IL287821A - Monitoring apparatus for electrical site - Google Patents

Description

MONITORING APPARATUS FOR ELECTRICAL SITE TECHNICAL FIELD The invention relates to the field of electrical sites management and monitoring.

BACKGROUND The term "electrical site" refers herein to a group of electrical appliances connected to different electrical branches, all powered by one or more electrical sources.

Fig. 1 is a prior art electrical site.

At unmanned times (night, weekend, etc.) the electrical site’s branches are divided into two groups: 1. Branches which do not carry current. 2. Branches which do carry current.

The risk of fire breakout, during unmanned time, comes from either group, if: Case 1: In a branch which does not carry current, some current, considered as fault current, has started to flow. (this includes the case where some appliances have been forgotten to be switched off during start of monitoring by the apparatus at the beginning of unmanned time) Case2: In a branch which already carries current the current has turned into fault current as a result of a fault in any of the appliances or the wires of the branch.

The monitoring apparatus according to this invention detects these two cases which pose a threat of fire and interrupts the current to the branch in order to prevent fire breakout.

-The detection of a fault current in case1 is simple and requires just to identify, by a current measuring mean, that current has started to flow. This is done by presetting some fixed threshold level which the current has to cross.

-The detection that the current in case2 has turned into a fault current is not simple and not obvious, as the current does not turn into fault current at fixed threshold level. (excluding leakage current that turns into fault current only when crossing a fixed threshold of 0.3 amp, therefore, a fixed threshold can be set in order to detect a leakage current fault).

The monitoring apparatus uses analytical methods to detect when a current in case2 becomes a fault current and as such has a potential to cause fire.

Case2 fault currents, when started are characterized by: 1. non-linear current fast rise that can stabilize within normal current limits of the branch or can continuously increase. 2. Creation of excessive heat that can ignite fire. 3. No specific value of current that ignites fire. (Fire can start at any value of fault current) Therefore, in order to identify fault currents, simple solutions like setting a fixed current threshold which when crossed proves that a fault current has occurred, do not provide full protection since as stated above any fault current value can cause fire and it is not guaranteed that the fault current will cross a certain known pre-determined value before fire starts.

General information: By law, each electrical site branch is equipped by an automatically operated electrical switch 18 that cuts the current of the branch at a pre-determined threshold. The threshold is determined in accordance with the rated current carrying capability of each branch wires, this is in order to protect the branch electric wires from over-heating (and causing fire).

As stated above, it is not guaranteed that a fault current will exceed the rated current protection value for the branch wires and therefore, the electrical wires protection switches cannot not act as a protection mean against fire breakout.

More than that, even if the fault current rises and triggers the electrical wires protection switch 18, it may be already too late as fire may have already started.

One of the known huge fires, during night-time (un-manned time), which destroyed a whole 4 story building was a case2 fault current that was caused by rainwater that penetrated the electrical wires of an illuminated sign which was mounted on the roof of the building. The fault current ignited an ark between the wires and caused fire. The fault current was, as always, well below the rated current of the wires and therefore well below the thresholds of the electrical protection switch 18 for the electric sign. (The building was equipped with smoke detectors which were operated but it was too late.) Therefore, there has been a long felt need for a solution that that can early detect (early as possible) fault currents before they ignite fire. A solution that will also provide a fast power cut-ff.

SUMMARY A monitoring apparatus for early detection of fault current with potential fire causing, for an unmanned electrical site, including: - a current meter for each electrical branch, for providing current measurements; and - a controller for processing data provided by the current sensors in order to early detect developing fault currents which can lead to fire outbreak, all at currents which are well below the thresholds of the electrical switch 18. The early detection of fire potential currents is performed by continuously reading at predetermined short intervals (sampling) the branch current, calculating and monitoring the first order and second order time derivatives of the branch load current. The calculation of the derivatives is performed by digital approximation methods well known within the art.

BRIEF DESCRIPTION OF THE DRAWINGS Embodiments, features, and aspects of the invention are described herein in conjunction with the following drawings: Fig. 1 is a prior art electrical site.

Fig. 2 is the electrical site of Fig. 1 including a monitoring apparatus according to one embodiment of the invention.

Fig. 3 is an exemplary graph of current measured on one of the electric branches of Fig. 1.

The drawings are not necessarily drawn to scale.

DETAILED DESCRIPTION The invention will be understood from the following detailed description of embodiments of the invention, which are meant to be descriptive and not limiting. For the sake of brevity, some well-known features are not described in detail.

The reference numbers have been used to point out elements in the embodiments described and illustrated herein, in order to facilitate the understanding of the invention. They are meant to be merely illustrative, and not limiting. Also, the foregoing embodiments of the invention have been described and illustrated in conjunction with apparatuses, which are meant to be merely illustrative, and not limiting.

Fig. 2 is the electrical site of Fig. 1 including a monitoring apparatus according to one embodiment of the invention.

A prior art electrical site 12 includes an electrical source 14; an electrical distribution panel 16 for distributing the electricity to branches 22A, 22B, etc., each including an automatically operated electrical protection switch 18; and consumers/appliances 20 on each branch, such as air conditioners.

By this invention electrical site 12 include an electrical monitoring apparatus 10 which according to one embodiment of the invention includes a current meter 24, such as a current transformer, for each electric branch 22A; and a controller 26, for automatically controlling and processing data from the meters 24 and for cutting the electric branch through switch 19, and for early alerting of the occurrence of fire potential fault currents.

Electrical monitoring apparatus 10 may include for each current meter 24, an additional electric supply 64 including an electrical source 66 for flowing electrical current as controlled (70) by controller 26, and further including an electrical winding 68, for flowing the current from electrical source 66 to be measured by current meter 24.

Controller 26 triggers electrical source 66 from time to time, and receives from the current meter 24 the measurement, thus expecting this current value, thus ensuring current meter 24 works properly.

Controller 26 considers the readings at night or at un­ manned times only, for not considering changes rendered by use of man.

Controller 26 is further configured to not consider changes rendered from known control sensors, such as thermostats activating air conditioners.

Fig. 3 is an exemplary graph of current versus time measured on one of the electric branches of Fig. 2.

Controller 26 is configured to calculate and determine the digital approximations of first order and second order time derivatives of the current according to the last three, or more, readings (samples).

As a non-limiting example, suppose controller 26 reads electric current measurements at intervals 56 which are time intervals of tenths of a second, then controller 26 determines the approximation of the first and second derivatives according to a period 34A which constitutes of 3 successive measurements on two intervals of 0.1 sec.

Suppose the 3 measurements of load current for period 34A ending at time 0.3 are: 9.1 amp at time 0.1; 9.2 amps at time 0.2; and 9.3 amps at time 0.3.

Controller 26 computes a first difference 61 of rise from time 0.1 to time 0.2, being of 0.1 amp per 0.1sec (=1amp/sec) (which is an approximation of the first order derivative at time 0.1); and a second difference 61 of rise from time 0.2 to time 0.3 being of 0.1 amp per 0.1sec (=1amp/sec). (which is an approximation of the first order derivative at time 0.2) The difference between these two differences (which is an approximation of the second order derivative at time 0.1) is zero as being both 0.1 amp per 0.1 second (62A), and thus even though the current difference of the rise is positive, (i.e. the approximation of first order derivative at time 0.1 is positive).

Controller 26 is configured to allow this steady current rise rate (28A) up to some set absolute value (58B), reason being, the second order derivative (approx. of) is zero which means that the current is rising relatively slowly and is not a clear indication of a developing fault current. However, controller allows an additional absolute current rise over time to no more than 10% (value is changeable) or any absolute preset value (58B) before the current is interrupted. This acts as a safety threshold.

Example: If the current 9.3amp at time 0.3 rises at the same steady state of 0.1amp per 0.1sec (62A) and exceeds after time (1 sec) the absolute limit value of 10.0 amp (58B), controller 26 will interrupt the current at the end of period 34B, which is time 1.0 sec, and will give an alert (28B).

Suppose in a different case, the 3 measurements of a period 34C ending at time 0.4 are: 9.2 amps at first reading at time 0.2; 9.3 amps at second reading at time 0.3; and 9.6 amps at third reading at time 0.4.

These reading provide a difference 61 of 9.3 -9.2=0.1 amp between the second and first reading, and a difference 61 of 9.6 -9.3=0.3 amp between the third and second reading, thus giving an approximation for the first order derivative at time 0.2 the value of 0.1 amps per 0.1 sec (=1 amp/sec) and at time 0.3 the value of 0.3 amp per 0.1 sec (=3 amp/sec). Thus the approximation of the second order derivative at time 0.2 is therefore 0.2 amp/0.1 sec2 ( = 2 amp/sec2) which is not zero.

In contrast to periods 34A, the positive differences 61 in current rise of period 34C are not equal (62B) i.e. the approximation of the first order derivatives at time 0.2 and at - time 0.3 are not equal thus resulting in a non-zero positive second order derivative (approx. of) of 2 amp/sec2 at time 0.2, thus controller 26 may be programmed to cut automatically and immediately the branch current at time 0.4, even though the current 58A at time 0.4 is only 9.6 amps which is a small change in the absolute value of 0.4amp only. The existence of non-zero positive second order derivative implies that the current is a fast-rising non-linear current which is a clear indication of a developing fault current.

Thus, as seen above, the use of the current time derivatives which are calculated (approximated) by a digital computer, constitutes an analytical method for early detection of developing fault currents that can lead to fire in electrical sites during unmanned times. This without use of any fixed threshold value for detecting the fault current.

By the use of first order derivative and second order derivative along with the value at a point of time, an approximating second order polynomial can be constructed in order to facilitate the calculation for the predicted current around that point of time.

The determination that the three last readings are part of a function including a second order polynomial function may be applied by approximation of Taylor series.

Thus, in one aspect, the invention is directed to a monitoring apparatus (10) for an electrical site (12) during unmanned times, including:

Claims (10)

15 - CLAIMS What is claimed is:

1. A monitoring apparatus (10) for an electrical site (12), comprising: - a current meter (24) for measuring current for each electrical branch (22A) of said electrical site (12); and - a controller (26) for reading said measured currents, for alerting and for cutting any of said electrical branches (22A), said alerting being determinable by first, second and third readings being three sequential last readings of said current measurements over equal time intervals (56) therebetween, wherein said third reading is larger than said second reading, and said second reading is larger than said first reading.

2. The monitoring apparatus (10) according to claim 1, wherein said determination of said electrical fault comprises: current difference between said third and second current readings, being equal to current difference between said second and first current readings, wherein said third current reading is a pre-determined current (58B).

3. The monitoring apparatus (10) according to claim 1, wherein said determination of said electrical fault comprises a non-linear function of the current, said non­ linear function comprising: 16 - current difference between said third and second current readings, being larger (62B) than current difference between said second and first current readings.

4. The monitoring apparatus (10) according to claim 3, wherein said largement of said current differences comprises a function comprising a square function.

5. The monitoring apparatus (10) according to claim 3, wherein said function is obtained via a Taylor series.

6. The monitoring apparatus (10) according to claim 1, wherein said first, second and third readings are while said electrical site (12) is not operated by man.

7. The monitoring apparatus (10) according to claim 1, wherein said first, second and third readings are while not being changeable by known control of an appliance (20) of said electrical site (12).

8. The monitoring apparatus (10) according to claim 1, further comprising: - an electric winding (68) attached to said current meter (24), said electric winding (68) for supplying additional current to said current meter (24) via said controller (26), thereby said controller (26) supplies additional current and measures thereof, thereby ensuring propriety of said current meter (24). 17 -

9. The monitoring apparatus (10) according to claim 1, wherein said measured currents and said threshold current refer to load current.

10. The monitoring apparatus (10) according to claim 1, wherein said measured currents and said threshold current refer to leakage current.