Abstract:
A method for controlling fault interruption in a high voltage electrical energy transmission system semiconductor high voltage electrical energy transmission switching system which includes a multiple stack of power semiconductor SCR thyristor circuit sections arranged for single phase or three-phase operation with each section of the stack comprising; a pair of input and output terminals, a plurality of switching thyristors connected in series between the input and output terminals, an auxiliary power transformer having at least three windings with first and second windings interconnecting the input and output terminals of one section to the respective output and input terminals in each adjacent section of the stack such that the voltage difference between the first and second transformer windings is limited to the maximum voltage across the section. The system also includes an AC/DC power supply connected to the third winding of the auxiliary power transformer and a gate drive circuit for controlling the firing of theswitching thyristors in said section.comprising the steps of connecting in tandem a multiple stack of power semiconductor SCR thyristor circuit switching sections. The a gate drive circuit is connected to the gate terminals of the commutation SCR thyristor&#39;s for controlling the firing of the commutation SCR thyristor&#39;s in each section and the firing angle of the SCR thyristor&#39;s in each section is changed between 0° and 180° to allow for downstream protection coordination.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to a semiconductor high voltage switching system and method for transmitting and distributing electrical power at very high voltage.  
         BACKGROUND OF THE INVENTION  
         [0002]    The transmission and distribution of electrical power benefits from the use of high power semiconductor thyristor switches for the distribution of high voltage DC and for the commutation of AC at high voltage. To produce the voltage levels needed to provide power at voltage levels above 10 kv and up to 200 k volts or higher it is commonplace to combine a multiple number of Silicon Controlled Rectifiers SCR&#39;s (hereafter referred to as “thyristors”) in series to form a semiconductor “valve”. Conventional high voltage systems are designed using such semiconductor valves. To test the operation of a system using a semiconductor thyristor valve at multi kv voltage levels involves considerable safety considerations and makes the system very expensive to manufacture.  
           [0003]    It is therefore the primary objective of the present invention to simplify the design and manufacture of a semiconductor high voltage electrical energy transmission switching system and to provide a high voltage semiconductor commutation circuit for single and/or three phase operation.  
           [0004]    It has been discovered in accordance with the principals of the present invention that by forming a series stack of free floating circuit sections each composed of a predetermined number of semiconductor SCR thyristors in series, auxiliary power can be transferred through the stack of circuit sections by incorporating an auxiliary power transformer in each circuit section arranged such that the voltage difference between the power transformer windings applied to the opposite ends of each circuit section will be equal to the maximum voltage across each circuit section of the stack. Using this arrangement the voltage across each section can be limited to a voltage of, e.g. 10 k volts, which reduces the cost of the semiconductor and firing transformer elements and simplifies the testing of the circuit sections during manufacture and use. The number of sections in a stack will depend on the desired voltage to be applied across each section and the line voltage.  
           [0005]    Additional benefits include a reduction in the number of spare parts (the switching system for different voltage levels are built from the same building block) and ease of maintenance in that a faulty section need only be replaced.  
           [0006]    Each free floating circuit section in each phase may include any number of unidirectionally series connected thyristors. Alternatively, the thyristors in each section may be bi-directionally connected, i.e., connected in an anti-parallel arrangement to allow current to pass in both directions and to permit forced commutation to turn off the thyristors if this is needed. Forced commutation is used to limit the fault current in a section before it rises to a level which will cause substantial damage to circuit breakers and other components (transformer, busses, etc.) elsewhere in the power system and/or to the components in the section. In the preferred forced commutation switch circuit embodiment of the present invention, applicable to single and three phase operation, the number of commutating capacitors in the switch circuit is reduced by a factor of two compared to conventional commutation switching circuits using semiconductor thyristors in which each semiconductor thyristor requires its own commutating capacitor. This is also true for the number of commutation inductors which is also reduced by a factor of two thereby further reducing the cost and weight/size of the section. For three-phase operation, each phase has a multiple stack of identical free floating circuit sections.  
         SUMMARY OF THE INVENTION  
         [0007]    The semiconductor high voltage electrical energy transmission switching system of the present invention includes a multiple stack of power semiconductor SCR thyristor circuit sections arranged for single phase or three-phase operation with each section of the stack comprising; a pair of input and output terminals, a plurality of switching thyristors connected in series between the input and output terminals, an auxiliary power transformer having at least three windings with first and second windings interconnecting the input and output terminals of one section to the respective output and input terminals in each adjacent section of the stack such that the voltage difference between said first and second transformer windings is limited to the maximum voltage across said section, an AC/DC power supply connected to a third winding of said auxiliary power transformer and a gate drive circuit for controlling the firing of said switching thyristors in said section. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    Other advantages and objects of the present invention will become apparent from the following detailed description when read in conjunction with the accompanying drawings in which:  
         [0009]    [0009]FIG. 1 is a schematic block diagram of a multiplicity of free floating sections in the switching system of the present invention stacked together in electrical series;  
         [0010]    [0010]FIG. 2 is an enlarged schematic block diagram of a single section in Fig.  
         [0011]    [0011]FIG. 3 is a schematic of one embodiment of a single section of FIG. 2 showing the circuit arrangement for an HVDC or other converter DC/AC application;  
         [0012]    [0012]FIG. 4 is another embodiment of the schematic circuit diagram of FIG. 3 for bi-directional operation;  
         [0013]    [0013]FIG. 5 is a circuit diagram of the arrangement of FIG. 4 modified in accordance with the present invention to achieve forced commutation;  
         [0014]    [0014]FIG. 6(A) is a circuit diagram of the pre-charge circuit in FIG. 5;  
         [0015]    [0015]FIG. 6(B) is another circuit arrangement of the pre-charge circuit in FIG. 5; and  
         [0016]    [0016]FIG. 7(A-C) are functional block diagrams of alternate versions for distributing power between the sections of a multiple stack arrangement for use in a single or three-phase power distribution system in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]    As can be seen in the arrangement of FIG. 1, a multiplicity of free floating sections  10  are coupled to one another using a first pair of terminals labeled “pwr top” and “aux top” at the top end of each section  10  and a second pair of terminals labeled “pwr bot” and “aux bot” at the bottom end of each section. The sections  10  are connected in tandem to form a single stack connected together in electrical series as will be evident from FIGS. 2 through 5 which will be explained in more detail hereafter. Each of the sections  10  receive auxiliary power from another auxiliary transformer (not shown) powered from a conventional line supply which is connected to the pair of terminal connections marked “pwr top” and “aux top” at the top end of the stack. Power is transferred through each section of the stack from the upper terminals of each section to the lower pair of terminals “pwr bot” and “aux bot” from whence power is transferred to the upper terminal “pwr top” and “aux top” in the next adjacent section in sequence. Control signals are fed to each section  10  from a common main controller  12 . The connection from the main controller  12  to each section  10  may be via a bi-directional fiber optic link using duplex fiber optic cables  14  as shown in FIG. 1 or by means of a single bi-directional cable and multiplexor (not shown). The main controller  12  transmits gating signals to the sections  10  to turn them on and off and in the opposite direction receives signals from the respective sections  10 .  
         [0018]    The details of each section  10  in the multiple stack is shown in FIGS. 2 through 6 respectively. As indicated in FIG. 2 the section  10  has four parts inclusive of parts  1 ,  2 ,  3  and  4  respectively. Part  1  is a thyristor switching circuit containing power SCR&#39;s and voltage sharing and voltage limiting components such as resistive-capacitive snubbers and may also include varistors. The thyristor switching circuit may further include commutation means for bi-directional flow and means to achieve forced commutation. Part  2  includes an auxiliary power distribution transformer T. Part  3  is a control circuit  15  which, in turn, may be controlled from the main controller  12 . Part  4  is power supply  16  of conventional design. The use of a control circuit  15  for supplying gate signals to fire the power SCR&#39;s is of itself conventional and does not need an independent description for one skilled in the art to understand. The auxiliary power distribution transformer T has three windings A, B and C with the two windings A and B connected to the “pwr top” and “pwr bot” terminals and the third winding C connected to the power supply  16 . The third winding C isolates the power supply  16  and control circuit  15  in the section  10  in which they are connected. The first and second windings A and B of the auxiliary power transformer T interconnect the input and output terminals of a section  10 , i.e., interconnects the opposite terminals of one section  10  to the respective output and input terminals in each adjacent section of the stack such that the maximum voltage difference between the first and second transformer windings A and B is equal to the maximum voltage across the section  10 . In this way power is transferred through the sections of the stack and the voltage across the auxiliary transformer T is limited by the voltage across each section.  
         [0019]    One preferred thyristor switching circuit arrangement for section  10  for use in an HVDC (high voltage direct current) application or for a converter DC/AC application is shown in FIG. 3. In this current arrangement the thyristor switching circuit includes three SCR&#39;s  18  connected to one another in series, an auxiliary power transformer T, an AC/DC power supply  16  and a control circuit  15 . Each SCR  18  is connected to a voltage limiting RC snubber  19 , a firing pulse transformer assembly  20  and a break over diode  21  (“BOD” ). The firing pulse transformer assembly  20  and break over diode  21  are connected in series across the cathode to anode terminals of the SCR  18  with the pulse transformer  20  also connected to the gate terminal of the SCR  18 . Should the voltage across an SCR  18  exceed a safe level this causes the appropriate BOD diode  21  to turn on which fires the SCR  18  in series therewith. Once fired the pulse transformer assembly  20  to which the SCR  18  is connected is bypassed. The pulse transformer assembly  20  itself is conventional and includes a firing pulse transformer, diode, resistor, etc. (not shown) as is well known to those skilled in the art. The firing of the SCR  18  in this manner saves it from destructive voltage breakdown.  
         [0020]    It is preferred to connect the control circuit  15  to a common point  22  representing the cathode of the middle SCR  18 . In this way the voltage between the control circuit  15  and any of the gates of the SCR&#39;s  18  is limited by the voltage across one SCR  18  which also limits the voltage across the pulse transformer assembly  20 . Since the section  10  is free floating, i.e., not connected to ground potential and the common point  22  is tied to the cathode of the middle SCR  18 , the control circuit  15  is at the same potential permitting a temperature sensor such as an RTD (not shown) to be used to monitor the temperature of the SCR  18 . The voltages across each of the SCR&#39;s  18  in the section  10  can all be monitored using resistive dividers (not shown) to detect a short circuit. This can be directly reported to the main controller  12  through the control circuit  15 . Moreover, the primary voltage across each gate terminal to the pulse transformer  20  can also be monitored to detect an open gate condition and this information can likewise be transmitted to the main controller  12  through the control circuit  15 .  
         [0021]    [0021]FIG. 4 is a circuit diagram of a single section  10  in which the thyristor switch is a bi-directional valve including a first series pair of SCR&#39;s  30  and  31  connected in an anti-parallel arrangement with a second series of SCR&#39;s  32  and  33  to allow current to pass in both directions. Two pairs of series connected SCR&#39;s are used in this circuit instead of the three series connected SCR arrangement of FIG. 3 to limit the voltage across the pulse transformer  20  to the peak voltage across the SCR to which that pulse transformer  20  is connected. The common point  35  of this circuit is located between each of the two pair of series connected SCR&#39;s  30 ,  31  and  32  and  33  respectively. A snubber circuit  19  composed of a resistor R and capacitor C (similar to FIG. 3) is connected from the common point  35  to the “pwr top” and to the “pwr bot” terminals at each opposite end of the section  10 . The auxiliary transformer T, the AC/DC power supply  16  and the control circuit  15  is identical to the arrangement in FIG. 3.  
         [0022]    The arrangement of FIG. 4 is shown modified in FIG. 5 to achieve forced commutation of the main thyristor SCR&#39;s TH 1 -TH 4 . For simplicity all of the components of FIG. 4 which are equivalent to their counterpart in FIG. 5 are identified with the same reference numbers except for the main and commutating thyristors which are labeled with the prefix “TH”. For further clarity only one thyristor TH 3  is shown with its gate terminal connected to a pulse transformer assembly  20  and BOD diode  21  with the understanding that each of the other thyristor&#39;s TH 1 , TH 2  TH 4  and TH 12  also have their gate terminals connected to a pulse transformer assembly  20  which, in turn, is in series with BOD diode  21  in an arrangement equivalent to that of FIG. 3. Each pulse transformer assembly  20  in FIG. 5 is connected to a gate drive circuit terminal GT 3  of the gate drive circuit  15  which, in turn, receives on/off signals from the main controller  12 .  
         [0023]    To achieve forced commutation in the circuit arrangement of FIG. 5 additional thyristors TH 5  through TH  10  are used as well as commutating capacitors C 1  and C 2  and commutating inductances L 1  and L 2 . The commutating capacitors C 1  and C 2  may be recharged through a recharging circuit which includes additional thyristors TH 11  and TH  12  or the pre-charge circuits  40  and  41 . In either case the recharging circuit includes the voltage limiting components R 1 -R 6 , capacitors C 3 -C 8  and varistor RV 1 . The methodology for recharging the commutating capacitors C 1  and C 2  through the semiconductor SCR&#39;s TH 11  and TH 12  or by means of the pre-charge circuits  40  and  41  will be explained in greater detail hereinafter.  
         [0024]    The main current in FIG. 5 is carried by the main SCR&#39;s TH 1 -TH 4 , while the capacitors C 1  and C 2  are charged with the polarity as shown. Should the current exceed a preset limit (or di/dt is exceeded) the main controller  12  will, in turn, issue a turn-OFF command generating gate signals which fire the commutating SCR&#39;s. Depending on the direction of current flow through the main SCR&#39;S. TH 1 -TH 4  either. TH 5 , TH 7 , TH 10 , TH 9  (for the positive main current, i.e., through TH 1  and TH 3 ) or through TH 6 , TH 7 , TH 10 , TH 8  (for the negative main current flow i.e., through TH 2  and TH 4 ).  
         [0025]    The circuit can operate without the commutating SCR&#39;s TH 7  and TH 10 , but then the voltage applied to the commutating SCR&#39;s TH 5 , TH 6 , TH 7 , TH 8  will be almost twice the voltage across the main SCR&#39;s (the main SCR&#39;s voltage plus a charged capacitor C! or C 2  voltage). This would necessitate the series connection of two devices for all commutation SCR&#39;s bringing the total number of them to 8. By using two additional SCR&#39;s TH 7  and TH 10  we reduce the total number of commutating SCR&#39;s from 8 to 6 with related cost and weight/size savings. These additional SCR&#39;s are fired every time we need to force commutate the current, regardless of current direction.  
         [0026]    If we assume that the main current is positive, i.e., the SCR&#39;s TH 1  and TH 3  are conducting then FIG. 5 provides two commutation SCR discharging circuits inclusive of a first discharging circuit including elements TH 5 , TH 7 , TH 11 , L 1 , C 1  and a second discharging circuit including elements TH 10 , TH 9 , TH 3 , C 2 , L 2 . The capacitors C 1  and C 2  discharge through these two circuits. When capacitor currents reach the level of load current (current flowing through the main SCR&#39;s TH 1  and TH 3 ), the current through the SCR&#39;s TH 1  and TH 3  goes to zero and they are turned off. This switches the load current into the commutation circuit formed from elements TH 5 , TH 7 , L 1 , Cl, C 2 , L 2 , TH 10  and TH 9 . The current flow causes the voltage across the capacitors to go to zero and then recharge them with the opposite polarity. When the voltage across the capacitors reach the breakdown voltage of the varistor RV 1 , the current switches into the varistor RV 1 . At that point the current through all of the commutating SCR&#39;s TH 5 -TH 10  goes to zero and they are turned off. The load current now flows only through the varistor RV 1  .Once the energy is dissipated in the varistor the current decays to zero. At this point the turn-off process is completed but for the need to change the voltage polarity across the capacitors C 1  and C 2 . This can be done using either the precharge circuits  40  and  41  or the SCR&#39;s TH 11  and TH 12 . The disadvantages of using the precharge circuits  40  and  41  is a long time delay (of at least a few seconds) and power dissipation in the auxiliary power circuit. By using the SCR&#39;s TH 11  and TH 12 , the capacitors C 1  and C 2  will recharge in a few hundred microseconds with virtually no energy dissipation. Once TH 11  and TH 12  are fired there are two recharge circuit configurations with the first inclusive of elements TH 1 , C 1  and L 1  and the second inclusive of elements TH 12 , L 2  and C 2  respectively. Following discharge the capacitors C 1  and C 2  will then recharge with opposite polarity.  
         [0027]    [0027]FIG. 6A shows the details of one embodiment of each of the precharge circuits  40  and  41 . Capacitor C is one of the capacitors C 1  and C 2  from FIG. 5. As shown in FIG. 6 these capacitors charge through the transformer TR and a diode bridge D 1 -D 4 . An optional inductor L is used to limit charging current to avoid collapsing the voltage on the auxiliary power distribution chain although its function can be performed alternatively by the leakage inductance of the transformer TR&#39;s. Resistor R is needed to protect current flowing through the diodes D 1 -D 4  when the capacitor voltage changes the polarity during commutation cycle; it is also can be used to limit the charging current instead of (or combined with ) inductance L. The primary winding of the transformer TR is connected to the auxiliary power source, the third winding of the transformer T from FIG. 5.  
         [0028]    [0028]FIG. 6B shows a circuit variation of FIG. 6A with additional antiparallel connected SCR&#39;s TH 13  and TH 14  used on the primary winding side of transformer TR. By gating the SCR&#39;s TH 13  and TH 14  the charging process can be stopped when the capacitor voltage reaches a predefined value. In this way the capacitor voltage is independent of auxiliary voltage level and the charging cycle does not have a long tail.  
         [0029]    FIGS.  7 A- 7 C show the details for auxiliary power distribution through the stack of multiple sections  10 . FIG. 7A shows the simplest method of auxiliary power distribution in which the auxiliary power is fed through high voltage transformer TRAUX at one end of the sections&#39;string. This has the disadvantage in that power has to go through many sections in series suffering a substantial voltage drop due to section&#39;s auxiliary transformer internal impedances. In FIG. 7B the voltage drop is reduced by one half by feeding the power at the center of the string. In FIG. 7C two feed inputs at ¼ and ¾ positions along the string are used to reduce the voltage drop by four times.  
         [0030]    Under normal conditions the circuit of FIG. 5 operates as a thryristor switch with main SCR&#39;s TH 1 -TH 4 . They are turned on with gradually decreasing firing angle to:  
         [0031]    1. reduce switching transients (“soft start”), and  
         [0032]    2. avoid turning into the fault by monitoring the current during the soft start process.  
         [0033]    Once the circuit is successfully turned on the main SCR&#39;s conduct during the whole period until a fault condition is experienced or the normal turn-off command is received. If a normal turn-off command is received the firing angle of the main SCR&#39;s can be increased gradually to reduce the voltage (“soft stop”) and then the pulses are removed, or the pulses can be removed immediately without changing the firing angle.  
         [0034]    If the fault condition is detected (high current and/or high di/dt), gating signals fire the commutating SCR&#39;s the gating signals are removed from the main SCR&#39;s TH!-TH 4  and the circuit goes through the current turn-off process as explained above .  
         [0035]    If a fault interruption occurs in a high voltage electrical energy transmission system including the switching circuit of the present invention arrangement of FIG. 5 permits the main SCR&#39;s TH 1 -TH 4  to be fired so as to generate a programmed number of high current bidirectional pulses to allow for “downstream protection coordination” which allows to isolate the fault. Moreover, the amplitude of the pulses may be regulated by changing the firing angle of the main SCR&#39;s TH 1 -TH 4  similar to the soft start as described above so that the let through current is limited to a predetermined maximum value which will protect against the damaging of circuit breakers and other circuit elements. This can be accomplished by measuring the let through current (using the controller to monitor current) at different firing angles to achieve the desired current amplitude regardless of fault impedance. In this way both current limiting is achieved with the benefit of corresponding reduction in equipment stresses, improved service life as well as downstream protection coordination.