stator heating effects.

A single 27/59 can do the additional babysitting, with the 27/47/59 combination riding herd on all combinations of voltage abnormalities.

In transmission systems with distance relaying, healthy VT secondary voltages are sometimes assured [as much as reasonably possible] with a 60 or 60Q {voltage-balance} relay comparing sets of three-phase voltages from two VT sets.

One caveat for voltage-only monitoring seems to be that running motors my have a tendency to “generate” the missing phase and fool 47 devices. In that case, current-balance [negative-sequence current] relaying is applicable, but a single 46 device is not reliable for group motor installations. A desirable application for a 47 relay seems to be inhibiting motor starting and, given process limitations, can conceivably be applied to group installations.

As far as the cooked RTU power supply, was it DC or AC powered?

Ferroresonance is a strange one. There is a short discussion in IEEE C57.105 §7. [ Index at

http://standards.ieee.org/reading/ieee/std_public/description/dtransformers/C57.105-1978_desc.html

]

 

[jghrist]

I don't think we need anything too sophisticated. The protection would be at the 25 kV bus of the 100-25 kV distribution substation. Any motors would be on the distribution lines at customers and would not significantly hold up voltages. I'm thinking of time-delay undervoltage relays or phase sequence relays. Two phases will be at about half voltage, one would be at full voltage. Negative-sequence voltage will equal positive-sequence voltage.

The RTU supplies that failed were 120 vac (with battery backup). There were two RTU's at the switching station; both power supplies failed. At least one had indications of flashovers internally. There is also a station 125V battery (not used for the RTU's). The ac breaker serving the battery charger tripped. The station ac supply is from a 100000-120 volt power VT on the source side of the station 100 kV breaker.

There are indications of flashover on some 100 kV post insulators. After the operator opened the 100 kV breaker to de-energize the line to the 100-25 kV distribution station, he heard arcing sounds around the incoming 100 kV line, but did not see anything. The incoming 100 kV line also serves two industrial customers ahead of the switching station. My theory is that the delta windings of the industrial customers delta-wye transformers ended up in series with the capacitance to ground of the open phase of the transmission line, causing a ferroresonant situation.

Balancing this theory is the fact that power engineers tend to blame every unknown problem either on ferroresonance or harmonics. %-)

 

[stevenal]

SEL has this to say about it:

http://www.selinc.com/appguide/9526.pdf

Application is detecting high side fuse operation. Seems like a much more likely occurance than the open phase, no fault current situation you described.

We have not implemented this scheme yet, but would consider using it on a fuse protected transformer. I think I would opt for tripping, since this situation could be harmful to all three phase customers and two thirds of the single phase customers. Manned response to a SCADA alarm would be too slow to prevent damage.

 

1- The best thing would be to install phase sensing relays for all electrical rotating machinery.In case of failure of on or more phases it opens the main contactor of the equipment.

2- May be a neutral voltage sensing relay on the Y side could help because it would respond to the sudden un balance in the phases.

 

[Bung]

We had a similar situation - burned off bonds on one phase of a 33kV supply to an industrial load, and we also had some interesting discussions about who should have seen what and taken what action, protectionwise.

I've never come across any utility practice in my experience where this scenario is specifically protected. There are arguments for and against, and the arguments against utilities trying to do it get more compelling as you move down in voltage levels (better to keep some customers on etc etc, life support machines...).

but if customers are concerned about it, its a matter of only a few dollars to put volatagge unbaance relays on critical plant.

 

[jghrist]

The SEL scheme certainly would work. Unfortunately, the client has all electromagetic relays in the station, so an additional relay would be required. Normally, an SEL-251 uses phase-to-ground voltages. I wonder why similar logic could not be used with phase-to-ground. Two phases would be 50%, one at 100%. The logic equation could look for any phase greater than 30% and less than 70%.

Without a microprocessor overcurrent relay available, a more direct and less expensive approach would be to use a voltage unbalance relay (like a GE MIV) and trip after a time delay on negative-sequence voltage. I'd want to make sure that there is enough time delay to prevent operation for a fault on a distribution feeder before the feeder tripped on overcurrent.

Best thing might be to put relays on all rotating equipment, but this is a substation that serves thousands of customer owned pieces of rotating equipment. Not only are three-phase motors affected by the negative-sequence, but single phase motors (like everyone's home air conditioner and clothes washer and refrigerator) on the two half-voltage phases are also affected. Most of these would trip on thermal overload, but some would be damaged or fail.

Neutral sensing relays (voltage or current) would not work because there is no zero-sequence, only positive- and negative-sequence voltages and currents.

The only difference between applying the protection on breaker- or circuit switcher-protected transformers and on fuse-protected transformers is the probability of occurence of an open phase on the high side. It can happen on either.

 

[busbar]

Folks at SoCal Edison did a paper in 1992 that may possibly be of some use.

Transmission voltage recovery delayed by stalled air conditioner compressors
Williams, B.R.; Schmus, W.R.; Dawson, D.C.
Power Systems, IEEE Transactions on , Volume: 7 Issue: 3 , Aug. 1992
Page(s): 1173 -1181

Abstract at: { aaaaaggghh!! }

http://ieeexplore.ieee.org/search97...or=Williams,+B.R.;+Schmus,+W.R.;+Dawson,+D.C.

 

[jbartos]

Suggestion to busbar: The posted link appears to need access rights.

 

[jbartos]

Suggestion: The low voltage electronic overload relays have a feature that senses singlephasing at three-phase motors feeds.
The medium voltage multifunction protection systems have this option (or appropriate ANSI Device No. ) available too.

 

[electricpete]

I haven't followed the whole discussion. I tend to agree with gulloona that the unbalance protection should be provided as close to the individual motors as possible. (It might be in the form of current or voltage unbalance or negative sequence relays.) That way they are protected for a wider range of problems which might not be detectable further upstream in the system.

I believe that unbalance protection is a standard protection for large motors.... should be in IEEEC37.96. It sounds to me like the customer's motors were not adequately protected (the customer protection bears some of the blame).

 

[stevenal]

Jghrist

Another thought: How is voltage regulated? If three phase tap changers or regulators are used, they will try to correct all three phases based on a single phase input. If line to neutral sensing is used, then two thirds of the time the otherwise unaffected phase will end up too high. If l-l sensing is used, then high voltage occurs every time on that phase. Controllers will block on potential loss, but the threshold is not usually adjustable.

 

[jghrist]

Single phase regulators are used. SCADA event reports show that the load side voltage of two phases were low at between 61 and 65 volts (on a 120 volt base) during the incident. There is no SCADA point for regulator tap, but there is a maximum 10% boost. The third phase did not reach a limit voltage and did not generate an event. This is consistent with an open phase on the primary of the delta-wye transformer.

 

[jbartos]

Suggestion: Visit

http://www.abb.com/global/abbzh/abb...2&m=v&e=us&c=b3587b772e41c72ec12569d0003ba819

http://www.tde.alstom.com/p-c/bases...fdc134a628135906c1256842003308fd?OpenDocument

PD 932 DescriptionNumerical distance protection
with supplementary functions
and bay unit for control and monitoring
Product RangePG88 Introduction
.

The PD 932 integrated distance protection and control unit is a one-box solution for protection and control. The unit's protection functions provide selective short-circuit protection, ground fault protection and overload protection in medium- and high-voltage systems. The systems can be operated as impedance-grounded, resonant-grounded or isolated-neutral systems. The multitude of protection functions incorporated into the unit enable the user to cover a wide range of applications in the protection of cable and line sections, transformers and motors.

The control functions are designed for the control of up to six electrically operated switchgear units equipped with electrical check-back signaling located in the bay of a medium-voltage substation or a non-complex high-voltage station. External auxiliary devices are largely obviated through the integration of binary inputs and power outputs that are independent of auxiliary voltages, by the direct connection option for current and voltage transformers and by the comprehensive interlocking capability. This simplifies handling of the protection and control technology for a switchbay from planning to commissioning.

During operation, the user-friendly interface facilitates setting of the unit and promotes safe operation of the substation by preventing non-permissible switching operations.

Control Functions
· Control and monitoring of up to six switchgear units
· Selection from over 200 pre-defined bay types
· Bay interlock
· Local control and LCD display with a selection between the diagrams and lists of the
- Bay Panel
- Measured Value Panel and
- Signal Panel

Protection Functions
· Overcurrent fault detection logic
· Undervoltage fault detection logic
· Underimpedance fault detection logic with load blinding
· Distance measurement with polygonal or circular tripping characteristics
· Four distance stages, including one that can be used as a special stage
· Six timer stages, including two backup timer stages
· Directional voltage memory
· Measuring-circuit monitoring
· Backup overcurrent-time protection
· Protective signaling
· Auto-reclosing control (three-pole)
· Definite-time overcurrent protection, four stages
· Inverse-time overcurrent protection, single-stage
· Time-voltage protection, with overvoltage and undervoltage protection
· Thermal overload protection
· Switch on to fault protection
· Circuit breaker failure protection
· Ground fault direction determination by steady-state power evaluation
· Transient ground fault direction determination (optional)
· Ground fault protection signaling
· Ground fault tripping
· Limit value monitoring
· Programmable logic

All main functions are individually configurable and can be disabled or enabled by the user as desired. By means of a straight-forward configuration procedure, the user can adapt the device flexibly to the scope of protection required in each particular application. Due to the powerful, freely configurable logic of the device, special applications can be accommodated.

Global Functions
· Comprehensive self-monitoring
· Parameter subset selection
· Operating data recording (time-tagged signal logging)
· Overload data acquisition
· Overload recording (time-tagged signal logging)
· Ground fault data acquisition
· Ground fault recording (time-tagged signal logging)
· Fault data acquisition
· Fault recording (time-tagged signal logging
together with fault value recording of
- the three phase currents
- the residual current
- the three phase-to-ground voltages and
- the neutral displacement voltage)

The PD 932 is of modular design. The pluggable modules are housed in a robust aluminum case and electrically connected via an analog and a digital bus printed circuit board.

Inputs / Outputs
· 4 current-measuring inputs
· 4 voltage-measuring inputs
· 4 binary signal inputs (optical couplers)
with freely configurable function assignment
· 8 or 14 output relays
with freely configurable function assignment
· 6 binary signal inputs (optical couplers) and 6 output relays for the control of 3 switching devices
or
12 binary signal inputs (optical couplers) and 12 output relays for the control of 6 switching devices
or
8 binary signal inputs (optical couplers)
with freely configurable function assignment for individual signals

The nominal voltage range of the optical coupler inputs is 24 to 250 V DC without internal switching.
The auxiliary voltage input for the power supply is a wide-range design as well.

The nominal voltage ranges are 48 to 250 V DC and 100 to 230 V AC.
All output relays are suitable for both signals and commands.

Control and Display
· Local control panel with LCD display
(16 lines of 21 characters each with a resolution of 128 x 128 pixels)
· 16 LED indicators,
13 of which allow freely configurable function assignment
· PC interface
· Communication interface for connection to a substation control system (optional)

Information exchange is via the local control panel, the PC interface and the optional communication interface.
The communication interface conforms to IEC 60 870-5-103 or, alternatively, to IEC 870-5-101 or Modbus. Using this interface, the PD 932 may be integrated into a substation control system.

Associated ProductsDP 362
FPC . [PD 932]

PD 932 in flush-mounting case 40 T Main FunctionControl & Monitoring, Distance protection . ANSI Code21N, 21P Sub ANSI Codes27, 50BF, 50N, 50P, 50Q, 51N, 51P, 51Q, 59, 60, 62, 67N, 67P, 67Q, 79, 85