Double Ended Power Distribution Panel
I have a situation at my facility where we have two plants and their respective 4.16kV power systems operating off of two different utility transformers. On the main switchgear lineup for each distribution system there is a tie breaker to conntect one power system to the other in the event that we loose one of our utilty transformers this allows us to run both plants at reduced load. The tie breaker system is designed only to have on transformer operating when the tie breakers are closed, or in other words the system is not intended to run with the utility transformer paralleled.
The 2 main breakers and tie-breakers are vaccum breakers that have a kirk key interlock system in place to ensure that the ties and both mains are never all closed at once putting transformers in parallel.I am aware of all the hazards and percautions involved with putting transforers in parallel (tap changes, circulating currents, increased fault capacity etc, phase seqencing etc.) Most of these percautions with the exception of phase synchronizing are for transformers paralled for an extended duration. I am curious what options I may have to set these tie-breakers up in a 'make before break' transfer scheme in order to keep the power supply seemless to the plant that is already running. If I have a phase synchronizing relay on both voltage sources is it possible to momentarily parallel both sources before interrupting one of the mains only as quickly as it would take for the breakers to open and close from a relay operation?One of the issues we have is that if we need to shut down one of the utility transformers in order to switch the plant load to the other transformer we have to shut the entire plant down in order to close the tie breakers and then bring the plant back up again. This interrupts production and is generally not acceptable.
.Alternative to Double-Ended Substations. Enclosed power switchgear. Removable (draw-out) type switching and interrupting devices. Electrical Distribution Systems Critical Facilities Round Table 12th Quarterly Membership Meeting June 2, 2006 David D.
I am looking for what options may exist for making this transfer seemless by closing both tie breaker first before opening the main and briefly paralleling the two transformers under a relay controlled operation.Is this even an option without getting into a larger order of other issues with the transformers themsleves? RE: Tie Breaker Closing Make Before Break (Electrical) 17 Aug 10 14:17.
If I have a phase synchronizing relay on both voltage sources This is used when one of the sources may be a variable frequency such as a generator.On a utility/utility the phase synchronizing relay will prove that incoming phases have not been switched during an outage.Your two most important issues may be back feeding into a utility circuit and fault current levels.Due to possible liability issues should an unauthorized back feed cause damage, injury or death, it is STRONGLY recommended that you follow David's advice and contact your supply utility.Bill-'Why not the best?' Jimmy Carter RE: Tie Breaker Closing Make Before Break. Momentary closed transition transfer schemes using breakers is very common, although it takes appropriate controls and engineering, similar to those found in closed transition ATSs.I have designed it many times, but on new or replacement systems. Retrofitting existing could be a challenge but can be done.It is simple in theory but takes experience and care in practice to implement. Basically a phase check relay, transfer scheme with relays, timers and/or PLCs plus an independent watchdog timer circuit to separate the two sources, if the transition is not completed within a prescribed time, not exceeding 0.5 second. Other checks and balances have to be in place to avoid a mishap based on your system. Where it gets a little complex is allowing manual operation, when automation fails or in a bypass mode.
Locking out after a fault, etc. Some hardwired interlocks still needs to be in place all the time.You would need approval of the utility copay, if at least one of the sources is them.Rafiq BulsaraRE: Tie Breaker Closing Make Before Break (Electrical) 17 Aug 10 15:36. David:That is not surprising. In fact that would make your utility company more lenient, not requiring a watchdog timer, which is a backup and a last ditch effort to separate the two sources if the normal transfer mechanism get stuck. This is over and above normal operation required to be completed in 100ms.Each utility co. Has own rules.
What I stated is the published policy of more than one local utility companies in Northeast, in fact available on their websites as part of the application for paralleling interconnections.Rafiq BulsaraRE: Tie Breaker Closing Make Before Break (Electrical) 17 Aug 10 20:21. David:It is not re-tripping the same device. It is an entirely independent circuit to trip a 'different' device to break the circuit, IF the primary arrangement FAILS. This is a backup requirement on top of the a 'normal' closed transition throw-over controls.The different device typically is upstream breaker feeding an ATS or one of the other two breakers in a main-tie-main setup.Plus I do not make the rules, nor do I work for a utility company. It is a requirement in our region, nonetheless. Most ATS mfr offer it as an option, the good ones offer it as a standard feature.Rafiq BulsaraRE: Tie Breaker Closing Make Before Break (Electrical) 17 Aug 10 22:05. Devices that do just this are readily available, making switchover times in 30ms quite possible.
A lot depends on the breaker switching times. High speed bus transfer it's called. It is quite common in power plants to switch the aux. Power supply from an external transformer to the generator once it is online. Or in petrochemical industries to switch without interruption from one feeder to the other.Especially in the last case, more interesting is switching over from supply A to B when A fails, provided you have separate feeders upstream.
If you have enough spinning reserves in the form of motors to keep the voltage up for a very short time you can just synchronize on the fly onto the other feeder and resume operation. Depending on the inrush current you might have to load shed some, but it's better as going black.The one I used once was a SUE3000 by ABB. And a REC316 once, but those are end-of-life by now. Not sure about other manufacturers. I've seen some hand build solutions in old power plants as well.
From my experience setting the things up requires so much tweaking and timing that you're better off with something tried and tested. RE: Tie Breaker Closing Make Before Break (Electrical). Thanks for the great information guys. I agree that as a first step I need to discuss this with the utility to see what their requirments are. I am only in the brainstorming stages at this point.One thing that I am concerned about with the momentary paralleling of the two transformers is the increased fault capacity. If there was a fault at a downstream location in one of the plants during the brief paralleling period this would subject our equipment to fault currents exceeding their ratings.
I would think this is something to be genuinly concerned about? It will also increase Arc Flash levels but when looking at these two I would think equipment failure is the greater of the two evils.After seeing some of the responses I did a little more research and looked into other 'High Speed Bus Transfers'. It looks like as an option to a closed transition transfer there are an 'open transition fast transfer' and an 'open transition in-phase transfer'. Does anyone have any experience with these two methods in particular to an application where we are switching a whole plant as I desire? Would these open transfer schemes be more desirable over the closed transfer scheme? Is one scheme economically better as opposed to the other?I appreciate everyone comments and input. RE: Tie Breaker Closing Make Before Break (Electrical) 18 Aug 10 17:04.
Fast bus transfer is a high-speed open transition typically used when there are large induction motors involved. I have experience with it at large fossil power plants where auxiliary loads are transferred between transformers after synchronizing.This is a tricky scheme to implement and involves a fair amount of complex interlocking and special high-speed breaker status contacts.
It is generally designed in from the ground up. The Beckwith Electric website used to have some good technical papers on this subject.David CastorRE: Tie Breaker Closing Make Before Break. We've used the fast transfer schemes on several power plants.
The first few were not successful because the motor loads were too small and drove high speed pumps with low inertia. The motors couldn't hold the voltage long enough for successful transfers.The most recent one on a 7.2 kV Main-Tie-Main arrangement works well in one direction and OK in the other. Going A-B it is imperceptible - no lights blink and the UPS doesn't even alarm. B-A always gets a momentary alarm out of the downstream UPS and a noticeable THUMP from the four 2.5 MVA 600V dry transformers in the electrical room, but the motors don't seem to notice. We lost one 230 kV feeder which killed the A Main but no loads were lost.Momentary paralleling is not allowed on this system, but we do use simultaneous trip and close signals to speed the transfer on power loss. (All upstream protection relays send a transfer initiate to the downstream gear.)All transitions are simultaneous trip and close, supervised by several features in the two Beckwith relays.
There is a lot of logic available for breaker failure and control schemes. For example, the transfer delay feature lets you initiate transfer but delay it for 15 seconds while you get away from the gear out of the blast zone. But it's not cheap.I have installed many rudimentary Momentary Paralleling systems, with no known issues so far. (If all three breakers are closed for 0.5 seconds, then trip the selected breaker.) But my esteemed colleagues on this site have convinced me that is not a good design.RE: Tie Breaker Closing Make Before Break (Electrical). RcwilsonVery intersting point about issues with the transfer on a system with small inertia loads. It sounds like part of the sucess for implementing an open transfer scheme depends on the motor loads holding the bus voltage to an appropriate level during the couple cycle transition? Is there a way of analytically determining before hand weather or not the motors on the system will be large enough to hold up bus voltage, or is this something that has to be determined through testing or trial and error.
I have previous fault reports which show our bus voltage for several cycles after the feeder breaker has opened. Is this sort of information adequate for determining sucess?It sounds like both closed transition and open transition may be equally as complex however an open transition may be the safer of the two options. Do others agree? Would the cost for implementing this on an existing system be about the same.I have seen some of these Beckwith papers and understand that they make a pretty good relay for this fast transfer scheme.
I'm not sure about others but I'm thinking ASCO may make something as well.The one question that I have is how non-motor loads are effected during an open trasnition. Most of the papers I have seen talk about motor loads transfering seemlessly however I have not seen how this transfer can effect electronic controls,PLC's etc. For such transfers. There was one comment above regarding a UPS but I'm curious if otehrs have seen such effects?RE: Tie Breaker Closing Make Before Break (Electrical) 19 Aug 10 08:46.
David:No, it has to be independent of PLC, powered from engine battery power and a separate discrete digital timer. No controls that plays part in normal operation transfer are part of this watchdog timer circuit. As they could be the culprit of the malfunction to begin with. Visit website of UI/ Northeast Utilities. They include a circuit diagram in their application's appendix.They call is 'extended paralleling protection'.
It is primarily intended, I believe (you have to ask them), to prevent sitting in closed transition mode for extended period of time, when two sources are in sync by default, such as when two transformers fed by the same utility source (even if separate feeders), like a spot network.Rafiq BulsaraRE: Tie Breaker Closing Make Before Break (Electrical) 19 Aug 10 09:22. Very intersting point about issues with the transfer on a system with small inertia loads. It sounds like part of the sucess for implementing an open transfer scheme depends on the motor loads holding the bus voltage to an appropriate level during the couple cycle transition?Not just voltage. Phase angle as well. Once the primary source is opened, the motor's generated emf begins to slip behind that of the supply voltage.
At some point, this phase difference reaches 180 degrees, making the subsequent inrush worse than locked rotor MVA. The system may not like that much. Motors even less so. RE: Tie Breaker Closing Make Before Break (Electrical) 19 Aug 10 13:32.
The Beckwith relay monitors the phase angle and voltage differences across the open breaker and sends the close signal anticipating the breaker closing time, similar to a good auto-synchronzier.A regular synch check relay is too slow to use in this application.By closing in phase, the excessive torques are avoided. If the motor voltage drops below a threshold (usually 30% nominal voltage) where the torque excursions would be within motor design parameters, a breaker close signal is given. If the in-phase close doesn't work, the minimum voltage or dead bus logic closes the breaker. There are many options that can be configured.A big portion of the cost for fast transfer systems is the engineering investigation to determine all of the settings and logic.
The relays themselves are not cheap either. I'd recommend calling a couple of suppliers and getting some estimates for both the investigation and the actual hardware.RE: Tie Breaker Closing Make Before Break (Electrical) 20 Aug 10 13:47.
In the low voltage field, we've had this situation a few times before, and it was dropped every time except on one high profile building where the decision was to use the generators as an intermediate point of supply:1. Installation is connected to TX12. Start-up gensets and synchronize with TX1.3.
Switch load to gensets by opening connection to TX1.4. ATS is closed transition.5.
Pole-mounted single-phase transformer with three-wire 'split-phase' secondary. On the three secondary terminals, the center tap is grounded with a short strap to the transformer case.A split-phase or single-phase three-wire system is a type of electric power distribution. It is the equivalent of the original Edison three-wire direct-current system. Its primary advantage is that it saves conductor material over a single-ended single-phase system, while only requiring a single phase on the supply side of the distribution transformer.The two 120 V AC lines are supplied to the premises from a transformer with a 240 V AC winding which has a connected to ground.
This results in two 120 V AC line voltages which are out of phase by 180 degrees with each other. The system neutral conductor is connected to ground at the transformer center tap. 240 V AC can be obtained by connecting the load between the two 120 V AC lines. 2A supplying a three-wire distribution system has a single-phase input (primary) winding. The output (secondary) winding is center-tapped and the connected to a grounded. As shown in Fig.
Either end to center has half the voltage of end-to-end. 2 illustrates the diagram of the output voltages for a split-phase transformer. Since the two phasors do not define a unique direction of rotation for a, a split single-phase is not a two-phase system.In the United States and Canada, the practice originated with the DC distribution system developed.
By connecting pairs of lamps or groups of lamps on the same circuit in series, and doubling the supply voltage, the size of conductors was reduced substantially.The line to neutral voltage is half the line-to-line voltage. Lighting and small appliances requiring less than 1800 watts may be connected between a line wire and the neutral. Higher wattage appliances, such as cooking equipment, space heating, water heaters, clothes dryers, air conditioners and electric vehicle charging equipment are connected across the two line conductors. This means that (for the supply of the same amount of power) the current is halved. Hence, smaller conductors may be used than would be needed if the appliances were designed to be supplied by the lower voltage. 5If the load were guaranteed to be balanced, then the neutral conductor would not carry any current and the system would be equivalent to a single-ended system of twice the voltage with the line wires taking half the current. This would not need a neutral conductor at all, but would be wildly impractical for varying loads; just connecting the groups in series would result in excessive voltage and brightness variation as lamps are switched on and off.By connecting the two lamp groups to a neutral, intermediate in potential between the two live legs, any imbalance of the load will be supplied by a current in the neutral, giving substantially constant voltage across both groups.
The total current carried in all three wires (including the neutral) will always be twice the supply current of the most heavily loaded half.For short wiring runs limited by conductor, this allows three half-sized conductors to be substituted for two full-sized ones, using 75% of the copper of an equivalent single-phase system.Longer wiring runs are more limited by voltage drop in the conductors. Because the supply voltage is doubled, a balanced load can tolerate double the voltage drop, allowing quarter-sized conductors to be used; this uses 3/8 the copper of an equivalent single-phase system.In practice, some intermediate value is chosen. For example, if the imbalance is limited to 25% of the total load (half of one half) rather than the absolute worst-case 50%, then conductors 3/8 of the single-phase size will guarantee the same maximum voltage drop, totalling 9/8 of one single-phase conductor, 56% of the copper of the two single-phase conductors.Balanced power In a so-called balanced power system, an isolation with a center tap is used to create a separate supply with conductors at a balanced Vnom/2 with respect to ground.
The purpose of a balanced power system is to minimize the noise coupled into sensitive equipment from the power supply.Unlike a three-wire distribution system, the grounded neutral is not distributed to the loads; only line-to-line connections at 120 V are used. A balanced power system is used only for specialized distribution in audio and video production studios, sound and television broadcasting, and installations of sensitive scientific instruments.The U.S. Provides rules for technical power installations. The systems are not to be used for general-purpose lighting or other equipment, and may use special sockets to ensure only approved equipment is connected to the system. Additionally, technical power systems pay special attention to the way the distribution system is grounded.A risk of using a balanced power system, in an installation that also uses 'conventional' power in the same rooms, is that a user may inadvertently interconnect the power systems together via an intermediate system of audio or video equipment, elements of which might be connected to different power systems.Applications Europe In, 230/400 V is most commonly used.
However, 230/460 V, three-wire, systems are used to run farms and small groups of houses when only two of the high-voltage conductors are used. A split-phase final step-down transformer is then used, with the centre-tap earthed and the two halves usually supplying different buildings with a single phase supply, although in the UK a large farm may be given a 230-0-230 (nominal) supply.In the, electric tools and portable lighting at larger construction and demolition sites are governed by BS7375, and where possible are recommended to be fed from a centre-tapped system with only 55 V between live conductors and the earth (so called CTE or Centre Tap Earth, or 55-0-55). This reduced low voltage system is used with 110 V equipment. No neutral conductor is distributed.
In high hazard locations, additional double pole protection may be used. The intention is to reduce the electrocution hazard that may exist when using electrical equipment at a wet or outdoor construction site, and eliminate the requirement for rapid automatic disconnection for prevention of shocks during faults. Portable transformers that transform single-phase 240 V to this 110 V split-phase system are a common piece of construction equipment.
Power Distribution Panel Concept
Generator sets used for construction sites are equipped to supply it directly.An incidental benefit is that the filaments of 110 V used on such systems are thicker and therefore mechanically more rugged than those of 240 V lamps.North America This three-wire single phase system is common in North America for residential and light commercial applications. Circuit typically have two hot wires, and a neutral, connected at one point to the grounded center tap of a local transformer). Single pole feed 120 volt circuits from one of the 120 volt busses within the panel, or two-pole circuit breakers feed 240 volt circuits from both busses. 120 V circuits are the most common, and used to power and outlets, and most residential and light commercial direct-wired lighting circuits. 240 V circuits are used for high-demand applications, such as, electric, and electric vehicle.
These use or outlets that are deliberately incompatible with the 120 V outlets.Wiring regulations govern the application of split-phase circuits. Since the neutral (return) conductor is not protected by a fuse or circuit breaker, a neutral wire can be shared only by two circuits fed from opposite lines of the supply system.
Electrical Distribution Panel
Two circuits from opposing lines may share a neutral if both breakers are connected by a bar so that both trip simultaneously ( NEC 210.4), this prevents 120 V from feeding across 240 V circuits.Railways In split-phase electric power is also used on some railways. The center tap is grounded, one pole is fed with an overhead wire section, while the other wire is used for another section.in the between New York and Boston also uses split-phase power distribution. Two separate wires are run along the track, the contact wire for the locomotive and an electrically separate feeder wire. Each wire is fed with 25 kV with respect to ground, with 50 kV between them. Along the track balance the loads between the contact and feeder wires, reducing resistive losses.In the UK are using autotransformers on all new 50 Hz electrification, and (as of 2014) are converting many old booster transformer installations to autotransformers, to reduce energy losses and exported electromagnetic interference, both of which increase when longer, heavier, or faster trains are introduced, drawing higher peak current from the supply. Note that booster transformers only 'boost' the return of traction current through its intended path, the 'return conductor', rather than randomly through the earth, and do not boost, but rather reduce, the available voltage at the train, and introduce additional losses. The autotransformer system enforces the traction return current taking its intended path, while reducing the transmission losses, and therefore achieves both required objectives, of controlling return current leakage to earth and ensuring low energy loss, simultaneously.
Power Distribution Panel Wiring
There is an initial cost penalty, because the previous return conductor, insulated to a fairly modest voltage, must be replaced by an anti-phase feeder, insulated to 25 kV, and the autotransformers themselves are larger and more expensive than the previous booster transformers.See also.References.