DESC:TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to an electronic trip unit for circuit breaker. More particularly, the present invention provides an improved electronic trip unit for fast tripping of circuit breaker during low current ground faults at the instant of closing the breaker.

BACKGROUND AND THE PRIOR ART

Molded Case Circuit Breaker consist of the Electronic Trip unit which comprises of various blocks like power supply, signal sensing, signal conditioning , data processing, Memory management, User interface, data communication, configuration switches for setting the adjustment parameters of the trip unit and tripping mechanism.

An electronic protection device can protect a load from fault events by monitoring the current drawn by the load and disconnecting the load from its power source upon detecting a fault event. The electronic protection device can also be self-powered, that is, the device can be powered from the same current that the device monitors. Self-powered electronic protection devices are conventionally powered from a rectified secondary Winding of a current transformer (CT), with the monitored current sent through a primary winding of the CT.

Conventional current sensing systems for electronic trip units employ a pair of current transformer and Rogowski coil in each phase and in the neutral, if a neutral is used. The current transformer outputs are applied to respective full-wave bridge rectifiers for self powering the trip circuit. The rectified output of each line is then filtered by storage capacitor and Rogowski outputs are applied to signal conditioning circuits for current measurement. A signal related to the measured RMS current is then applied to the trip circuit which causes circuit breaker tripping when measured current of a given magnitude exists for given times.

A circuit protection device includes a trip mechanism and at least one capacitor configured to store electrical energy and to provide the electrical energy to the trip mechanism. The circuit protection device also includes a controller communicatively coupled to the at least one capacitor and configured to measure voltage of at least one capacitor, compare the measured voltage to a threshold, and output a signal indicative of the comparison.

It is important in ground fault protection to have three essential functions basically combined into a single unit; namely, overload protection, short circuit protection, and ground fault protection. Trip units for circuit breakers are used to automatically operate the circuit breaker under fault current conditions. The time required for the circuit breaker to open will depend on the fault current magnitude and nature. Ground fault current magnitudes depend on the system grounding method. Solidly and low impedance grounded systems may have high levels of ground fault currents. These high levels typically require line tripping to remove the fault from the system which can be easily achieved. However, high-impedance ground fault tripping is critical in self powered protection devices because the time taken to trip this fault at the instant of closing the breaker depends on the storage capacitor voltage. At lower currents the storage capacitor takes more time to charge and hence delayed tripping of the fault. Conventionally the CT turns ratio is increased to increase the secondary current, so that the storage capacitor is charged quickly and hence fast tripping is achieved even at low currents. However this method requires modifications in CT design, current ratings of electronic components which involve increase in size and cost.

Hence there is a need of inexpensive solution for fast tripping of the circuit breaker during low current ground fault condition at the instant of closing the breaker.

OBJECTS OF THE INVENTION

A basic object of the present invention is to overcome the disadvantages/drawbacks of the known art.

Another object of the present invention is to provide an improved electronic trip unit for fast tripping of circuit breaker during low current ground faults at the instant of closing the breaker.

These and other advantages of the present invention will become readily apparent from the following detailed description read in conjunction with the accompanying drawings.

SUMMARY OF THE INVENTION

The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the present invention. It is not intended to identify the key/critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concept of the invention in a simplified form as a prelude to a more detailed description of the invention presented later.
In an aspect of the present invention, there is provided an arrangement for fast tripping of self powered circuit breaker in low current ground fault, said arrangement comprising:
a power supply connected to a load;
plurality of sensor(s) means;
a trip mechanism circuit for tripping said circuit breaker;
a signal sensing and conditioning circuit;
a controller means;
a divider circuit;
wherein a capacitive switching circuit comprises at least two storage capacitor(s) connected in series and parallel configuration with plurality of switch(es) and a switching mean, said capacitive switching circuit communicatively coupled with said controller means to facilitate the tripping in said circuit breaker during low current ground fault condition.

Other aspects, advantages, and salient features of the invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses exemplary embodiments of the invention.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

The following drawings are illustrative of particular examples for enabling methods of the present invention, are descriptive of some of the methods, and are not intended to limit the scope of the invention. The drawings are not to scale (unless so stated) and are intended for use in conjunction with the explanations in the following detailed description.

Figure 1 illustrates the block diagram of an electronic protection device

Figure 2 illustrates the block diagram of a conventional trip circuit.

Figure 3 illustrates the block diagram of a new trip circuit.

Figure 4 illustrates the initial position of the capacitor switches at the instant of closing the breaker.

Figure 5 illustrates the final position of the capacitor switches at the instant of closing the breaker.

Persons skilled in the art will appreciate that elements in the figures are illustrated for simplicity and clarity and may have not been drawn to scale. For example, the dimensions of some of the elements in the figure may be exaggerated relative to other elements to help to improve understanding of various exemplary embodiments of the present disclosure.
Throughout the drawings, it should be noted that like reference numbers are used to depict the same or similar elements, features, and structures.

DETAILED DESCRIPTION OF THE INVENTION

The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of exemplary embodiments of the invention as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. In addition, descriptions of well-known functions and constructions are omitted for clarity and conciseness.

The terms and words used in the following description and claims are not limited to the bibliographical meanings, but, are merely used by the inventor to enable a clear and consistent understanding of the invention. Accordingly, it should be apparent to those skilled in the art that the following description of exemplary embodiments of the present invention are provided for illustration purpose only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.

It is to be understood that the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise.

By the term “substantially” it is meant that the recited characteristic, parameter, or value need not be achieved exactly, but that deviations or variations, including for example, tolerances, measurement error, measurement accuracy limitations and other factors known to those of skill in the art, may occur in amounts that do not preclude the effect the characteristic was intended to provide.

Accordingly, present invention provides an improved electronic trip unit for fast tripping of circuit breaker during low current ground faults at the instant of closing the breaker without changing CT design by implementing series and parallel switching of the storage capacitors.
High-impedance ground fault tripping is critical in self powered protection devices because the time taken to trip this fault at the instant of closing the breaker depends on the storage capacitor voltage. At lower currents the storage capacitor takes more time to charge and hence delayed tripping of the fault. Conventionally (figure 1) the CT turns ratio is increased to increase the secondary current, so that the storage capacitor is charged quickly and hence fast tripping is achieved even at low currents. However this method requires modifications in CT design, current ratings of electronic components which involve increase in size and cost.

In the presented Invention use of switched capacitors in series and parallel combination (Figure 3) provides 1.3 times faster tripping compared to the conventional method (Figure 2) during low current ground fault condition at the instant of closing the breaker. In addition, this method improves the quality factor of storage capacitor and hence its life time.

The present invention provides an electronic trip unit, which employs, as major components, a power supply, a signal conditioning circuit for each line, data processing, data communication, Memory management, User interface, and configuration switches for setting the adjustment parameters of the trip unit. The circuit is applicable to both three and four wire systems. Opening of the circuit breaker is accomplished by a conventional mechanical mechanism which is activated by an electromagnetic latching mechanism triggered by the electronic trip unit.

The power supply section consists of output from current transformer, Bridge rectifier, Capacitor switching circuit, and voltage regulator section for microcontroller, communication section, and display section. As mentioned previously there is a need of inexpensive solution for fast tripping of the circuit breaker during low current ground fault condition at the instant of closing the breaker, here incorporated a capacitive switching circuit instead of single storage capacitor. The advantage of fast tripping during low ground fault currents is achieved by switching the storage capacitors in series and parallel configuration.

Microcontroller/microprocessor (MCU), Where said MCU measures the voltage of storage capacitors, Where said MCU commands the current that flows from said full wave bridge rectifier circuit network to said storage capacitors, said MCU controls the switches in the capacitive network by turning them ON or OFF. In this way the storage capacitors can be charged to the required voltage 1.3 times faster and hence fast tripping can be achieved.

Figure 1 is a block diagram of an electronic protection device 14 configured to provide protective functions to a protected load 3 connected to a power source 1. The power source 1 can be a polyphone alternating current (AC) power source. The power source 1 can include three conductive lines 9, 10, 11 for providing three phases of a supply current, and can be arranged according to a delta or wye configuration. In a configuration, the protected load 3 can be an inductive motor load. The electronic protection device 14 includes a first sensor 4, a second sensor 5, a third sensor 6, a controller 7, and a trip mechanism 8.

The controller 7 receives the outputs of the three sensors 4, 5, 6. While the power source 1 is illustrated having three conductors carrying three phases of a supply current, the present disclosure applies to implementations having polyphase power sources with supply currents having more than three or fewer than three phases, such as a single phase power source.
The controller 7 is connected to the trip mechanism 8 and is configured to actuate the trip mechanism 8 responsive to detecting a fault event. Actuating the trip mechanism 8 disconnects the protected load 3 from the power source 1 by activating a switch 2.
In an implementation of the present disclosure, the trip mechanism 8 can be actuated by sending current through an actuator coil. The current can be sent through the actuator coil by discharging energy in the storage capacitors C1, C2 by biasing a transistor to complete a circuit including the energy storage and the actuator coil. The actuator coil can be a portion of a solenoid, and the mechanical movement due to actuation of the solenoid can cause a mechanical lever, rod, linkage, or rotating element to open electrical contacts in the trip mechanism 8, either directly or indirectly.

The controller 7 can be a microcontroller, an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), or another electronic device suitable for receiving signals indicative of an electrical characteristic (e.g., current or voltage) of the power source 1. The controller 7 can analyze the received signals to determine whether a fault condition has occurred, and causes the trip mechanism 8 to actuate by issuing the trip signal 12.

In an implementation, the electronic protection device 14 can be self-powered, meaning that the electronic components of the electronic protection device 14 are powered by the same current or voltage that the electronic protection device 14 is monitoring. In other Words, by self powered, it is meant that the electronic protection device 14 does not have an independent power supply, but rather derives its power from the conductive line or lines 9, 10, 11 it is monitoring.

The self-powered electronic protection device 14 includes a current transformer (CT) and Rogowski 4 outputs. The current carried by the first conductive line 9 can be configured to flow through a primary winding of the CT and Rogowski, such that the current in the CT and Rogowski secondary Windings are proportional to the current flowing through the first conductive line 9. The CT secondary Winding has a first and second output connected to a rectifier 15. The rectifier 15 rectifies the alternating current from the CT secondary Winding to provide a direct current (DC) power source to the remaining electronic components within the self-powered electronic protection device 14.

In an implementation of the self-powered electronic protection device 14, the switch S3 is used to regulate the DC bus voltage 17 of the trip mechanism circuit. Closing the switch S3 causes DC current to flow back to the anode of the rectifier 15. The controller 7 can be configured to periodically close and open the switch S3 by issuing the switch signal to provide a desired DC bus voltage 17 for the trip mechanism circuit.

Figure 2 is a block diagram of a conventional trip circuit 14 for a self-powered electronic protection device. The circuit 14 includes a rectifier 15 configured as a diode bridge rectifier for rectifying AC current from the CT secondary Winding. The rectifier 15 includes four rectifying diodes D1, D2, D3, and D4. The first output of the CT secondary Winding is connected to the anode of the first rectifying diode D1 and the cathode of the fourth rectifying diode D4. The second output of the CT secondary Winding is connected to the anode of the second rectifying diode D2 and the cathode of the third rectifying diode D3. The cathodes of the first rectifying diode D1 and the second rectifying diode D2 are both connected to the DC bus line 17, and the anodes of the third rectifying diode D3 and the fourth rectifying diode D4 are both connected to the common ground. The rectifier 15 provides a DC power source from the CT secondary Winding to the trip mechanism circuit 8, and DC- DC regulator 16. The DC bus voltage 17 is built by charging the storage capacitors C1. The DC- DC regulator 16 is used for stepping down the Bus voltage 17 to lower voltage for controller 7, communication section and display section.

Figure 3 shows the new method used for quickly charging the storage capacitor during low current ground faults at the instant of closing the breaker. Here the DC bus voltage 17 is built by charging the storage capacitors C1, C2 of equal capacitance using switches S1 and S2. Switches S1 and S2 are used for quick charging of the Storage capacitors. The potential divider circuit R1 and R2 is used to sense the DC bus voltage 17 for operating the Switches S1, S2, and S3.

Figure 4 shows the initial position of the switches S1 and S2 of storage capacitors C1 and C2. In this position capacitors C1, C2, and diode D5 are connected in series and switches S1 and S2 are in open position. Diode D5 used to protect the short circuit of capacitor C2 when switch S2 is closed. The moment circuit breaker 2 closed, the current transformer (CT) starts charging the series connected capacitors C1 and C2. As the capacitance of C1 and C2 are same, both will charge to the same voltage. The voltage across capacitor C2 is sensed by potential divider circuit R1, and R2. The sensed signal 19 is given to the controller 7, which in turn will give command to switches S1, S2, and S3. Figure 5 shows position of the switches S1 and S2 when the voltage across capacitor C2 is greater than half of the DC bus voltage 17. In this position capacitor C1, and C2 are connected in parallel and Switches S1, and S2 are in closed position, where as Diode D5 is reverse biased. Here switch S3 is used only to regulate the DC bus voltage 17. By connecting capacitors initially in series and then in parallel, the time taken to charge the storage capacitors C1, and C2 to the required Dc bus voltage 17 is reduced to 70% and hence fast tripping is achieved during low current ground faults at the instant of closing the breaker. In addition, as two parallel capacitors are used, the effective ESR of the storage is reduced by half and hence reduction in losses, which in turn improves the quality factor of the storage capacitor.

BENEFITS
· In the present invention, the circuit breaker trips the fault 1.3 times faster compared to the conventional method during low current ground fault condition at the instant of closing the breaker.
· 50% reduction in the ESR of storage capacitor and hence reduction in resistive power loss.
· Improves the quality factor of capacitor and hence its life.
,CLAIMS:1. An arrangement for fast tripping of a self powered circuit breaker in low current ground fault condition, said arrangement comprising:
a power supply connected to a load;
plurality of sensor(s) means;
a trip mechanism circuit for tripping said circuit breaker;
a signal sensing and conditioning circuit;
a controller means;
a divider circuit;
wherein a capacitive switching circuit comprises at least two storage capacitor(s) connected in series and parallel configuration with plurality of switch(es) and a switching mean, said capacitive switching circuit communicatively coupled with said controller means to facilitate the tripping in said circuit breaker during low current ground fault condition.
2. The arrangement as claimed in claim 1, wherein charging of said storage capacitor(s) is facilitate by ON or OFF switching of said switches by using said controller means.

3. The arrangement as claimed in claim 1, wherein connection of said storage capacitor(s) initially in series and then in parallel reduces the charging time, therefore facilitate the tripping in said circuit breaker.

4. The arrangement as claimed in claim 1, wherein said controller means analyzing the signal received from said power supply through said sensor means and said capacitive switching circuit for determining the fault condition.

5. The arrangement as claimed in claim 3, wherein said controller means actuating said trip mechanism by issuing a trip signal, therefore disconnecting said load from the power source by activating at least one of said switch in said capacitive switching circuit.

6. The arrangement as claimed in claim 1, wherein said trip mechanism is actuated by sending current through an actuator coil by discharging energy in said storage capacitor(s).

7. The arrangement as claimed in claim 1, wherein one of said switch regulating the DC bus voltage of said trip mechanism.

8. The arrangement as claimed in claim 1, wherein said controller means selected from a group comprising of a microcontroller, an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) and other electronic device.