Abstract:
A method for the protection of double circuit electrical power lines protected by local and remote circuit breakers is performed by a local protection apparatus and comprises sensing electrical quantities measured at the local end of both circuits of the double circuit lines, monitoring the sensed electrical quantities to detect occurrence of a fault, delaying operation of the local circuit breakers on both circuits for a time period sufficient to allow detection of operation of a remote circuit breaker by its effect on the electrical quantities on both circuits, and if the effect of a fault on a circuit of the double circuit line persists after remote circuit breaker operation, triggering operation of a local circuit breaker to isolate the fault.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to improvements in protection apparatus for the protection of electrical power lines, in particular protection apparatus enabling fast non-communication protection of power lines having double circuit configurations.  
         BACKGROUND OF THE INVENTION  
         [0002]    Two of the Applicant&#39;s previous inventions in this field are disclosed, for example, in published United Kingdom patent application numbers GB 2,341,738 A and GB 2,341,737 A, to which the reader is referred for background to the present invention.  
           [0003]    Known fault protection techniques for power lines (including power cables) can be broadly classified into two categories, unit and non-unit protection techniques. Each type has its own limitations. These prior art techniques will be described with reference to FIGS.  1 ( a ) to ( c ), in which CB and the symbol X represent a circuit breaker, CT represents a current transformer, and VT represents a voltage transformer.  
           [0004]    The technique of non-unit protection, e.g., distance protection, has the disadvantage that it can not completely cover the entire protection zone, which is normally defined as the whole length of a line section. FIG. 1( a ) shows such a line section extending between the ends ‘S’ and ‘R’. One of the main reasons for incomplete coverage is that the distance protection apparatus (loosely known as a “relay”) only has one measurement point ‘M 1 ’, as shown in FIG. 1( a ), at which to sense the power line&#39;s electrical parameters. Hence, overlapping zones of protection are provided. For instance, Zone  1 , which normally covers up to 80% of the total line section length, extends from the relay location to a point known as the “Reach Point”. The relay operates instantly if a fault such as F 1  is inside Zone  1  i.e., between the relay measurement point M 1  and the reach point. For a fault beyond the Zone  1  reach point, for example ‘F 2 ’ in FIG. 1( a ), delayed operation of the relay has to occur. Normally a time delay of 0.3 to 0.4 seconds is required in setting of the relay operating time to cover the next zone, known as Zone  2 . Zone  2  is normally arranged to cover up to 50% of the shortest adjacent line section beyond the protected line section, as shown in FIG. 1( a ). Consequently, Zone  2  does not have a well-defined boundary.  
           [0005]    On the other hand, unit protection techniques, such as current differential protection, have a clearly defined protection zone as shown in FIG. 1( b ), because they use information about a fault F 1  sensed by relays at both ends S and R of a protected line section, based on measurement points ‘M 1 ’ and ‘M 2 ’, one point for each relay. A communication link is used to transmit information about the system condition from one end of the protected line to the other(s); it can also be arranged to initiate or prevent tripping (opening) of the remote circuit breaker. With this technique, the relays at both ends of the protected line section can instantly trip the entire line section, using circuit breakers CB, when a fault F 1  occurs on the line, but neither relay will respond to a fault F 2  outside the protected zone. However, this technique requires expensive and sophisticated communication link(s) between the ends S and R of the protected section. Therefore, the reliability of the technique will also depend on the reliability of the communication link and device.  
           [0006]    Applicant&#39;s previously published United Kingdom patent application number GB 2,341,738 A, relates to power line protection using distance protection techniques operating in instant and delayed operation modes. Using suitable algorithms, the invention enables power line protection apparatus or relays to make the decision as whether to operate the associated circuit breaker(s) instantly, or after a delay, depending on the system and fault conditions. The technique, in fact, provides the protected zone with a clearly defined boundary using one point measurement only. In other words, it is a unit protection apparatus which does not need a communication link.  
           [0007]    [0007]FIG. 1( c ) of the present specification will be used as an example to describe the prior invention of United Kingdom patent application number GB 2,341,738 A as an introduction to the present invention. FIG. 1( c ) shows a multi-section power line system with the protection apparatus RS 1  and RR 1  installed at each end S and R of the protected line section IL- 1 . BS 1  and BR 1  are three-phase circuit breakers used to protect line section IL- 1 . RR 2  and BR 2  are respectively the protection apparatus and the three-phase circuit breaker responsible for the protection of line section EL- 2 , which is outside the protected zone IL- 1 . The protection zone of the protection apparatus RS 1  covers the line section IL- 1 , while the fault detection of RS 1  covers both Zone  1  and Zone  2  from the end ‘S’.  
           [0008]    The protection apparatus or relay uses the previously stated distance technique (described in relation to FIG. 1( a )) as the means for fault detection and protection. The basic principle of the distance protection technique is well known and will not be reviewed here. There are two operating modes of the protection apparatus RS 1 , the delayed operation mode and the instant operation mode. Here the word ‘operation’ means to open or close circuit breaker(s), which could be one phase of the three phase circuit breaker BS 1  (single-phase tripping or closing) or all three phases of the circuit breaker concurrently (three-phase tripping or closing). The protection apparatus makes a decision as whether the associated circuit breaker(s) should open instantly or after a delay, depending on the system and fault condition.  
           [0009]    With reference to FIG. 1( c ), for a fault F 1  occurring on section IL- 1  within the protection Zone  1  of both protection apparatus RS 1  and protection apparatus RR 1 , RS 1  and RR 1  will operate to trip the line section IL- 1  instantly. However, for a fault F 2  occurring close to one end of the line section IL- 1  but inside it, or a fault F 3  occurring close to one end of the line section IL- 1  but outside it on line section EL- 2  and within the Zone  2  reach of the protection apparatus RS 1 , the relays RR 1  or RR 2  which are close to the fault point will quickly detect the fault and instantly trip corresponding circuit breakers BR 1  if the fault is on line IL- 1 , or BR 2  if the fault is on line EL- 2 . The protection apparatus RS 1  at the end S will first assess the severity of the fault&#39;s impact on the power line system to decide which operation mode is to be used in this fault condition, i.e., whether to trip the associated circuit breakers BS 1  instantly, or after a delay. A criterion for the severity of a fault with respect to the levels of the faulted voltage and current signals can be derived and set in advance. If the preset criterion is met, RS 1  should enter the instant operation mode. Otherwise, the delayed operation mode will be adopted. Depending on the system and the fault condition to be protected against, the protection apparatus can also be set to operate under instant operation mode only or delayed operation mode only.  
           [0010]    In the delayed operation mode, the protection apparatus RS 1  will wait while detecting the breaker operation at the end R. The tripping of the remote breaker BR 1  means that the fault is inside the protected zone or on the remote busbar associated with the protected line section IL- 1 . In both cases, RS 1  will issue a tripping command to isolate the protected line section IL- 1 . However, RS 1  will not respond if the remote circuit breaker BR 2  outside the protected line operates to isolate the fault from line section IL- 1 .  
           [0011]    In the delayed operation mode the protection apparatus is able to offer fast operation for most system and fault conditions, but its response can be slow for some system and fault conditions, such as balanced fault, single pole tripping and no pre-fault current flow. These system and fault conditions can be covered by the instant operation mode of the protection apparatus, in which the protection apparatus will first trip the circuit breaker BS 1  at end S and then detect the operation of the remote breaker BR 1 . An in-zone fault can be assumed if the end R circuit breaker BR 1  on the protected line IL- 1  opens and the fault is still persistent. This effectively means that the remote relay RR 1  has detected an in-zone fault and tripped its associated circuit breaker BR 1  on the faulted line section IL- 1 . In this situation, the protection apparatus RS 1  is inhibited from an immediate reclosing operation. Otherwise, when protection apparatus RS 1  detects an out-of-zone operation of the remote circuit breaker BR 2  and the fault is isolated from the protected line section IL- 1 , RS 1  will issue a reclosing command and close its associated circuit breaker BS 1 . The disadvantage of this instant operation mode is that operation of the circuit breaker may on occasion be required for a fault outside the protected section.  
         SUMMARY OF THE INVENTION  
         [0012]    An object of the invention is to offer fast operation for all system and fault conditions for double circuit line systems without having to use an instant operation mode.  
           [0013]    In its broadest aspect, the invention provides a method of protecting double circuit electrical power lines protected by local and remote circuit breakers, the method comprising the steps of: sensing electrical quantities measured at the local end of both circuits of the double circuit lines, monitoring the sensed electrical quantities to detect occurrence of a fault, delaying operation of the local circuit breakers on both circuits for a time period sufficient to allow detection of remote circuit breaker operation by its effect on the electrical quantities on both circuits, and if an effect of a fault on a circuit of the double circuit line persists after remote circuit breaker operation, triggering operation of a local circuit breaker to isolate the fault.  
           [0014]    The invention also provides protection apparatus for carrying out the method, the protection apparatus being located at a local end of a double circuit electrical power line for protection of both circuits thereof over a protected zone extending from the local end to a remote end of the power line, the protection apparatus being interfaced with both circuits through current and voltage transducers and being equipped with distance protections for each circuit separately, or with cross differential protection for both circuits.  
           [0015]    In more detail, the invention provides a method of protecting a multi-phase electrical power line system, the system comprising: a double circuit power line having local and remote ends; local and remote circuit breaker means near respective ends of each circuit of the double circuit power line; and local and remote protection apparatus linked to respective local and remote circuit breaker means to trigger operation thereof; at least one further electrical power line connected to the remote end of the double circuit power line, and further remote circuit breaker means on the further electrical power line, the further remote circuit breaker means being linked to further remote protection apparatus.  
           [0016]    The method is performed by the local protection apparatus of the double circuit power line and comprises the steps of. sensing electrical quantities on both circuits of the double circuit line at a single local location on each line, and detecting the presence of an electrical fault on the system by analyzing changes in the sensed electrical quantities on both circuits of the double circuit line. If a fault is detected, operation of a local circuit breaker means is triggered only: (a) after a time period sufficient to allow detection of remote circuit breaker operation by analyzing changes in the sensed electrical quantities on both circuits of the double circuit line, and (b) if the effect of the fault on the double circuit line persists after remote circuit breaker operation.  
           [0017]    It is preferred that the sensed electrical quantities include phase currents and quantities representative of changes in levels and angles of the phase currents. Sensed electrical quantities representative of changes in levels and angles of the phase currents preferably comprise superimposed currents and zero, negative and positive sequence currents.  
           [0018]    Correct detection of remote circuit breaker operation may be facilitated by the steps of predefining a time window within which operation of remote circuit breakers can be expected, and ignoring any changes in unfaulted phase currents outside the predefined time window; operation of a remote circuit breaker may be further confirmed by the step of comparing magnitudes of superimposed current signals generated by remote circuit breaker operation with magnitudes of superimposed current signals generated by a fault.  
           [0019]    To derive the superimposed current, the local protection apparatus may sense the electrical quantities at a sampling frequency which is an integer multiple of a power system frequency, delay the sampled current values in a memory means of the local protection apparatus for one cycle of the power system frequency and subtract the delayed sample from the most recent current sample.  
           [0020]    In the preferred embodiment, the sensed electrical quantities further comprise first and second ratio signals, the first ratio signal representing a change in the zero and negative sequence currents with respect to change in the positive sequence current, and the second ratio signal representing a change in the positive sequence quantity with respect to its pre-fault value. Once a fault has been detected, the method can use the first and second ratio 
           [0021]    signals to ascertain system and fault conditions, thus: 
           [0022]    (a) if the first ratio signal has a zero value, the system is ascertained to be in electrical balance with either no fault present or a balanced fault present,  
           [0023]    (b) if the second ratio signal has a value greater than a predefined threshold value, an unbalanced fault is ascertained to be present, and  
           [0024]    (c) if the first ratio signal has a value greater than a predefined threshold value, the system is ascertained to be electrically unbalanced with an unbalanced fault present. 
           [0025]    In one embodiment of the invention, the method comprises a distance protection technique. In an alternative embodiment, the method comprises a cross-differential protection technique.  
           [0026]    A protection apparatus suitable for the distance protection technique comprises two parts located at the local end of the double circuit line section, each part being responsible for the protection of an individual line of the double circuit line and interfacing with their respective lines through current and voltage transducers, a two-way communication channel being arranged to connect the two parts of the protection apparatus for information exchange.  
           [0027]    A protection apparatus suitable for either technique comprises a single part located at the local end of the double circuit line section, the single part being responsible for the protection of both lines and interfacing with both lines through current and voltage transducers and equipped with distance protections for each line separately, or with cross differential protection for both lines.  
           [0028]    Further aspects of the invention will be apparent from a study of the following description and claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]    Embodiments of the invention will now be described by way of example only, with reference to the drawings, in which:  
         [0030]    [0030]FIG. 1( a ) is an equivalent circuit for a typical distance protection scheme in accordance with the prior art;  
         [0031]    [0031]FIG. 1( b ) is an equivalent circuit for a typical unit-protection scheme in accordance with the prior art;  
         [0032]    [0032]FIG. 1( c )is an equivalent circuit of a power line system used to illustrate operation of applicant&#39;s prior art protection apparatus;  
         [0033]    FIGS.  2 ( a ) and  2 ( b ) are equivalent circuits of power line systems having double circuit configurations used to demonstrate the invention embodied in the protection apparatus RS 1 /RS 2  or RS 12 ;  
         [0034]    [0034]FIG. 3( a ) is a diagram to illustrate a technique for the derivation of super-imposed current signals;  
         [0035]    [0035]FIG. 3( b ) is a graphical representation of waveforms of super-imposed current signals;  
         [0036]    FIGS.  4  to  14  comprise graphs showing RMS values of the phase currents, sequence currents, Ratio (Ratio  1  or Ratio  2 , as explained later) and super-imposed current signals in double circuit system configurations under a variety of different fault conditions, using the fast operation mode of the invention protection apparatus RS 1 /RS 2  or RS 12  of FIG. 2;  
         [0037]    [0037]FIG. 4 illustrates a single phase (phase ‘a’) to earth fault outside the protection zone of the invention protection apparatus, using three-phase (i.e., three-pole) operation of the circuit breakers;  
         [0038]    [0038]FIGS. 5 a  and  5   b  illustrate a single phase (phase ‘a’) to earth fault inside the protection zone of the invention protection apparatus, and with pre-fault current flow, using three-phase operation of the circuit breakers;  
         [0039]    [0039]FIGS. 6 a  and  6   b  illustrate a single phase (phase ‘a’) to earth fault inside the protection zone of the invention protection apparatus, but without pre-fault current flow, using three-phase operation of the circuit breakers;  
         [0040]    [0040]FIG. 7 illustrates a single phase (phase ‘a’) to earth fault outside the protection zone of the invention protection apparatus, using single-phase (i.e., single-pole) operation of the circuit breakers;  
         [0041]    [0041]FIGS. 8 a  and  8   b  illustrate a single phase (phase ‘a’) to earth fault inside the protection zone of the invention protection apparatus and with pre-fault current flow, using single-phase (i.e., single-pole) operation of the circuit breakers;  
         [0042]    [0042]FIGS. 9 a  and  9   b  illustrate a single phase (phase ‘a’) to earth fault inside the protection zone of the invention protection apparatus but without pre-fault current flow, using single-phase (i.e., single-pole) operation of the circuit breakers;  
         [0043]    [0043]FIG. 10 illustrates a three phase (phase ‘a’, ‘b’ and ‘c’) to earth fault outside the protection zone of the invention protection apparatus, using three-phase operation of the circuit breakers;  
         [0044]    [0044]FIGS. 11 a  and  11   b  illustrate a three phase (phase ‘a’, ‘b’ and ‘c’) to earth fault inside the protection zone of the invention protection apparatus and with pre-fault current flow, using three-phase operation of the circuit breakers;  
         [0045]    [0045]FIGS. 12 a  and  12   b  illustrate a three phase (phase ‘a’, ‘b’ and ‘c’) to earth fault inside the protection zone of the invention protection apparatus but without pre-fault current flow, using three-phase operation of the circuit breakers;  
         [0046]    [0046]FIGS. 13 a  and  13   b  illustrate a cross-circuit fault (phase ‘a’ of circuit IL- 1  to phase ‘b’ of circuit IL- 2 ) inside the protection zone of the invention protection apparatus, using single-phase operation of the circuit breakers; and  
         [0047]    whereas FIGS.  4  to  13  particularly relate to the system configuration of FIG. 2( a ), FIG. 14 particularly relates to the system configuration of FIG. 2( b ) and illustrates a single phase (phase ‘a’) fault outside the protection zone of the invention protection apparatus, using three-phase operation of the circuit breakers. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0048]    Basic Principles  
         [0049]    The present invention provides a protection apparatus that is designed to offer fast protection of double circuit line systems without the need for data communication from any remote protection apparatus. It therefore extends the delayed operation mode described in United Kingdom patent application number GB 2,341,738 A. The protection apparatus of the present invention relies on techniques such as distance or cross-differential protection techniques for the detection of the occurrence of a fault, and more importantly, for the detection of remote circuit breaker operation, using quantities derived from both circuits of the double circuit lines.  
         [0050]    The cross differential protection technique is also a non-unit protection technique for application to double circuit lines only. The technique works by calculating the differential in current between each of the parallel phases of a double circuit line. In principle this difference tends to zero for an unfaulted line. Any differential current between the lines implies a fault, while the magnitude of this differential will be positive for a fault on one of the lines in the double circuit and negative for a fault on the other line. As in the case of the distance protection technique, the cross differential technique cannot offer high-speed protection for the entire protection zone since there is a zone at the remote end of the line which cannot be protected by the cross differential technique alone.  
         [0051]    FIGS.  2 ( a ) and  2 ( b ) show respectively first and second typical configurations of double circuit line systems, which will be used to describe a protection apparatus according to the present invention. As in FIG. 1, the symbol x represents a circuit breaker. As shown in FIG. 2 a , the protected section of the double circuit system consists of two line sections, IL- 1  and IL- 2 . BS 1 , BS 2 , BR 1  and BR 2  are circuit breakers employed to protect each line section respectively. BR 3 , BR 4  and BR 5  are employed for the protection of the line sections EL- 3  to EL- 5 , which are outside the protected double circuit section. In the explanation given below, the distance technique is used to demonstrate the invention. According to the invention, there are two alternative implementations of the protection apparatus (A) and (B), both based on the principle shown in FIG. 2 a.    
         [0052]    In the first implementation (A), there are two separate parts RS 1  and RS 2  of the local protection apparatus, both located near end S of the double circuit line section. Using the distance protection technique at end ‘S’, each part RS 1  and RS 2  is responsible for the protection of an individual line of the double circuit, IL- 1  and IL- 2  respectively. Protection apparatus RS 1  interfaces with line IL- 1  through current and voltage transducers (not shown) and RS 2  interfaces likewise with line IL- 2 . A two-way communication channel, indicated by the double-headed zig-zag arrow C, is arranged to connect the two parts of the protection apparatus for information exchange.  
         [0053]    In the second implementation (B), just one protection apparatus RS 12  at the end ‘S’ of the parallel circuit is responsible for the protection of both lines IL- 1  and IL- 2  of the double circuit. In this case, the protection apparatus interfaces with both lines through current and voltage transducers and is equipped with distance protections for each line separately, or with cross differential protection for both lines.  
         [0054]    The examples given below will apply to both cases, although form (B) is used for demonstration purposes.  
         [0055]    With reference to FIG. 2( a ), for a fault F 2  or F 3  occurring close to end ‘R’ outside Zone  1  and inside Zone  2  of the invention protection apparatus RS 12 , the relays RR 1 , RR 2  or RR 3  etc., which are close to the fault point, will quickly detect the fault. The relays RR 1 , RR 2  or RR 3  will then instantly trip their corresponding circuit breakers BR 1  (if the fault is on line IL- 1 ) or BR 2  (if fault is on line IL- 2 ) or BR 3  (if fault is on line EL- 3 ). The invention protection apparatus RS 12  at the end ‘S’ will wait and detect whether a circuit breaker BR 1 , BR 2  or BR 3  on the section IL- 1 , IL- 2  or EL- 3  at the remote end R has tripped or not. If one of the remote circuit breakers BR 1  or BR 2  on a protected section operates and the system is still on an unbalanced condition due to the continued presence of the fault, the protection apparatus RS 12  will issue a tripping command to BSl or BS 2  to isolate the protected line section IL- 1  or IL- 2 . Otherwise, protection apparatus RS 12  will give no response if a remote circuit breaker outside the protected line (for example, BR 3 ) operates to isolate the Zone  2  fault from line sections IL- 1  and IL- 2 .  
         [0056]    The technique for determining whether a fault is inside the protected zone is based on detecting operation of remote circuit breakers on both circuits.  
         [0057]    As will be evident from FIG. 2 a , although the opening of the remote in-zone circuit breaker BR 1  for a fault F 2  on IL- 1  near busbar ‘R’ may not cause changes in the current detected on IL- 1  at end ‘S’, it will cause a change in the double circuit system configuration. This configuration change will result in a significant change in the levels and angles of the phase currents on the healthy (i.e., unfaulted) circuit IL- 2 , and these changes will occur within a short time period from inception of the fault. It is these changes in current that the invention protection apparatus relies on to detect operation of the remote circuit breakers for different system and fault conditions. The above-mentioned sequence and superimposed current quantities are used in detecting the changes in the levels and angles of the current signals. For an explanation of sequence and superimposed current and voltage quantities and their derivation, the reader should consult prior United Kingdom patent publication GB 2,341,738 A and “Power System Protection”, Vol. 4: “Digital Protection and Signalling”, edited by The Electricity Training Association, The Institution of Electrical Engineers, London, UK, 1995.  
         [0058]    This invention technique overcomes the limitations of prior art technology used to detect the operation of the remote breaker, in that it can guarantee fast operation for system and fault conditions such as: (i) three phase or three phase to earth faults, (ii) no pre-fault loading on line; and (iii) single pole tripping.  
         [0059]    The time delay from inception of the fault to opening of the remote circuit breaker will be the summation of the response times of the remote relay or protection apparatus and the remote circuit breaker. Since this delay will vary slightly depending on the response time of the relay and circuit breaker to different fault conditions, a time window can be predefined within which the operation of a remote circuit breaker can be expected. Any changes in the level of the healthy (i.e., unfaulted) phase current outside this predefined window will be ignored. This will ensure the correct detection of the remote circuit breaker operation. At the same time, the magnitudes of the superimposed current signals generated by circuit breaker operation can be compared with that of superimposed signals generated by a fault, from which the circuit breaker operation can be further confirmed.  
         [0060]    Algorithm Used  
         [0061]    (a) Ratio Signals for Detection of System Balanced Operating Condition.  
         [0062]    Two ratio signals are used to detect whether a system is in a balanced operation or not. A first ratio signal expresses the change in the zero and negative sequence quantities with respect to the change of the positive sequence quantity and is used as the main criterion to detect whether the system is in a balanced operating condition or not. This Ratio  1  signal is given in equation (1):  
                 R   1          (   k   )       =           S   2          (   k   )       +       S   0          (   k   )             S   1          (   k   )                 (   1   )                               
 
         [0063]    where S 1 (k), S 0 Sk), S 2 (k)are the RMS values of the positive, zero and negative sequence quantities and k is the sampling instant.  
         [0064]    To cover some special fault conditions, such as three phase and three phase to earth faults, a second ratio signal, Ratio  2 , which expresses the change in the positive sequence quantity during a fault period with respect to its pre-fault value, is used as an additional criterion to detect whether the system is in a balanced operating condition or not. This Ratio  2  signal is given as:  
                 R   2          (   k   )       =           S   1          (   k   )       -       S     1      pre            (   k   )             S     1      pre            (   k   )                 (   2   )                               
 
         [0065]    where S 1pre (k) is the RMS value of the pre-fault positive sequence quantity.  
         [0066]    The signal R 1  will be of zero value under no fault or no unbalanced fault conditions, since negative and zero sequence quantities are not present then, but will be increased to a level well above zero if an unbalanced fault is present on the system. Under the unbalanced fault condition, the signal R 2  will increase to a level and return to zero after the clearance of the fault from the system. The theoretical values for the signal R 1  will be ‘ 1 ’ for a phase-to-phase fault and ‘ 2 ’ for phase-to-earth fault with remote circuit breaker open. The level of signal R 2  will be well above ‘ 1 ’, depending on the pre- and post-fault system conditions. As a result, a preset threshold above zero level (say between 0.2 and 0.4) can be used to determine the system fault condition by examining whether the one of the two ratio signals R 1  and R 2  has exceeded it or not.  
         [0067]    (b) Super-Imposed Signals  
         [0068]    The invention technique also uses super-imposed signals to detect the change in magnitude and angle of the healthy phase currents caused by operation of a remote circuit breaker. As shown in FIGS.  3 ( a ) and  3 ( b ), f 0  is the system frequency, Ts is the time period of the RMS value of the super-imposed current signal and Tb is the time of circuit breaker operation. The super-imposed current is derived by delaying the sampled current values in memory for one cycle; and the super-imposed current is then formed by subtracting the delayed sample from the most recent current sample. This technique assumes that the sampling frequency (i.e., the frequency at which measurements are taken by current and voltage transducers on lines IL- 1  and IL- 2 ) is an integer multiple of the power system frequency and so there will be a whole-number of samples taken per power cycle. If this is so, then a one cycle delay can be implemented by delaying the samples in a fixed number of memory locations. The super-imposed current signal is defined as follows: 
         Is(k)=I(k)−I(k−n)  (3) 
         [0069]    where k is the sampling time instant; n is the number of samples per cycle; I(k) is the healthy phase current; I(k−n) is the one-cycle-delayed healthy phase current; and Is(k) is the healthy phase super-imposed current signal. They are derived from the angle difference between pre- and post-circuit breaker operations. The theoretical aspect of the super-imposed quantities is well documented in the literature, (see, e.g., the above-mentioned “Power System Protection”, Vol.4, IEE, London, UK) and will therefore not be repeated here.  
       EXAMPLES  
       [0070]    The functioning of protection apparatus according to the invention under various fault conditions will now be further explained with reference to FIGS.  4  to  14 , in which Ia, Ib and Ic represent the three phase currents, I 1 , I 0  and I 2  represent the positive, zero and negative sequence currents respectively, Ias, Ibs and Ics represent the three phase super-imposed currents given in equation (3), and the ratio signals, Ratio  1  and Ratio  2  are as given in Equations (1) and (2). The derivation of the sequence quantities was fully explained in prior United Kingdom patent application number GB 2,341,738 A.  
         [0071]    Case 1 Single Phase to Earth Fault with Three Pole Tripping  
         [0072]    Out-of-Zone Fault  
         [0073]    FIGS.  4 ( 1 )- 4 ( 3 ) show the responses for an out-of-zone (i.e., outside the zone protected by protection apparatus RS 1 /RS 2  or RS 12 ) single phase (‘a’) to earth fault F 3  occurring near busbar ‘R’ on line section EL- 3  as shown in FIG. 2 a . T 1  is the time of fault inception, and T 2  is the time when the three phase circuit breaker BR 3  opens. As in FIG. 3, Ts is the time period of the superimposed current signal. For this external fault position, the responses from the measurements on both IL- 1  and IL- 2  sections will be identical, therefore only one set of three FIGS. is used here to demonstrate the responses obtained at the ‘S’ end of both lines IL- 1  and IL- 2  of the double circuit. FIG. 4( 1 ) shows the level of the three phase current signals,  4 ( 2 ) shows the sequence current signals, and  4 ( 3 ) shows the Ratio and super-imposed current signals. As shown in FIG. 4, during the normal balanced operation before the fault inception, the three phase current signals are of the same magnitudes and the zero and negative sequence currents remain as zero. There is a significant increase in the faulted phase current I a  and consequently an increase in both the negative I 2  and zero I 0  sequence currents and the Ratio  1  signal after the fault inception at time T 1 . The protection apparatus RR 3  on line section EL- 3  at the end ‘R’ detects the fault, issues a tripping command and subsequently opens the associated circuit breaker BR 3 . After the opening of the circuit breaker BR 3  at time T 2 , the fault is isolated from the protected section IL- 1  and IL- 2 , and the system resumes balanced operation. Consequently, the faulted phase current I a  returns to its pre-fault level and the negative current, the zero sequence current, and their associated Ratio signal drop back to zero as shown in FIGS.  4 ( 2 ) and  4 ( 3 ). The protection apparatus RS 12  (or RS 1 /RS 2 ) is microprocessor-controlled and is programmed so that the current/time characteristics shown in FIG. 4 inhibit the protection apparatus RS 12  at the end ‘S’ from making a tripping decision for this out-of-zone fault condition. In this case, although for a time Ts the super-imposed current Ibs and Ics increase to a high level caused by the remote circuit breaker operation as shown in FIG. 4( 3 ), the Ratio  1  signal returns zero, thereby indicating that the system has returned to balanced operation. Therefore, the fault is identified as an out-of-zone fault.  
         [0074]    It should be mentioned here that for an external fault condition, the pre-fault current flow does not affect the performance of the protection apparatus. For an internal fault condition, however, the operation of the protection apparatus will be affected by the flow of the pre-fault current since it relies on the detection of the change in level of the post-fault currents to detect the operation of the remote in-zone circuit breaker.  
         [0075]    It should be also mentioned here that the fault itself also generates super-imposed currents that are not shown in FIG. 4 since this particular information is not used by the protection apparatus.  
         [0076]    Typical In-Zone Fault with Pre-Fault Current Flow  
         [0077]    In FIG. 5, T 1  is the time of fault inception, T 2  is the time when the three phase circuit breaker BR 1  opens and T 3  the time when the three phase circuit breaker BS 1  opens. FIGS. 5 a ( 1 ),  5   a ( 2 ) and  5   a ( 3 ) show the responses at end S obtained from the measurements and derived quantities for the device installed on line IL- 1  and FIGS. 5 b ( 1 ),  5   b ( 2 ) and  5   b ( 3 ) show those for line IL- 2 .  
         [0078]    [0078]FIGS. 5 a  and  5   b  show the corresponding responses of the protection apparatus for a typical in-zone single phase to earth fault (‘a’-‘e’) near busbar ‘R’ on the line section IL- 1  during the periods of pre-fault and post-fault with breaker operations. In this case, there is a pre-fault current flow between ends ‘S’ and ‘R’.  
         [0079]    For line section IL- 1 , the phase and sequence signals are shown in FIGS. 5 a ( 1 ) and  5   a ( 2 ) respectively and the Ratio and super-imposed current signals are shown in FIG. 5 a ( 3 ). For line section IL- 2 , the phase and sequence signals are shown in FIGS. 5 b ( 1 ) and  5   b ( 2 ) respectively and the Ratio and super-imposed current signals are shown in FIG. 5 b ( 3 ). T 1  is the time of fault inception. After the opening of the remote end circuit breaker BR 1  at time T 2 , the phase ‘b’ and ‘c’ of line section IL- 1  becomes open circuit at end ‘R’. Consequently, I b  and I c  drop to zero as shown in FIG. 5 a ( 1 ). However, the fault on phase ‘a’ has not been cleared by the remote circuit breaker operation since the fault is inside the protected zone. The faulted phase current I a  and the negative current, zero sequence current and their associated Ratio signal on IL- 1  remain at a high level.  
         [0080]    This change also produces super-imposed current signals for the time Ts as shown in FIGS. 5 a ( 3 ) and  5   b ( 3 ). The protection apparatus RS 12  at the end ‘S’ detects both the changes in the magnitudes of the unfaulted phased currents as shown in FIG. 5 a ( 1 ) and the super-imposed current signals through the thresholds set as shown in both FIG. 5 a ( 3 ) and  5   b ( 3 ). At the same time, the Ratio signal for IL- 1  remains at a high level, which means that the line IL- 1  is still in an unbalanced operating condition. Consequently, the operation of remote circuit breaker BR 1  inside of the protected zone is detected for this in-zone fault condition. RS 12  therefore issues a tripping command and subsequently circuit breaker BS 1  opens to isolate the faulted line section IL- 1  at time T 3 .  
         [0081]    In the invention, the protection apparatus detects the operation of the remote circuit breaker within a short time window ΔT after the fault inception as shown in FIG. 5 a . The ΔT is centered around a time period which is the summation of the response time of the remote relay and the remote circuit breaker. Any disturbance which may be caused by various reasons outside the predefined time window will not be taken into consideration by the protection apparatus to ensure the correct operation can be obtained for various system and fault conditions.  
         [0082]    As shown in FIG. 5 a , the time delay from the fault inception at time T 1  to the opening of the circuit breaker BS 1  at time T 3  mainly consists of 2 time periods, t 12  and t 23 . Period t 12  is the elapsed time from the fault inception to the operation of the end ‘R’ circuit breaker BR 1 , which is the time taken by relay RR 1  to detect the presence of a fault (this normally takes about one power frequency cycle) and the time taken for circuit breaker BR 1  to open (this normally takes between one to three cycles depending on the circuit breaker used). Period t 23  consists of the time taken for the protection apparatus RS 12  to identify the operation of circuit breaker BR 1 , plus the response time of the circuit breaker BS 1 . In the case of instant operation, the time taken from fault inception to the opening of breaker BS 1  at the end ‘S’ will be the same as the time period t 12 . Therefore, the total time delay will be the time period of t 23 , which is approximately 2 to 4 cycles depending on the circuit breaker used.  
         [0083]    In-Zone Fault, with no Pre-Fault Current Flows  
         [0084]    In FIG. 6, T 1  is the time of fault inception, T 2  is the time when the three phase circuit breaker BR 1  opens and T 3  the time when the three phase circuit breaker BS 1  opens. FIGS. 6 a ( 1 ),  6   a ( 2 ) and  6   a ( 3 ) are for the signals sensed on line ‘IL- 1 ’ and FIGS. 6 b ( 1 ),  6   b ( 2 ) and  6   b ( 3 ) are for the signals sensed on line ‘IL- 2 ’.  
         [0085]    [0085]FIGS. 6 a  and  6   b  show the responses of the protection apparatus for the same in-zone fault condition as that in the above case. However, there is no pre-fault current flow between ends ‘R’ and ‘S’ in this case. As shown in FIG. 6 a ( 1 ), the fault occurred at T 1 , causing an increase in the faulted phase current and subsequently the sequence current and the Ratio signal. After opening the remote in-zone circuit breaker BR 1  at T 2 , the unfaulted phase currents I b  and I c  keep at a very low level. At the same time, there is little change in the level of the ratio signal, as shown in FIG. 6 a ( 3 ). This makes the identification of the exact time of the remote breaker operation impossible if the measurement from the line IL- 1  is relied upon. However, when examining the response obtained from line IL- 2 , the increase in the super-imposed current quantity is clearly evident at the time of opening of the remote circuit breaker BR 1  as shown in FIG. 6 b ( 3 ). At the same time, there is a significant drop in the faulted phase current Ia (FIG. 6 b ( 1 )) and the ratio signal as shown in FIG. 6 b ( 3 ), which indicates that the fault is not on IL- 2 . This change, together with the high level of the ratio signal on IL- 1 , as shown in FIG. 6 a ( 3 ), clearly indicates that the operation of BR 1  and the fault is on the line section IL- 1 . Hence, the protection apparatus RS 12  detects the internal fault condition and so issues a trip command to open the circuit breaker BS 1  to isolate the faulted line section IL- 1  at time T 3 .  
         [0086]    Case 2-Single Phase Earth Fault with Single Pole Tripping  
         [0087]    Out-of-Zone Fault  
         [0088]    In FIG. 7, T 1  is the time of fault inception and T 2  is the time when the faulted phase of the circuit breaker BR 3  opens. FIG. 7 shows the responses for a phase-to-earth (‘a’-‘e’) out-of-zone fault close to busbar ‘R’ on line section EL- 3 . As shown in FIG. 7( 1 ), the system enters an unbalanced operation condition after the fault inception at time T 1  and there is a significant increase in the faulted phase current I a . The opening of the circuit breaker BR 3  at time T 2  clears the fault on line section IL- 1  and IL- 2 , and the negative sequence current associated with the fault drops to zero. Although the super-imposed currents lbs and Ics increase to a high level due to the operation of circuit breaker BR 3 , the Ratio returns to zero as shown in FIG. 7( 3 ) and the system then returns to a balanced operation condition, which inhibits the protection apparatus RS 12  at the end ‘S’ from making a tripping decision.  
         [0089]    Typical In-Zone Fault with Pre-Fault Current Flow  
         [0090]    In FIG. 8, T 1  is the time of fault inception, T 2  is the time when the three phase circuit breaker BR 1  opens, and T 3  the time when the faulted phase of the circuit breaker BS 1  opens. FIGS. 8 a ( 1 ),  8   a ( 2 ) and  8   a ( 3 ) are for the device installed at line ‘IL- 1 ’and FIGS. 8 b ( 1 ),  8   b ( 2 ) and  8   b ( 3 ) are for line ‘IL- 2 ’.  
         [0091]    [0091]FIGS. 8 a  and  8   b  show the response of the protection apparatus to a single phase (‘a’) in-zone fault on line section IL- 1  near busbar R for line IL- 1  and IL 2  respectively. As shown in FIG. 8 a ( 1 ), the opening of phase ‘a’ of circuit breaker BR 1  at time T 2  neither clears the fault on line section IL- 1 , nor changes the level of the unfaulted phase currents and their associated super-imposed currents. There is no change of the ratio signal derived from IL- 1  as shown in FIG. 8 a ( 3 ). As a result, there is no clear indication from the measurements of quantities on line section IL- 1  as to when the faulted phase of circuit breaker BR 1  opens. However, when examining the response obtained from IL- 2 , the significant change in the level of the faulted phase current and the super-imposed current resulting from the BR 1  operation is clearly evident as shown in FIG. 8 b ( 1 ) and  8   b ( 3 ). Hence, the protection apparatus RS 12  detects this fault condition and issues a tripping command and subsequently opens phase ‘a’ of the circuit breaker BSl at time T 3 , so isolating the faulted phase on section IL- 1 .  
         [0092]    In-Zone Fault, with no Pre-Fault Current Flows  
         [0093]    In FIG. 9, T 1  is the time of fault inception, T 2  is the time when the three phase circuit breaker BR 1  opens, and T 3  the time when the faulted phase of the circuit breaker BS 1  opens. FIGS. 9 a ( 1 ),  9   a ( 2 ) and  9   a ( 3 ) are for the device installed at line ‘IL- 1 ’ and FIGS. 9 b ( 1 ),  9   b ( 2 ) and  9   b ( 3 ) are for line ‘IL- 2 ’.  
         [0094]    [0094]FIGS. 9 a  and  9   b  show the response of the protection apparatus to the same in-zone fault condition as that of FIG. 8 with no pre-fault current flows. Similarly to the above case, although there is no clear indication as when the faulted phase remote breaker opens from the quantities derived from line IL- 1 , the significant changes in the faulted phase current and the super-imposed current signals derived from line IL- 2  clearly indicate when the remote circuit breaker BR 1  operates, and the continuous high level of the ratio  1  signal on IL- 1  indicates that the fault is on line section IL- 1 . Therefore, the protection apparatus makes a correct decision to isolate the fault by opening phase ‘a’ of the circuit breaker BS 1  at T 3 .  
         [0095]    Case 3 Three Phase Fault with Three Pole Tripping  
         [0096]    Out-of-Zone Fault  
         [0097]    In FIG. 10, T 1  is the time of fault inception, and T 2  is the time when the three phase circuit breaker BR 3  opens.  
         [0098]    [0098]FIG. 10 shows the responses for a three phase (‘a’-‘b’-‘c’) out-of-zone fault close to busbar ‘R’ on line section EL- 3 . As shown in FIG. 10( 1 ), there are significant increases in the three phase fault currents after the fault inception at time T 1 . In this case the Ratio  1  remains as zero and Ratio  2  increases to a high level since the fault is a balanced fault. The opening of the circuit breaker BR 3  at time T 2  clears the fault on line section IL- 1  and IL- 2 . Although the super-imposed current Ias, lbs and Ics increases to a high level due to the operation of BR 3 , the Ratio  2  signal returns to zero as shown in FIG. 10( 3 ) and the system then returns to a normal, operation condition, which inhibits the protection apparatus RS 12  at the end ‘S’ from making a tripping decision.  
         [0099]    Typical In-Zone Fault with Pre-Fault Current Flow  
         [0100]    In FIG. 11, T 1  is the time of fault inception, T 2  is the time when the three phase circuit breaker BR 1  opens, and T 3  the time when the three phase circuit breaker BS 1  opens. FIGS. 11 a ( 1 ),  11   a ( 2 ) and  11   a ( 3 ) are for the device installed at line ‘IL- 1 ’ and FIGS. 11 b ( 1 ),  11   b ( 2 ) and  11   b ( 3 ) are for line ‘IL- 2 ’.  
         [0101]    [0101]FIGS. 11 a  and  11   b  show the response of the protection apparatus to a three phase (‘a’-‘b’-‘c’) in-zone fault on line section IL- 1  near busbar R for line IL- 1  and IL- 2  respectively. As shown in FIG. 11 a ( 1 ), the opening of the circuit breaker BR 1  at time T 2  neither clears the fault on line section IL- 1 , nor changes the level of the faulted phase current. There is also no change of the Ratio  2  signal derived from IL- 1  as shown in FIG. 11 a ( 3 ). Therefore, the quantities derived on line section IL- 1  give no clear indication as to when the remote circuit breaker BR 1  opens. However, when examining the response obtained from IL- 2 , the significant change in the level of the faulted phase current and the super-imposed current resulting from the operation of BR 1  is clearly evident in FIG. 11 b ( 1 ) and  11   b ( 3 ). Hence, the protection apparatus RS 12  detects this fault condition and issues a tripping command to open the circuit breaker BS 1  at time T 3 , so that the faulted section IL- 1  is isolated.  
         [0102]    In-Zone Fault, with no Pre-Fault Current Flows  
         [0103]    In FIG. 12, T 1  is the time of fault inception, T 2  is the time when the three phase circuit breaker BR 1  opens, and T 3  the time when the three phase circuit breaker BS 1  opens. FIGS. 12 a ( 1 ),  12   a ( 2 ) and  12   a ( 3 ) are for the device installed at line ‘IL- 1 ’ and FIGS. 12 b ( 1 ),  12   b ( 2 ) and  12   b ( 3 ) are for line ‘IL- 2 ’.  
         [0104]    [0104]FIGS. 12 a  and  12   b  show the response of the protection apparatus to the same in-zone fault condition as that shown in FIG. 11 but with no pre-fault current flows. Similarly to the above case, although there is no clear indication from the quantities derived from line IL- 1  as to when the remote breaker opens, the significant changes in the faulted phase current and the super-imposed current signals derived from line IL- 2  clearly indicate the instant when the remote circuit breaker BR 1  operates and the continuous high level of the ratio signal on IL- 1  indicates that the fault is on line section IL- 1 . Therefore, the protection apparatus makes a correct decision to isolate the fault by opening circuit breaker BS  1  at T 3.    
         [0105]    Case 4-Cross Circuit Fault with Single Pole Tripping  
         [0106]    In FIG. 13, T 1  is the time of fault inception, T 2  is the time when the three phase circuit breaker BR 1  opens, and T 3  the time when the faulted phase of the circuit breaker BS 1  opens. FIGS. 13 a ( 1 ),  13   a ( 2 ) and  13   a ( 3 ) are for the device installed at line IL- 1  and FIGS. 13 b ( 1 ),  13   b ( 2 ) and  13   b ( 3 ) are for line IL- 2 .  
         [0107]    [0107]FIGS. 13 a  and  13   b  show the response of the protection apparatus to a cross-circuit in-zone fault between phase ‘a’ of IL- 1  and phase ‘b’ of IL- 2  near busbar R for line IL- 1  and IL- 2  respectively. As shown in FIG. 13( 1 ), both phase ‘a’ of circuit breaker BR 1  and phase ‘b’ of BR 2  open at time T 2 , since the fault involves two circuits. The remote circuit breaker operations do not clear the fault on line sections IL- 1  and IL- 2 . Although there is only a slight change in the levels of the faulted phase current signals and the ratio signals when the remote breaker opens at the time T 2 , the significant increases in the super-imposed current signals detected at both IL- 1  and IL- 2 , together with the high level of the ratio signals, indicates that the fault is an in-zone cross-circuit fault. Consequently, the protection apparatus RS 12  detects this fault condition, issues a tripping command and subsequently opens the faulted phase of circuit breaker BS 1  (phase ‘a’) and BS 2  (phase ‘b’) at time T 3 , so that the faulted phases of IL- 1  and IL- 2  are isolated.  
         [0108]    Case 5 Single Phase-to-Earth Fault on Out-of-Zone Line of Double Circuit Configuration  
         [0109]    [0109]FIG. 2( b ) shows a system configuration different from that of FIG. 2( a ), in which the remote out-of-zone line is also of a double circuit configuration. The effect of an in-zone fault on Il- 1  or IL- 2  for the FIG. 2( b ) system configuration will be essentially the same as for the FIG. 2( a ) system configuration, since any such fault will be separated from the remote out-of-zone sections by opening of the remote in-zone circuit breaker BR 1  or BR 2 . Hence, all the above examples for the in-zone fault cases are applicable to this configuration, therefore only the response to an external fault such as F 3  is examined here.  
         [0110]    In FIG. 14, T 1  is the time of fault inception, and T 2  is the time when the remote, out-of-zone, three phase circuit breaker BR 3  opens. FIG. 14 shows the response of the protection apparatus RS 12  to a single phase-to-earth fault on the line section EL- 3  near busbar R, as shown in FIG. 2( b ). Again, the responses for line IL- 1  and IL- 2  at end S will be identical for this external fault condition, so only one figure. is used here to demonstrate the performance of the protection apparatus. As shown in FIG. 14, the opening of the remote circuit breaker BR 3  at T 2  does not return the system to an balanced operation condition since the source at end ‘S’ still feeds the fault through line EL- 4  and the busbar connection at the end ‘T’. However, immediately the circuit breaker BR 3  has opened, the fault becomes outside of the Zone  2  reach of the protection apparatus RS 12 , which therefore is able to detect the change in the fault condition and refrain from operation.  
         [0111]    Further Features of the Invention  
         [0112]    It should be understood that the levels of the phase and sequence currents shown in the graphs of all the above examples are of nominal values for demonstration purposes. However, the protection method and apparatus of the invention works under all levels of system voltages, source parameters, pre-fault and post-fault load flow conditions.  
         [0113]    The equations (1) to (2) only show a digital algorithm to calculate the ratio signals which are used as the criterion to determine system operation conditions, but the invention can also be based on algorithms and criteria expressed in different forms.  
         [0114]    The equation (3) only shows one method of using the super-imposed current signals to detect the operation of the remote circuit breaker, but the invention can also be based on algorithms and criteria expressed in different forms.  
         [0115]    In case the remote relay fails to detect a fault or a circuit breaker fails to respond for an out-of-zone fault, the protection apparatus will also have a backup protection function in its software. Because the fault is still on the line, in most circumstance the apparatus will trip the line and wait for a further chance to reclose when a remote out-of-zone fault is cleared by backup protection. Alternatively, a further delay can be arranged to wait for the backup protection of the remote relay to operate, from which can be judged whether a fault is inside the protected zone and the correct action can be taken.