Energy Measurement in Distribution Transformers
Field of the Invention
This invention relates to distribution transformer metering and particularly relates to current transformers for energy measurement applications of Distribution Transformers (DTs)
Background and Prior Art Description
The advent of distribution transformer (DT) metering requires the DTs to be metered for energy accounting purposes. Current transformers are preferably used to monitor energy consumption in the DTs.
The current transformers conventionally in use for DT metering applications are of the following types.
CT chamber with busbars: (Fig. 1)
A CT chamber 10 is introduced in the load circuit and installed near DT 20 on the Double Pole (DP) structure, so that one set of cables 11 are connected to DT bushings 12 and load cable 13 is connected to the outgoing side of the chamber 10. Multicore cables 14 comprising two secondary terminals of CT and one terminal for the potential signal per phase are brought out from the chamber and connected to a meter 15 for measurement purposes
The drawbacks of this system are:
1. Additional requirement of cables, lugs, sockets and connectors
2. Introduces two additional joints and hence more voltage drop in the load circuit
3. High Inventory cost due to more no. of material to be kept in inventory
4. High Cost
5. Mounting cost is also high due to extra requirement of iron.
6. More space is required
7. No. of joints are prone to loss of energy and prone to failure
8. Special skills and tools are required for installation
9. Chances of wiring errors and associated measurement errors are high.
10. Shutdown time required for installation is more, due to which there is a high " loss of opportunity to serve".
11. Heavy maintenance costs including painting and anti-rusting treatment
12. Tapping the potential signal is problematic and is not fool-proof.
Bar type of CT: (Fig. 2)
A bar type current transformer 30 is connected in between the LT bushings 12 and Load cables 13. A 3 core cable 34 comprising of two secondary terminals of the CT and one terminal for the potential signal are brought out from the CT 30 and connected to the meter 15 for the measurement purposes
The drawbacks of this system are:
1. Introduces one additional joint and hence more voltage drop in the load circuit
2. Cost is high due to requirement of busbars
3. Special skills are required for installation
4. Shutdown time required for installation is more, due to which there is a high " loss of opportunity to serve".
5. Low clearance spaces between the CTs
6. Extra weight of the CTs mounted on bushings causes leakage and results in failure of bushings/ transformer
7. Aesthetically it is not so good as CT chamber installation.
Ring type of CT: (Figs. 3 and 4)
Fig. 3 shows a first conventional ring type configuration. A window type or ring type CT 40 is inserted over the cables 13 and left free in the air, with secondary wires 44b connected to the meter. For a voltage pick-off, a separate wire 44a is connected on transformer terminals.
In a second conventional ring-type configuration shown in Fig. 4, the CT 40 rests on the bushing 12 and a separate wire for voltage signal is required per phase an hence, the voltage signal wire 44a is normally connected to the studs & nuts of the bushing 12. Another wire 44b runs from each CT 40 to the meter 15 for measurement.
Drawbacks of this system are:
1. Irregular flux linkage results as the conductor is not in the exact centre of the CT core. This contributes to some error in measurement.
2. Low clearance spaces between the CTs. This becomes worse when the number of load cables is high.
3. Aesthetically it is not pleasuring.
4. Chances of breakage of voltage signal wire are high.
5. Number of wires increases 44a & 44b causing high probability of wiring errors and associated measurement errors.
6. Resting of CT on insulator reduces the creepage distance and hence, chances for flashover increases.
7. Tapping of potential / voltage wire creates a scope for contact loss and improper contact of main conductor lugs with the bushing stud due to the voltage signal wire
Summary of the Invention
With the above drawbacks in mind, it is an object of the present invention to provide an improved arrangement for the reliable energy measurement of existing distribution transformers.
Accordingly, from a first aspect the present invention provides a metering apparatus for: a distribution transformer having an electrical bushing assembly, which comprises an electrical terminal stud arranged to provide an output of the distribution transformer; wherein the apparatus comprises a current monitoring arrangement comprising a current transformer for detecting the current in the output of the distribution transformer and arranged to be secured in a predetermined positional relationship to the terminal stud.
Preferably, the positional relationship is such that a central axis of the terminal stud and a central axis of the current transformer are on the same common axis. With this arrangement there is uniform flux linkage.
In one embodiment, a locating means which is attached to the current transformer, is arranged to secure the current transformer in the positional relationship with respect to the terminal stud.
In a second embodiment, a hole in the center of the current transformer is of a size to match the cross sectional dimension of the terminal stud.
In a third embodiment, the current transformer is provided with a clamping mechanism comprising means for clamping the current transformer to the terminal stud.
In a fourth embodiment, the current transformer is provided with a clamping mechanism comprising means for clamping the current transformer for Busbar.
It should be noted that the current transformer can be adapted to suit any particular shape or composition of terminal stud.
With the above configuration, the drawbacks of the prior art are overcome and the present invention provides an effective installation system with no breakage of distribution circuits to connect the current transformer, uniform flux linkage and no interference/ breakage of a wire for voltage signal, all features offered at a low cost when compared to the existing ones as mentioned in the prior art.
From a second aspect there is provided a low cost low tension (LT) instrument transformer providing current and voltage signals both in a single unit for DTs energy metering purposes on LT end, which do not add any joints in the distribution circuits and hence contact losses concerned with the joints. It is joint-less in primary, contact loss in tapping of potential signal is avoided, easy and hassle free in installation requiring no special tools and having universal installation adaptability.
The present invention can be implemented as a retrofit measure in any type of DT in an easy manner, and there is no need to change the DT. That is, the present invention is compatible with a conventional DT. The current transformer can be manufactured in various shapes such as circular, oval, square and rectangular shapes for convenience. The CT can be cast in EPOXY resin, cycloeliphatic resin or encapsulated in a suitable plastic grade material.
The following advantages are provided by the present invention:
1. Shutdown time required is less due to which loss of opportunity to serve is reduced.
2. Can be manufactured with ferrite core as well as cold rolled grain orientated (CRGO) cores.
3. Can be encapsulated in Epoxy resin, cycloeliphatic resin or a suitable plastic grade material.
4. Uniform flux linkage of the core makes the measurement more accurate by stabilizing the orientation of the current transformer with respect to the terminal stud such that the terminal stud passes through the center of the current transformer.
5. Voltage pick-off arrangement is in-built with the device.
6. Primary circuit does not need any modification.
7. Arrangement is universal and can be perfectly mounted on any size of the transformer.
Brief Description of Drawings
In order that the present invention be more readily understood embodiments thereof will be
described with reference to the accompanying figures in which:
Fig. 1 shows a conventional CT chamber type of installation;
Fig. 2 shows a conventional bar type installation;
Fig. 3 shows a conventional first ring type installation;
Fig. 4 shows a conventional second ring type installation;
Fig. 5 shows apparatus according to a first embodiment of the present invention;
Fig. 6a shows a basic view of the current transformer used in Fig. 5;
Fig. 6b shows a detailed view of the connection on one terminal of the distribution
transformer shown in Fig. 5;
Fig. 6c shows a side view of the arrangement in Fig. 6b;
Fig. 7a shows a current transformer arrangement attached to one terminal of a distribution
transformer according to a second embodiment of the present invention;
Fig. 7b shows a current transformer according to a second embodiment of the present
invention;
Fig. 8a shows a current transformer arrangement according to a third embodiment of the
present invention;
Fig. 8b shows a current transformer arrangement according to the third embodiment of the
present invention attached to a terminal stud of a distribution transformer;
Fig. 8c shows a current transformer arrangement attached to a cable according to a third
embodiment of the present invention;
Fig. 8d shows a current transformer arrangement according to the third embodiment of the
present invention attached to a conductor;
Fig. 8e shows a current transformer arrangement according to the third embodiment of the
present invention attached to a busbar;
Fig. 9 shows a current transformer arrangement according to a fourth embodiment of the
present invention attached to a busbar;
Detailed Description of the Present Invention
The present invention adopts the standard distribution transformer which appears in conventional systems and adapts the mounting configuration of the current transformers so that the orientation of the current transformer in respect of a terminal stud, which provides an output of the distribution transformer, is stable and provides uniform flux linkage. It should be noted that any particular arrangements may be provided to stabilize the current transformer in respect to the stud of the distribution transformer, but Figs. 5 and 6 show a first embodiment which can be considered the preferred embodiment of the present invention.
A typical distribution transformer 20 (as shown in conventional systems of Figs 1 to 4 described hereinbefore) comprises four terminals each arranged to receive a conductor 13. The conductors 13 are coupled to a stud 52 of a bushing 12 which is provided on each terminal.
The terminal stud 52 is in the form of a tubular elongate member which in this embodiment is of constant cross section along it entire length. However, the invention is not restricted to this composition and various cross sections and lengths for the stud 52 can be used in the invention. The length of the terminal stud 52 should be sufficient to mount both a current transformer 50 and a conductor cable 13.
The current transformer 50 is provided in order to detect the current and comprises a core 50a which may be manufactured with ferrite or CRGO silicon steel (cold rolled grain orientated). A secondary winding (not shown) is wound on the core 50a and an opening 50b is provided through the axial center of the core 50a. The diameter of the opening 50b is such that the terminal stud 52 can pass through the opening 50b.
A locating member 51 is attached to the CT 50 and is arranged to provide stable orientation of the CT 50 with respect to the stud 52 which receives the CT 50. The locating member 51 is preferably in the form of a brass plate which has an opening 51a. The dimensions of the opening are such as to match or to be slightly greater than the cross sectional dimension of the terminal stud 52 such that the terminal stud 52 is able to fit in the opening 51a. The current transformer 50 is coupled to the stud 52 of bushing 12 such that the stud 52 is arranged to pass through the opening 51a of the locating member and in this manner the stud 52 and opening 51a correspond with each other.
The locating member 51 is releasably attached to the stud 52 by means of a fastening member such as a nut 54 which secures the locating member to the stud 52. Any particular fastening member may be utilized to secure the locating member 51 to the stud 52.
The orientation of the locating member 51 is such that the central axis of the locating member
51 is the same as the central axis of the current transformer 50 and the central axis of both are
the same as the axis A of the terminal stud 52 when the installation is present on the stud 52.
The terminal stud 52 is also arranged to receive a conductor 13 similar to the conductor shown in Fig. 2 and 3. The conductor is attached near the exposed end of the stud 52 by means of a pair of nuts which secure the conductor to the stud 52. Therefore the stud 52 is sufficiently long enough to receive the current transformer 50, locating member 51, the conductors 13 and the various nuts which secure the elements on the stud 52. (Refer fig. 6b and 6c)
It will be appreciated that the circumference of the opening 51a of the locating member may be inserted to the stud 52 so that the locating member is screwed into position on the stud 52. Accordingly the nuts will also be threaded to secure the elements.
A second embodiment of the present invention will be described with reference to Fig. 7a and 7b. As shown, a stud 62 which may be of a similar cross sectional shape and size as the stud
52 of the previous embodiment is provided as an output of the distribution transformer 20 via
the bushing 12.
In this embodiment, CT 60 comprises a core 60a and an opening 60b through the axial center of the CT 60. The opening 60a is adapted to be a size and shape similar to the stud 62 so that the CT 60 can be directly received by the stud 52. As with the previous embodiment, the central axis of the CT 60 is the same as the central axis of the terminal stud 62.
Further, circumference 60c of the opening 60b is threaded and at least a portion of the terminal stud 62 also has a threaded outer surface so that the CT 60 can be effectively screwed into the stud 62. The CT 60 would be held in position where the threaded portion of the stud 62 ends. However a threaded nut 64 could be provided adjacent the CT to further secure the CT 60 on the stud 62.
A third embodiment is shown in Fig. 8a, 8b,8c, 8d and 8e. In this embodiment, a CT comprises a core 70a and an opening 70b to receive a terminal stud 72 of a distribution transformer (Fig. 8b)/ cable 73 (Fig. 8c)/ conductor 74 (Fig 8d) or busbar 75 (Fig. 8e). A clamping mechanism 71a, b, c ,d, e, f is attached to one side of CT core 70a and the CT core 70a is arranged to receive the terminal stud 72/ cable 73/ conductor 74/ busbar 75 through the other side of the core 70a, through the opening 70b and out from the one side of the core 70a and clamped on the one side by the clamping means. With this arrangement the alignment of the central axis of the CT 70 with respect to the axis of the terminal stud/ cable/ conductor/ busbar will not depend on the actual properties of the opening as with the first and second embodiment but is controlled by the alignment which is chosen by a user to clamp the CT core 70a to the terminal stud 72/ cable 73 / conductor 74/ busbar 75.
The clamping mechanism comprises a first and second rigid member 71a, 71b arranged opposite on one side of the core 70a and opposite one another across the opening 70b. The first rigid member 71a comprises a clamping member 71c which is movable towards a clamping member 71d of the second rigid member similarly the second rigid member 71b comprises a clamping member 71d which is movable towards the clamping member 71c of the first rigid member and when the terminal stud /cable/conductor/busbar is received through the opening 70b, the respective ends of each clamping member 71c, 71d abut the terminal stud/ pierce the surface of the terminal stud/ cable/ conductor/ busbar and clamping
member 71c of the first rigid member 71a and clamping member 71 d of the second rigid member 71b is tightened by means of a bolt-head 71 e and 71 f on the rigid member 71c thereby securing the CT 70a to the stud/ cable/ conductor/ busbar. It will be appreciated that both clamping members 71c, 7Id may be movable towards each other.
It will be apparent from the above that many different configurations may be used to secure the CT to the terminal stud of the distribution transformer. Many different types of terminal studs can be used with the CT and the openings of the CT can be varied accordingly.
In addition, the mounting means of the CT to the stud/ cable/ conductor/ busbar which is described hereinbefore as a locating member 51, opening 60a, or clamping mechanism are arranged to provide a voltage pick off which is directly taken from the conductive part of the terminal stud/ cable/ conductor/ busbar. Therefore, the voltage and current may be monitored using a single installation. The wires 77 (Fig. 6a, 6b, 7a, 7b, 8a, 8b, 8c, 8d/8e) from the single installation would be connected to a processing means 15 in order to provide accurate values for voltage and current in the distribution transformer.
A fourth embodiment is shown in Fig. 9. In this embodiment, a CT comprises a core and an opening through the axial centre of the CT to receive a busbar. A pair of parallel locating members 76 is provided in the opening and the distance between the locating members is preferably marginally greater than the width of busbar. Each of the parallel locating members is provided with atleast one clip/clamps to hold the busbar at the predetermined position with respect to the CT.

We Claim:
1. A metering apparatus for a distribution transformer having an electrical bushing assembly, which comprises an electrical terminal stud arranged to provide an output of the distribution transformer, wherein the apparatus comprises a current monitoring arrangement comprising a current transformer for detecting the current in the output of the distribution transformer and arranged to be secured in a predetermined positional relationship to the terminal stud.
2. The metering apparatus according to claim 1 wherein the predetermined positional relationship is such that a central axis of the terminal stud and a central axis of the current transformer are on the same common axis.
3. The metering apparatus of claim 1 or 2 wherein the current monitoring arrangement comprises means for mounting the arrangement in the predetermined positional relationship to the terminal stud without any alternation in the load circuit.
4. The metering apparatus of claim 3 wherein the mounting means is arranged to monitor the voltage in the distribution transformer.
5. The metering apparatus of claim 3 or 4, wherein the mounting means is arranged to pick off the voltage from the terminal stud/ cable/ conductor/ busbar.
6. The metering apparatus of claim 3, 4 or 5 wherein the mounting means comprises a locating member attached to the current transformer and arranged to secure the current transformer in the predetermined positional relationship.
7. The metering apparatus of claims 3, 4 or 5 wherein the mounting means comprises a clamping mechanism comprising means for clamping the current transformer in the predetermined positional relationship.
8. The metering apparatus of claim 1 or 2 wherein the cross sectional shape of the terminal stud is the same as the shape of an opening in the current transformer.
9. The metering apparatus of claim 8 wherein the terminal stud and the opening of the current transformer are threaded such that the current transformer is arranged to be screwed onto the terminal stud.
10. The metering apparatus of any preceding claim wherein the terminal stud has a varied cross section along its length.
11. The metering apparatus for a distribution transformer having an electrical bushing assembly substantially as herein described with reference to the accompanying drawings.