WO2002059628A1

WO2002059628A1 - Electronic transformer system

Info

Publication number: WO2002059628A1
Application number: PCT/IL2001/000062
Authority: WO
Inventors: Yoram Waltuch
Original assignee: Solcon Industries Ltd.
Priority date: 2001-01-22
Filing date: 2001-01-22
Publication date: 2002-08-01
Also published as: WO2002059628A8; AU2001228769A1

Abstract

An electronic potential transformer comprising: a. a transmitter connected to the high voltage mains, and comprising a voltage divider for generating a low voltage proportional to the measured high voltage, means for measuring the amplitude of the low voltage and for encoding the measurements into digital signals and an optical transmitter. b. a receiver comprising an optical receiver, signal processor for decoding the received signals; c. optical fiber cables connected between the transmitter and receiver. A power supply system for supplying electrical power to circuits under high voltage, comprising: a. a primary wire connected to a power supply generating a periodically varying current; b. a plurality of ferromagnetic cores with secondary coils wound thereon, wherein the primary wire passes through all the cores and each secondary coil is connected to a power supply; and c. mechanical support means for keeping the primary wire and the ferromagnetic cores at a predefined relative orientation therebetween.

Description

Electronic transformer system

Technical Field

This invention concerns systems for transferring electric power and signals to/from high voltage circuits.

The invention relates in particular to such systems which include electronic means for achieving a reliable high voltage insulation.

Background Art

Prior art high voltage systems present difficulties in implementing Potential transformers and power supply systems for thyristor firing.

At present, Potential transformers (high voltage transformers) are used for transferring electrical signals from circuits under high voltage. For example, a transformer is used to measure the voltage at high voltage or medium voltage power lines. Such measurements may be necessary, for example in a factory or for other users of high voltage mains. For a three phase power mains, a three phase transformer is required as well.

The Potential transformer is used to reduce the voltage to a lower voltage, that may be directly measured using standard voltmeters. For example, a 6,600 Volt line (6.6kV) may be reduced to 110 V.

In another application, solid state switches such as thyristors are used to control the power to a load, such as an electric motor. The power is controlled by changing the duty cycle the thyristor is turned on. To implement such a controller, it is necessary to measure the precise phase of the input voltage. Also, a replica of the high voltage waveform may be required. The control circuits operate at a low voltage, thus the high voltage input has to be reduced to a lower voltage. Again, Potential transformers are used to reduce the voltage, usually of the three phase type.

A problem with presently used high voltage transformers is the expensive insulation required. Bulky and expensive insulation is required to protect the low voltage circuits from the high voltage inputs.

For higher voltages, the transformer becomes more bulkier, expensive and difficult to implement.

The cost and complexity further increase for a three phase transformer.

A problem with implementing high voltage transformers is the Partial Discharge effect. Minute imperfections in the insulator or small asperities in the conductors may create inhomogenities in the electrical field. The electric field may have higher values at specific locations, thus creating a local stress. Over time, such imperfections may result in damage to the insulation.

A breakdown of the insulation may result, with possible disastrous results to personnel and equipment.

Accordingly, stringent specifications have been defined regarding Partial Discharge, such as those indicated in the European Standard EN50178, HD625.1 SI & One for transformers. It may be very difficult and expensive to design, produce and test transformers that are Partial Discharge approved.

Thus, it may be very difficult and expensive to use transformers for measuring the voltage at high voltage power lines or for measuring the phase of such high voltages, or for creating a replica of the original waveform.

A prior art substitute to the Potential Transformer is to use a resistor voltage divider to reduce the high voltage to a lower voltage.

In each phase, a power resistor divider is connected between the phase voltage and ground (Chassis, Earth).

High voltage resistors are used, capable of withstanding the high voltage as required. The low voltage output is protected with Varistors, Gas arresters or other overvoltage clamping devices.

It is then connected, without Medium Voltage Galvanic insulation, to the low voltage control section.

This commonly used solution, although allowed by some standards, does not present a very good safety margin. Thus, for example, if the ground connection is imperfect or incapable of maintaining a ground voltage when a breakdown occurs, then the low voltage output remains unprotected.

Another prior art solution to the Potential transformer problem is to use a capacitive voltage divider - usually inserted in the insulator stubs. This solution presents disadvantages similar to those in the abovedetailed resistive voltage divider. To keep a reasonable accuracy, the output load VA is limited to a very small value. For resistive divider this is a fraction of 1 VA. For capacitive divider, a very small fraction of 1 VA.

Yet another prior art solution to the Potential transformer problem is in using a commercial three phase or two one-phase potential transformers (PTs). A common, standard way to measure medium voltages is to use ferromagnetic potential transformers, which are accurate and have a high voltage insulation.

However, these devices are bulky and expensive.

It is difficult to get PTs approved for Partial Discharge. Such PTs are very expensive, especially for the higher voltages.

Together with the Medium Voltage (MV) fuses, the PTs require relatively large, expensive space in the switchgear enclosure.

Prior art patents apparently do not offer satisfactory solutions to the above problems.

Thus, US Patent No. 5,859,529 discloses a resistance divider having the a dielectric strength that is suitable for the high voltage in use, followed by an operational amplifier. The primary of an inductive transformer is connected to the output of the operational amplifier, with the secondary winding of the transformer forming the undervoltage side of the voltage transformer. The structure has a lower cost, since resistors having the required dielectric strength can be obtained at lower cost. A problem with this prior art structure is the basic assumption that the lower part of the voltage divider is at zero potential. Although theoretically correct, this assumption may prove wrong in practice, for example in case there is a breakdown in the upper part of the high voltage divider. If there is a breakdown in one of the resistors under high voltage, which may happen in practice, then a high voltage is connected to the low voltage resistor, operational amplifier and low voltage transformer.

Such a breakdown may have disastrous results, such as complete damage to the low voltage components and possible danger to personnel.

A problem in this prior art system is the lack of true, real high voltage insulation between the high voltage section and the low voltage section.

The high voltage resistor is the only means of protection from the high voltage, and this may prove insufficient.

Moreover, there is no separation between the high voltage ground and the low voltage ground. Thus, the low voltage ground is itself unprotected. In case of failure or breakdown, the low voltage ground wire may itself transfer a dangerously high voltage.

PCT/AU 99/00107 discloses a power monitoring apparatus including a capacitive divider and a power monitoring module. The module receives signals from a current sensor and a voltage sensor, computes the power and transmits its value over an optical fiber link, using an optical signal encoder.

The optical fiber link provides the high voltage insulation.

The apparatus is connected between a high voltage conductor and a ground terminal. The apparatus transfers power measurements, rather than the instantaneous samples of voltage as in the present invention. The measurement depends on a connection to a high voltage ground, which may be problematic.

Further advantages of the present invention over prior art will become apparent upon reading the present disclosure and the related drawings.

Another problem in high voltage applications is the implementation of a power supply system for thyristors firing. It may be difficult to transfer electrical power to circuits at high voltage, for example to the circuits that control the solid state switches.

To control a high voltage load, high voltage solid state switches are usually used, for example thyristors. Each solid state switch may require a low voltage power supply for the switch activation circuit. Although a low voltage is required, the problem is the high voltage potential of the circuit, since the circuit is connected to a solid state switch that is at a high voltage. Each of the solid state switches is at a different high voltage potential. Thus, there is a high voltage between the solid state switches, this requiring high voltage insulation therebetween.

Thus, a transformer with high voltage insulation is required to supply electrical power to the solid state switch circuits.

For higher voltages, several thyristors in series may be used in each phase. Accordingly, a transformer insulated for high voltage may be required for each such circuit. For higher currents, several thyristors in parallel, further complicating the insulation task.

For example, in a typical 6.6 kV three-phase application, 6 SCR units are used in each phase, for a total of 18 SCR circuits. Thus, 18 high voltage transformer secondaries are required, with each transformer secondary being insulated from low voltage and from each other.

The disadvantages in such prior art systems may include, for example:

1 . A large number of high voltage transformers are required. Each transformer is expensive, as it has to withstand the high voltage in the system. Moreover, each transformer may have to comply with the Partial Discharge requirements as detailed above, an expensive and difficult problem.

2. Complex high voltage wiring - each transformer has two high voltage output wires that have to be connected to the corresponding SCR circuit. The path of each wire has to be carefully designed and implemented, to present partial discharge or large local electrical fields.

The wiring has to be performed by experts, to prevent possible errors in wiring. Such errors may have possible disastrous results, and may result in the immediate destruction of the high voltage system.

3. Expensive wiring - to reduce the Partial Discharge effect, special wires can be used, that is wires that are especially devised for such use. These wires should preferably be of the Corona-free type. 4. Difficult testing - because of the multitude of high voltage transformers and high voltage wires, in a rigid, non-modular system. The normal test is to short inputs and output bars together, and conduct the test in such a way, as the wires are all at the same potential and the test is not failed, even with regular wires.

Prior art systems may use one of several approaches to the power supply system for thryristor firing:

1 . Firing through separate, insulated pulse transformers

Many pulse transformers may be required, one each for each thyristor. Each transformer has to be insulated to medium voltage or high voltage requirements, according to application.

Moreover, special structure and techniques are required for achieving Partial Discharge Free pulse transformers. This structure was popular in the past, prior to the advent of fiber-optic firing.

2. Transformer with multiple secondary circuits for power supplies Each power supply, for every thyristor firing circuit, is fed from one secondary of a transformer having a plurality of secondary circuits.

A disadvantage of this system is that each winding of the transformer has to be insulated for Medium Voltage from the core and from the other windings. This requirement results in a relatively big and expensive transformer.

Furthermore, it is very difficult and expensive to get a Partial Discharge approved transformer, as required for example in the European Standard EN50178. For a modular construction, one transformer per phase is used. For example, in a soft starter with 18 thyristors (a common structure for a 6.6kV starter), 36 medium voltage wires are required between the transformers and the firing power supplies.

Disadvantages of using the above method include, inter alia: a. Expensive, because of the required wiring time b. Vulnerable to human mistakes. The multitude of wires increases the probability of errors. c. Requires careful and extensive testing d. Further complicates the task of designing a Partial Discharge free system.

Anyway, this is an accepted method in prior art, that is compatible with electro-magnetic or fiber optic-controlled firing and allows for simple diagnostics using LEDs.

3. Individual, small transformer per thyristor

To eliminate the MV wiring, the system may use an individual, small transformer per thyristor, with only one secondary. A disadvantage of this scheme is that several transformers may be required. Each transformer must be insulated for MV.

It may be very difficult and expensive to achieve a Partial Discharge-approved transformer. The wiring may be difficult, with Partial Discharge problems.

In both the abovementioned methods 2 and 3, this is also an accepted method in prior art, that is compatible with fiber optic firing or electro-magnetic timed firing. It allows for simple diagnostics using LEDs. 4. No transformer at all

The individual thyristor firing power supplies are fed directly from MV mains, usually through the snubber circuit of each thyristor.

Using this method, the power supplies are charged very fast, when the thyristors are turned off.

This structure increases the number of required wires per thyristor.

This is also an accepted method in prior art, that is compatible with fiber optic firing, although there may not be enough power for diagnostics LEDs. In some devices, a fiber optic feedback is used for indication.

It is an objective of the present invention to provide for systems and methods for transferring electric power and signals to/from high voltage circuits, using electronic means for overcoming the abovedetailed deficiencies.

Disclosure of Invention

It is an object of the present invention to provide a system for transferring electric power and signals to/from high voltage circuits. The novel systems include electronic means for achieving the high voltage insulation as required in high voltage systems.

This object is achieved by an electronic transformer as disclosed in claim 1. In accordance with the invention, the object is basically accomplished by providing an electronic transformer which includes means for transferring electrical signals, indicative of the measured high voltage, over fiber optic cables. The fiber optic cable has an inherent capability to withstand the high voltage present in the system, without the Partial Discharge effect. Thus, the fiber optic link replaces the high voltage transformer in transferring information relating to the instantaneous value of the high voltage, in each of the three phases present.

This structure may be used to create, in real time, voltage wave shapes which are identical or similar to the high voltage wave shapes, but at a lower voltage, of 120 Vac for example.

Moreover, the novel structure, unlike prior art potential transformers, can provide a significant level of output power, in the order of tens of VA.

The new EPT can be either used for measuring the amplitude and phase of the high voltage signal, or for acquiring the entire waveform, to get an accurate replica of the waveshape. The latter is achieved by performing a plurality of measurements of the high voltage signal during each cycle thereof. For example, in one embodiment, two high voltage waveforms are sent at a much higher rate than its frequency, i.e. 10 kHz vs. 50 Hz.

It is another object of the present invention to provide reliable means for precise transfer of the measured voltage, using a digital modulation method used for encoding and decoding the value of the measured high voltage. According to still another aspect of the present invention, a novel structure is used to transfer electric power to the solid state switch firing circuits. The novel structure is based on a novel current transformer structure with a plurality of ferromagnetic circuits mounted on a common primary wire. The novel structure replaces the high voltage transformers now in use in the power supply system for thyristor firing.

An important application of the novel technology is in medium voltage soft starters. The novel potential transformer and power supply can be used in a medium voltage starter or a temperature controller, for example.

The novel electronic potential transformer (EPT) can be used to replace electromagnetic PT in medium and high voltage measurements for industry switchgear and also for utility companies.

The novel high frequency toroids power supply can also be used in other medium and high voltage apparatus, such as variable speed drives, temperature controllers, etc.

It is possible to use just the novel EPT or the toroids power supply in various applications each.

Various applications can also use both the above novel systems together.

Further objects, advantages and other features of the present invention will become obvious to those skilled in the art upon reading the disclosure set forth hereinafter. Brief Description of Drawings

The invention will now be described by way of example and with reference to the accompanying drawings in which:

Fig. 1 illustrates the structure of a high voltage soft starter system.

Fig. 2 details the functional structure of the transmitter in an electronic potential transformer.

Fig. 3 details the functional structure of the receiver in an electronic potential transformer.

Fig. 4 details the physical structure of the transmitter in an electronic potential transformer.

Fig. 5 illustrates the structure of a power supply system for thyristor firing.

Fig. 6 details the current and voltage in the power supply system.

Fig. 7 details one embodiment of the ferromagnetic circuits in the power supply loop.

Fig. 8 details another embodiment of the ferromagnetic circuits in the power supply loop. Fig. 9 yet details another embodiment of the ferromagnetic circuits in the power supply loop.

Modes for Carrying out the Invention

A preferred embodiment of the present invention will now be described by way of example and with reference to the accompanying drawings.

Fig. 1 illustrates an example of a high voltage soft starter system. The system may also be used for other power control applications in medium or high voltage systems.

In a typical system, the input power is the high voltage input mains 11 , three phase. The electric power is applied to a load 13, for example a three phase electric motor.

Three solid state switches 12 are located between the power input 11 and the load 13.

The solid state switches 12, using for example series-connected thyristor pairs, are turned ON for part of each cycle of the AC voltage, to regulate the power to the load.

A controller 14 activates switches 12 during a predefined duty cycle of the input voltage, to control the power to the load 13.

A potential transformer 2 is used to convert a high voltage or medium voltage input 11 to a low voltage. The low voltage is then applied to controller 14. The phase of the input voltage is used to determine the precise timing for activating the solid state switches 12.

In one embodiment, the potential transformer 2 comprises a transmitter 22, connected to a receiver 26 through a fiber optic cable assembly 24. The fiber optic cable assembly 24 may comprise one or more cables. The structure of the potential transformer 2 is further detailed in the subsequent disclosure.

The low voltage may also be applied to other protection and/or commercial, standard power meter devices. These devices require accurate replicas of the medium or high voltage waveforms, at voltage levels of 110-120V.

Prior art systems use a resistive voltage divider with their own protective devices. Such a divider, which is not insulated, may not be usable with standard power meters and/or protective devices.

The new EPT can be either used for measuring the amplitude and phase of the high voltage signal, or for acquiring the entire waveform, to get an accurate replica of the waveshape. The latter is achieved by performing a plurality of measurements of the high voltage signal during each cycle thereof. In another embodiment, the above can be performed by high frequency sampling of the high voltage signal.

The complete voltage waveforms can be used (together with the current measurements) for calculating the power consumption. The power consumption information may be used for indication as well as for torque-related control features.

Many commercial protection devices as well as power meters need an exact replica of the three phase voltage sine waves. These standard devices normally require Potential Transformers.

There is also a power supply 3, which supplies electric power to the thyristor firing circuits in solid state switches 12.

One problem in such a closed circuit control system is in the implementation of the potential transformer 2 and its related wiring. As detailed above, it is difficult to achieve a potential transformer qualified to high voltage insulation and partial discharge requirements. Such transformers are bulky and expensive.

According to the present invention, the problem is solved with a new electronic potential transformer (EPT) and using fiber optic cables to transfer the measured high voltage values to the controller 14. These measurements indicate the real time, instantaneous value of the high voltage.

Furthermore, the electronic potential transformer 2 is divided into two parts, a transmitter and a receiver, with fiber optic cables therebetween. The voltage measurement circuits are in the EPT transmitter which, in a preferred embodiment, is connected directly to the mains bussbars 11 , thus eliminating the high voltage wires. The structure may also be used to provide mechanical support to the bus bars. In another embodiment, the EPT transmitter is located close to the mains bussbars 11 , using wires to connect therebetween.

The EPT receiver includes circuits for generating a replica of the high voltage three phase input 11 , at a low voltage that is directly usable in controller 14. The low voltage may also be used in other standard, commercial protection devices or power meters. The EPT receiver circuits are preferably located in the controller 14 itself or close thereto. One embodiment of the new electronic potential transformer 2 is detailed below, see for example Figs. 2, 3, 4 and the related disclosure.

Thus, the new EPT eliminates the high voltage wiring required in prior art for that task, since the transmitter is directly mounted over the input power lines 11 , and there are only the optical fiber cables connecting the transmitter with the receiver.

Furthermore, a reliable high voltage insulation is achieved, since the ferromagnetic transformer used in prior art is eliminated altogether. There is no transformer insulation to worry about, just the optical fiber cables with their excellent high voltage insulation.

Optical fibers are a preferred embodiment for various reasons, among them the excellent high voltage insulation and their high reliability and resistance to interference. Other noncontact channels may be used rather than the optical fibers, for example a radio channel or ultrasound or laser link or a combination thereof. These channels use waves rather than conducting wires for transferring the information relating to the measured high voltage.

The electronic potential transformer can be used in a one-phase system, a two-phase or a three-phase system.

The transmitter can include one, two or three measurement channels corresponding to one, two or three phase mains. Each channel comprises a high voltage divider, means for measuring the amplitude of the low voltage and for encoding the measurements into digital signals indicative thereof. Thus, for a three phase system , either a two-transformer or a three-transformer unit may be used.

For three-phase measurements, either an open delta or a "Y" scheme can be used. In the former case, no ground connection is required, as only voltages between phases is sampled. In the latter case, each of the three phases is measured relative to neutral or ground.

The transmitter needs not be connected to ground at all, so partial discharge to ground is eliminated altogether in the new structure.

The new electronic potential transformer 2 can achieve compliance with partial discharge specifications, at a lower cost and with a smaller volume implementation. Thus, the novel potential transformer and power supply can be used in a medium voltage starter, for example. A soft starter can benefit from the new novel electronic potential transformer (EPT) which provides a reliable high voltage insulation. A soft starter can also advantageously use the new power supply for the thyristor circuits.

The electronic potential transformer has many uses, for example in measuring the voltage of medium and high voltage mains. At present, potential transformers are widely used to that purpose, by the utility company and in industry.

Fig. 2 details the functional structure of one embodiment of the transmitter in an electronic potential transformer.

The high voltage input 11 comprise three mains input phases 11 1 , 112, 113. In one embodiment of the invention, phase 112, for example, may be taken as the common voltage to the measuring circuit. The voltages of the other two phases 11 1 , 113 are measured relative to phase 112. Only two voltages are measured, as illustrated. The three phases are then reconstructed using the "open Delta" method as known in the art.

At the output of the receiver, one or three phase mains are provided, without the neutral, as this is the common method in use today. The illustrated embodiments are not intended to limit the scope of the present invention.

The invention may be used to provide three voltages. In this case, the zero may be reconstructed, for example, using a resistive "Y" structure in the receiver output, as known in the art. A voltage divider 21 1 is used to convert the input phase 11 1 (relative to phase 112), to a lower voltage.

High voltage resistors are used in the voltage dividers, as illustrated. For higher voltages, a larger number of resistors can be used, thus the voltage on each resistor will be within the allowed limits.

The above structure may be used for any voltage, even with very high voltages, to hundreds of kilovolts (kV) . For these high voltages, the transmitter is preferably enclosed within an insulated tube, that is filled with the SF6 gas.

In another embodiment, a capacitive voltage divider may be used.

A voltage measuring unit 221 , may also include voltage encoding or modulating means.

An analog modulation means may be used, such as pulse width modulation (PWM) . Other types of signal modulation as known in the art may also be used. In another implementation, an analog to digital converter (ADC) may be used. Preferably, a digital signal is generated, which is indicative of the measured voltage.

An electro-optical (EO) transmitter unit 231 , converts the digital signals indicative of the measured voltage to optical signals or pulses, which are transmitted over fiber optic cable 241 to a receiver.

Other noncontact (wave) media may be used for transferring the signals, such as a laser beam, radio waves or ultrasonic waves, for example. Similarly, the other phase measurement uses a voltage divider 213 to convert the input phase 113 (relative to phase 112), to a lower voltage. It is then applied to voltage measuring unit 223, thence to EO transmitter unit 233, which converts the measured voltage to optical signals transmitted over fiber optic cable 243.

A voltage divider 212 is used to convert the input phase 11 1 (relative to phase 112) to a lower voltage, which is then applied through a transformer 222 to a power supply 225, used to supply the DC voltage (Vcc) for the operation of the measurement electronics, such as units 221 , 231 , 223 and 233. The transformer may be used to supply a relatively large current, while the divider 212 operates at a low current to allow using lower power resistors. For example, the transformer may receive an about 400 Volt voltage and convert it to lower values as desired.

The transformer does not need a special insulation. One edge of the primary is at a potential similar to that of the secondary. The Zener diode prevents the transformer primary voltage from being too high.

A separate voltage divider 212 is used for the power supply, rather than using one of the existing dividers 21 1 and 213, to prevent possible inaccuracies because of power supply loading.

In another embodiment of the invention (not shown), the two voltages from voltage dividers 21 1 and 213 are fed to a multiplexer and a common encoder or analog to digital converter (ADC). The two measured voltages can then be serially transmitted over a common fiber optic cable, using a multiplexing scheme for the two measured voltages.

The transmitter of the electronic potential transformer (EPT) is located close to the input mains 11 1 , 112, 113, to eliminate altogether the need for high voltage wires. The output to the receiver is over the optical fiber cables 241 and 243.

Thus, using fiber optic cables to transmit the voltage measurement information, a precise measurement of two high line voltages is performed, while also achieving excellent insulation between the high voltage input and the low voltage output of the device.

The new EPT can be either used for measuring the amplitude and phase of the high voltage signal, or for acquiring the entire waveform, to get an accurate replica of the waveshape. The latter is achieved by performing a plurality of measurements of the high voltage signal during each cycle thereof, such as high frequency sampling.

Fig. 3 details the functional structure of one embodiment of the receiver in an electronic potential transformer. The receiver may be located anywhere in the low voltage section or within the controller.

This embodiment illustrates signals reconstruction for a PWM transmission. There are two identical receiver channels, thus only one will be detailed. Other methods and systems may be used instead, for example another modulation method in lieu of PWM, or various methods of signal encoding. The optical signals can be transferred from the EPT transmitter over the fiber optic cable 241 to the receiver, and are converted to electrical signals in an EO receiver unit 251 .

The signals or pulses are applied to a voltage decoding unit comprising, for PWM in this example, units 261 to 264. The amplifier with hysteresis 261 is used to reconstruct the digital transmitted signals. The integrator 262 is used to demodulate the PWM signals, that is to generate a sawtooth signal whose amplitude is proportional to the pulse width at its input (a triangular signal can also be used, for example).

The sample and hold 263 keeps the voltage at the end of the integration period, thus eliminating the need for a low pass filter. The timing unit 264 coordinates and synchronizes the operation of the above units. It is also used to block amplifier outputs for the first few cycles, to eliminate DC voltage to the output transformer. Initially, the transformer current may be exceedingly high, until the capacitor is charged.

A filter with AC coupling 271 may be used to filter out the DC level and to smooth up the demodulated signals.

The output stage, can be implemented with power amplifier 281 as illustrated, with/without an output transformer 291 .

The other phase measurement channel has an identical structure, as illustrated. In another embodiment (not shown), where an ADC with serial output has been used in the EPT transmitter, a corresponding DAC with a serial input may be used at the receiver.

Feedback means may be added, for compensating the voltage drop in output transformers.

A power supply 31 is used to supply electrical power to the abovedetailed electronic circuits in the EPT receiver.

Either one or three phase mains are provided at the receiver output, with or without the neutral.

Fig. 4 details one embodiment of the physical structure of the transmitter in an electronic potential transformer (EPT) .

The transmitter box 41 is made of an insulating material such as glass epoxy, mounted over the high voltage input bus bars, three phase mains comprising input phases 11 1 , 112, 113.

The voltage dividers 21 1 , 212, 213 are mounted in the box 41 , preferably on one printed circuit board, with direct connection to the input phases 11 1 ,112, 113. This structure eliminates altogether the high voltage wiring to the potential transformer, as the EPT is mounted directly on the high voltage mains. This can achieve great savings relative to prior art, while concurrently reducing the danger of transformer failures. The voltage measuring units 221 , 223 are located close to the voltage dividers, and are connected to the EO transmitter units 231 , 233.

In one embodiment, they convert the input voltage waveform into high frequency PWM pulses, at the output of the modulators.

The fiber optic cable 241 , 243 are coupled to connectors in the EO transmitter units 231 , 233.

A power supply 225 is also mounted in box 41 and is connected to the third voltage divider 212.

This structure allows for easy mounting of the EPT transmitter over the input high voltage mains, without any high voltage wiring.

The voltage dividers 21 1 , 212, 213 may include optional means for low voltage testing: input connectors connected to part of the voltage divider, allow the application of lower voltages (for example 220 to 690 volts) for testing purposes.

Since just part of the voltage divider is used, the lower voltage will generate the same output voltage as the high voltage input divided by the full attenuation of the divider.

Thus, the EPT transmitter can be reliably and easily tested, using a lower voltage. This is a great improvement over prior art, where ferromagnetic transformers were used, and a multitude of wires had to be disconnected and reconnected for testing purposes. Testing of prior art potential transformers required a lot of work, and could result in possible mistakes in wiring, that could damage the system. These prior art disadvantages are eliminated in new EPT.

Using the novel structure, it is easier to test the soft starter, at low voltage.

Fig. 5 illustrates the structure of a novel power supply system for thyristors firing, illustrated as unit 3 in Fig. 1.

A primary loop wire 51 with a self inductance 52 is driven with solid state switches 53, for example MOSFETs. The solid state switches 53, for example MOSFETs , are concurrently turned ON or OFF.

Diodes 54 are used for closing the loop for coil current when switches 53 are OFF. The second MOSFET and the diodes are used to keep the voltage constant at T1 , while energy is returned to the input supply (or capacitor). This increases the efficiency of the circuit, thus reducing the heating of the MOSFET switches. These components also help reduce the amount of generated EMI.

Wire 51 passes through ferromagnetic toroids 55, for example made of ferrite.

A secondary coil 56 having N wounds, is mounted on each toroid 55.

A secondary power supply 57, may include a rectifying diode and LPF (low pass filter) capacitor as shown.

The number of toroids can be changed according to the required isolated power circuits.

As a minimum, a single power supply with a wire 51 running in the total three phases can be used. It is also possible to use more power supplies and wires. Graph 61 illustrates the current in the primary. Each cycle comprises a linearly rising current stage Ton, a current decreasing stage T1 and an OFF current stage Toff.

During stage Ton (with switches 53 ON) , the rising current induces an about fixed voltage in the secondary coil. Thus, a fixed voltage (secondary power supply) is generated. The power supply voltage is stored, at about a constant value, in the capacitor.

This voltage is used in the thyristor firing circuits (not shown), to drive the thyristor ON as required. This voltage can also be used for indicating LED circuits, for protection circuits, etc.

Thus, the power supply system includes means for generating a generally sawtooth-shaped current, including a time period of about a fixed slope current for generating a constant voltage in the secondary.

A primary power source 501 , may include a (not shown) DC power source (a primary or a secondary battery) or an AC source as known in the art.

The power supply system preferably also includes means for synchronizing the power supply with the thyristors firing signals.

This is the purpose of the circuit - to generate electrical power for the thyristor circuits, so that the circuit power increases when the firing pulse arrives. There is a signal to tell the power supply "now a high current is needed" , this signal is for example for a firing period of about 2 mSec, while the cycle time of the power supply may be of the order of magnitude of about 15 microsec.

The novel toroids power supply may be used to transfer medium or high amounts of energy to a few secondaries, which can be located relatively far from each other and are insulated from each other.

Fig. 6 details the current and voltage in the power supply system. The graph 62 illustrates the current in the primary, with stage 621 of rising current, stage 622 of decreasing current, and stage 623 of about zero current.

The graph 63 illustrates the induced voltage in the secondary, with stage 631 of about fixed output voltage being used in the secondary power supply.

In another embodiment, other waveforms may be used rather than the sawtooth waveform. For example, a high frequency sinusoidal waveform may be used. Preferably, a frequency of about 10 kHz or higher is used.

The frequency and amplitude of the sawtooth signals may change, responsive to the current consumption in the secondaries. Fig. 7 details one embodiment of the ferromagnetic circuits in the power supply loop.

A primary loop wire 51 is connected to a primary power supply unit 50, including a (not shown) power source, with optional solid state switches and diodes, to generate a varying current as required. The wire 51 passes through the centers of a plurality of ferromagnetic toroids 55, for example made of ferrite. A secondary coil 56 having N wounds, is mounted on each toroid 55, and is connected to a secondary power supply 57. The power supply 57 generates the required power supply voltage for the (not shown) thyristor firing circuits and other circuits, responsive to the changing current in wire 51 .

The system further includes mechanical supports 71 for the primary loop wire 51 . Supports 71 , which are made of an insulating material, keep the loop wire in place, so that wire 51 passes through the center of each toroid 55, to achieve a maximum clearance 72 between wire 51 and toroid 55.

Preferably, the central wire 51 is inserted in a glass epoxy isolated tube, at least in locations where the wire is supported by supports 71 . Preferably, each insulator tube is supported by two units 71 . The electrical insulation is thus improved, to withstand tens of kV or more. The above structure may achieve both higher electrical insulation and mechanical support.

The toroids can be mounted on a printed circuit board, and do not touch the central wire, the insulation tubes, nor the support means 71 . Preferably, wire 51 is made of a Corona-free wire. In a preferred embodiment, the primary wire 51 is contained in two glass epoxy tubes 51 1 and 512 as shown, with a shrink tube 513 connecting the rigid tubes 51 1 and 512.

Tubes 51 1 and 512 provide both precise mechanical support and positioning, as well as electrical insulation.

A mechanical support 71 is preferably attached close to each end of the tubes 51 1 and 512.

Furthermore, it is easy to install the wire by passing it through both the tubes 51 1 and 512. A shrink tube 513 may be used to connect the two tubes 51 1 and 512, to form a contiguous insulation tube. The internal wire 51 can be easily inserted therein, then the tube is bent as shown to form the primary wire loop.

Preferably, this structure is also used in the primary loop in the structures illustrated in Figs. 8 and 9, for example.

The exact shape of the central wire can be designed according to the location of the individual firing circuits.

In a preferred embodiment, a primary wire 51 is used for each phase. The number of toroids per phase may vary from 1 to 6.

A modular structure is thus achieved, wherein each phase may be disconnected or replaced, independently of the other phases.

This embodiment may be applied to the system illustrated in Figs. 7, 8 and 9.

Different values of N may be required, for additional or different loading. In another embodiment, the number of toroids may vary from 1 to 18, with one current wire 51 being used for the three phases together.

For a 13.8 kV starter, the number of thyristors may be doubled, to 12 per phase or 36 units altogether.

As a general model, if N is identical for all the toroids, then each toroid circuit gets the same current, and its voltage is related to its loading. Therefore, protection Zener diodes with a correct power ratings are preferably used to prevent overvoltage.

The difference between the source voltage and the sum of the reflected voltage causes the linear increase in current, based on the loop inductance. When the switches are switched OFF, the difference is negative, thus creating a constant rate of decrease in the current, while still supplying current to the loads and returning energy to the primary capacitor.

Furthermore, each toroid is preferably located on a plane normal to the wire 51 , for achieving the maximal distance between wire 51 and toroid 55, together with the insulation for the wire and/or the insulated tubes.

This spacing 72 achieves the high quality, high voltage insulation between primary and secondary, as required.

For higher voltages, the inner diameter of toroids 55 can be increased, thus increasing the clearance between toroid 55 and primary wire 51 .

The novel structure can be used with one phase, two phase or three phase circuits. As can be seen in Fig. 7, the number of high voltage wires is dramatically reduced: in prior art each thyristor unit required several high voltage wires, whereas in the novel structure the toroids are mounted on the printed circuit board and there is just one wire 51 that passes through the toroids of all the power supplies, for all the thyristor units in use.

The novel structure can achieve significant savings in labor, including for example the wiring design, production, testing and debugging. Moreover, the reliability of the unit is improved, as the probability of a high voltage breakdown is reduced.

Fig. 8 details another embodiment of the ferromagnetic circuits in the power supply loop.

The structure is similar to that illustrated in Fig. 7 above, with the addition of ferromagnetic toroids 55 mounted on both sides of the primary loop wire 51 . A secondary coil 56 having N wounds, is mounted on each toroid 55 and is connected to a secondary power supply 57, which generates the required power supply voltage for the (not shown) thyristor firing circuits.

In a preferred embodiment, the primary wire 51 is contained in two glass epoxy tubes 51 1 and 512 as shown, with a shrink tube 513 connecting the rigid tubes 51 1 and 512.

A mechanical support 71 is preferably attached close to each end of the tubes 51 1 and 512. The U-shaped structure of Fig. 7, including the glass epoxy tubes and the shrink tube, can be used together with Corona-free wire, to achieve a partial discharge-free structure.

It may be desirable to minimize the internal area 73 delimited by the wire 51 , to minimize the voltages and/or currents being induced into nearby circuits or electrically conducting parts.

Fig. 9 yet details another embodiment of the ferromagnetic circuits in the power supply loop.

This is an example of a structure that may be used to minimize the internal area 73 delimited by the wire 51 . Such a structure may be used to minimize the induced voltages and/or currents in nearby circuits or electrically conducting parts.

This is done by keeping the outgoing and returning current paths in the wire 51 as close as possible, except in the vicinity of the toroids 55, where both wire paths are generally kept at a fixed distance from the toroid, to achieve the high voltage insulation as desired.

Thus, according to the present invention, the primary wire 51 is laid in such a way to keep a minimum predefined distance from the toroids 55, for both the outgoing as well as for the return legs of wire 51 . Other embodiments of the present invention are possible. For example, the toroids 55 can be replaced with rectangular cores of a ferromagnetic material, or a C~core, for example. As long as the minimal distance between wire 51 and core 55 is kept, a reliable high voltage insulation is achieved.

It will be recognized that the foregoing is but one example of an apparatus and method within the scope of the present invention and that various modifications will occur to those skilled in the art upon reading the disclosure set forth hereinbefore.

Claims

1. An electronic potential transformer comprising: a. transmitter means connected to the high voltage mains, and comprising a voltage divider for generating a low voltage proportional to the measured high voltage, means for encoding the low voltage waveform into digital signals indicative thereof, and wave transmitter means for generating wave signals responsive to the digital signals; b. receiver means comprising wave receiver means for receiving wave signals and for generating electrical signals responsive thereto, signal processing means for decoding the received electrical signals to regenerate the measured voltage ; and c. wave media means connected between the transmitter and receiver, for transferring the wave signals from the transmitter to the receiver.

2. The electronic potential transformer according to claim 1 , wherein the wave transmitter means comprise optical transmitter means for generating optical signals responsive to the digital signals, the wave receiver means comprise optical receiver means for receiving optical signals and for generating electrical signals responsive thereto, and the wave media means comprise optical fiber means.

3. The electronic potential transformer according to claim 1 , wherein the wave transmitter and receiver means comprise radio, laser or ultrasonic wave means.

4. The electronic potential transformer according to claim 1 , wherein the transmitter includes two or three measurement channels, and wherein each channel comprises a high voltage divider, means for measuring the waveform of the low voltage and for encoding the measurements into digital signals indicative thereof.

5. The electronic potential transformer according to claim 4, wherein each of the measurement channels is connected to a corresponding optical transmitter means for generating optical signals.

6. The electronic potential transformer according to claim 4, wherein the measurement channels are multiplexed onto a common optical transmitter means for generating optical signals.

7. The electronic potential transformer according to claim 1 , wherein the encoding means in the transmitter uses a pulse width modulation scheme for voltage encoding.

8. The electronic potential transformer according to claim 1 , wherein the encoding means in the transmitter uses analog to digital converter means for voltage encoding.

9. The electronic potential transformer according to claim 1 , wherein the transmitter is contained within an insulated case with means for direct attachment to the high voltage mains, further including means for connecting the three phases high voltage to the voltage dividers in the transmitter, to reduce the required high voltage wiring.

10. A power supply system for supplying electrical power to circuits under high voltage while reducing the number of high voltage wires, comprising: a. a primary wire connected to a power supply comprising means for generating a periodically varying current; b. a plurality of ferromagnetic cores with secondary coils wound thereon, wherein the primary wire passes through all the cores and each secondary coil is connected to a power supply; and c. mechanical support means for keeping the primary wire and the ferromagnetic cores at a predefined relative orientation therebetween, with the primary wire passing over the center of each core and wherein the primary wire is about normal to the plane of each core.

11 . The power supply system according to claim 10, wherein each ferromagnetic core is a toroid.

12. The power supply system according to claim 10, wherein each core has an about rectangular or a C-core shape.

13. The power supply system according to claim 10, wherein the path of the primary wire is laid so as to minimize the internal area of the wire, while keeping a predefined distance from each ferromagnetic core.

14. The power supply system according to claim 10, wherein the power supply includes means for generating a a generally sawtooth-shaped current, including a time period of about a fixed slope current for generating a constant voltage in the secondary.

15. The power supply system according to claim 14, wherein the power supply includes means for synchronizing the time period of about a fixed slope current with the firing trigger signal for the thyristors.

16. The power supply system according to claim 10, wherein the distance between the primary wire and the ferromagnetic core is set to a value corresponding to the high voltage insulation required.

17. The power supply system according to claim 10, wherein the primary wire is inserted in a glass epoxy insulated pipe.

18. The power supply system according to claim 10, wherein the primary wire is made of a Corona-free wire.

19. The power supply system according to claim 10, further including means for supplying power to one phase, two phase or three phase circuits.

20. A soft starter comprising: a. a high voltage input mains; b. a solid state switch means connected between the input mains and an output to a load, further including control circuits and having control inputs for controlling the output power delivered to the load; c. means for sampling the voltage at the high voltage input mains; and d. controller means for receiving the input voltage samples, for computing an instantaneous power to be delivered to the load and for controlling the solid state switch means responsive to the instantaneous power required, further including connections to the solid state switch means and to the means for sampling the voltage at the high voltage input mains.

21 . The soft starter according to claim 20, wherein the means for sampling the voltage at the high voltage input mains comprise an electronic potential transformer comprising: a. transmitter means connected to the high voltage mains, and comprising a voltage divider for generating a low voltage proportional to the measured high voltage, means for measuring the amplitude of the low voltage and for encoding the measurements into digital signals indicative thereof, and optical transmitter means for generating optical signals responsive to the digital signals; b. receiver means comprising optical receive means for receiving optical signals and for generating electrical signals responsive thereto, signal processing means for decoding the received electrical signals to generate the measured voltage ; and c. optical fiber cable means connected between the transmitter and receiver, for transferring the optical signals from the optical transmitter to the optical receiver.

22. The soft starter according to claim 20, wherein the control circuits in the solid state switch means further include a power supply system for supplying electrical power to circuits under high voltage, comprising: a. a primary wire connected to a power supply comprising means for generating a periodically varying current; b. a plurality of ferromagnetic cores with secondary coils wound thereon, wherein the primary wire passes through all the cores and each secondary coil is connected to a power supply; and c. mechanical support means for keeping the primary wire and the ferromagnetic cores at a predefined relative orientation therebetween, with the primary wire passing over the center of each core and wherein the primary wire is about normal to the plane of each core.