Power receiving system

Abstract: In a power receiving system, a network transformer (7), a protector breaker (10) and a current detector (9) are connected between the load side of each of a plurality of network lines (1A, 1B, 1C) and a network bus (2), a breaker (13) is connected to the primary side of the network transformer, the current detector being operative to detect an accident current (2i) on the secondary side of the network transformer, and relays (14, 15) or an operation controller (20) is provided which is responsive to the detection signal to open the breaker and protector breaker to switch off accident currents (1i, 2i) on the primary and secondary sides of the network transformer, whereby the protective range is so expanded as to cover a line section between the network transformer and the protector breaker and the scale of the overall power receiving system is reduced.

Description: BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates to a power receiving system which makes improvements especially on a line section between a network transformer and a protector breaker. DESCRIPTION OF THE PRIOR ART In high-rise buildings, the load distribution of power increases in the vertical direction and the load capacity is very large. Further, publicly important facilities are accommodated in these buildings and reliability and safety of power service are strongly asked for. The high-rise buildings are often built in a crowded area in cities and therefore the installation area is desired to be as small as possible. For a place in which a large load is concentrated to one location, in general, a network power receiving system having high reliability of power service is considered to be the most suited. In a network power receiving system in which power from a power source substation is received through a 22 KV or 33 KV distribution network of two to four lines, there are provided on the primary side of each network transformer a power receiving breaker and on the secondary side a fuse and a protector breaker wherein the protector breakers have their load sides connected in parallel to a network bus, and power is fed to various loads through several trunk lines. With the above network power receiving system, in the event that a short-circuiting accident occurs on, for example, the primary side or secondary side of a network transformer, an accident circuit is disconnected and power is fed through a sound line to prevent the load from suffering power failure. Therefore/this type of system ensures the high reliability of power service which meets customers' needs and, recently, has been in high demand. In the network power receiving system as exemplified in Fig. 2 of US patent 4,873,601, however, in the event that an accident takes place on the secondary side of the network transformer, the network fuse is fused to protect only a network line section between the network fuse and the protector breaker, giving rise to disadvantages that the protective range is narrow, a network fuse dedicated to high voltage must be prepared and the scale of the network power receiving equipment is increased. SUMMARY OF THE INVENTION An object of the invention is to provide a power receiving system in which the protective range is so expanded as to cover a line section between the network transformer and the protector breaker and in which the scale of the network power receiving equipment is reduced. This object is met by a power receiving system according to claim 1. Preferred embodiments are disclosed in the depending claims. In an embodiment of a power receiving system according to the invention, a network transformer, a protector breaker and a current transformer are connected in each of a plurality of network lines between respective power source busses and a network bus, a brealer is connected to the primary side of the network transformer, and an accident current on the secondary side of the network transformer is detected by the current transformer to provide a detection current which energizes a relay to cause it to open the breaker and protector breaker so as to switch off current on the primary side of the network transformer, whereby the breaker used can be of low interruption capacity to reduce the scale of the network power receiving equipment and the protective range can be expanded to cover a line section between the network transformer and protector breaker. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a skeleton connection diagram of a network power receiving system for high voltage according to an embodiment of the invention. Fig. 2 is a skeleton connection diagram of a network power receiving system for high voltage according to another embodiment of the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the invention will now be described by referring to a network power receiving system for high voltage as shown in Fig. 1. Referring to Fig. 1, three network lines 1A, 1B and 1C are connected to a network bus 2. Connected to the network bus 2 are load cables 5 each connected with a take-off breaker 3 and a circuit breaker 4. The take-off breakers 3, circuit breakers 4 and load cables 5 are housed in a take-off switch gear (for example, a power-board) 6. Each of the network lines 1A, 1B and 1C is connected with a network transformer 7. Capacity and % impedance of the network transformer 7 are 2000KVA and 7.5%. In the network transformer 7, a primary voltage of about 22000V and a primary current of about 52A are transformed into a secondary voltage of about 415V and a secondary current of about 2800A. In each of the network lines 1A, 1B and 1C, a current transformer 9 and a protector breaker 10 are connected between the secondary side of network transformer 7 and the network bus 2. Connected in each of the network lines 1A, 1B and 1C, between the primary side of the network transformer 7 and respective power source buses 11A, 11B and 11C are a load switch 12 and a vacuum breaker 13. Each of the power source buses 11A, 11B and 11C is connected with a power source side breaker 11. The load switch 12 operatively switches off voltage (about 22000V) and small current (52A) on the primary side in each network line 1A, 1B or 1C. The vacuum breaker 13 is operatively opened by an accident current flowing in each network line 1A, 1B or 1C between the secondary side of network transformer 7 and the protector breaker 10 close to the network bus 2. Connected to the secondary side of the current transformer 9 are first and second relays 14 and 15. The first relay 14 is connected to the protector breaker 10 and vacuum breaker 13. The second relay 15 is connected to the protector breaker 10. The first relay may be energized by a setting value corresponding to, for example, about 50000A. The second relay 15 may be energized by a setting value corresponding to, for example, about 20000A. The first and second relays 14 and 15 are activated by counter-flow currents. In other words, the second relay 15 is activated by a smaller accident current than that for the first relay 14. A protector switch gear board (for example, a power-board) 16 houses the electrical components, for example, the protector breaker 10, load switch 12, vacuum breaker 13 and so on connected in each of the network lines 1A, 1B and 1C between the load switch 12 and network bus 2. The load switch 12 may be replaced by the vacuum breaker 13 combined with a breaker. The protector switch gear board 16 is also provided for each of the network lines 1B and 1C but is omitted in the drawing. An operation controller 20 as shown in Fig. 2 may substitute for the relays 14 and 15. Operation and effect of the network power receiving system according to the invention will now be described. (1) In the event that an accident takes place at an accident point X1 on the secondary side of the network transformer 7 in the network line 1B, first and second accident currents li and 2i flow to the accident point X1. The first accident current li is a forward current which flows from the power source bus 11B to the network transformer 7. When neglecting other impedance than that of the network transformer 7, the current value of the first accident current li can be expressed by the following equation: li= capacity (KVA) of network transformer 7/√3 × primary voltage (KV) of network transformer 7 × % impedance value (%) = 2000(KVA)/√3 × 22(KV) × 7.5(%) = 699.83 ≒ 700(A) The second accident current 2i is a current which flows in opposition to the first accident current li. The second accident current 2i is a resultant current of currents from the network lines 1A and 1C which join in the network line 1B and therefore it is of a far larger current value than that of the first accident current li. Given that the network lines 1A and 1C are under the same condition, the second accident current 2i is twice as large as the current in each of the networks 1A and 1C and can be expressed by the following equation: 2i= (capacity (KVA) of network transformer 7 in 1A × 2)/(√3 × secondary voltage (KV) of network transformer 7 × % impedance value (%)) = (20000(KVA) × 2)/(√3 × 0.415(KV) × 7.5(%)) = 74199.8 ≒ 74200(A) When this counter flow of the second accident current 2i of 74199.8 ≒ 74200 (A) flows, the first relay 14 (corresponding to 50000 A) and the second relay 15 (corresponding to 20000 A) are energized and an output signal from the relay 14 is applied to the vacuum breaker 13 and protector breaker 10 with an output signal from the second relay 15 applied to the protector breaker 10, so that the vacuum breaker 13 and protector breaker 10 are opened to interrupt the first and second accident currents li and 2i. After interrupting the first accident current li, the vacuum breaker 13 opens the load switch 12 to prevent application of the circuit voltage to the vacuum breaker 13 during a long interval of time. Accordingly, a line section between the network transformer 7 and the protector breaker 10 can be protected to extend the protective range as compared to the conventional protective range by which the line section between the fuse and the protector breaker can be protected. In addition, since the vacuum breaker 13 can equivalently switch off the current on the secondary side of the network transformer 7 by switching off the current (700A) on the primary side of network transformer 7 smaller than the secondary side current, a breaker of low interruption capacity which is compact and inexpensive can be used as the vacuum breaker 13, thus attaining scale reduction and cost reduction of the network power receiving equipment. Similar effects can also be attained by applying the teachings of the invention to a circuit in which a vacuum breaker and a current transformer are respectively connected in the primary and secondary side circuits of a transformer, an accident current in the secondary side circuit is detected by the current transformer and a relay is energized by a detected current to open the vacuum breaker. (2) On the other hand, in the event that an accident takes place at an accident point X2 on the primary side of the network transformer 7 in the network line 1B, third and fourth accident currents 3i and 4i flow to the accident point X2. The third accident current 3i activates the power source side breaker 11 and is consequently interrupted. The third accident current 3i is determined depending on the power source capacity of the power source substation and is a very large current. The fourth accident current 4i is a current in opposition to the third accident current 3i and is different from the third accident current 3i in that it is a resultant current of currents from other network lines than the accident network line 1B, that is, the network lines 1A and 1C, and joining in the network line 1B. The fourth accident current counter flows to the accident point X2 through the network transformer 7 connected in the network line 1B. Through calculation, this fourth accident current 4i can be expressed by the following equation: 4i= (capacity (KVA) of network transformer 7 in 1B × 2)/[√3 × secondary side voltage (KV) of network transformer 7 × {(1+1/2) × % impedance value (%)}] = 20000 (KVA)/(√3 × 0.415 (KV) × (1+1/2) × 7.5(%)) = 24733 = 24700 (A) where in the coefficient (1+1/2) of the % impedance value (%), 1 is an impedance value of the network line 1B and 1/2 is an impedance value of the respective network lines 1A and 1C, from which currents flow and joint in the network line 1B, When the fourth accident current 4i of 24700 (A) counter flows to the accident point X2, the first relay 14 (corresponding to 50000 A) is not energized but the second relay 15 (corresponding to 20000 A) is energized, so that the protector breaker 10 is opened to interrupt the fourth accident current 4i of 24700 (A). The third accident current 3i flows to the load switch 12 and vacuum breaker 13 before the power source side breaker 11 is activated. Since the first relay 14 is not energized, the vacuum breaker 13 does not interrupt the primary current of the network transformer. Accordingly, the vacuum breaker 13 used can be of low interruption capacity and low voltage and can be compact and inexpensive. (3) The operation controller 20 shown in Fig. 2 can substitute for the first and second relays 14 and 15 to reduce the scale of the network power receiving system. An accident current 5i flowing to an initial accident point X3 owing to, for example, a layer short caused in the secondary coil of the network transformer connected in the network line 1B is smaller than a normal current 6i flowing in each of the other network lines 1A and 1C. The normal current 6i and accident current 5i detected by the current transformers 9 are applied to the operation controller 20 to cause it to determine that a current smaller than the normal current 6i is the accident current to produce decision output signals by which the protector breaker 10 in the network line 1B is interrupted and thereafter the load switch 12 is opened, thereby preventing the network transformer 7 from being burnt. (4) A delay device 17 is additionally provided between the vacuum breaker 13 and load switch 12 in order that the load switch 12 can be opened by means of the delay device 17 after the vacuum breaker 13 is opened. Thus, the accident current on the secondary side of the network transformer 7 is detected by the current transformer 9 associated therewith and then interrupted by the vacuum breaker 13. Accordingly, the vacuum breaker 13 operatively interrupts only the accident current on the primary side, and a small current to the vacuum breaker 13 and voltage of the network line are switched off by the load switch 12, thereby ensuring that a vacuum contactor for simply switching off the small current can be used as the vacuum breaker 13 and in addition the scale reduction and cost reduction of the network power receiving equipment can be attained. In an alternative, a breaker is provided on the primary side of a network transformer and a current transformer is provided on the secondary side so that after detection of an accident current by the current transformer, the breaker may be opened. As described above, according to the invention, an accident current on the secondary side of the transformer is detected by the current detector and the breaker on the primary side of the transformer is opened by a detection signal. As a result, the breaker is required to interrupt only the primary side current smaller than the secondary side current and therefore this breaker can be of low interruption capacity to reduce the scale of the power receiving equipment and to expand the protective range such that it covers a line section between the transformer and the breaker.

Claims: A power receiving system comprising at least a network transformer (7) connected in a network line (1A, 1B, 1C), current detecting means (9) connected to the secondary side of said network transformer, controlling means (14, 15; 20) responsive to a detection signal of said current detecting means which is representative of an accident current on the secondary side of said network transformer, first breaking means (12, 13) connected to the primary side of said network transformer, and second breaking means (10) connected to the secondary side of said network transformer (7), said controlling means (14, 15 ; 20) being connected to control opening of said second breaking means (10) in accordance with said detection signal, characterized in that : the interruption capacity of said first breaking means is smaller than that of said second breaking means (12, 13), and said controlling means (14, 15; 20) is also connected to control opening of said first breaking means (12, 13) in accordance with said detection signal. The system according to claim 1, wherein the secondary side of said network transformer (7) is connected by said network line (1A, 1B, 1C) to a network bus (2). The system according to claim 1 or 2, wherein said controlling means includes first and second relays (14, 15) or an operation controller (20). The system according to any of claims 1 to 3, wherein said first breaking means includes a load switch (12) and/or a vacuum breaker (13). The system according to claim 4, further comprising a delay device (17) connected to said load switch (12) and said vacuum breaker (13) to open said load switch later than said vacuum breaker. The system according to any of claims 1 to 5, wherein said second breaking means includes a protector breaker (10). The system according any of claims 1 to 6, wherein said current detecting means includes at least a current transformer (9). The system according to any of claims 1 to 6, wherein said controlling means (14, 15; 20) opens said second breaking means (10) in accordance with a detection signal, which is representative of an accident current caused in said network transformer (7) and being smaller than a normal current flowing in a network line.

Inventors: IZUMI TOSHIAKI, SUGITANI SINZI, TAKEMATSU KOUTAROU, TANIMIZU TORU, WAKASA FUMIO, IZUMI, TOSHIAKI, SUGITANI, SINZI, TAKEMATSU, KOUTAROU, TANIMIZU, TORU, WAKASA, FUMIO
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1991
Main classification: H02H3
IPC codes: H02H3, H02H7