Indian Patents. 268948:METHOD AND APPARATUS FOR IMPROVING OPERATIONAL RELIABILITY DURING A LOSS OF A PHASE VOLTAGE

Title of Invention	METHOD AND APPARATUS FOR IMPROVING OPERATIONAL RELIABILITY DURING A LOSS OF A PHASE VOLTAGE
Abstract	The present invention is directed to a method and apparatus for improving operational reliability during a loss of a phase voltage (LOV) in a multi-phase power system, wherein a voltage representative of the LOV phase is calculated. The representative voltage is used for computing a reference polarizing voltage suitable for a protection unit.

Full Text	WO 2006/124743 PCT/US2006/018666 1 E20050140 METHOD AND APPARATUS FOR IMPROVING OPERATIONAL RELIABILITY DURING A LOSS OF A PHASE VOLTAGE Cross Reference To Related Application This application claims the priority of U.S. provisional patent application Ser. No. 60/680,719 filed on May 13, 2005, entitled "A METHOD FOR IMPROVING OPERATIONAL RELIABILITY DURING A LOSS OF SINGLE PHASE VOLTAGE" the contents of which are relied upon and incorporated herein by reference in their entirety, and the benefit of priority under 35 U.S.C. 119(e) is hereby claimed. BACKGROUND OF THE INVENTION [0001] The present invention relates to a multi-phase power system and in particular to the reliability of that system during a loss of phase voltage. [0002] As widely known in the art, many protective devices, such as relays, circuit breakers, various types of monitoring, supervising and fault detection units, are used in power system. One basic task of these devices is to allow electrical power to be distributed in a reliable manner and to adequately protect either the transmission lines and the loads or equipments connected therewith, e.g. generators, motors, from hazardous conditions which could lead to malfunction, severe damage, failure, etc. [0003] Often, the protection of power system elements is based on accurate measurement of three-phase voltages in order to provide reliable fault detection and breaker operations and thus minimize power system disruptions. In these cases, incorrectly measuring one or more of the three-phase voltages by a protective unit may result in technical shortcomings, e.g. erroneous trips (breaker operations) and/or clearing more of the power system than desired. [0004] For example a common failure that causes incorrect voltage measurement is when there is a downed line, or due to an equipment failure in an end-user's facility, or when one or more fuses protecting a three-phase voltage transformer (VT) secondary circuit blow. In the latter case, protective relays connected to that secondary circuit would measure zero voltage if a secondary phase is isolated (only phase-to-ground connections) or there exist a non-zero coupled value if there are phase-to-phase connections in the secondary circuit. WO 2006/124743 PCT/US2006/018666 2 E20050140 Conditions, other than blown fuses, may also occur where one or more secondary phase voltages are unintentionally removed from the protective relay. [0005] In the electrical power system industry operating in this abnormal state may for example be referred to as "single-phasing", or as fuse failure, or as "loss-of-potential (LOP)", or more commonly as "loss-of-voltage (LOV)" and this last definition ("LOV") will be used hereinafter to refer to the loss of voltage in a single phase. [0006] It is clear that when an LOV state occurs, adequate control of voltage dependent measuring units need to be carried out so as to reduce potential detrimental effects on the whole power system and/ or on the various pieces of equipment. [0007] For example, with protection measuring units, such as distance and impedance units, e.g. distance relays, functioning is based on calculating the impedance seen by the units themselves; since the impedance measured is directly dependent on the voltage(s). Thus an LOV condition will adversely affect obtaining correct measurements and therefore missing one or more phase voltages will lead to misoperations, such as untimely tripping. [0008] The same considerations apply also in the case of the so-called directional units, i.e. protection measuring devices that determine the direction of the current flow in an AC circuit and are used to supervise for example an overcurrent relay in order to let it trip only in the desired direction. These directional units can perform their supervision task by comparing the angular relationship between the current in the protected circuit and an independent voltage source. Since the current can vary significantly for various types of faults, in order to determine directionality the independent voltage may be used as a reference or polarizing quantity. This reference voltage may be not correct during an LOV state, and hence the directional units will not be able to provide the required supervision and may contribute to a misoperation. [0009] A further example of the negative impact that the occurrence of an LOV state may have on power line protection is the operation of a recloser. A recloser opens and closes multiple times when a fault condition exists in an attempt to clear the fault. Should the fault condition continue to exist, the recloser opens and remains open until reset manually. The recloser enters a "lock out" WO 2006/124743 PCT/US2006/018666 3 E20050140 state when this occurs. Automatic reclosing requires synchronization or comparison of voltage on each side of an open line circuit breaker. These comparisons are often accomplished by comparing the voltage from one phase VT on each side of the open breaker. If an LOV condition exists on either VT, reclosing may operate incorrectly and prevent automatic system restoration, cause a line outage, or possibly cause circuit breaker damage. Thus, the location of the LOV may impact the protection functions provided by the recloser. [0010] In the past, in order to improve reliability under an LOV state, various solutions have been devised varying depending on several aspects of the specific applications, such as type, number and technology of protecting systems, available options provided by the units, architecture of the power system, etc. These known solutions, even though providing some improvements may not be entirely satisfactory as they may, when an LOV condition occurs, either give rise to false trip or not allow for local fault clearing, or require the use of costly redundant protection system. [0011] Thus it is desirable to provide a solution which improves operational reliability during an LOV state. SUMMARY OF THE INVENTION [0012] In accordance with the present invention, a method for improving operational reliability during a loss of a phase voltage in a multi-phase power system, in particular a three-phase power line, is provided. In accordance with the method, when one of the phases is under an LOV state, a fictitious voltage representative of this LOV state phase is calculated and a reference polarizing voltage is computed suitable for any protection unit associated with the power system. The invention also provides a multi-phase power system, such as a three- phase power line, for allowing operational reliability during a loss of a phase voltage (LOV), comprising a computing device having therein program code usable by the computing device itself, wherein the program code comprises code configured to: identify the phase which is in an LOV state; calculate a fictitious voltage representative of the voltage of said phase in an LOV state; and calculate a reference polarizing voltage. Further, the present invention also provides a computer program product for allowing reliable operation during a loss of a phase WO 2006/124743 PCT/US2006/018666 4 E20050140 voltage (LOV) in a multi-phase power system, wherein the computer program product comprises computer usable program code which is configured to: identify the phase which is in an LOV state; then to calculate a fictitious voltage representative of the voltage of said phase in an LOV state; and finally to calculate a reference polarizing voltage. BRIEF DESCRIPTION OF THE DRAWINGS [0013] The features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: Fig. 1 illustrates an example of a power system wherein a three-phase protective relay is operatively connected to a three-phase bus or line voltage; Fig. 2 is an operational block diagram schematically representing an embodiment of the method according to the present invention; Fig. 3 shows a fault oscillography for a practical test based on pre-fault and fault quantities recorded for a forward fault at a remote bus; DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS [0014] It should be noted that in order to clearly and concisely disclose the present invention, the drawings may not necessarily be to scale and certain features of the invention may be shown in somewhat schematic form. [0015] In the following description and illustrative examples, the method according to the present invention is described by making particular reference to its functioning with directional protection units without intending in any way to limit its scope and potential field of application. [0016] Fig. 1 illustrates a power system apparatus, globally indicated with reference number 1, wherein a primary three-phase power line (or bus) 2 is operatively connected to a protective device 3, preferably a computing device. Such a protective computing device 3 can be constituted for example by any suitable electronic relay. [0017] The protective relay 3 comprises a microprocessor unit 4 with a program code which is embedded therein and is suitable to carry out the method of the present invention, and a memory unit 5 which can be directly incorporated WO 2006/124743 PCT/US2006/018666 5 E20050140 within or operatively connected with the microprocessor unit 4 itself. The protective relay 3 is operativeiy connected to the phases of the power line, i.e. phase A, phase B and phase C, through voltage transformers (VT) that step down the primary system quantities to values that enable practical and safe implementation of the relay 3 itself. Some equivalent alternatives may be adopted for the connection, for example current transformers can be used in addition with or in alternative to voltage transformers VT. The secondary system circuit indicated at 6, which is normally used by protective devices, such as the relay 3, for their measures and appropriate operations, is suitably fused by means of secondary circuit fuses 7 in order to protect the voltage transformer secondary winding and circuit. [0018] Referring now also to Fig. 2, the relay 3 receives the voltage signals 10 from the phases and, under normal operating conditions, i.e. an LOV state is not occurring, the microprocessor unit 4 computes at block 11 the respective voltage values (VA for phase A, VB for phase B, Vc for phase C). These values are recorded in the memory unit 5 and are continuously updated by repetitive calculations. [0019] When an LOV state occurs on one phase, for example because a fuse 7 blows due to a fault on the secondary circuit 6, the relay 3 through its microprocessor unit 4 first identifies (block 12) which phase is under an LOV- state. One possible manner suitable for the identification of the phase under an LOV-state is disclosed for example in US patent 5,883,578 whose disclosure is fully incorporated herein by reference. [0020] Once the phase under an LOV state is identified, the microprocessor unit 4 alarms and calculates (at block 13 for phase A, at block 14 for phase B, at block 15 for phase C) a fictitious voltage which is representative of the voltage of the phase in an LOV state. The fictitious voltage is recorded (block 16) in the memory unit 5 and is continuously updated by repetitive calculations. [0021] Then, a reference polarizing voltage is calculated. This reference polarizing voltage can be used by the relay 3 and its incorporated logic/functions or it can be provided to any protection unit operatively associated with or incorporated in the multi-phase power system, so as to allow an improved operation reliability under some operating conditions, namely occurrence of a fault WO 2006/124743 PCT/US2006/018666 6 E20050140 on one of the other phases, e.g. either phase A or C, which are not subject to a LOV state. Suitable protection units can be for example a unit for monitoring, supervising, detecting, tripping etc. [0022] In the method according to the invention, the step of calculating said reference polarizing voltage can be carried out either by means of zero voltage sequence calculations or of negative voltage sequence calculations, and in particular the following equations can be used: where a is a complex operator and VA, VB and Vc are pre-fault voltages during an LOV state and with the assumption that with a perfectly balanced three-phase system Vo (zero sequence) and V2 (negative sequence) are equal to zero. [0023] For perfectly balanced systems the results using either the zero or the negative sequence voltage equation would be substantially equal; however, there might be some differences if some unbalance exists. In these cases, a purposive selection can be done; for example, the voltage of the lost phase can be preferably calculated by using the zero sequence equation, e.g. for zero sequence directional polarization, and the computation using the negative sequence equation could be preferably used, e.g. for negative sequence directional polarization. [0024] Hence, according to the method of the present invention, when an LOV state occurs on a phase, the above mentioned fictitious voltage representative of the LOV-state phase (VA1 when the LOV is on phase A, or VB' when the LOV is on phase B, or Vc when the LOV is on phase C) is calculated based on the recorded voltages (VA and VB when the LOV is on phase C, or VA and Vc when the LOV is on phase B, or VB and Vc when the LOV is on phase A) of the other phases in a non-LOV state; in particular the following equations are used for these calculations: WO 2006/124743 PCT/US2006/018666 7 E20050140 LOV on Phase A LOV on Phase B LOV on Phase C [0025] The calculated fictitious representative voltage(s) (VA1, or VB', or Vc) are advantageously recorded in the memory unit 5 (block 16 of Fig. 2) and continuously updated. [0026] Preferably, it is assumed that there is no significant phase angle shift for single-line-to-ground-faults of the non-faulted phase voltages between the pre-fault and fault states. This is a function of system grounding and is expected on the effectively grounded transmission systems. [0027] Therefore, during an LOV state with a fault occurring on one of the healthy phases, i.e. not in a LOV state, the reference polarizing voltage is computed (block 17 of Fig. 2) on the basis of the calculated fictitious voltage (VA', or VB1, or Vc) representative of the LOV-state phase and of the voltages measured on the other healthy phases during the fault. In particular the appropriate reference polarizing voltage may be computed as follows: LOV on phase A, fault on phases B or C WO 2006/124743 PCT/US2006/018666 8 E20050140 LOV on phase B, fault on phases A or C LOV on phase C, fault on phases A or B where Va, Vb and Vc are the respective healthy phase voltages during the fault and VA', VB' and Vc' are the respective computed pre-fault and fault phase memory voltages of the phase in the LOV state. [0028] The method according to the invention can be helpful even in the worst condition where the LOV state condition and a fault condition occur both on the same phase. In this case, the measurement is significantly reduced or near zero and in the present method the fictitious LOV phase voltage is assumed equal to zero. As it will be demonstrated in more details in the example which follows, the standard calculation for 3Vo and 3V2 without the computed memory phase voltage will be still valid for the purpose of obtaining useful information about the direction of the fault. [0029] Accordingly, for the conditions described directly above: LOV and fault on phase A: LOV and fault on phase B: WO 2006/124743 PCT/US2006/018666 9 E20050140 LOV and fault on phase C: [0030] There will be now described an example comparing the values registered in a practical test and those calculated according to the method of the present invention. [0031] The following example is based on pre-fault and fault quantities recorded for a forward fault at a remote bus. The related fault oscillography is displayed in Figure 3 and the voltage and current secondary values are shown in Table A below. Table A. Secondary Pre-fault and Fault Quantities for the Fault of Figure 3 Pre-fault Magnitude Angle Magnitude Angle VA 66.87 359 IA 2.16 358 vB 66.94 238 IB 2.40 232 Vc 66.34 118 Ic 2.16 108 3V0 0.73 21 3l0 0.08 66 3V2 1.34 98 3I2 0.56 111 Fault Magnitude Angle Magnitude Angle va 47.4 0 la 8.98 285 vb 66.55 239 Ib 1.48 205 Vc 64.88 117 Ic 3.35 96 3V0 16.93 177 3IO 5.99 277 3V2 20.01 169 3I2 9.15 269 As can be inferred from Table A and Figure 3, the fault can be identified as a single phase A-to-ground fault based on current values. However, there appears WO 2006/124743 PCT/US2006/018666 10 E20050140 to be some involvement with phase C based on the current on phase C during the fault and the fact that the negative sequence current, 3I2, is considerably larger than the zero sequence current, 3l0, since the two sequence currents would normally be equal for a single phase-to-ground fault. [0032] For this example, an LOV state will be applied to each phase by setting that phase voltage to zero. Then using the above equations, both the zero and negative sequence polarizing voltages used during the LOV period will be computed and compared with the original values recorded. LOV on phase A Zero Sequence Polarizing Voltage Negative Sequence Polarizing Voltage LOV on Phase B Zero Sequence Polarization Voltage Negative Sequence Polarizing Voltage WO 2006/124743 PCT/US2006/018666 11 E20050140 LOV on Phase C Zero Sequence Polarization Voltage Negative Sequence Polarization Voltage [0033] Table B below shows the results comparing the actual polarizing voltages, 3V0 and 3V2, with the computed polarizing voltages, 3Vo and 3V2', using the LOV memory phase voltages, VA', VB' or Vc'. Table B Comparison of Actual and LOV Computed Polarizing Voltages Polarizing No LOV Simulated LOV on Phase Table B Comparison of Actual and LOV Computed Polarizing Voltages Polarizing No LOV Simulated LOV on Phase WO 2006/124743 PCT/US2006/018666 12 E20050140 Voltage (Actual Results) A B C 3V0 16.93611" 63.73e179 17.92179 18.296177 3V2 20.01e159 68.1e177 21.5e171 19.69179 [0034] It can be observed that there is very little difference when the LOV state is on a non-faulted phase, B or C. When the fault occurs on the LOV phase A then the polarizing voltage magnitude is substantially greater, but the polarizing voltage angle, which is the quantity used for polarization, is still accurate enough to achieve proper directional sensing for this fault. [0035] It is also noted that for this case the fault was not a true single phase-to-ground fault, but the analysis shows that correct operation for the LOV state will occur with this adaptive approach. [0036] Hence, as evident from the above example, the operational reliability of protection units can be significantly improved while protection is in an LOV state by using the method according to the invention. [0037] Likewise, in order to compute ground distance measurement during a single-phase LOV state, the voltage on the lost phase of the secondary circuit can be calculated in the same manner as done for the directional units using the method according to the invention and in particular, the zero and negative sequence voltage equations. [0038] For example, in the case of cross-polarized ground distance measurement units, the operating principal is based on comparing the phase relationship of two phasor quantities, according to the following equations: WO 2006/124743 PCT/US2006/018666 13 E20050140 Where V0P = Operating voltage VP0L = Polarizing voltage VXG = Faulted phase - to - ground voltage VYX = Cross Phase - to - phase voltage lx = Faulted phase current /o = Zero sequence current Z1, = Positive sequence line impedance Zo = Zero sequence line impedance Zc = Distance reach setting, ohms [0039] Trip occurs when VOP leads VPOL. [0040] It can be easily observed that the polarizing voltage VPOL will be in error if either of the two non-faulted phases is in an LOV state, and the distance unit would not operate accurately. In this case the reference polarizing voltage, VPOL, can be also computed according to the present method and in particular by using the LOV phase memory voltage, as per the following equations: LOV on Phase A LOV on Phase B LOV on Phase C WO 2006/124743 PCT/US2006/018666 14 E20050140 [0041] Thus, if the LOV state occurs on phase B and fault occurs on phase A or C, computation of the reference polarizing voltage is calculated based on the voltages Va and Vc recorded during the fault and on the memorized voltage VB. [0042] It is obvious that the operating quantity, VOP, cannot be computed with the similar pre-fault memory voltage because it is dependent on the accurate measurement of the phase-to-ground fault voltage, VXG- In this case there are options that could be provided: block operation, force the unit to overreach, convert the unit to operate based on directional overcurrent with pre-set setting, etc. [0043] Although the method of the present invention has been described with particular regard to directional units, it should be appreciated that it may be used with any other suitable voltage dependent protection unit, be it a measuring unit, or a detecting unit, or supervising unit, a tripping unit, etc. Further, it can be easily implemented with or within any suitable type or multi-phase power system or apparatus using a computing device, and as will be appreciated by one of skill in the art, it may be embodied as the method described, or as a system, or as a computer program product. Furthermore, the present invention may take the form of a computer program product on a computer-usable or computer-readable medium having computer-usable program code embodied in the medium. The computer-usable or computer-readable medium may be any medium that can contain, store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device, etc. Non exhaustive examples of the computer-readable medium would include: a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an optical fiber, a portable compact disc read-only memory (CD-ROM), an optical storage device, a transmission media such as those supporting the Internet or an intranet, or a magnetic storage device. Computer program code for carrying out operations of the present invention may be written in any suitable WO 2006/124743 PCT/US2006/018666 15 E20050140 programming language, such as object oriented programming languages e.g. Java, Smalltalk, C++ or the like, or may also be written in more conventional procedural programming languages, such as the "C" programming language. [0044] Hence, the present invention also deals with a computer program product for allowing reliable operation during a loss of a phase voltage (LOV) in a multi-phase power system. The computer program product comprises computer usable program code configured to: - identify the phase which is in an LOV state; - calculate a fictitious voltage representative of the voltage of said phase in an LOV state; and - calculate a reference polarizing voltage. [0045] The present invention also encompasses a multi-phase power system for allowing operational reliability during a loss of a phase voltage (LOV), comprising a computing device having therein program code usable by the computing device itself, wherein the program code comprises code configured to: - identify the phase which is in an LOV state; - calculate a fictitious voltage representative of the voltage of said phase in an LOV state; and - calculate a reference polarizing voltage. The program code residing in the computing device or usable as or with the above indicated program product is preferably configured so as to calculate the reference polarizing voltage either by means of zero voltage sequence calculations or of negative voltage sequence calculations. In particular, the program code is configured to calculate the fictitious voltage on the basis of the voltages of the phases other than said phase in an LOV-state. Further, the program code is configured in such a way that, when a fault occur on one of the phases other than the phase in an LOV-state, the calculation of the reference polarizing voltage is carried out on the basis of: the pre-calculated fictitious voltage representative of the voltage of the phase in an LOV state; and of the voltages which are suitably detected during the fault on the phases other than the phase in an LOV-state. The program code preferably comprises code which is configured to assume the fictitious voltage representative of the voltage of said phase in an LOV state equal to zero when a fault occurs on the phase under an LOV-state, and/or to assume no WO 2006/124743 PCT/US2006/018666 16 E20050140 significant phase angle shift for single-line-to-ground-faults between pre-fault and fault condition on the phases in a non-LOV-state. [0046] It is to be understood that the description of the foregoing exemplary embodiment(s) is (are) intended to be only illustrative, rather than exhaustive, of the present invention. Those of ordinary skill will be able to make certain additions, deletions, and/or modifications to the embodiments) of the disclosed subject matter without departing from the spirit of the invention or its scope, as defined by the appended claims. WO 2006/124743 PCT/US2006/018666 17 E20050140 What is claimed is: 1. A method for improving operational reliability during a loss of a phase voltage (LOV) in a multi-phase power system, the method comprising: - identifying the phase which is in an LOV state; - calculating a fictitious voltage representative of the voltage of said phase in an LOV state; and - calculating a reference polarizing voltage. 2. The method of claim 1, wherein said reference polarizing voltage is calculated by means of zero voltage sequence calculations. 3. The method of claim 1, wherein said reference polarizing voltage is calculated by means of negative voltage sequence calculations. 4. The method of claim 1, further comprising: - recording the voltages of the phases during normal operating conditions. 5. The method of claim 1, further comprising: - recording the calculated fictitious voltage representative of the voltage of said phase in an LOV state. 6. The method of claim 4, wherein said fictitious voltage is calculated on the basis of the recorded voltages of the phases other than said phase in an LOV-state. 7. The method of claim 5, wherein, when a fault occurs on one of said phases other than the phase in an LOV-state, said reference polarizing voltage is calculated on the basis of the recorded fictitious voltage representative of the voltage of said phase in an LOV state and of the voltages measured during the fault on said phases other than the phase in an LOV-state. 8. The method of claim 1 wherein, when a fault occurs on the phase which is in an LOV state, further comprises: WO 2006/124743 PCT/US2006/018666 18 E20050140 - assuming said fictitious voltage representative of the voltage of said phase in an LOV state equal to zero. 9. The method of claim 1, further comprising: - assuming no significant phase angle shift between pre-fault and fault condition on the phases in a non-LOV-state. 10. A computer program product for allowing reliable operation during a loss of a phase voltage (LOV) in a multi-phase power system, said computer program product comprising computer usable program code configured to: - identify the phase which is in an LOV state; - calculate a fictitious voltage representative of the voltage of said phase in an LOV state; and - calculate a reference polarizing voltage. 11. A computer program product according to claim 10, wherein said computer usable program code is configured to calculate said reference polarizing voltage by means of zero voltage sequence calculations. 12. A computer program product according to claim 10, wherein said computer usable program code is configured to calculate said reference polarizing voltage by means of negative voltage sequence calculations. 13. A computer program product according to claim 10, wherein said computer usable program code is configured to calculate said fictitious voltage on the basis of the recorded voltages of the phases other than said phase in an LOV- state. 14. A computer program product according to claim 10, wherein said computer usable program code is configured to calculate said reference polarizing voltage on the basis of the fictitious voltage representative of the voltage of said phase in an LOV state and of the voltages measured during the fault on the phases other than the phase in an LOV-state, when a fault occurs on one of said phases WO 2006/124743 PCT/US2006/018666 19 E20050140 other than the phase in an LOV-state. 15. A computer program product according to claim 10, wherein said computer usable program code comprises code configured to assume said fictitious voltage representative of the voltage of said phase in an LOV state equal to zero when a fault occurs on the phase which is in an LOV state. 16. A computer program product according to claim 10, wherein said computer usable program code comprises code configured to assume no significant phase angle shift between pre-fault and fault condition on the phases in a non-LOV-state. 17. A multi-phase power system for allowing operational reliability during a loss of a phase voltage (LOV), comprising a computing device having therein program code usable by said computing device, said program code comprising code configured to: - identify the phase which is in an LOV state; - calculate a fictitious voltage representative of the voltage of said phase in an LOV state; and - calculate a reference polarizing voltage. 18. A multi-phase power system according to claim 17, wherein said program code is configured to calculate said reference polarizing voltage by means of zero voltage sequence calculations. 19. A multi-phase power system according to claim 17, wherein said program code is configured to calculate said reference polarizing voltage by means of negative voltage sequence calculations. 20. A multi-phase power system according to claim 17, wherein said program code is configured to calculate said fictitious voltage on the basis of the recorded voltages of the phases other than said phase in an LOV-state. WO 2006/124743 PCT/US2006/018666 20 E20050140 21. A multi-phase power system according to claim 17, wherein said program code is configured to calculate said reference polarizing voltage on the basis of the fictitious voltage representative of the voltage of said phase in an LOV state and of the voltages measured during the fault on the phases other than the phase in an LOV-state, when a fault occurs on one of said phases other than the phase in an LOV-state. 22. A multi-phase power system according to claim 17, wherein said program code comprises code configured to assume said fictitious voltage representative of the voltage of said phase in an LOV state equal to zero when a fault occur on the phase which is in an LOV state. 23. A multi-phase power system according to claim 17, wherein said program code comprises code configured to assume no significant phase angle shift between pre-fault and fault condition on the phases in a non-LOV-state. The present invention is directed to a method and apparatus for improving operational reliability during a loss of a phase voltage (LOV) in a multi-phase power system, wherein a voltage representative of the LOV phase is calculated. The representative voltage is used for computing a reference polarizing voltage suitable for a protection unit.

Full Text

WO 2006/124743 PCT/US2006/018666
1
E20050140
METHOD AND APPARATUS FOR IMPROVING OPERATIONAL RELIABILITY
DURING A LOSS OF A PHASE VOLTAGE
Cross Reference To Related Application
This application claims the priority of U.S. provisional patent application
Ser. No. 60/680,719 filed on May 13, 2005, entitled "A METHOD FOR
IMPROVING OPERATIONAL RELIABILITY DURING A LOSS OF SINGLE
PHASE VOLTAGE" the contents of which are relied upon and incorporated herein
by reference in their entirety, and the benefit of priority under 35 U.S.C. 119(e) is
hereby claimed.
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a multi-phase power system and in
particular to the reliability of that system during a loss of phase voltage.
[0002] As widely known in the art, many protective devices, such as relays,
circuit breakers, various types of monitoring, supervising and fault detection units,
are used in power system. One basic task of these devices is to allow electrical
power to be distributed in a reliable manner and to adequately protect either the
transmission lines and the loads or equipments connected therewith, e.g.
generators, motors, from hazardous conditions which could lead to malfunction,
severe damage, failure, etc.
[0003] Often, the protection of power system elements is based on
accurate measurement of three-phase voltages in order to provide reliable fault
detection and breaker operations and thus minimize power system disruptions. In
these cases, incorrectly measuring one or more of the three-phase voltages by a
protective unit may result in technical shortcomings, e.g. erroneous trips (breaker
operations) and/or clearing more of the power system than desired.
[0004] For example a common failure that causes incorrect voltage
measurement is when there is a downed line, or due to an equipment failure in an
end-user's facility, or when one or more fuses protecting a three-phase voltage
transformer (VT) secondary circuit blow. In the latter case, protective relays
connected to that secondary circuit would measure zero voltage if a secondary
phase is isolated (only phase-to-ground connections) or there exist a non-zero
coupled value if there are phase-to-phase connections in the secondary circuit.

WO 2006/124743 PCT/US2006/018666
2
E20050140
Conditions, other than blown fuses, may also occur where one or more secondary
phase voltages are unintentionally removed from the protective relay.
[0005] In the electrical power system industry operating in this abnormal
state may for example be referred to as "single-phasing", or as fuse failure, or as
"loss-of-potential (LOP)", or more commonly as "loss-of-voltage (LOV)" and this
last definition ("LOV") will be used hereinafter to refer to the loss of voltage in a
single phase.
[0006] It is clear that when an LOV state occurs, adequate control of
voltage dependent measuring units need to be carried out so as to reduce
potential detrimental effects on the whole power system and/ or on the various
pieces of equipment.
[0007] For example, with protection measuring units, such as distance and
impedance units, e.g. distance relays, functioning is based on calculating the
impedance seen by the units themselves; since the impedance measured is
directly dependent on the voltage(s). Thus an LOV condition will adversely affect
obtaining correct measurements and therefore missing one or more phase
voltages will lead to misoperations, such as untimely tripping.
[0008] The same considerations apply also in the case of the so-called
directional units, i.e. protection measuring devices that determine the direction of
the current flow in an AC circuit and are used to supervise for example an
overcurrent relay in order to let it trip only in the desired direction. These
directional units can perform their supervision task by comparing the angular
relationship between the current in the protected circuit and an independent
voltage source. Since the current can vary significantly for various types of faults,
in order to determine directionality the independent voltage may be used as a
reference or polarizing quantity. This reference voltage may be not correct during
an LOV state, and hence the directional units will not be able to provide the
required supervision and may contribute to a misoperation.
[0009] A further example of the negative impact that the occurrence of an
LOV state may have on power line protection is the operation of a recloser. A
recloser opens and closes multiple times when a fault condition exists in an
attempt to clear the fault. Should the fault condition continue to exist, the recloser
opens and remains open until reset manually. The recloser enters a "lock out"

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state when this occurs. Automatic reclosing requires synchronization or
comparison of voltage on each side of an open line circuit breaker. These
comparisons are often accomplished by comparing the voltage from one phase
VT on each side of the open breaker. If an LOV condition exists on either VT,
reclosing may operate incorrectly and prevent automatic system restoration,
cause a line outage, or possibly cause circuit breaker damage. Thus, the location
of the LOV may impact the protection functions provided by the recloser.
[0010] In the past, in order to improve reliability under an LOV state,
various solutions have been devised varying depending on several aspects of the
specific applications, such as type, number and technology of protecting systems,
available options provided by the units, architecture of the power system, etc.
These known solutions, even though providing some improvements may not be
entirely satisfactory as they may, when an LOV condition occurs, either give rise
to false trip or not allow for local fault clearing, or require the use of costly
redundant protection system.
[0011] Thus it is desirable to provide a solution which improves operational
reliability during an LOV state.
SUMMARY OF THE INVENTION
[0012] In accordance with the present invention, a method for improving
operational reliability during a loss of a phase voltage in a multi-phase power
system, in particular a three-phase power line, is provided. In accordance with the
method, when one of the phases is under an LOV state, a fictitious voltage
representative of this LOV state phase is calculated and a reference polarizing
voltage is computed suitable for any protection unit associated with the power
system. The invention also provides a multi-phase power system, such as a three-
phase power line, for allowing operational reliability during a loss of a phase
voltage (LOV), comprising a computing device having therein program code
usable by the computing device itself, wherein the program code comprises code
configured to: identify the phase which is in an LOV state; calculate a fictitious
voltage representative of the voltage of said phase in an LOV state; and calculate a
reference polarizing voltage. Further, the present invention also provides a
computer program product for allowing reliable operation during a loss of a phase

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voltage (LOV) in a multi-phase power system, wherein the computer program
product comprises computer usable program code which is configured to: identify
the phase which is in an LOV state; then to calculate a fictitious voltage
representative of the voltage of said phase in an LOV state; and finally to calculate
a reference polarizing voltage.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The features, aspects, and advantages of the present invention will
become better understood with regard to the following description, appended
claims, and accompanying drawings where:
Fig. 1 illustrates an example of a power system wherein a three-phase
protective relay is operatively connected to a three-phase bus or line voltage;
Fig. 2 is an operational block diagram schematically representing an
embodiment of the method according to the present invention;
Fig. 3 shows a fault oscillography for a practical test based on pre-fault and
fault quantities recorded for a forward fault at a remote bus;
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0014] It should be noted that in order to clearly and concisely disclose the
present invention, the drawings may not necessarily be to scale and certain
features of the invention may be shown in somewhat schematic form.
[0015] In the following description and illustrative examples, the method
according to the present invention is described by making particular reference to
its functioning with directional protection units without intending in any way to limit
its scope and potential field of application.
[0016] Fig. 1 illustrates a power system apparatus, globally indicated with
reference number 1, wherein a primary three-phase power line (or bus) 2 is
operatively connected to a protective device 3, preferably a computing device.
Such a protective computing device 3 can be constituted for example by any
suitable electronic relay.
[0017] The protective relay 3 comprises a microprocessor unit 4 with a
program code which is embedded therein and is suitable to carry out the method
of the present invention, and a memory unit 5 which can be directly incorporated

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within or operatively connected with the microprocessor unit 4 itself. The
protective relay 3 is operativeiy connected to the phases of the power line, i.e.
phase A, phase B and phase C, through voltage transformers (VT) that step down
the primary system quantities to values that enable practical and safe
implementation of the relay 3 itself. Some equivalent alternatives may be adopted
for the connection, for example current transformers can be used in addition with
or in alternative to voltage transformers VT. The secondary system circuit
indicated at 6, which is normally used by protective devices, such as the relay 3,
for their measures and appropriate operations, is suitably fused by means of
secondary circuit fuses 7 in order to protect the voltage transformer secondary
winding and circuit.
[0018] Referring now also to Fig. 2, the relay 3 receives the voltage signals
10 from the phases and, under normal operating conditions, i.e. an LOV state is
not occurring, the microprocessor unit 4 computes at block 11 the respective
voltage values (VA for phase A, VB for phase B, Vc for phase C). These values are
recorded in the memory unit 5 and are continuously updated by repetitive
calculations.
[0019] When an LOV state occurs on one phase, for example because a
fuse 7 blows due to a fault on the secondary circuit 6, the relay 3 through its
microprocessor unit 4 first identifies (block 12) which phase is under an LOV-
state. One possible manner suitable for the identification of the phase under an
LOV-state is disclosed for example in US patent 5,883,578 whose disclosure is
fully incorporated herein by reference.
[0020] Once the phase under an LOV state is identified, the
microprocessor unit 4 alarms and calculates (at block 13 for phase A, at block 14
for phase B, at block 15 for phase C) a fictitious voltage which is representative of
the voltage of the phase in an LOV state. The fictitious voltage is recorded (block
16) in the memory unit 5 and is continuously updated by repetitive calculations.
[0021] Then, a reference polarizing voltage is calculated. This reference
polarizing voltage can be used by the relay 3 and its incorporated logic/functions
or it can be provided to any protection unit operatively associated with or
incorporated in the multi-phase power system, so as to allow an improved
operation reliability under some operating conditions, namely occurrence of a fault

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on one of the other phases, e.g. either phase A or C, which are not subject to a
LOV state. Suitable protection units can be for example a unit for monitoring,
supervising, detecting, tripping etc.
[0022] In the method according to the invention, the step of calculating said
reference polarizing voltage can be carried out either by means of zero voltage
sequence calculations or of negative voltage sequence calculations, and in
particular the following equations can be used:

where a is a complex operator and VA, VB and Vc are pre-fault
voltages during an LOV state and with the assumption that with a perfectly
balanced three-phase system Vo (zero sequence) and V2 (negative sequence) are
equal to zero.
[0023] For perfectly balanced systems the results using either the zero or
the negative sequence voltage equation would be substantially equal; however,
there might be some differences if some unbalance exists. In these cases, a
purposive selection can be done; for example, the voltage of the lost phase can
be preferably calculated by using the zero sequence equation, e.g. for zero
sequence directional polarization, and the computation using the negative
sequence equation could be preferably used, e.g. for negative sequence
directional polarization.
[0024] Hence, according to the method of the present invention, when an
LOV state occurs on a phase, the above mentioned fictitious voltage
representative of the LOV-state phase (VA1 when the LOV is on phase A, or VB'
when the LOV is on phase B, or Vc when the LOV is on phase C) is calculated
based on the recorded voltages (VA and VB when the LOV is on phase C, or VA
and Vc when the LOV is on phase B, or VB and Vc when the LOV is on phase A)
of the other phases in a non-LOV state; in particular the following equations are
used for these calculations:

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LOV on Phase A

LOV on Phase B

LOV on Phase C

[0025] The calculated fictitious representative voltage(s) (VA1, or VB', or Vc)
are advantageously recorded in the memory unit 5 (block 16 of Fig. 2) and
continuously updated.
[0026] Preferably, it is assumed that there is no significant phase angle
shift for single-line-to-ground-faults of the non-faulted phase voltages between the
pre-fault and fault states. This is a function of system grounding and is expected
on the effectively grounded transmission systems.
[0027] Therefore, during an LOV state with a fault occurring on one of the
healthy phases, i.e. not in a LOV state, the reference polarizing voltage is computed
(block 17 of Fig. 2) on the basis of the calculated fictitious voltage (VA', or VB1, or
Vc) representative of the LOV-state phase and of the voltages measured on the
other healthy phases during the fault. In particular the appropriate reference
polarizing voltage may be computed as follows:
LOV on phase A, fault on phases B or C

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LOV on phase B, fault on phases A or C

LOV on phase C, fault on phases A or B

where Va, Vb and Vc are the respective healthy phase voltages during the fault
and VA', VB' and Vc' are the respective computed pre-fault and fault phase
memory voltages of the phase in the LOV state.
[0028] The method according to the invention can be helpful even in the
worst condition where the LOV state condition and a fault condition occur both on
the same phase. In this case, the measurement is significantly reduced or near
zero and in the present method the fictitious LOV phase voltage is assumed equal
to zero. As it will be demonstrated in more details in the example which follows,
the standard calculation for 3Vo and 3V2 without the computed memory phase
voltage will be still valid for the purpose of obtaining useful information about the
direction of the fault.
[0029] Accordingly, for the conditions described directly above:
LOV and fault on phase A:

LOV and fault on phase B:

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LOV and fault on phase C:

[0030] There will be now described an example comparing the values
registered in a practical test and those calculated according to the method of the
present invention.
[0031] The following example is based on pre-fault and fault quantities
recorded for a forward fault at a remote bus. The related fault oscillography is
displayed in Figure 3 and the voltage and current secondary values are shown in
Table A below.

Table A. Secondary Pre-fault and Fault Quantities for the Fault of Figure 3
Pre-fault Magnitude Angle Magnitude Angle
VA 66.87 359 IA 2.16 358
vB 66.94 238 IB 2.40 232
Vc 66.34 118 Ic 2.16 108
3V0 0.73 21 3l0 0.08 66
3V2 1.34 98 3I2 0.56 111

Fault Magnitude Angle Magnitude Angle
va 47.4 0 la 8.98 285
vb 66.55 239 Ib 1.48 205
Vc 64.88 117 Ic 3.35 96
3V0 16.93 177 3IO 5.99 277
3V2 20.01 169 3I2 9.15 269
As can be inferred from Table A and Figure 3, the fault can be identified as a
single phase A-to-ground fault based on current values. However, there appears

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to be some involvement with phase C based on the current on phase C during the
fault and the fact that the negative sequence current, 3I2, is considerably larger
than the zero sequence current, 3l0, since the two sequence currents would
normally be equal for a single phase-to-ground fault.
[0032] For this example, an LOV state will be applied to each phase by
setting that phase voltage to zero. Then using the above equations, both the zero
and negative sequence polarizing voltages used during the LOV period will be
computed and compared with the original values recorded.
LOV on phase A
Zero Sequence Polarizing Voltage

Negative Sequence Polarizing Voltage

LOV on Phase B
Zero Sequence Polarization Voltage

Negative Sequence Polarizing Voltage

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LOV on Phase C
Zero Sequence Polarization Voltage

Negative Sequence Polarization Voltage
[0033] Table B below shows the results comparing the actual polarizing
voltages, 3V0 and 3V2, with the computed polarizing voltages, 3Vo and 3V2', using
the LOV memory phase voltages, VA', VB' or Vc'.
Table B Comparison of Actual and LOV Computed Polarizing Voltages
Polarizing No LOV Simulated LOV on Phase
Table B Comparison of Actual and LOV Computed Polarizing Voltages
Polarizing No LOV Simulated LOV on Phase

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Voltage (Actual
Results) A B C
3V0 16.93611" 63.73e179 17.92179 18.296177
3V2 20.01e159 68.1e177 21.5e171 19.69179
[0034] It can be observed that there is very little difference when the LOV
state is on a non-faulted phase, B or C. When the fault occurs on the LOV phase
A then the polarizing voltage magnitude is substantially greater, but the polarizing
voltage angle, which is the quantity used for polarization, is still accurate enough
to achieve proper directional sensing for this fault.
[0035] It is also noted that for this case the fault was not a true single
phase-to-ground fault, but the analysis shows that correct operation for the LOV
state will occur with this adaptive approach.
[0036] Hence, as evident from the above example, the operational
reliability of protection units can be significantly improved while protection is in an
LOV state by using the method according to the invention.
[0037] Likewise, in order to compute ground distance measurement during
a single-phase LOV state, the voltage on the lost phase of the secondary circuit
can be calculated in the same manner as done for the directional units using the
method according to the invention and in particular, the zero and negative
sequence voltage equations.
[0038] For example, in the case of cross-polarized ground distance
measurement units, the operating principal is based on comparing the phase
relationship of two phasor quantities, according to the following equations:

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Where
V0P = Operating voltage
VP0L = Polarizing voltage
VXG = Faulted phase - to - ground voltage
VYX = Cross Phase - to - phase voltage
lx = Faulted phase current
/o = Zero sequence current
Z1, = Positive sequence line impedance
Zo = Zero sequence line impedance
Zc = Distance reach setting, ohms
[0039] Trip occurs when VOP leads VPOL.
[0040] It can be easily observed that the polarizing voltage VPOL will be in
error if either of the two non-faulted phases is in an LOV state, and the distance
unit would not operate accurately. In this case the reference polarizing voltage,
VPOL, can be also computed according to the present method and in particular by
using the LOV phase memory voltage, as per the following equations:
LOV on Phase A

LOV on Phase B

LOV on Phase C

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[0041] Thus, if the LOV state occurs on phase B and fault occurs on phase
A or C, computation of the reference polarizing voltage is calculated based on the
voltages Va and Vc recorded during the fault and on the memorized voltage VB.
[0042] It is obvious that the operating quantity, VOP, cannot be computed
with the similar pre-fault memory voltage because it is dependent on the accurate
measurement of the phase-to-ground fault voltage, VXG- In this case there are
options that could be provided: block operation, force the unit to overreach,
convert the unit to operate based on directional overcurrent with pre-set setting,
etc.
[0043] Although the method of the present invention has been described
with particular regard to directional units, it should be appreciated that it may be
used with any other suitable voltage dependent protection unit, be it a measuring
unit, or a detecting unit, or supervising unit, a tripping unit, etc. Further, it can be
easily implemented with or within any suitable type or multi-phase power system
or apparatus using a computing device, and as will be appreciated by one of skill
in the art, it may be embodied as the method described, or as a system, or as a
computer program product. Furthermore, the present invention may take the form
of a computer program product on a computer-usable or computer-readable
medium having computer-usable program code embodied in the medium. The
computer-usable or computer-readable medium may be any medium that can
contain, store, communicate, propagate, or transport the program for use by or in
connection with the instruction execution system, apparatus, or device, etc. Non
exhaustive examples of the computer-readable medium would include: a portable
computer diskette, a hard disk, a random access memory (RAM), a read-only
memory (ROM), an erasable programmable read-only memory (EPROM or Flash
memory), an optical fiber, a portable compact disc read-only memory (CD-ROM),
an optical storage device, a transmission media such as those supporting the
Internet or an intranet, or a magnetic storage device. Computer program code for
carrying out operations of the present invention may be written in any suitable

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programming language, such as object oriented programming languages e.g.
Java, Smalltalk, C++ or the like, or may also be written in more conventional
procedural programming languages, such as the "C" programming language.
[0044] Hence, the present invention also deals with a computer program
product for allowing reliable operation during a loss of a phase voltage (LOV) in a
multi-phase power system. The computer program product comprises computer
usable program code configured to:
- identify the phase which is in an LOV state;
- calculate a fictitious voltage representative of the voltage of said phase in an
LOV state; and
- calculate a reference polarizing voltage.
[0045] The present invention also encompasses a multi-phase power system
for allowing operational reliability during a loss of a phase voltage (LOV),
comprising a computing device having therein program code usable by the
computing device itself, wherein the program code comprises code configured to:
- identify the phase which is in an LOV state;
- calculate a fictitious voltage representative of the voltage of said phase in an LOV
state; and
- calculate a reference polarizing voltage.
The program code residing in the computing device or usable as or with the above
indicated program product is preferably configured so as to calculate the reference
polarizing voltage either by means of zero voltage sequence calculations or of
negative voltage sequence calculations. In particular, the program code is
configured to calculate the fictitious voltage on the basis of the voltages of the
phases other than said phase in an LOV-state. Further, the program code is
configured in such a way that, when a fault occur on one of the phases other than
the phase in an LOV-state, the calculation of the reference polarizing voltage is
carried out on the basis of: the pre-calculated fictitious voltage representative of the
voltage of the phase in an LOV state; and of the voltages which are suitably
detected during the fault on the phases other than the phase in an LOV-state.
The program code preferably comprises code which is configured to assume the
fictitious voltage representative of the voltage of said phase in an LOV state equal
to zero when a fault occurs on the phase under an LOV-state, and/or to assume no

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significant phase angle shift for single-line-to-ground-faults between pre-fault and
fault condition on the phases in a non-LOV-state.
[0046] It is to be understood that the description of the foregoing exemplary
embodiment(s) is (are) intended to be only illustrative, rather than exhaustive, of the
present invention. Those of ordinary skill will be able to make certain additions,
deletions, and/or modifications to the embodiments) of the disclosed subject matter
without departing from the spirit of the invention or its scope, as defined by the
appended claims.

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What is claimed is:
1. A method for improving operational reliability during a loss of a phase
voltage (LOV) in a multi-phase power system, the method comprising:
- identifying the phase which is in an LOV state;
- calculating a fictitious voltage representative of the voltage of said phase in
an LOV state; and
- calculating a reference polarizing voltage.

2. The method of claim 1, wherein said reference polarizing voltage is
calculated by means of zero voltage sequence calculations.
3. The method of claim 1, wherein said reference polarizing voltage is
calculated by means of negative voltage sequence calculations.
4. The method of claim 1, further comprising:
- recording the voltages of the phases during normal operating conditions.
5. The method of claim 1, further comprising:
- recording the calculated fictitious voltage representative of the voltage of said
phase in an LOV state.
6. The method of claim 4, wherein said fictitious voltage is calculated on the
basis of the recorded voltages of the phases other than said phase in an LOV-state.
7. The method of claim 5, wherein, when a fault occurs on one of said
phases other than the phase in an LOV-state, said reference polarizing voltage is
calculated on the basis of the recorded fictitious voltage representative of the
voltage of said phase in an LOV state and of the voltages measured during the fault
on said phases other than the phase in an LOV-state.
8. The method of claim 1 wherein, when a fault occurs on the phase which is
in an LOV state, further comprises:

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- assuming said fictitious voltage representative of the voltage of said phase in an
LOV state equal to zero.
9. The method of claim 1, further comprising:
- assuming no significant phase angle shift between pre-fault and fault condition on
the phases in a non-LOV-state.
10. A computer program product for allowing reliable operation during a loss
of a phase voltage (LOV) in a multi-phase power system, said computer program
product comprising computer usable program code configured to:
- identify the phase which is in an LOV state;
- calculate a fictitious voltage representative of the voltage of said phase in an LOV
state; and
- calculate a reference polarizing voltage.

11. A computer program product according to claim 10, wherein said
computer usable program code is configured to calculate said reference
polarizing voltage by means of zero voltage sequence calculations.
12. A computer program product according to claim 10, wherein said
computer usable program code is configured to calculate said reference
polarizing voltage by means of negative voltage sequence calculations.
13. A computer program product according to claim 10, wherein said
computer usable program code is configured to calculate said fictitious voltage on
the basis of the recorded voltages of the phases other than said phase in an LOV-
state.
14. A computer program product according to claim 10, wherein said
computer usable program code is configured to calculate said reference polarizing
voltage on the basis of the fictitious voltage representative of the voltage of said
phase in an LOV state and of the voltages measured during the fault on the phases
other than the phase in an LOV-state, when a fault occurs on one of said phases

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other than the phase in an LOV-state.
15. A computer program product according to claim 10, wherein said
computer usable program code comprises code configured to assume said
fictitious voltage representative of the voltage of said phase in an LOV state equal
to zero when a fault occurs on the phase which is in an LOV state.
16. A computer program product according to claim 10, wherein said
computer usable program code comprises code configured to assume no
significant phase angle shift between pre-fault and fault condition on the phases in
a non-LOV-state.
17. A multi-phase power system for allowing operational reliability during a
loss of a phase voltage (LOV), comprising a computing device having therein
program code usable by said computing device, said program code comprising
code configured to:
- identify the phase which is in an LOV state;
- calculate a fictitious voltage representative of the voltage of said phase in an LOV
state; and
- calculate a reference polarizing voltage.
18. A multi-phase power system according to claim 17, wherein said
program code is configured to calculate said reference polarizing voltage by
means of zero voltage sequence calculations.
19. A multi-phase power system according to claim 17, wherein said
program code is configured to calculate said reference polarizing voltage by
means of negative voltage sequence calculations.
20. A multi-phase power system according to claim 17, wherein said
program code is configured to calculate said fictitious voltage on the basis of the
recorded voltages of the phases other than said phase in an LOV-state.

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21. A multi-phase power system according to claim 17, wherein said
program code is configured to calculate said reference polarizing voltage on the
basis of the fictitious voltage representative of the voltage of said phase in an LOV
state and of the voltages measured during the fault on the phases other than the
phase in an LOV-state, when a fault occurs on one of said phases other than the
phase in an LOV-state.
22. A multi-phase power system according to claim 17, wherein said
program code comprises code configured to assume said fictitious voltage
representative of the voltage of said phase in an LOV state equal to zero when a
fault occur on the phase which is in an LOV state.
23. A multi-phase power system according to claim 17, wherein said
program code comprises code configured to assume no significant phase angle
shift between pre-fault and fault condition on the phases in a non-LOV-state.

The present invention is directed to a method and apparatus for improving operational reliability during a loss of a phase voltage (LOV) in a multi-phase power system, wherein a voltage representative of the LOV phase is calculated. The representative voltage is used for computing a reference polarizing voltage suitable for a protection unit.

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Patent Number

268948

Indian Patent Application Number

3871/KOLNP/2007

PG Journal Number

40/2015

Publication Date

02-Oct-2015

Grant Date

24-Sep-2015

Date of Filing

10-Oct-2007

Name of Patentee

ABB TECHNOLOGY AG

Applicant Address

AFFOLTERNSTRASSE 44, CH-8050 ZURICH

Inventors:

#	Inventor's Name	Inventor's Address
1	PRICE ELMO	5525 DUNROVEN WAY, DAWSONVILLE, GEORGIA 30534

PCT International Classification Number

H02H 3/253

PCT International Application Number

PCT/US2006/018666

PCT International Filing date

2006-05-12

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	60/680719	2005-05-13	U.S.A.