Classifications

• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
  • H02J3/00—Circuit arrangements for AC mains or AC distribution networks
  • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
  • H02J3/40—Synchronising a generator for connection to a network or to another generator
  • H02J3/42—Synchronising a generator for connection to a network or to another generator with automatic parallel connection when synchronisation is achieved
• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
  • H02J9/00—Circuit arrangements for emergency or stand-by power supply, e.g. for emergency lighting
  • H02J9/04—Circuit arrangements for emergency or stand-by power supply, e.g. for emergency lighting in which the distribution system is disconnected from the normal source and connected to a standby source
  • H02J9/06—Circuit arrangements for emergency or stand-by power supply, e.g. for emergency lighting in which the distribution system is disconnected from the normal source and connected to a standby source with automatic change-over, e.g. UPS systems
  • H02J9/062—Circuit arrangements for emergency or stand-by power supply, e.g. for emergency lighting in which the distribution system is disconnected from the normal source and connected to a standby source with automatic change-over, e.g. UPS systems for AC powered loads
• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
  • H02J9/00—Circuit arrangements for emergency or stand-by power supply, e.g. for emergency lighting
  • H02J9/04—Circuit arrangements for emergency or stand-by power supply, e.g. for emergency lighting in which the distribution system is disconnected from the normal source and connected to a standby source
  • H02J9/06—Circuit arrangements for emergency or stand-by power supply, e.g. for emergency lighting in which the distribution system is disconnected from the normal source and connected to a standby source with automatic change-over, e.g. UPS systems
  • H02J9/068—Electronic means for switching from one power supply to another power supply, e.g. to avoid parallel connection

Definitions

• the invention relates generally to the field of power management systems, and more specifically to method of synchronizing power supplies for fast bus transfer in a power system.
• High-speed Bus Transfer System (HSTS) scheme is applied to transfer critical loads from one source to another source during disturbances.
• the loads predominantly induction motors
• the Main source On failure of the Main source, the loads are transferred to a Standby source. Attempts are typically made to re-connect the load to the Standby power as quickly as possible. This is to ensure that the voltage, frequency and phase angle differences across the breaker poles are within synchronizing limits, to reduce shock to the induction motors and other connected loads. This is called 'Fast' transfer. Though this is the most desirable type of transfer, it may be a challenge to achieve this.
• the motors spin down further till the voltage collapses to a very low voltage, say 20%, before attempting to reconnect to a standby source.
• the challenge in such scenario is related to dealing with heavy starting current surge, mechanical shocks to connected loads etc.
• the invention describes methods to improve the probability of successful reconnection of two electrical sub power systems.
• Applications are mainly in extending power supplies to critical loads in industrial systems but the methods can also be used for EHV systems to resynchronize two major power systems faster, before stability is irretrievably lost waiting for synchronizing to happen.
• the methods can be applied when there are no local means to control the speed/frequency and voltage of both of the systems to be connected. In such systems, it is usual to wait for synchronization condition to occur and an attempt is made to reconnect the systems.
• the present invention proactively modifies one or both of the system phases / voltages by use of additional components for phase shifting or/and voltage corrections to derive a set of voltages which can be brought in phase with each other's voltages much earlier, improving the chances to reconnect the two systems under the control of an intelligent electronics device (IED) in the electrical power system.
• IED intelligent electronics device
• a method for using an Intelligent Electronic Device (IED) to synchronize power and connect a first sub power system with a second sub power system by use of at least one switching device between the first sub power system and the second sub power system in an electrical power system comprises the steps of: a) Monitoring power supply parameters for the first sub power system and the second sub power system;
• the method is illustrated for a power system where the first sub power system is an incomer system and the second sub power system is a load system.
• the method is illustrated for a power system where the first sub power system or the second sub power system is a power generating system.
• the monitored power supply parameters includes measured electrical voltage, current and frequency parameters, and derived information from computing performed in the IED using the measured electrical voltage, current and frequency parameters including at least one of rate of change of electrical parameters, time to operate the switching device, and time to perform phase shifting, differences in magnitude and phase angles and other such parameters that can be computed or measured in the power system.
• the instance for operating the switching device(s) to connect the first sub power system and the second sub power system is identified by analyzing available instances for synchronization resulting from having phase shifting or voltage magnitude correction carried out in the first sub power system or the second sub power system.
• the method illustrates phase shifting or/and voltage correction step carried out in anticipation for synchronizing power supplies on connection of the first sub power system with the second sub power system at the selected one instance by the IED for making a synchronized connection includes cross switching (cross arrangement) of one or more phases in the electrical power system or making phase/voltage magnitude corrections using a phase shifting transformer in the electrical power system.
• the method illustrates identifying the instance for operating the switching device(s) is carried out by the IED within an AC slip cycle time period from the instance identifying a need for connecting the first sub power system with the second sub power system (eg through external command or input).
• the method provides for a fast bus transfer providing number of opportunities to make synchronized connection in the power system.
• the method illustrates identifying the instance for operating the switching device(s) at an instance at which voltage in the load system is more than about 70% of the voltage in the incomer system i.e. the difference in voltage between the two electrical sub system is less than 30%.
• a suitable correction to the voltage level through use of transformers, tap changers or load shedding can be used to provide voltage correction for fast and efficient bus transfer.
• a synchronizing system in an electrical power system to synchronize and connect a first sub power system with a second sub power system comprises of: a) An Intelligent Electronic Device (IED) that monitors power supply parameters for the first sub power system and the second sub power system and identifies at least one instance for connecting the two sub systems.
• IED also selects a suitable one instance for transfer and issues commands for synchronizing, and connecting the first sub power system with the second power system at the selected instance of time;
• At least one switching device to connect the first sub power system and the second sub power system on receiving the command from the IED.
• the switching device can be a circuit breaker or disconnector switch with appropriate interlocks;
• the system is illustrated to identify one or more instances for operating the switching device(s) within a full slip cycle time period that is within the time from the starting of power cycle from 0 degree coincidence to 360 degree coincidence.
• the time of transfer can be well within the full slip cycle time period from the instance the IED identifies a need for connecting the first sub power system with the second sub power system.
• the synchronizing system in an electrical power system is illustrated with switching device(s) that is a circuit breaker(s) or a disconnector switch(es).
• the means for phase shifting or/and voltage correction in the first sub power system or the second sub power system to enable fast and efficient synchronization between the first sub power system and the second sub power system at the identified instance of time is carried out by operating the switching device or a phase shifting transformer such that at the time of connection, efficient bus transfer is achieved.
• Figure 1 embodies the overall principles underlying the invention
• Fig. 2 is a one line diagram of a typical supply scheme to Unit Auxiliary Bus, feeding a number of motor load of a major Thermal Power Unit;
• Fig. 3 shows the limits of voltage and phase angle difference when closing to a load bus
• Fig. 4 illustrates the limits of phase angle for close command from IED
• Fig.5a illustrates variations in voltage magnitude and phase angle of the motor load bus in comparison with a standby source voltage, referring to a particular Phase-A of a three phase system
• Fig. 5b illustrates a limit of motor load bus voltage 70% of the nominal voltage magnitude when the angle and magnitude of the motor load bus keeps varying with respect to standby source voltage
• Fig 5c illustrates variations in voltage magnitude, frequency and phase angle of load bus with respect to a standby system voltage
• Fig. 6a illustrates the phase A voltage of standby source along with all three phase voltages of the motor bus
• Fig 6b illustrates variations of differences in voltage magnitude, frequency and the phase angle of load bus of all three phases with respect to the standby system voltage phase A;
• Fig 7 illustrates a three breakers arrangement to connect the bus phases ABC of standby source with the motor load bus with ABC connection along with CAB or BCA in that sequence as alternatives;
• Fig. 8 illustrates a single breaker and three disconnectors arrangement to achieve synchronized fast bus transfer
• Fig. 9 illustrates a two position high speed disconnector / switch arrangement to connect the incoming phases ABC with the motor load bus in ABC, CAB phases in that order with flexibility of changeover as well as reconnection back to the system with inherent interlocks at disconnector level to avoid asynchronous closing;
• Fig. 10 illustrates a three position high speed disconnector / switch arrangement to connect the incoming phases ABC with the motor load bus in ABC, CAB and BCA phases in that order with full flexibility of changeover as well as reconnection back to the system with inherent interlocks at disconnector level to avoid asynchronous closing;
• Fig. 11 is the one line diagram of a typical supply scheme to a load bus from two incomers with phase switching facility controlled by an Intelligent Electronic Device (IED).
• IED Intelligent Electronic Device
• the present invention improves the prior art of 'Transfer at first phase coincidence' technique.
• the proposed method can be used to manipulate the Standby source voltage phases, either by way of switching or phase shifting or a combination of such action as required to reconnect source to the load bus as quickly as possible.
• the manipulation can be done in either or both of the systems (source and load), but description further considers manipulation only of one side (Source) of the power system for clarity of description.
• Figure 1 illustrates a part of power system 100 having a main source 110 and a standby source 120. Breakers 52G indicated with numeral 130 and 52S indicated with numeral 140 are provided to connect the main source 110 or the standby source 120 to a load bus 170 as the need maybe for the source connection.
• the invention introduces phase shifting components 150 and 160 to achieve fast bus transfer.
• One of the methods to manipulate the incoming supply phase angle can be a rearrangement (cross- switching) of the three phases. This gives the possibility of three choices of incoming voltage phases which are 120 degree apart for possibility to synchronize the load bus with incoming source. Other multiple of 30degrees angles are possible using Delta / Wye windings in the incoming supply. Other angles are also possible using a combination of windings. Should the system be single phase, it is possible to look at 180 phase rearrangement at very least.
• the invention allows for manipulations of voltage magnitude as well to achieve high speed re-connection though phase switching can be regarded as one of the economical way to achieve a quicker first phase co-incidence transfer and has been described in detail for a three phase system.
• the three phases A,B and C of a typical power system may be considered as three individual voltage sources, coupled together such that the phase angles are separated by 120degrees.
• phase angles are separated by 120degrees.
• phase A,B and C of one system are synchronized with phases A, B and C of another in the same order.
• the method of the invention proposes a flexibility in the switchgear connection arrangements such that, phases A,B and C of a system can be synchronized with phases B,C and A (also / alternatively phases C, A and B), in addition to conventional phases ABC of the second system.
• phase switching components phase shifting devices
• IED programmed intelligent electronics device
• FIG. 2 illustrates an exemplary supply arrangement 200 to auxiliary loads of a major thermal generating unit.
• the auxiliary loads comprise predominantly of major induction motors 210 driving Boiler feed pumps, Induced Draft fans, Forced Draft fans etc., and are connected to the Unit Auxiliary Bus 220.
• the Unit Auxiliary Bus gets energy from a station transformer 230.
• the Unit Auxiliary Bus gets transferred to an Unit Auxiliary Transformer 240. This is accomplished by closing breaker 52G, 250 momentarily paralleling the two sources and then tripping the breaker 52S, 260.
• the operations of the breakers 250 and 260 are managed by an IED 280.
• breaker 52G remains closed and breaker 52S remain open.
• the breaker 52G trips cutting off supply to the Unit Auxiliary Bus 220. It is however essential to keep running the auxiliary motors connected to the auxiliary bus.
• the High Speed Bus Transfer scheme gets primed at the same time when trip command is issued to the breaker 52G, 250 and thus the IED identifies a need for bus transfer. Based on various criteria considered by the IED 280, the closing command will be issued to 52S, 260. Assuming simultaneous command going out to breakers 52G and 52S, should the trip time of 52G is longer than closing time of 52S, there could be a momentary paralleling of the breaker before transfer occurs. This may not be always allowed depending on the fault type in the generator system. In most cases, the trip time of 52G, 250 is expected to be less. This invariably results in a momentary loss of power on the unit bus from both sources. However the connected major induction motors back emf would keep the bus voltage alive during this duration, characteristically dropping down in frequency and voltage, depending on various factors including connected loads, stored magnetic energy in the stator and rotor of the induction motors etc.
• Figure 3 depicts phasor relationships for power supply parameter signals in the exemplary electrical system 200 illustrated in Figure 2.
• the figure shows VS (stand by source voltage), 310 as a steady phasor and is typically in step with the main source voltage VG (also shown as 310).
• the phasor VM depicted by numeral 320 represents the status of load bus voltage, after it has been disconnected from the main source VG but yet to be connected to standby source VS. During this transition, typical of a motor bus voltage running down in the absence of any source, the phasor VM would decay both in magnitude as well as frequency as illustrated in the phasor.
• the phasor VM which was coincident with and equal to VG, has started separating and is falling behind VS.
• the standby source can be connected to the load bus only when: 1) VM does not fall behind VS by a critical angle cpL depicted by numeral 330; 2) The voltage difference magnitude is less than 30% (appx.), illustrated by the numeral 325; and 3) The frequency difference is within limits at the time of closure. The time required for the breaker to close has to be accounted accordingly in any close command to the breaker to ensure closure of breaker at the desired time (illustrated by numeral 340).
• a prediction has to be done as to the magnitude of phase angle difference and voltage difference, based on the measured signal values as well as their rate of changes ( V, dV/dt, f, ⁇ and dcp/dt).
• the rate of change of signals dV/dt and dcp/dt depend on a number of factors including the number of motors connected to the bus, how much they are loaded and the type of loads etc. These factors are considered to select a suitable instance for bus transfer.
• the close command shall be given such that, allowing for the time of breaker to close, the VM phasor 410 would be 1) 'In phase' with VS 420; 2) The voltage difference magnitude is less than 30% (appx.) as VM phasor is depicted to be between the region encompassed by numerals 430 and 440 ; and 3) the rate of frequency fall 450 and voltage fall 460 are within acceptable limits.
• FIG. 5a illustrates an exemplary phasor A of standby source (VSA), 503 having a region 505, and loci of phasor A, 507, of load bus (VM A ) with respect to time, from the moment the load bus gets separated from the main source.
• Time scale is indicated along the path of the loci at 510, 515 and 517 to corresponding to lOOmS, 200mS, and 300mS respectively.
• Figure 5b is a reproduction of initial portion of loci of VM A 507 in Figure 5a, but with the limit of voltage approximately 70% superposed as a circle 520. It may be observed that as the magnitude of VM A decreases, the magnitude falls below 70% even when the phase angle difference is increasing beyond about 90degrees. It may be observed that beyond this point of time (525), voltage would only keep decreasing and there is no way the two systems can be synchronized.
• Figure 5c shows the absolute values of difference in frequency (Af 530, difference in voltage (AV) 540 and the phase angle difference ( ⁇ V SA- VMA) 550, 560, between the two voltage phasors as a function of time.
• Fast Transfer possibility occurs immediately after loss of power source (565) and "First Phase co-incidence Transfer” when the angle between the two phasors is about 360 degrees (567).
• the desired criteria to have the bus transfer carried out within 30% difference in voltage is represented for reference with numeral 570 in the figure.
• Another opportunity at residual or fixed time, when the load voltage has reduced to residual value or near zero value occurs much later and is depicted with numeral 580 in the figure.
• Figure 6a illustrates phasors corresponding to phase A for standby source VSA (610) and the load bus VM A (620)as well as the phasors for B and C phases of load bus VM B and VM C (625, 627), captured at an instance of time.
• phase A of the load bus (VM A ) is having significant phase difference with respect to phase A of the standby source.
• C phasor of load bus VM C is ahead of VSA and is closing in on it with a small difference in phase (630). If only a re-connection can be made from phase C of the load bus to phase A of the incoming source (as well as suitably for other two phases as well), a fast transfer is established.
• Figure 7 illustrates a schematic for connecting the standby source (710) to the load bus (720).
• the schematic shows a set of circuit breakers 730 to connect the standby source in the same phase sequence with the load.
• the set of breakers (52AC) represented with numeral 740 are breakers to allow phase A of the standby source to be routed to phase C of the load bus to achieve synchronization for bus transfer within 360 degree wait period.
• the set of breakers (52AB) depicted with numeral 750 are provided to allow phase A of the standby source to be routed to phase B of the load bus for fast bus transfer.
• Figure 8 illustrates another embodiment of the invention where instead of using additional breakers as shown in Figure 7, a common breaker 810 with disconnector arrangements (820, 830, 840) is shown. This arrangement is more economical. The disconnectors have to have quick operating times.
• FIG. 9 A simplified schematic of the overall switching scheme with a two position high speed isolator is shown in Figure 9. Note that the two position disconnector 89M (910) is used with the main source (905) path in series with the main source breaker 52M (920). Disconnetor 89S (930) is a similar one used in series with the breaker 52S (940) in the standby source (907) path. Both disconnectors can be ganged or interlocked so that they both can be in position AA (945) or AC (947).
• phase ABC of the sources are connected to phases ABC of the load bus.
• phase AC phases ABC of the sources are connected to phases CAB of the load bus.
• breaker 52M (920) is closed, 52S (940) is open and 89M/89S (910, 930) are in position AA (945).
• the disconnector position is changed immediately to AC (947).
• An attempt can now be made to close the 52S breaker (940) to reconnect the load bus with the system at 120 degree coincidence as described earlier.
• the breakers 52M could be closed prior to opening 52S with momentary paralleling.
• the load bus voltage is 120 degree behind the Main source voltage during running conditions.
• FIG. 10 represents a schematic with addition of a third switching element AB (1010).
• the cycle is complete to the extent that further load transfer can be achieved with the first breaker 52M should an additional transfer become essential.
• the disconnectors (910, 930) can be provided with suitable mechanical couplings or interlocks (1020) to support the above function.
• FIG 11 illustrates a block diagram with an IED (1110) to enable fast bus transfer.
• the IED shown in Figure 11 is programmed to issue switching commands to breakers / disconnectors (1115), check synchronizing etc. to achieve full or part implementation of the schemes suggested in this invention.
• the IED voltage inputs from the main incomer (1120), standby incomer (1130) and the load bus (1140) are provided as an input to the IED for computations related with voltage phase synchronization and magnitude limits for fast bus transfer.
• Single phase input is sufficient since the other phase voltages can be internally derived by the IED using 120 degree phase rotation but additional phase inputs might improve the measurement process and also aid detect other abnormalities in the system as a whole.
• the IED is provided with binary inputs and outputs to monitor and control all the associated switching devices (not shown in the figure).
• the inputs and outputs can be given through conventional analog inputs, binary inputs and outputs or through communication channels using either IEC61850 or other methods.
• the IED may also have additional binary inputs and / or outputs (not shown in the figure) to monitor the status of motors and other loads connected to the bus so as to trip out less critical loads during transfer if necessary.
• the IED is shown to control and operate circuit breakers and disconnector switches (1150) as per any of the configurations described earlier and other commonly known configurations to the person skilled in the art.
• Receive command or input (external) to initiate transfer i.e. sense or determine the need for bus transfer (to connect the two sub power systems) based on power system parameter inputs received or receive external command from another IED in the system.
• synchro-check condition to be checked for phases A of source and B of the load and suitable switching action initiated 240 degree transfer accomplished d.
• synchro check condition to be checked for phases A of source and A of the load and suitable switching action initiated 360 degree transfer accomplished
• step 4 if the decay rates of voltage and frequency are determined to be too fast for any of the above steps, wait for the voltage to drop below 30% to initiate closure of the breaker at residual condition or after a fixed time.
• phase shifting transformer to provide necessary shift in phase angles (eg. at supply side or load side) or voltage magnitude correction in anticipation of synchronized power supply at the instance of having the bus transfer and then enable connection between the two sub power systems.
• the invention can be also applied to higher voltage power systems during resynchronizing or auto- reclosing.
• two power systems could be connected through a weak link.
• the link may open out during transient fault or abnormal operating conditions including power swing conditions.
• the challenge is often to reconnect the systems through either manual or auto-reclosing.
• the prior art would wait out for the two systems to go through their own internal dynamics before the voltage across respective phases of the systems are within acceptable limits when a close attempt may be made.
• the present invention allows to proactively modify the system voltages that are synchronized, thereby improving the chances for a successful reconnection of two systems before the two systems well before their parameters drift too far apart causing permanent separation of the systems. The latter might cause brown outs and black outs in individual systems.
• the principles of the invention in general can be applied to connect two power systems in an electrical network or grid.

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• Engineering & Computer Science (AREA)
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• Emergency Management (AREA)
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• 2013
  • 2013-10-01 WO PCT/IB2013/059037 patent/WO2014053989A2/en not_active Ceased
  • 2013-10-01 CN CN201380051515.8A patent/CN104813558B/zh active Active
  • 2013-10-01 EP EP13776581.4A patent/EP2904680B1/en active Active
• 2015
  • 2015-04-01 US US14/676,517 patent/US9923375B2/en active Active

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