Patent Application
20170344039
Embodiments described herein relate generally to voltage regulators, and more particularly to systems, methods, and devices for controlling a gated semiconductor device in an arcless tap changer of a voltage regulator.
Tap changers are used in medium voltage auto-transformer-based voltage regulators to regulate the output voltage. Medium voltage auto-transformer-based voltage regulators typically operate in the range of 2 kV AC to 30 kV AC. The tap changers regulate the output voltage by compensating for input voltage changes and load (current) induced changes. These tap changers typically have autotransformer windings with eight stationary taps.
Whenever a movable contact moves off of one stationary tap (i.e, breaks), arcing can occur due to current flow and circuit inductance. Additionally, whenever a moveable contact moves onto a stationary tap (i.e. makes), arcing can also occur. This arcing process causes contact wear and causes contaminants, such as carbon, to be added to the insulating medium, typically a dielectric fluid referred to as transformer oil, which reduces the useful life of the tap changer within a voltage regulator. Maintenance of substation equipment to replace voltage regulators requires extensive bypass procedures to maintain power on the distribution circuit and has associated costs. A tap changer that eliminates arcing (referred to herein as an arcless or non-arcing tap changer) eliminates arcing from occurring during a tap change operation by shunting one movable contact. Arcless tap changers therefore are desirable because they extend the life of tap changer contacts by eliminating electrical arcing during a tap switching operation and they reduce the likelihood of electrical failures within the voltage regulator due to carbon buildup in the dielectric fluid.
A voltage regulator with an arcless tap changer is described in prior U.S. Pat. No. 3,617,862 (“the '862 patent”), which is hereby incorporated herein by reference. The arcless tap changer in the '862 patent utilizes an arrangement of capacitors and a pair of silicon controlled rectifiers (SCRs) 80 and 81 that gradually turn off current to the movable contacts in the tap changer thereby eliminating, arcing when a tap change operation occurs. The arrangement of the power circuit and the SCRs 80 and 81 in the '862 patent requires continuous gate current flowing to the SCRs 80 and 81 when load current flows through both auxiliary switches AB and AD. However, this requirement for continuous gate current to the SCRs 80 and 81 imposes limitations on the voltage regulator if the intended application needs to span a large current range. For example, if the minimum load current for gating the SCRs is 10 amps and the minimum SCR trigger current is 100 milliamps, these values require a turns ratio of less than 100 for the current transformers supplying gate current. However, the current transformers in the power circuit of the voltage regulator need to be rated to handle fault currents of 10's of kA. Fault current can initiate at the crest of the power signal wave prior to any saturating effects, and the secondary current would be correspondingly large.
An additional limitation with the power circuit shown in the '862 patent is that the gate-cathode junction of the SCRs cannot dissipate more than 2 watts continuously during the time of high load current. This constraint on the power dissipation of the SCRs requires that gate current be limited to modest levels in the range of a few 100 milliamps. On the other hand high levels of gate current in the range of 1-3 amps are required to cope with high rates of change of anode current when the SCRs are initially conducting due to the opening of an auxiliary contact AB or AD as shown in the power circuit of the '862 patent. These two requirements are not compatible and affect reliability when continuous gate currents are employed in the power circuit of the '862 patent. A further drawback of the power circuit in the '862 patent is that the use of continuous, high-current gate control circuits requires that the rectified power for the control circuits be filtered by large capacitors, such as electrolytic capacitors. Large capacitors, such as electrolyte capacitors, are largely incompatible with the hot-oil environment of a voltage regulator tank.
Therefore, an improvement is needed to voltage regulators, such as the one shown in the '862 patent, so that the current transformer turns ratio can be increased in order to have a range of manageable secondary currents, while also having a useful gate current.
In general, in one aspect, the disclosure relates to controlling a tap changer of a voltage regulator. The present disclosure is an improvement on the voltage regulator described in the '862 patent in that the power circuit and the SCRs are controlled differently for improved performance. The tap changer comprises a voltage source terminal, a plurality of stationary taps, a first movable contact, and a second movable contact. The first movable contact is coupled to a first switch and a first current transformer of the voltage regulator. The second movable contact is coupled to a second switch and a second current transformer of the voltage regulator. The voltage regulator further comprises a first control circuit coupled to the first current transformer, the second current transformer, and a first silicon controlled rectifier (SCR) and a second control circuit coupled to the first current transformer, the second current transformer, and the second silicon controlled rectifier (SCR). The first SCR and the second SCR are oriented anti-parallel to each other and in parallel with a voltage load terminal. Each of the first control circuit and the second control circuit comprise a first rectifier, second rectifier, a gating switch that supplies a gating signal to an SCR gate, and a positive voltage detector coupled to an SCR anode and providing a signal to the gating switch.
In another aspect, the disclosure can relate to a method for operating a voltage regulator comprising the steps of supplying a load current from a source terminal to a load terminal through a tap changer of the voltage regulator. The tap changer comprising a first movable contact in contact with a first stationary tap and a second movable contact also in contact with the first stationary tap. The first movable contact is coupled to a first switch and a first current transformer of the voltage regulator. The second movable contact is coupled to a second switch and a second current transformer of the voltage regulator. The voltage regulator further comprises a first control circuit coupled to the first and second current transformers and a first silicon controlled rectifier (SCR) and a second control circuit coupled to the first and second current transformers and the second silicon controlled rectifier (SCR). The method further includes the steps of initiating a tap change of the voltage regulator by opening the first switch thereby causing the first SCR and the second SCR to each assume load current dependent on the polarity of the load current at the time of the switch opening through the action of the gating signals to be applied to a gate of the first SCR and a gate of the second SCR. In the next step of the method, the gating signal is removed from the gate of the first SCR and the gate of the second SCR causing the first SCR and the second SCR to turn off when the load current next equals zero. Once the first SCR and the second SCR have turned of the tap changer moves the first movable contact from the first stationary tap to a second stationary tap. After the movement of the movable contact is compete, the first switch of the voltage regulator is closed and the first control circuit and the second control circuit are prepared to apply a gating signal to a gate of the first SCR and a gate of the second SCR.
In yet another aspect, the disclosure relates to controlling a tap changer of a voltage regulator using gated semiconductor devices. The tap changer composes a voltage source terminal, a plurality of stationary taps, a first movable contact, and a second movable contact. The first movable contact is coupled to a first switch of the voltage regulator. The second movable contact is coupled to a second switch of the voltage regulator. The voltage regulator further comprises a first control circuit coupled to a first current sensor, a power supply and a first gated semiconductor device and a second control circuit coupled to a second current sensor, the power supply, and a second gated semiconductor device. The first gated semiconductor device and the second gated semiconductor device are oriented anti-parallel to each other and in parallel with a voltage load terminal. Each of the first control circuit and the second control circuit comprise a rectifier, a gating switch that supplies a gating signal to the gated semiconductor device, and a positive voltage detector coupled to an anode of the gated semiconductor device and providing a signal to the gating switch.
These and other aspects, objects, features, and embodiments will be apparent from the following description and the appended claims.
The drawings illustrate only example embodiments of non-arcing tap changers of a voltage regulator and are therefore not to be considered limiting of its scope, as non-arcing tap changers of voltage regulators may admit to other equally effective embodiments. The elements and features shown in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the example embodiments. Additionally, certain dimensions or positionings may be exaggerated to help visually convey such principles. In the drawings, reference numerals designate like or corresponding, but not necessarily identical, elements.
The example embodiments discussed herein are directed to systems, apparatuses, and methods of controlling a tap changer of a medium voltage transformer-based voltage regulator. The present disclosure is an improvement on the voltage regulator described in the '862 patent in that the power circuit and the SCRs are controlled differently for improved performance.
While example embodiments are described herein as being directed to voltage regulators used in medium voltage electric distribution systems of a power grid, example embodiments can also be used with voltage regulators in other types of systems. As described herein, a user can be any person who interacts with a voltage regulator. Examples of a user may include, but are not limited to, a consumer, an electrician, an engineer, a lineman, a consultant, a contractor, an instrumentation and controls technician, an operator, and a manufacturer's representative.
In one or more example embodiments, a voltage regulator is subject to meeting certain standards and/or requirements. Examples of entities that set and/or maintain such standards can include, but are not limited to, the International Electrotechnical Commission (IEC), the National Electric Code (NEC), the National Electrical Manufacturers Association (NEMA), and the Institute of Electrical and Electronics Engineers (IEEE). Example embodiments are designed to be used in compliance with any applicable standards and/or regulations.
As described herein, communication between two or more components of an example voltage regulator is the transfer of any of a number of types of signals. Examples of signals can include, but are not limited to, power signals, control signals, communication signals, data signals, instructions, and status reporting. In other words, communication between components of example voltage regulators can involve the transfer of power (e.g., high levels of current, high levels of voltage), control (e.g., low voltage, low current), and/or data.
Any component described in one or more figures herein can apply to any subsequent figures having the same label. In other words, the description for any component of a subsequent (or other) figure can be considered substantially the same as the corresponding component described with respect to a previous (or other) figure. For any figure shown and described herein, one or more of the components may be omitted, added, repeated, and/or substituted. Accordingly, embodiments shown in a particular figure should not be considered limited to the specific arrangements of components shown in such figure.
Example embodiments of systems and methods for controlling a switching module of a voltage regulator will be described more full hereinafter with reference to the accompanying drawings, in which example voltage regulator systems are shown. Voltage regulator systems may, however, be embodied in many different forms and should not be construed as limited to the example embodiments set forth herein. Rather, these example embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of voltage regulator systems to those of ordinary skill in the art. Like, but not necessarily the same, elements (also sometimes called components) in the various figures are denoted by like reference numerals for consistency.
Terms such as “first” and “second” are used merely to distinguish one component (or part of a component or state of a component) from another. Such terms are not meant to denote preference or a particular orientation. Also, the names given to various components described herein are descriptive of one embodiment and are not meant to be limiting in any way. Those of ordinary skill in the art will appreciate that a feature and/or component shown and/or described in one embodiment (e.g., in a figure) herein can be used in another embodiment (e.g., in any other figure) herein, even if not expressly shown and/or described in such other embodiment.
Referring now to
Voltage regulator 200 includes a preventive transformer 237, comprising a first and second winding, that provides an impedance to prevent short-circuits during tap change. In alternate embodiments, reactors other than a preventive transformer may be implemented. In the example shown in
The two secondary coils of the first current transformer 246 and the second current transformer 248 sense current and provide power to the first control circuit 244 and the second control circuit 245. The first current transformer 246 and the second current transformer 248 can have non-linear characteristics and are not intended for current measurement because they are designed to span a large range of primary current, for example 10 to 2000 amperes of continuous current and even larger ranges for short duration surge currents. The ratio of turns between the primary coil and the secondary coils in the first current transformer 246 and the second current transformer 248 is greater than 100 in certain example embodiments and 1000 or greater in additional example embodiments.
The first control circuit 244 controls the first silicon controlled rectifier (“first SCR”) 241 and the second control circuit 245 controls the second silicon controlled rectifier (“second SCR”) 240. The connections between the first control circuit 244 and the first and second current transformers 246 and 248, and the connections between the second control circuit 245 and the first and second current transformers 246 and 248 are simplified in
The voltage regulator 300 differs from the voltage regulator 200 in several aspects. First, instead of two current transformers supplying power to the control circuits 344 and 345 voltage regulator 300 comprises an alternative power supply 352 which may derive power from a single current or a voltage transformer with two isolated outputs. As shown in
Voltage regulator 300 further differs from voltage regulator 200 in that Hall effect sensors 346 and 348 are used to sense current in conductors 350 and 351, respectively. The Hall effect sensors 346 and 348 are coupled to the control circuits 344 and 345 and provide signals indicating when the current turns on and off in the conductors 350 and 351. The signals from the Hall effect sensors 346 and 348 can be used in the control circuits 344 and 345 as a substitute for the signals provided by the current transformers in voltage regulator 200. It should also be understood that in alternate embodiments of the disclosure other sensing devices can be used.
Lastly, voltage regulator 300 differs from voltage regulator 200 in that gated semiconductor devices 340 and 341 are used in place of SCRs 240 and 241. Gated semiconductor devices 340 and 341 operate in a similar manner to SCRs 240 and 241 in that they are coupled to the control circuits 344 and 345. The explanation of SCRs 240 and 241 herein is applicable to gated semiconductor devices 340 and 341. Examples of different types of gated semiconductor devices that can be implemented in voltage regulator 300 include insulated gate bipolar transistors, integrated gate-commutated thyristors, gate turn of thyristors, other wide-bandgap semiconductor devices, or a combination of the foregoing described semiconductor devices.
Referring now to
Unlike the prior art voltage regulator described in the '862 patent wherein a continuous gating current to the SCR interrupters was required, in the control circuit 244 gating of the SCRs takes place only when necessary so that the capacitor 420 is not continuously drained. The control circuit 244 is connected to the first SCR 240 at the gate 428, cathode 432 and anode 434. The gating signal to the first SCR 240 is controlled by the signal AND gate 414. The signal AND gate 414 is arranged to turn on a gating switch 22 only when both the first and second current transformers 246 and 248 have current flow, and there is a positive voltage at the anode 434 of the first SCR 240. The control circuit 244 detects the voltage at the anode 434 and conditions the voltage received from the anode at signal conditioner 436. If the positive voltage detector 438 detects a positive voltage from the conditioned voltage received from the anode 434, the voltage is applied to the signal AND gate 414.
When the voltage regulator 200 is in operation and first switch 247 begins to open, both the first and second current transformers 246 and 748 will have current flow and the positive voltage detector 438 may detect a positive voltage at the anode 434 of the first SCR 241. This combination of three signals at the signal AND gate 414 will exist when there is a potential for incipient arcing at the tap changer. Upon receiving this combination of three signals, the signal AND gate 414 will turn on the gating switch 422 thereby providing a gating current through resistor 424 to the gate 428 of the first SCR 241 when the first SCR 241 has a positive anode voltage. Similarly, control circuit 245 would provide gating current to the gate of the second SCR 240 when the first and second current transformers 246 and 248 have current flow and the control circuit 245 detects a positive voltage at the anode of the second SCR 240. Providing the gating current to the first SCR 241 or the second SCR 240 then eliminates arcing when moving the movable contact 232 of the tap changer. In some example embodiments, at least a voltage of 10 V across first switch 247 with simultaneous current is needed tor arcing to occur. The example embodiments described herein are designed such that the gating switch 422 is turned on when a voltage of 2 V exists across the first switch 247 or the second switch 249.
The example control circuit 244 also includes a diode 418 and a shunt regulator 416 placed in parallel to the capacitor 420 to regulate the capacitor voltage and to drain away excess current from the first and second current transformers 246 and 248. The example control circuit 244 also includes a snubber 430 disposed between the cathode 432 and the anode 434 to handle transient voltages and a resistor 426 disposed between the gate 428 and the cathode 432.
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The foregoing example embodiments provide several improvements over prior art voltage regulators. The example embodiments described herein are able to handle higher surge currents and higher rates of current change at the SCR. The example embodiments also use components that require less power, have lower cost, and greater compatibility with the hot-oil environment of a voltage regulator. These improvements result in a voltage regulator with greater reliability than that found in the prior art.
Although embodiments described herein are made with reference to example embodiments, it should be appreciated by those skilled in the art that various modifications are well within the scope of this disclosure. Those skilled in the art will appreciate that the example embodiments described herein are not limited to any specifically discussed application and that the embodiments described herein are illustrative and not restrictive. From the description of the example embodiments, equivalents of the elements shown therein will suggest themselves to those skilled in the art, and ways of constructing other embodiments using the present disclosure will suggest themselves to practitioners of the art. Therefore, the scope of the example embodiments is not limited herein.
