FIELD OF THE INVENTION
The present invention relates to three phase common enclosure type gas insulated switchgear, particularly to a three phase common enclosure type gas insulated current transformer module.
BACKGROUND OF THE INVENTION
Gas Insulated Substations (GIS) are widely used due to their environmental and operational advantages. GIS are replacing the conventional yard substations particularly in highly polluted and climatically harsh industrialized environments, including in urban areas to conserve space and to ensure reliable power supply. The gas insulated metal-clad modules of circuit breakers (CB), disconnector switches (DS), earth switches (ES), current transformers (CT), voltage transformers (VT), surge arresters (SA), bus bars and terminal equipment from different substation configurations. Each of these equipment operates at a gas pressure in the order of less than lOOkPa to 750kPa depending on type of a selected module and voltage class. In GIS, the live parts are insulated from the grounded metal encapsulation using sulphur hexafluoride (SF6) gas insulation and the equipment is hermetically sealed. The chambers are fabricated from non-magnetic stainless steel or Aluminium to make them corrosion resistant and to ensure low gas permeation rate.
Gas insulated current transformer is an important module of high voltage gas insulated switchgear designed for various current measuring tasks and current measuring ranges. This means current transformer cores of different sizes and quantities are required in a gas insulated current transformer. In gas insulated equipment, the current transformer core is slipped onto a core support tube

which in turn installed in current transformer chamber by means of suitable flange connection. This core support tube must not be continuous to achieve required class of measurement accuracy.
Gas insulated current transformers comprise current transformer cores that are located on core support tubes and annularly enclose the conductor through which current passes. The current transformer housing is filled with SF6 gas which is under a pressure of few bars. The current carrying conductor is held by insulators particularly cone insulators or disc insulators or partition insulators or non-communicated insulators. These insulators are supported on the gas insulated switchgear housing. The current flowing through the current carrying conductor needs to be measured by using CT cores of different sizes depending on nature of current like normal current or short circuit current etc. primary to secondary current ratio and accuracy class of measurement.
In conventional CTs, cylindrical member or core housing is made in two parts and contains screw type flange for connecting these two parts in the center. These two parts are tapered towards screw type flange such that outside diameter of the screw type flange is reduced. The main disadvantage of this design is that gaseous spacing between central HT conductor and enclosure of GIS is reduced in the tapered region and result to high electric field strength which may result to electrical flashovers. Alternatively, two part cores are used but are less accurate and more expensive than one piece CT cores. One piece current transformers core need to be slipped on the core housing. To meet this requirement, there is a patent (US 7417847) using an installation flange for core housing which is tightened against CT enclosure with necessary insulation and sealing

arrangement. The disadvantage of this system is application of seal as insulation between core housing and CT enclosure.
OBJECTS OF THE INVENTION
It is therefore an object of the invention is to propose a compact three phase current transformer for metal-clad gas insulated switchgear.
Another object of the invention is to propose a compact three phase current transformer for metal-clad gas insulated switchgear, in which a support-cum-guided insulator is integrated to core-support housing and connected to the transformer enclosure.
A still another object of the invention is to propose a compact three phase current transformer for metal-clad gas insulated switchgear, in which electric field controlled shield is integrated to the core support pipes to act as an electric flashover-free current transformer.
Yet another object of the invention is to propose a compact three phase current transformer for metal-clad gas insulated switchgear, in which the induced currents generated in the core support due to primary current is eliminated.
A further object of the invention is to propose a compact three phase current transformer for metal-clad gas insulated switchgear, in which the transformer enclosure is integrated with a reducer assembly to be compatible to other Gas insulated switchgear modules.

SUMMARY OF THE INVENTION
The current transformer module is used to measure the current flowing through the switchgear equipment during normal service conditions and during abnormal conditions like fault. According to the invention, a three phase compact current transformer is provided which consists of central conductor and multiple numbers of cores in each phase which is supported by cylindrical member. The assembly of cores and cylindrical members is placed in a grounded metallic chamber. This chamber is connected to other modules like circuit breaker and disconnector switch of the gas insulated switchgear. When current flows through central conductor (Primary), secondary current is induced in cores depending on the ratio of primary and secondary turns. For accurate measurement of current, the cylindrical member is not connected to the grounded metallic chamber at least on one end. One end of cylindrical member is connected to the grounding metallic chamber through a support member. Hence, a three-phase core support housing with electric field controlled shield assembly, and a three-phase support-cum-guided insulator are provided to the CT transformer module.
BRIEF DESCRIPTION OF THE ACCOMPNAYING DRAWINGS
The invention is described with the help of accompanying Figures 1 to 8, where:
Figure 1: Shows a Conventional Current transformer (CT).
Figure 2 : Shows a three phase current transformer assembly with support
insulator and HT conductors.
Figure 3 : Shows a three phase Core support housing of a CT of figure 2.
Figure 4 : Shows a three phase core support housing with electric field controlled
shield assembly according to the invention.

Figure 5 : Shows a three phase support-cum-guided insulator according to the invention.
Figure 6 : Shows a three phase core support housing with electric field controlled shield and Support-cum-Guided Insulator according to the invention.
Figure 7 : Shows a three phase current transformer housing according to the invention.
Figure 8 : Shows a three phase current transformer assembly according to the invention.
DETAIL DESCRIPTION OF THE INVENTION
In the present invention, the primary (high tension) conductors [01] of all three phases, are made of high conductivity materials and are held by support insulators [02] made of filled epoxy [02]. The support insulator [02] is of three phase type and comprising at least three HT inserts [03] to commensurate with three phases of gas insulated current transformer. Figure 2 shows a three phase current transformer assembly with HT conductors [01] and three phase support insulator [02]. When current flows through the central primary conductor [01] of current transformer module, a significant current magnitude is induced in core assemblies [04] depending on ratio of primary and secondary turns. The core assemblies [04] are located on a core support housing [05] which is made from material of higher mechanical strength. There is one core support housing [05] for each phase of the current transformer. The core support housing [05] consists of three phase core support pipes [06] and a core support plate [07].

The three phase core support pipes are core pipe A [06-A], core pipe B [06-B] and core pipe C [06-C] respectively. These core support pipes [06] are integrated to the core support plate [07] in equilateral triangle fashion or square fashion or equivalent to formulate a three phase system. At the interface of these components a matching profile with corresponding radii is made to ensure flashover-free design of the three phase current transformer. Figure 3 shows a core support housing.
The configuration of the core support housing [05] is very critical as high electric field is generated between the central primary conductor and the core support assembly. To control these electric field levels at higher system voltages, core support pipes [06] are terminated at either ends with electric-field controlled shields [08] to overcome flashovers during its type testing. At the same time, core support housing [05] is constructed to accommodate multiple numbers of cores of different sizes as required. The core support pipe is supported by a core support plate [07] at one end and a support-cum-guided insulator [09] is used to support at other end. For accurate measurement of current, the core support housing [05] is not connected directly to a CT grounded enclosure [10] at either of its terminals. Hence, one end of core support housing [05] is directly connected to the CT grounded enclosure [10] through a core support holder [11] and other end is supported by the CT grounded enclosure [10] through the support-cum-guided insulator [09]. Figure 4 shows the core support housing [05] with the integrated electric-field controlled shields [08]. The support-cum-guided insulator [08] is constructed in such a way that the electrostatic field distribution between the HT conductor [01] and the core support housing [05] does not get disturbed due to its presence and ensure a flashover-free current transformer even for system voltages for example, 420 kV class and above. The core

assemblies [04] are placed in an annular space between the core support housing [05] and the CT grounded enclosure [10]. The electric field controlled shields [08] also act as a stopper for restricting movement of the core assemblies [04]. A plurality of insulated spacers are also provided between the core assemblies. Figure 5 shows a support-cum-guided insulator [08] of the three phase compact current transformer assembly.
Figure 6 shows the three phase current transformer housing assembly. The CT grounded enclosure [10] consists of three parts, first one is a reducer assembly [12] connected to modules of gas insulated substation for example, disconnector switch, earthing switch etc. The main function of the reducer assembly is not only to be compatible to other modules of gas insulated switchgear but also to act as a guide-cum-stopper to the three phase support-cum-guided insulator[08]. The second one is straight chamber [13], which had provision for supporting core support housing [05] and a secondary wiring box [14]. The terminals of the core assemblies are connected to control equipment through the secondary wring box [14]. The secondary wiring box [14] consists of connectors for electrical connection between gaseous region and air media. A straight chamber is selected corresponding to circuit breaker module. A CT Expander [15], which is required to integrate the current transformer module to all GIS modules except circuit breaker module, is optionally provided.
Figure 8 shows a three phase current transformer module for GIS application, with the core support housing [05] integrated to electric field controlled shields [08] and the assembly is connected to the CT grounded enclosure [10] at one end and at other end it is supported through three phase support-cum-guided insulator [08]. Precisely, the support-cum-guided insulator [08] is tightened to the core support housing [05] against the CT reducer [12] portion of the CT grounded enclosure [10].

A compact three phase current transformer for metal-clad gas insulated
switchgear, comprising :
a plurality of primary conductors one each for all three phases made of
high conductivity materials;
a core support housing with core support pipes terminating at either end
with electric-field-controlled shields, the core support pipes being
supported at a first end with a core support plate, and at a second end by
a support-cum-guided insulator;
a core support holder connecting a first end of the core support housing
to a first end of a current transformer grounded enclosure and the
support-cum-guided insulator supporting a second end of said current
transformer grounded enclosure; and
a plurality of core assemblies located inside an annular space between the
core support housing and said grounded enclosure, with a plurality of
insulated spacers disposed between the core assemblies.
The three phase compact current transformer as claimed in claim 1, wherein the support-cum-guided insulator is constructed in such a way that the distribution of electrostatic field between the primary conductors and the core support housing does not get disturbed so as to operate the current transformer flashover-free mode.
The three-phase compact current transformer as claimed in claim 1 wherein the core support housing is integrated to the electric-field-controlled shields which further acts as a stopper to restrict movement of the core assemblies within the core support housing.

The three-phase compact current transformer as claimed in claim 1, wherein the grounded enclosure comprises a reducer assembly connectable to any of the other modules of gas insulated switchgear and also acts as a guide-cum-stopper to said guided insulator.
The compact three-phase current transformer as claimed in claim 4, wherein the grounded enclosure comprises a straight chamber with means for supporting the core support housing.
The compact three-phase current transformer as claimed in claim 4 or 5, wherein the grounded enclosure comprises a secondary wiring box having connectors for establishing electrical connection between the gaseous region and air media.
The compact three-phase current transformer as claimed in claim 4 to 6, optionally comprising a current transformer expander to allow integration of the current transformer module with the other modules of the gas insulated switchgear.
The compact three-phase current transformer as claimed in claim 1, wherein the support-cum-guided insulator is precisely connected to the core support housing against the reducer assembly.

ABSTRACT

The invention relates to a compact three phase current transformer for metal-clad gas insulated switchgear, comprising : a plurality of primary conductors one each for all three phases made of high conductivity materials; a core support housing with core support pipes terminating at either end with electric-field-controlled shields, the core support pipes being supported at a first end with a core support plate, and at a second end by a support-cum-guided insulator; a core support holder connecting a first end of the core support housing to a first end of a current transformer grounded enclosure and the support-cum-guided insulator supporting a second end of said current transformer grounded enclosure; and a plurality of core assemblies located inside an annular space between the core support housing and said grounded enclosure, with a plurality of insulated spacers disposed between the core assemblies.