DESC:Self Powered Circuit Breaker

Field of the invention

The present invention relates to circuit breakers and more particularly, to a self powered circuit breaker having a trip unit for fast tripping during critical faults like short circuit and low current ground fault conditions at the instant of closing the circuit breaker.

Background of the invention

An electronic protection device such as a circuit breaker protects a load from fault events by monitoring the current drawn by the load and disconnecting the load from a power source upon detecting the fault event. The circuit breaker consist of a trip unit which comprises various blocks for power supply, signal sensing, signal conditioning , data processing, memory management, data communication, a user interface and configuration switches for setting the adjustment parameters of the trip unit and a tripping mechanism.

The circuit breaker can also be self-powered by deriving the same current that it monitors. The self-powered circuit breaker is 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 the trip units employ a pair of current transformers (CT) and a Rogowski coil in each phase and in a neutral, if the neutral is used. The CT outputs are applied to respective full-wave bridge rectifiers for self powering a trip circuit. The rectified output of each line is then filtered by a storage capacitor and the Rogowski coil 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 the circuit breaker tripping when the measured current of a given magnitude exists for given times above a set threshold.

The trip units for the circuit breakers are used to automatically operate the circuit breaker under fault current conditions. As soon as the circuit breaker is closed, for the trip unit to be able to function properly with a microcontroller, it is necessary to wait a short but significant start-up time Tst, in which the electrical and electronic parts are subject to a transient that brings them up to steady-state conditions. Once this time has elapsed, the trip unit is able to perform the normal functions of protection and to control opening of the circuit by the circuit-breaker.

The time required for the circuit breaker to open depends 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 the self powered circuit breakers 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.

Thus, one of the data characterizing the trip unit is the start up time Tst of the trip unit itself. The start-up time Tst depends upon different factors which are listed below:

• power-supply start-up time (Tps) of the supply device of the trip unit defined above, the supply device having as an input a stretch of electrical network protected by the circuit breaker and as an output a supply adequate for the characteristics of the trip unit,
• characteristic time of stabilization of hardware components of the trip unit (Thw) necessary for operation of the microcontroller (for example, an oscillator),
• characteristic time of initialization of the software (Tsw) present in the microcontroller, and
• time for calculating the currents (Tc), i.e., the time necessary for processing the signal coming from current sensors in a form useful for generating the signal for actuation of the protections.

Thus, the start-up time Tst is given by the sum of these partial times, namely, Tst=Tps+Thw+Tsw+Tc. Hence, any failure or fault that may occur in the part of the system protected by the circuit-breaker during this time will prevent adequate interpretation of the Tst by the trip unit.

Attempts have been made in the art to introduce progressively more rapid systems and components capable of reducing the time Tst to values in the region of few milli seconds (ms) like 5 ms. However, just the reduction of Tst is insufficient to set the circuit-breaker in conditions of safety in the case of instantaneous short circuits where tripping is desirable in times in the region of 2 ms. There arises a problem that the detection delay of approx 1.4 ms causes an increase in the interrupting current at the time of interruption and the passing energy due to the interrupting current in an instantaneous tripping operation when a short circuit happens in the turn-on mode of the circuit breaker, which disadvantageously affects the interruption performance of the circuit breaker.

This is illustrated in further detail with the help of a graphical representation as shown in figure 1. Figure 1 is an example showing a relation of the interrupting currents (in kA) and the operating time (in ms) in case of instantaneous short circuit when the operation delay of 1.4 ms occurs in the over current detection with the estimated short-circuit current Ip defined as 30 kA. The short-circuit current (Ip) is 17 kA in the case of the 2 ms operating time without the 1.4 ms delay from the short-circuit current starting up to the interruption after an over current is detected. The short circuit current is 26 kA in case of the 3.4 ms operating time with 1.4 ms delay. The difference there between is about 9 kA.

Accordingly, there is a need of a self powered circuit breaker that overcomes the above mentioned drawbacks of the prior art.

Objects of the invention

An object of the present invention is to provide an inexpensive solution for fast tripping of a circuit breaker during critical fault conditions at the instant of closing the breaker.

Another object of the present invention is to reduce a start-up time for fast tripping of the circuit breaker during the critical fault conditions such as a short circuit fault and a low current ground fault at the instant of closing the breaker.

Yet another object of the present invention is to provide a self powered circuit breaker control method to ameliorate the performance during short circuit and ground fault conditions.

Summary of the invention

Accordingly, the present invention provides a self powered circuit breaker. The self powered circuit breaker is configured between a power source and a load. The load is connected to the power source through a first switch connected there between. The power source is selected from any one of a single phase power source and a multiphase power source. The self powered circuit breaker comprises at least one sensor, a trip unit and a trip mechanism.

The at least one sensor is connected to the power source to sense supply current flowing there through and generate outputs in response thereto. The at least one sensor includes a current transformer and a Rogowski coil. The current transformer and the Rogowski coil include a primary winding and two secondary windings.

The trip unit is connected to the at least one sensor. The trip unit includes a rectifier, a storage capacitor, a second switch, a first regulator, a second regulator, a control logic, a signal conditioning circuit and a controller.

The rectifier is adapted to receive and rectify the outputs of the secondary windings of the current transformer into a direct current (DC) power. The rectifier is a diode bridge rectifier that includes four rectifying diodes. The storage capacitor is operably connected to the rectifier to store the DC power generated therefrom and build up a DC bus voltage. The second switch is operably connected to the storage capacitor to regulate the DC bus voltage upon receiving a first signal from the controller.

The first regulator and the second regulator are adapted for stepping down the DC bus voltage to a lower voltage. The control logic is adapted to change a power supply configuration by switching between the first regulator and the second regulator upon receiving a second signal from the controller. The first regulator is a linear regulator used during a short circuit fault condition. The second regulator is a switched mode power supply regulator used during a low current ground fault condition and a normal condition.

The signal conditioning circuit is adapted to receive the outputs of the secondary windings of the Rogowski coil of the at least one sensor. The controller is operably connected to the signal conditioning circuit to receive conditioned outputs of the secondary windings of the Rogowski coil of the at least one sensor there through. The controller is adapted to generate the first signal for sending to the second switch, the second signal for sending to the control logic and to analyze the outputs to determine a fault condition and generate a third signal for sending to the trip mechanism in response to the fault condition. The controller is connected to an external clock to receive a clock supply therefrom.

The trip mechanism is operably connected to the controller to receive the third signal therefrom for actuation thereof during the fault condition to generate a fourth signal to activate the first switch for disconnecting the load from the power source thereby resulting in fast tripping of the self powered circuit breaker.

Brief description of the drawings

Other features as well as the advantages of the invention will be clear from the following description.
In the appended drawings:

Figure 1 shows a waveform diagram showing a relation of interrupting current values when an operation start delay happens in accordance with the prior art over current detecting circuits;

Figure 2 shows a block diagram of a self powered circuit breaker, in accordance with the present invention;

Figure 3 shows a circuit diagram of a trip unit for the self powered circuit breaker, in accordance with the present invention; and

Figure 4 shows a flow chart illustrating working of the trip unit for the self powered circuit breaker, in accordance with the present invention.

Detailed description of the invention

The foregoing objects of the present invention are accomplished and the problems and shortcomings associated with the prior art, techniques and approaches are overcome by the present invention as described below in the preferred embodiment.

The present invention provides a self powered circuit breaker (herein after ‘the circuit breaker’). The circuit breaker includes a trip unit that uses a hardware configuration to monitor the current at the instant of closing the circuit breaker and switches a power supply configuration upon detecting a specific fault condition. The trip unit reduces a wait time required for charging a capacitor to actuate the circuit breaker and thus reduces a start-up time to achieve fast tripping of the circuit breaker during critical faults like short circuit and low current ground fault conditions at the instant of closing the circuit breaker thereby ameliorating the performance.

The present invention is illustrated with reference to the accompanying drawings, throughout which reference numbers indicate corresponding parts in the various figures.

Referring now to figures 2 and 3, in one aspect, a circuit breaker (200) in accordance with the present invention is shown. The circuit breaker (200) is configured between a power source (110) and a load (120) for example, an inductive motor load. The power source (110) is a polyphase alternating current (AC) power source that includes one or more conductive lines/ conductors (herein after ‘the conductive line’) (not numbered). Specifically, the power source (110) is a multiphase power source having more than three conductive lines (110A, 110B and 110C) arranged in a delta or wye configuration for providing three phases of supply current to the load (120) as shown in figure 2. However, it is understood that the circuit breaker (200) can also be applicable to the multiphase power source having fewer than three conductive lines as well as to a single phase power source. The power source (110) is connected to the load (120) to provide supply current thereto via a first switch (130) connected there between. The circuit breaker (200) protects the load (120) from fault current conditions.

Specifically, the circuit breaker (200) is a self powered circuit breaker that derives power from the conductive line being monitored rather than using an independent power supply. Opening of the circuit breaker (200) is accomplished by a conventional mechanical mechanism (not shown) that is activated by an electromagnetic latching mechanism (not shown) that in turn is triggered by the trip unit (100). The circuit breaker (200) comprises at least one sensor (140), a trip unit (100) and a trip mechanism (90).

The at least one sensor (140) (herein after ‘the sensor (140)’) is connected to the power source (110) specifically, to the conductive line of the power source (110) to sense / monitor supply current flowing there through and generate outputs in response thereto. In an embodiment, the circuit breaker (200) includes three sensors (140A, 140B and 140C) connected to the respective conductive lines (110A, 110B and 110C) of the power source (110).

The sensor (140) includes a current transformer (herein after ‘the CT’) (not shown) and a Rogowski coil (herein after ‘the RC’) (not shown). The CT and the RC include a primary winding (not shown) and two secondary windings (not shown). The supply current carried by the conductive line flows through the primary windings of the CT and of the RC of the sensor (140) such that the current flowing through the secondary windings of the CT and the RC is proportional to the current flowing through the conductive line.

The secondary windings of the CT and the RC generate two outputs in response to the current flowing there through. Specifically, the outputs of the secondary winding of the CT of the sensor (140) are in the form of alternating current (AC). The outputs of the secondary winding of the CT are connected to the trip unit (100). In an embodiment, the trip unit (100) is applicable to both three and four wire systems.

The trip unit (100) is connected to the at least one sensor (140). The trip unit (100) comprises a rectifier (20), a storage capacitor (30), a second switch (40), a first regulator (50A), a second regulator (50B), resistors (55A, 55B), a control logic (60), a signal conditioning circuit (70) and a controller (80). Further, the trip unit (100) also comprises a power supply section (not numbered), a signal conditioning section (not numbered), a data processing section (not shown), a data communication section (not shown), a memory management section (not shown), a display / user interface section (not shown) and configuration switches (not shown) for setting adjustment parameters thereof.

Specifically, the outputs of the CT secondary windings are connected to the trip unit (100) via the rectifier (20) of the trip unit (100) and the outputs of the RC secondary windings are connected to the controller (80) via the signal conditioning circuit (70) of the trip unit (100).

The rectifier (20) is adapted to receive and rectify the outputs of the CT secondary windings of the CT into a direct current (DC) power. Specifically, the rectifier (20) is a diode bridge rectifier that includes four rectifying diodes for example, a first rectifying diode (20A) (herein after ‘the diode (20A)’), a second rectifying diode (20B) (herein after ‘the diode (20B)’), a third rectifying diode (20C) (herein after ‘the diode (20C)’) and a fourth rectifying diode (20D) (herein after ‘the diode (20D)’). One of the two outputs of the CT secondary winding is connected to an anode (not shown) of the diode (20A) and a cathode (not shown) of the diode (20C). Another output of the two outputs of the CT secondary windings is connected to an anode (not shown) of the diode (20B) and a cathode (not shown) of the diode (20D). The cathodes of the diodes (20A and 20B) are connected to a DC bus voltage (V1) alternatively referred as DC bus line or actuator voltage. The anodes of the diodes (20C and 20D) are connected to a common ground (not numbered). The rectifier (20) rectifies the AC outputs of the CT secondary windings to provide the DC power to the remaining electronic components within the circuit breaker (200).

The storage capacitor (30) is operably connected to the rectifier (20) to store the DC power generated therefrom. The storage capacitor (30) is charged to build up the stored DC power into the DC bus voltage (V1). The second switch (40) is operably connected to the storage capacitor (30) to regulate the DC bus voltage (V1) upon receiving a first signal (S1) from the controller (80). The second switch (40) is periodically opened and closed in response to the first signal (S1) to provide the DC bus voltage (V1) as desired for the trip mechanism (90). The second switch (40) when closed causes the rectified DC power to flow back to the anodes of the rectifier (20).

The regulator (50A/50B) is adapted for stepping down the bus voltage (V1) to a lower voltage (V2) for the controller section, the communication section and the display section. Specifically, the first regulator (50A) is a linear/ LDO regulator and the second regulator (50B) is a switched mode power supply/ SMPS regulator. A power supply configuration is capable of being switched to any of the first regulator (50A) and the second regulator (50B) depending on a fault condition such as a short circuit fault condition and a low current ground fault condition at the instant of closing the circuit breaker (200).

The power supply configuration at start-up is switched to the first regulator (50A) during the short circuit fault condition at the instant of closing the circuit breaker (200) and to the second regulator (50B) during the low current ground fault condition at the instant of closing the circuit breaker (200) for reducing the power supply start-up time and thereby achieving fast tripping of the circuit breaker (200).

Table 1
Current (A) DC bus voltage (V1) Buildup time (ms) Controller voltage Buildup time(ms)
First regulator (50A) Second regulator (50B) First regulator (50A) Second regulator (50B)
80 140 80 16 17
6000 2.4 2.5 0.80 0.90
10000 2.0 2.4 0.70 0.80
15000 1.2 2.0 0.4 0.80

The table 1 shows that the first regulator (50A) takes less time for building the required voltage in case of the short circuit fault condition and the second regulator (50B) takes less time for building the required voltage in case of the low current ground fault condition.

The control logic (60) is adapted to change the power supply configuration by switching between the first regulator (50A) and the second regulator (50B) upon receiving a second signal (S2) from the controller (80) during the closing of the circuit breaker (200). Specifically, the control logic (60) switches the power supply configuration to the first regulator (50A) by default at the instant of closing the circuit breaker (200) and to the second regulator (50B) during the low current ground fault condition and a normal condition.

The signal conditioning circuit (70) is adapted to receive the outputs of the secondary windings of the RC of the sensor (140).

The controller (80) is operably connected to the signal conditioning circuit (70) to receive conditioned outputs of the secondary windings of the Rogowski coil of the at least one sensor (140) there through. Specifically, the controller (80) is a microcontroller. However, it is understood that an application specific integrated circuit (ASIC), a microprocessor, a field programmable gate array (FPGA) or any other electronic device suitable for receiving signals indicative of an electrical characteristic (e.g., current or voltage) of the power source (110) can also be used in other alternative embodiments of the present invention. The controller (80) is further connected to an external crystal (95).

The external crystal (95) is a crystal oscillator used for supplying a clock to the controller (80) with an accurate and stable frequency. The quality factor for the external crystal (95) is typically on the order of 100,000 as compared to the quality factor of an internal oscillator for example a typical RC oscillator which is on the order of 100. The external crystal (95) therefore has much lower phase noise and much lower variation in the output frequency. Another reason for the external crystal (95) is choice of frequency. Crystals come in a wide range of frequencies whereas the internal oscillator is usually one frequency.

The controller (80) is adapted to generate and send the first signal (S1) to the second switch (40) and the second signal (S2) to the control logic (60). The controller (80) measures the currents at the starting as well as analyzes the outputs of the RC secondary windings of the sensor (140) to determine presence of the fault condition such as the short circuit condition and the low current ground fault condition. In response to the detection of the fault condition, the controller (80) generates a third signal (S3) for sending to the trip mechanism (90) required for opening the circuit breaker (200) contacts.

The trip mechanism (90) is operably connected to the controller (80) to receive the third signal (S3) therefrom for actuation thereof. The trip mechanism (90) is also operably connected to the rectifier (20) to receive the rectified DC power therefrom. In an alternate embodiment, the trip mechanism (90) is actuated by an actuator coil (not shown). The actuator coil sends current to the trip mechanism (90) by discharging energy in the storage capacitor (30) by biasing a transistor (not shown) to complete the circuit including the energy storage and the actuator coil. The actuator coil is a portion of a solenoid, and the mechanical movement due to the solenoid actuation causes a mechanical lever, a rod, a linkage, or a rotating element to open electrical contacts of the circuit breaker (200), either directly or indirectly. The trip mechanism (90) upon actuation generates a fourth signal (S4) to activate the first switch (130). The first switch (130) upon activation disconnects the load (120) from the power source (110) thereby resulting in opening the circuit breaker (200) and thus fast tripping of the circuit breaker (200).

Referring now to figure 4, in another aspect, a method (300) for tripping the circuit breaker (200) using the trip unit (100) of figures 2 and 3, in accordance with the present invention is described.

During a normal current condition, the controller (80) performs functions like communicating with other connected modules and displaying the measured signals like current, voltage, power, and energy consumption of the load (120).

At the instant of closing the circuit breaker (200), the trip unit (100) builds the required supply voltage to the controller (80) with the first regulator (50A). Since the power supply built up time is less, the controller (80) starts sampling the currents and computes the current values. If the measured current (Imeasured) is greater than a threshold current (IHTh) value then the controller (80) issues the third signal (S3) which is the case in instantaneous short circuit fault condition or else the controller (80) switches the power supply configuration to the second regulator (50B). If the measured current (Imeasured) is less than a threshold current (ILTh) value then the controller (80) checks whether the DC bus voltage (V1) is built up or not. If the DC bus voltage (V1) is available then the controller (80) issues the third signal (S3) or else waits till the DC bus voltage (V1) is available. This is described below in further details:

When the fault condition such as short circuit condition is present at the instant of closing the breaker, the controller (80) computes the current values quickly since the default first regulator (50A) is used at the starting. The controller (80) upon detecting the existence of the short circuit fault sends the third signal (S3) to the trip mechanism (90). The trip mechanism (90) upon receiving the third signal (S3) from the controller (80) gets actuated. Upon actuation, the trip mechanism (90) generates a fourth signal (S4) to activate the first switch (130). The first switch (130) upon activation disconnects the load (120) from the power source (110) thereby resulting in opening the circuit breaker (200) and thus fast tripping of the circuit breaker (200).

When the fault condition such as low current ground fault condition is present at the instant of closing the breaker, the controller (80) computes the current values quickly since the default first regulator (50A) is used at the starting. The controller (80) upon detecting the non-existence of short circuit fault sends the second signal (S2) to change the regulator configuration from the first regulator (50A) to the second regulator (50B) and the controller (80) again checks existence low current ground fault condition. If the fault condition is detected, the controller (80) then checks whether sufficient DC bus voltage (V1) is built up or not. Once the sufficient DC bus voltage (V1) is present, the controller (80) sends the third signal (S3) to the trip mechanism (90). The trip mechanism (90) upon receiving the third signal (S3) from the controller (80) gets actuated. Upon actuation, the trip mechanism (90) generates a fourth signal (S4) to activate the first switch (130). The first switch (130) upon activation disconnects the load (120) from the power source (110) thereby resulting in opening the circuit breaker (200) and thus fast tripping of the circuit breaker (200).

Advantages of the invention

1. The trip unit (100) achieves faster tripping during the short circuit fault condition as well as during the low current ground fault condition at the instant of closing the circuit breaker (200) as compared to the conventional methods.
2. The trip unit (100) uses different techniques of regulation based on supply current for reducing the power build up time and thus issuing early tripping.
3. The trip unit (100) uses the controller (80) for achieving a reliable digital control of switching between the regulators (50A, 50B). At the instant of closing the circuit breaker (200), the default regulator configuration means the first regulator (50A) is used and once the controller (80) detects the non-existence of short circuit fault, regulator configuration will be changed to the second regulator (50B) by the controller (80) using the second signal (S2).

The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the present invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the present invention and its practical application, and to thereby enable others skilled in the art to best utilize the present invention and various embodiments with various modifications as are suited to the particular use contemplated. It is understood that various omissions and substitutions of equivalents are contemplated as circumstances may suggest or render expedient, but such omissions and substitutions are intended to cover the application or implementation without departing from the spirit or scope of the claims of the present invention.

,CLAIMS:We claim:

1. A self powered circuit breaker (200), the self powered circuit breaker (200) configured between a power source (110) and a load (120), the load (120) being connected to the power source (110) through a first switch (130) connected there between, the self powered circuit breaker (200) comprising:
• at least one sensor (140) connected to the power source (110) to sense
supply current flowing there through and generate outputs in response thereto, the at least one sensor (140) having a current transformer and a Rogowski coil, the current transformer and the Rogowski coil having a primary winding and two secondary windings;
• a trip unit (100) connected to the at least one sensor (140), the trip unit
(100) having,
a rectifier (20) adapted to receive and rectify the outputs of the secondary windings of the current transformer into a direct current (DC) power,
a storage capacitor (30) operably connected to the rectifier (20) to store the DC power generated therefrom and build up a DC bus voltage (V1),
a second switch (40) operably connected to the storage capacitor (30) to regulate the DC bus voltage (V1) upon receiving a first signal (S1),
a first regulator (50A) and a second regulator (50B), the regulator (50A/50B) adapted for stepping down the DC bus voltage (V1) to a lower voltage (V2),
a control logic (60) adapted to change a power supply configuration by switching between the first regulator (50A) and the second regulator (50B) upon receiving a second signal (S2),
a signal conditioning circuit (70) adapted to receive the outputs of the secondary windings of the Rogowski coil of the at least one sensor (140), and
a controller (80) operably connected to the signal conditioning
circuit (70) to receive conditioned outputs of the secondary windings of the Rogowski coil of the at least one sensor (140) there through, the controller (80) adapted to generate the first signal (S1) for sending to the second switch (40), the second signal (S2) for sending to the control logic (60) and to analyze the outputs to determine a fault condition and generate a third signal (S3) in response thereto; and
• a trip mechanism (90) operably connected to the controller (80) to
receive the third signal (S3) therefrom for actuation thereof during the fault condition to generate a fourth signal (S4) to activate the first switch (130) for disconnecting the load (120) from the power source (110) thereby resulting in fast tripping of the self powered circuit breaker (200).

2. The self powered circuit breaker (200) as claimed in claim 1, wherein the power source (110) is selected from any of a single phase power source and a multiphase power source.

3. The self powered circuit breaker (200) as claimed in claim 1, wherein the rectifier (20) is a diode bridge rectifier having four rectifying diodes (20A, 20B, 20C and 20D).

4. The self powered circuit breaker (200) as claimed in claim 1, wherein the first regulator (50A) is a linear regulator used during a short circuit fault condition.

5. The self powered circuit breaker (200) as claimed in claim 1, wherein the second regulator (50B) is a switched mode power supply regulator used during a low current ground fault condition and a normal condition.

6. The self powered circuit breaker (200) as claimed in claim 1, wherein the controller (80) is connected to an external clock (95) to receive a clock supply therefrom.