CIRCUIT FOR STABILIZING AN AC POWER LINE

Abstract: The circuit consists of an oscillatory circuit (1) and a link element (2). The oscillatory circuit consists of a precharged capacitance (3), and inductance (4) and a switching element (5). When activated the oscillatory circuit generates positive and negative current half-cycles, each one triggered in synchronism to the line (P1, P2). Capacitance and inductance are dimensioned such that the period of the generated current half-cycles is less than or equal to the half-cycle period of the line. The oscillating voltage across the capacitance is connected to the line by the link element. The stabilizing current characteristics can be varied over a wide range.

Description: CIRCUIT FOR STABILIZING AN AC POWER LINE Technical Field The invention relates to a circuit for stabilizing an ac power line. One or several of these circuits are connected in parallel to the line. The circuit is activated if one or s more line parameters, especially the line voltage, have left or will predictably leave preset tolerance ranges. This is for example the case when loads with inrush currents are connected to the line, while clearing short circuits in the iine by circuit breakers or fuses, or during malfunctioning of power transmission devices. Weak lines are especially endangered. These kind of lines are for example lines involving long interconnections, or converter-fed lines, where in case of overload the converter disconnects automatically from the line. Background Art Devices for dynamically stabilizing ac power lines are known. As an example, rotating var-compensators are acting this way. In case of sudden change of the line parameters these are capable of exchanging large quantities of real and reactive power with the line and thereby compensating the disturbance. The disadvantage of rotating var-compensators lies in the large investment and maintenance costs, also their permanent losses are not neglectable. Static var-compensators, consisting of a capacitance and a parallel phase-controlled inductance, have too little stored energy for the given task and in addition inject harmonic currents in the line. Also known is converter-fed line-stabilization by switched, or for lower power linear controlled, semiconductor arrangements which for example are supplied by a capacitance- supported voltage source. As a disadvantage, their design for the required dynamic stabilizing power, which can go up to 30 times rated line power, would be extremely expensive. Moreover, switched semiconductor arrangements produce harmonics. Disclosure of Invention In the view of the foregoing it is the object of the invention to find a simple and robust circuit which is primarily used for dynamic line stabilization. The circuit according to the present invention consists of an oscillatory circuit and a link element. Current half-cycles are generated in the oscillatory circuit by a dc- precharged capacitance, an oscillatory inductance and a switching element. Each RECTIFIED SHEET (RULE 91) current half-cycle is individually triggered in synchronism to the line Capacitance and inductance are thus dimensioned that the period of the current half-cycles is less than or equal to the line half-cycie period, the oscillatory circuit is not mandatoπly tuned to line frequency Switching the line to this oscillatory circuit by the link element allows branching of stabilizing current to the line The coupling to the oscillatory circuit may be galvanically to the capacitance or inductively to the oscillatory inductance Triggering of the current half-cycles is done such that the voltage of the capacitance is approximately in-phase to the line voltage The link element can be built-up in bridge connection The circuit according to the invention has the advantage of a similar behaviour as a rotating var-compensator Stabilizing initial current and decay can be set over a wide range The circuit can also be used for damping of harmonics and oscillations in the line The circuit is of simple and robust design, there are no standby losses and while activated the circuit produces small harmonics It can be used for lines in the range of some few kVA up to some thousand MVA It profits from the steady development of capacitors towards more capacitance per volume Brief Description of Drawings Fig 1 shows the basic stabilizing circuit of the invention Fig 2 shows an embodiment of the switching elements of the circuit Fig 3 shows the principal electrical operation of the circuit Fig 4 shows an embodiment of the circuit comprising galvanic coupling to the capacitance Fig 5 shows an embodiment of the circuit comprising inductive coupling to the oscillatory inductance Fig 6 shows the connection to the line using a bridge connection in the link element Fig 7 shows the connection to a three-phase line using a bridge connection Fig 8 shows three circuits connected in delta to a three-phase line Fig 9 shows an embodiment of the capacitance by using polarized capacitors Fig 10 shows the use of elements of a pwm-converter for the circuit according to the invention RECTIFIED SHEET (RULE 91) Detaiiled Description and Preferred Embodiments Fig 1 shows the basic circuit of the invention The circuit consists of an oscillatory circuit 1 and a coupling of the line Pi, P 2 to the oscillatory circuit by means of the link element 2 The oscillatory circuit 1 consists of the capacitance 3, the oscillatory inductance 4 and the switching element 5 The switching element consists of two parallel switching branches 5a, 5b, each being capable of conducting in one direction and which are connected with opposite polarization The capacitance is held in precharged state in standby by a not shown charging device All the elements of the oscillatory circuit are connected such that they form a closed current loop In activated state the oscillatory circuit 1 generates current half-cycles The switching branches 5a, 5b of the switching element 5 are triggered such that positive and negative current half-cycles are generated in synchronism to the line By galvanic or inductive coupling of the line to the oscillatory circuit 1 stabilizing current is fed to the line via the link element 2 In activated state of the circuit the link element can be switched in synchronism to the line, if needed phase-controlled, but preferably it remains closed in both polarities while the circuit is being activated A simple embodiment of the switching element 5 is shown in Fig 2 This consists of two thyπstors 50a, 50b connected in anti-parallel The switch of the link element 2 can also consist of two anti-parallel thynstors 20a, 20b For further widening the range of dimensioning a link inductance 6 can be inserted in the link to the line The principal electrical operation is shown in Fig 3 The capacitance 1 is kept at a charging voltage V 0 This corresponds at least to the crest value of the rated line voltage Activating of the circuit is started by closing one of the switching branches, for example 5a, at the time ti, whereby the capacitance 3 starts a half-cycle swing with the oscillatory inductance 4 and the not shown line inductance This half-cycle swing produces a half-cycle current I in the capacitance The ratio of the inductances and the development of the disturbed line then determine the stabilizing current Is into the line Preferentially the oscillatory inductance is in the range of 25 100% of the sum of link inductance and line inductance At the end of the half-cycle swing the chosen switching branch 5a opens automatically or by being actively switched off By dimensioning of capacitance and inductances and taking into account the line RECTIFIED SHEET (RULE 91) inductance the period T LC of the oscillation is kept less than or equal to the period T N of the line. Therefore: T N > T LC = 2 CC where T L c period of oscillation T N period of line C capacitance L composition of oscillatory-, link- and line-inductance and: l max V 0 / vl7 C where l max maximum oscillatory current in first half-cycle Vo dc charging-voltage of capacitance It is essential that the circuit consisting of capacitance 3, oscillatory inductance 4, link inductance 6 and line inductance must not be tuned to line frequency. At the time ti + half line period the anti-parallel switching branch, for example 5b is closed, whereby the inverse polarized half-cycie is produced, and so on. The decay of the generated stabilizing current is determined by the losses of the circuit and the supply of real power into the line. The activated oscillatory circuit is switched such that the capacitance voltage is approximately in-phase with the line voltage. The triggering pulses for the switching branches 5a, 5b can be produced by an auxiliary circuit, generating an ideal line-voltage image. Such an image is realised in known circuits by use of phase-locked loop devices. Fig. 4 shows the embodiment of the circuit with galvanic coupling of the line to the capacitance 3. Fig. 5 shows an inductive coupling of the line to the oscillatory circuit. This is done by coupling to the oscillatory inductance 4 which is used as part of a transformer or autotransformer. The charging voltage of the capacitance 3 has to be adapted to the turns ratio. Because the capacitance 3 is precharged with a fixed polarity a period of up to 360° can elapse from starting of disturbance until first switching of the circuit. By using a bridge connection in the link element 2 as shown in Fig. 6, the maximum delay can be reduced to 180°. The branches of the bridge consist of the above mentioned RECTIFIED SHEET (RULE 91) switching branches, represented here by thyristor pairs 20a, 20b. During an activation of the circuit two diagonally arranged branches of the bridge stay closed. Fig. 7 shows the embodiment of the link element 2 as three-phase bridge for the connection to a three-phase line Pi, P 2 , P 3 . Thereby one circuit can selectively stabilize one of the three phases. For a simultaneous stabilizing of all three phases of a three-phase line an arrangement of three circuits according to Fig. 8 is needed. An arrangement in delta is shown, an arrangement in wye is also possible whereby the neutral line conductor can be involved. The width of the generated current half-cycles can be self-adapted during operation by means of tappings in the inductances, thereby achieving half-cycles as wide as possible. The activating of the circuit can be triggered by the decay of the grid voltage beyond a preset value, preferentially combined with a minimum timing threshold. It is also possible to start triggering if the value of the difference between line voltage and ideal line-voltage image exceeds a preset value, preferentially combined with a minimum timing threshold. As an alternative use, the circuit can be activated before an expected line disturbance. The circuit losses are then automatically supplied from the line. By proper dimensioning the circuit can be used in steady-state operation as harmonics filter and voltage-dip filter or, by slightly phase-shifting the triggering pulses to the line voltage, as low harmonics static var-compensator. The circuit is disconnected from line by one or several of the following processes: -voltage of capacitance 3 decays beyond preset value -voltage of capacitance 3 increases -line voltage increases -current in the link to the line falls beyond preset value -real power supplied in the line becomes negative. RECTIFIED SHEET (RULE 91) For highest ratings the switching elements 2, 5 are preferentially equipped with elements of the type silicon carbide (SiC). Selected components of the circuit may be kept in a superconducting state. Fig. 9 shows the use of polarized capacitors as elements of the capacitance 3. The capacitance is composed of series-connected polarized capacitors 3a, 3b which are arranged in same amount in each polarity and which are protected in each polarity by at least one parallel diode 30a, 30b. Fig. 10 shows the use of elements of a pwm-converter for the stabilizing circuit. In the shown example a part of the dc link capacitance is used as capacitance 3 of the circuit. A decoupling element 8 contains a thyristor or similar device 8a, a diode 8b and a resistor 8c. When detecting irregular line parameters the converter disconnects from the line, at the same time thyristor 8a is deactivated, which allows activating of the oscillatory circuit 1. The stabilizing current is produced by inductive coupling to the oscillatory inductance 4 and feeding via the switching link element 2 to the line connections of the converter Pi, P 2 . This allows to use at least part of the dc-link energy for the injection of a stabilizing current in the disturbed line. The resistor 8c serves for voltage equilibration of the dc-link capacitors before reclosing the thyristor 8a. RECTIFIED SHEET (RULE 91)

Claims: Claims 1. Circuit for stabilizing the voltage of an ac power line, characterized by an oscillatory circuit (1) comprising a precharged capacitance (3), an oscillatory inductance (4) and a switching element (5), said oscillatory circuit generating current half-cycles, each one triggered in synchronism to the line and said oscillatory circuit being connected to the line by a link element (2). 2. Circuit according to claim 1, characterized by a connection of the line to the capacitance (3) of the oscillatory circuit. 3. Circuit according to claim 1, characterized in that the connection to the line comprises a link inductance (6). 4. Circuit according to claim 1 , characterized by a transformer or autotransformer connection of the line to the oscillatory inductance (4). 5. Circuit according to claim 1 , characterized in that the capacitance (3) is precharged at least to a charging voltage corresponding to the crest voltage of the rated line voltage. 6. Circuit according to claim 1 , characterized in that the width of the generated current half-cycles is less or equal half line period, as governed by the values of capacitance and inductances. 7. Circuit according to claim 6, characterized in that the width of the current half- cycles is automatically controlled in operation by using tapping in the inductances. 8. Circuit according to claim 1, characterized by an embodiment of the switching elements (2, 5) as thyristors connected in anti-parallel. 9. Circuit according to claim 1 , characterized in that the link element (2) is composed in bridge-connection. RECTIFIED SHEET (RULE 91) ISA/EP 10. Circuit according to claim 1 , characterized by three identical circuits being connected in delta or wye to a three-phase line. 11. Circuit according to claim 1 , characterized by the generation of the triggering pulses for the switching element (5) in an ideal line-voltage image. 12. Circuit according to claim 1 , characterized in that during the stabilizing period the switching element (5) is actuated by pulses such that the generated voltage half- cycles at the capacitance (3) are in-phase to the line-voltage and that the link element (2) is in closed state during the stabilizing period. 13. Circuit according to claim 1 , characterized in that the closing to the line is activated by the decay of the line voltage beyond a preset value, preferentially combined with a minimum timing threshold. 14. Circuit according to claim 1 , characterized in that the closing to the line is actuated if the value of the difference between line-voltage and ideal line-voltage image exceeds a preset value, preferentially combined with a minimum timing threshold. 15. Circuit according to claim 1 , characterized in that the disconnection from the line is actuated by one or more of the following processes: -capacitance voltage decays beyond preset value -capacitance voltage increasing -line voltage increases -current in link falls beyond preset value -real power supplied in the line becomes negative. 16. Circuit according to claim 1, characterized by at least parts of one or more circuits being elements of a pwm-converter (7), when needed these elements being decoupled from the pwm-converter and the circuit stabilizing the pwm-output. RECTIFIED SHEET (RULE 91 ) ISA/EP

Inventors: JOHO REINHARD, JOHO, REINHARD
Applicants: JOHO REINHARD, JOHO, REINHARD
Application year: 1998
Main classification: H02J3
IPC codes: H02J3