Capacitor compensated thyristor controlled brake resistor

Abstract

A system (100) for stabilizing a power grid (102) includes a power generator (104) configured to provide power to the power grid (102), the power having an active power component and a reactive power component, and a power line (106) configured to transmit the power from the power generator (104) to the power grid (102). The system (100) includes a thyristor controlled braking resistor (TCBR) (108) arranged on the power line (106) and a capacitor (110) electrically connected in series with the TCBR (108). The TCBR (108) absorbs at least a portion of the reactive power component from the power generator (104) during a fault on the power line (106), and the capacitor (110) is configured to compensate for at least a portion of the reactive power component absorbed by the TCBR (108).

Description

TECHNICAL FIELD

[0001] The present disclosure relates to fault tolerance in an electrical grid. More particularly, the present disclosure relates to a thyristor controlled braking resistor for use during a fault in an electrical grid. BACKGROUND

[0002] An electrical grid comprises electrical generators that provide electrical power to loads. The generators are typically connected to the grid with power lines, i.e. cables that transmit electrical power from the generators to the loads of the grid. For example, in radial networks, a single generator feeds a load via a single power line.

[0003] Although power lines are typically configured to be resilient, if a power line fault occurs, the load of the generator can quickly be lost. In this case, the generator can trip, be damaged and / or be required to shut down.

[0004] In the event of a fault, a braking resistor can be used as a temporary load for the generator to avoid the generator tripping until the power line is repaired and / or reconnected. If the generator is to be shut down, this can be done in a controlled manner by ramping down the power using a thyristor controlled braking resistor (TCBR).

[0005] A TCBR consumes reactive power when in operation due to the internal inductance of the resistor, the reactive power consumption when the thyristors are triggered late, and the reactive power consumption of the transformer if used. This reactive power consumption can reduce the voltage on the grid, which in turn can force the generator to trip on undervoltage. An example of such a TCBR is disclosed in US Patent No. 5,198,745.

[0006] The reactive power absorbed by the TCBR can be compensated for by a parallel capacitor bank of mechanical or thyristor switches, thereby reducing the risk of the generator tripping. SUMMARY

[0007] Aspects of the present disclosure are provided to improve the stability of an electrical grid using a TCBR.

[0008] According to an aspect of the present disclosure, there is provided a system for stabilizing an electrical grid, the system comprising: a generator configured to provide electrical power to the electrical grid; and a power line configured to transmit the electrical power from the generator to the electrical grid.

[0009] The power grid can be a radial power grid, e.g. having a single generator feeding a load of the power grid via a single power line each. Alternatively, the power grid can be a ring or mesh network. The power grid provides power to a load, which can be constituted by a combination of loads, such as a power network of a city, a commercial property, or some other load, or a combination thereof.

[0010] The generator can be any generator suitable for providing power to the load to meet the demand of the load, which can vary greatly over the course of a day or a year. The generator can be driven by a renewable energy source, such as a wind turbine or a hydroelectric system, or some other electrical power source. The power line is configured so as to deliver power generated by the generator to the power grid, more specifically to the load on the power grid.

[0011] According to this aspect of the disclosure, the system further comprises a thyristor-controlled braking resistor (TCBR) arranged on the power line and configured to absorb power from the generator during a fault on the power line. The TCBR can comprise a braking resistor configured to draw an electrical load from the generator, and a thyristor electrically connected in series with the resistor, the thyristor being configured to control the electrical load drawn by the braking resistor. In general, the TCBR advantageously provides controllability of the current and power drawn from the generator, which can be controlled to a desired setpoint (e.g. by connection to a control unit or some other device).

[0012] The power generated by the generator has an active component and a reactive component, and the TCBR is configured to absorb, in its operation, at least a part of the reactive power component due to reactive power consumption by the internal inductance of the resistor and / or the thyristor when operating with a delay upon triggering.

[0013] Hence, in order to compensate for this reactive power absorption by the TCBR, as part of this aspect of the disclosure, it is proposed to provide a capacitor electrically connected in series with the TCBR, wherein the capacitor is configured to compensate for at least a part of the reactive power component absorbed by the TCBR. Depending on the current in the TCBR, the reactive power can be compensated to the required extent.

[0014] The reactive power consumed by the TCBR depends on a number of factors, such as the transformer reactance, the inductance in the TCBR circuit, and the control of the thyristor. From a general perspective, the compensation provided by the capacitor can be expressed as where X C is the reactance of the capacitor, and X L is the inductive reactance of the load. The specifics of the mathematical relationship between the reactive power absorbed by the TCBR and the reactive power compensated by the capacitor can vary between embodiments.

[0015] Providing series capacitors in this way improves upon previous solutions, such as shunt resistors, because the drawback of shunt resistors is that reactive power is compensated in discrete steps, resulting in undesirable steps in the grid voltage. The use of capacitors does not cause this step in the grid voltage because the compensation is scaled according to the current in the TCBR.

[0016] In some examples, the system may also include a transformer connected to the power line between the generator and the power grid.

[0017] In such an example, the capacitor can be placed between the transformer and the power grid, specifically on the primary side of the transformer. When the capacitor is placed on the primary side of the transformer, the voltage across the transformer can increase with the current, and the rated voltage of the transformer can also increase. For high voltage levels in the power grid, the series capacitor can preferably be insulated.

[0018] Alternatively, the capacitor can be placed between the transformer and the TCBR, i.e., on the secondary side of the transformer. When the capacitor is placed on the secondary side of the transformer, this advantageously provides lower insulation requirements for the series capacitor and the transformer; however, the voltage across the thyristor valve may increase with current. Optimization of the thyristor valve and resistor may become more complex.

[0019] The specific placement of the capacitor relative to the transformer may depend on the system. Reactive power compensation can also be provided by series capacitors in a TCBR system that does not include a transformer.

[0020] According to some example implementations, for further reliability of the system, the system may also include a bypass switch configured to bypass the capacitor and / or a variable resistor connected in parallel with the capacitor.

[0021] While the invention may have various modifications and alternatives, specific embodiments are illustrated by way of example in the accompanying drawings, as described in detail herein. However, it should be understood that the detailed description herein and the accompanying drawings are not intended to limit the invention to the specific forms disclosed. Rather, their purpose is to cover all modifications, equivalents, and alternatives falling within the scope of the appended claims.

[0022] Any references to prior art documents or comparative embodiments in this specification should not be construed as an admission that such prior art is well-known or constitutes part of common general knowledge in the art.

[0023] As used in this specification, the words “including,” “comprising,” and similar terms should not be interpreted in an exclusive or exhaustive sense. In other words, they are intended to mean “including, but not limited to.” Attached Figure Description

[0024] One or more embodiments will be described by way of example only and with reference to the following figures, in which:

[0025] Figure 1 A system for stabilizing a power grid according to an embodiment is schematically illustrated;

[0026] Figure 2 The diagram illustrates the use of in Figure 1 Example configuration of capacitors used in the system; and

[0027] Figure 3 The diagram illustrates the use of in Figure 1 An example configuration of a thyristor-controlled braking resistor used in the system. Detailed Implementation

[0028] The invention is described below by way of several illustrative examples. It should be understood that these examples are for illustration and explanation only and are not intended to limit the scope of the invention. Rather, the scope of the invention is defined by the appended claims. Furthermore, although examples may be presented in the form of individual embodiments, it will be appreciated that the invention also covers combinations of the embodiments described herein.

[0029] Figure 1 An example system 100 for stabilizing a power grid 102 according to an embodiment is shown.

[0030] The systems 100 shown are similar in at least the following respects: they include a generator 104 configured to supply power to a power grid 102, the power having active and reactive power components. The generator 104 is connected to the power grid 102 via a power line 106, which, depending on the specific implementation, may be one or more power lines.

[0031] A thyristor-controlled braking resistor (TCBR) 108 is connected in series with the generator 104 on the power line 106. An example configuration of the TCBR 108 will be described below. Figure 3 Describe it.

[0032] TCBR 108 serves as a temporary load for generator 104, for example, in the event of a fault in power line 106, which results in disconnection from the power grid 102. TCBR 108 is controllable (e.g., via a control unit connected to a thyristor) to control the power drawn from generator 104 according to the specific requirements of system 100, for example, to control it to a desired maximum or minimum setpoint.

[0033] As will be appreciated, the power from the generator 104 will have an active power component and a reactive power component, and the braking resistor included in the TCBR 108 is primarily configured to consume the active power component. If too much of the reactive power component is consumed by components of the TCBR 108 or other components in the system 100 (or stored without being returned), the voltage on the grid 102 can undesirably decrease. This in turn causes the generator 104 to trip out when the voltage is insufficient.

[0034] The system 100 also includes a capacitor 110 electrically connected in series with the TCBR 108. The capacitor 110 placed in series in this way allows the capacitor 110 to compensate for reactive power consumed by components of the TCBR 108. As a result, the voltage on the grid 102 does not undesirably decrease, and the risk of the generator 104 tripping out when the voltage is insufficient is reduced. The system 100 therefore becomes more robust and reliable.

[0035] The configuration of the capacitor 110 will be discussed in more detail below in relation to Figure 2

[0036] In some examples, the system can include a transformer 112 connected on the power line 106 between the generator 104 and the grid 102. The two illustrated systems 100 differ in the potential placement of the capacitor 110 in relation to the transformer 112. The transformer 112 can have a high voltage (HV) primary side and a medium voltage (MV) secondary side, with the voltage stepped up from the MV secondary side to the HV primary side before being distributed to the grid 102.

[0037] The capacitor 110 can be arranged in series on the primary HV side of the transformer 112, or in series on the secondary MV side of the transformer 112. In other words, the capacitor 110 can be arranged between the transformer 112 and the grid 102, or between the transformer 112 and the TCBR 108.

[0038] In the case where the capacitor 110 is arranged in series on the primary side of the transformer 112, the voltage on the transformer 112 can increase with current, and the rated voltage of the transformer 112 can increase accordingly.

[0039] Preferably, to accommodate the placement of the capacitor 110 on the HV side of the transformer 112, the series capacitor 110 can be insulated for the HV level of the grid 102.

[0040] ​With capacitor 110 connected in series on the secondary side of transformer 112, the voltage across the thyristor valve in TCBR 108 will increase with the current. Therefore, optimizing the thyristor valve and resistor(s) in TCBR 108 can become more complex. However, advantageously, since the series capacitor is placed on the MV side of transformer 112, lower insulation requirements can be imposed on the series capacitor 110.

[0041] Whether capacitor 110 is placed on the HV side or the MV side of transformer 112 depends on the system. Furthermore, as... Figure 1 As shown by the dashed line, system 100 may not include transformer 112. Even without transformer 112, capacitor 110 can still provide reactive power compensation for system 100 with TCBR. However, when transformer 112 is included, it can contribute to the dissipation (or storage without return) of reactive power.

[0042] Figure 2 The diagram illustrates the use of in Figure 1 Example configuration 200 of capacitor 110 used in the system.

[0043] In the illustrated configuration 200, capacitor 110 may have a bypass switch 114 arranged to bypass capacitor 110, for example, arranged to be connected across the terminals of capacitor 110 on power line 106. Therefore, reactive power compensation of capacitor 110 can be added and removed via control of bypass switch 114 (e.g., remote control).

[0044] The bypass switch 114 can take any form suitable for enabling electrical bypass of the capacitor 110 when the bypass switch 114 is closed, while allowing current to flow through the capacitor 110 when the bypass switch 114 is open. Providing the bypass switch 114 can further improve the safety and / or controllability of the system 100.

[0045] Alternatively or additionally, configuration 200 may further include a variable resistor 116 arranged in parallel with capacitor 110. The variable resistor 116 can advantageously provide further protection for capacitor 110, thereby further improving the safety and reliability of system 100.

[0046] Depending on the specifications required by system 100, such as in terms of safety, ratings, etc., configuration 200 may include one or both of bypass switch 114 and rheostat 116.

[0047] Figure 3 The diagram illustrates the use of in Figure 1 The example configuration of TCBR 108 used in System 100.

[0048] According to the illustrated example, the TCBR 108 can comprise a braking resistor 118 configured to draw an electrical load from the generator 104, and a thyristor 120 electrically connected in series with the resistor 118 and configured to control the electrical load drawn by the braking resistor 118.

[0049] Depending on the required specifications of the TCBR 108, the thyristor 120 can be one or more thyristors. For example, the thyristor 120 can comprise a plurality of thyristors 120 arranged in series to increase the voltage capacity.

[0050] As mentioned above, although the primary purpose of the braking resistor 118 when drawing an electrical load is to consume real power from the generator 104 (e.g. when the TCBR 108 is used as a temporary load for the generator 104), the TCBR 108 can also store or consume reactive power.

[0051] The TCBR 108 can consume a relatively large amount of reactive power (e.g. relative to the ideal reactive power consumption of zero) during its operation. This can be due to the internal inductance of the braking resistor 118 and / or the reactive power consumption of the thyristor(s) 120 when there is a delay in triggering the TCBR 108.

[0052] Due to the provision of the series capacitor 110, the amount of reactive power consumed by the TCBR 108 will be proportionally compensated. Thus, the reactive power compensated by the capacitor 110 is proportional to the reactive power consumed by the TCBR 108. As a result, the generator 104 can be prevented from tripping and the voltage of the power grid 102 will not be irregularly changed.

[0053] In some examples, the braking resistor 118 can comprise a further resistor 118a, so as to be constituted by a pair of resistors 118, 118a. In such examples, the thyristor(s) 120 can be arranged and electrically connected between the pair of braking resistors 118, 118a.

[0054] Providing an additional resistor 118a on the other side of the thyristor(s) 120 allows the resistance to be split into parts, i.e. two parts, which advantageously limits the short circuit current in the valves of the thyristor(s) 120.

[0055] In some further examples, a reactor (not shown) can be included in series with the thyristor(s) 120, thereby providing additional reactance to limit the derivative of the current through the valves of the thyristor(s) 120.

[0056] The resistors 118 and / or 118a can advantageously be selected to have a low internal inductance (e.g., to reduce reactive power consumption), a small temperature dependency (e.g., to prevent overheating during operation), and a high overload capability (e.g., to obtain better flexibility).

[0057] Although only one additional resistor 118a is shown, it should be understood that a larger number of additional resistors can be provided, e.g., on either side of the thyristor(s) 120. Also, as indicated by the dashed lines in Figure 3

[0058] The relationship between the resistance of the resistor(s) 118, 118a and the capacitance of the capacitor 110 can be determined based on the selected voltage to which the TCBR is connected (i.e., the secondary voltage of the transformer 112) and / or the reactive power that can be tolerated in the system 100 during operation.

[0059] In some examples, the TCBR 108 can further comprise a control unit 122 configured to control the thyristor(s) 120. For example, one control unit 122 can be provided for each thyristor 120.

[0060] The control unit 122 can itself be controlled locally or remotely, can control the thyristor(s) 120 locally or remotely, and allows the TCBR 108 to control the amount of power drawn from the generator 104 when the TCBR 108 is used as a temporary load for the generator 104. For example, the TCBR 108 can be controlled to a desired setpoint of the drawn electrical load via control of the thyristors 120 by the control unit 122. This setpoint can be predetermined or dynamically determined by some computing device, depending on the specific requirements of the system 100.

[0061] It should be understood that the various components and features disclosed in the foregoing description can be implemented in separate or combined form. Also, it should be understood that the foregoing disclosed embodiments are not exhaustive and that additional example embodiments can be within the scope of the appended claims. In any event, the scope of protection is defined by the claims that follow this disclosure.​

Claims

1. A system for stabilizing an electrical grid, comprising: a power generator configured to provide electrical power to the electrical grid, the electrical power having an active power component and a reactive power component; a power line configured to transmit electrical power from the power generator to the electrical grid; a thyristor-controlled braking resistor (TCBR) arranged on the power line and configured to control electrical power drawn from the power generator; a capacitor electrically connected in series with the TCBR; wherein: the capacitor is configured to compensate for at least a portion of the reactive power component absorbed by the TCBR.

2. The system of claim 1, further comprising: a transformer connected on the power line between the power generator and the electrical grid; wherein: the capacitor is arranged between the transformer and the TCBR, or between the transformer and the electrical grid.

3. The system of claim 1 or claim 2, further comprising: a bypass switch configured to bypass the capacitor.

4. The system of claim 1 or 2, further comprising: a varistor electrically connected in parallel with the capacitor.

5. The system of claim 1, wherein the TCBR comprises: a braking resistor configured to draw an electrical load from the power generator; a thyristor electrically connected in series with the resistor and configured to control the electrical load drawn by the braking resistor.

6. The system of claim 5, wherein: the thyristor is configured to be controlled by a control unit.

7. The system of claim 5 or claim 6, wherein: the braking resistor is composed of at least two resistors; and the thyristor is arranged between the at least two resistors.

8. An electrical grid comprising the system of any one of claims 1 to 7.

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