Voltage Transformers, Including VT Marshalling Box

13. PRIMARY PLANT 40

13.6 Voltage Transformers, Including VT Marshalling Box

High voltage inductive or capacitive voltage transformers used on Ergon Energy’s network must be from the current Period Contract and are listed in ANNEX B – PERIOD CONTRACT PLANT. If a special, non-standard installation is required to suit existing substation installations, e.g. to meet fault levels, VT accuracy class, pollution ratings, space restrictions etc., then this must be approved by Ergon Energy Substation Standards Group before proceeding with design and procurement. When specifying a voltage transformer from period contract items, the Designer must select the following:

 Inductive or Capacitive type (for 66kV units only)

Current Period Contract has IVT for 36kV and below and CVT for 123kV and above, and both types are available at 72.5kV. The preference is that IVTs be used for 66kV installations due to a

 A new CT marshalling box in accordance with the current ZSS Drawings must be installed. The exceptions to this replacement are:

- if the physical condition of the existing box is still satisfactory and internal space permits, then a new fit-out can be undertaken. The marshalling box fit-out must be in accordance with the current ZSS Drawings.

- if the existing marshalling box is used for the marshalling of other related bay circuitry and considerable design and construction would be required.

 Control cables between the current transformer and the current transformer marshalling box must be replaced in accordance with the ZSS Template Drawings.

 Control cables between the current transformer marshalling box and the substation protection and control panels should be replaced in accordance with the ZSS Template Drawings. The exception to this is if existing control cables are in accordance with the current Control Cable Standard and have sufficient length to terminate in new control box without intermediate junction boxes, or substantial construction work would be required to replace the control cables.

(b) Replacing two phases of a CT Installation

If the project scope is requiring only two phases of a CT Installation to be replaced, then the Designer should question the validity of this scope with the Project Manager, Project Sponsor and Asset Manager as it can be more economical and prudent to replace all three phases.

 Footing and structure must be assessed in accordance with Section 11.2.5 of this manual  The existing marshalling box may be re-used. There must be adequate space in the

existing marshalling box to ensure the termination of all new CT secondary cores. There is no requirement to upgrade the existing CT cores secondary wiring, including the installation of CT links to current standards.

 Control cables between the current transformer and the current transformer marshalling box must be replaced in accordance with the ZSS Template Drawings.

(d) Relay compensation

When retro fitting transformer differential protection relays that were externally compensated (e.g. delta connected line CT's), the CT circuits must be reconfigured to Star connected and

compensation undertaken in the differential relay. This includes the removal of interposing CT’s required for the compensation.

physical smaller size and weight. The CVTs are required if the installation requires Type 1 revenue metering where both secondary windings require a 0.2M accuracy class.

When CVTs are installed, a CVT monitor must be included as part of the secondary systems design. Refer to Section 15.10 for details.

VT Marshalling Boxes

A VT secondary marshalling box must be provided for each three phase set of VT’s. The

marshalling box must be made of either stainless steel or aluminium. The construction and layout of internally mounted equipment must be in accordance with the Substation Standards.

The marshalling box must also have a rotating handle, not a key type access mechanism. The secondary terminals of all VT secondary windings must be connected by a 4 core control cable to the terminals in the marshalling box.

Refer to EESS-10225-01 to -06 Stainless Steel VT Marshalling Box

VT Secondary Fusing and Earthing

 VT secondary circuits must be fused in the VT marshalling box. The VT terminal boxes must not contain any fuses.

 Each VT secondary winding must be fused in each of A, B, C phases and for the star connected windings, a link must be provided in the neutral using a fuse holder and copper link.

 Similarly a single phase VT secondary winding must be fused on the “a2” side and provided with a link on the “a1” side.

 All fuses and links must be clearly labelled with the circuit function and fuse size or ‘Link’ as appropriate.

 Where appropriate, warning labels must be fitted to fuses and links e.g. Distance Protection - Do Not Remove.

 The secondary circuits of all VT’s including auxiliary VT’s must be earthed. Any disturbance to the connections must not at any time allow the VT to be unearthed. The earth connection must be made as close as possible to the VT’s, with only one earth connection in each circuit.  In general the earthing must be by means of a solid connection between the earth bar and

neutral. The following earth connections must be made:

- Three phase voltage transformer circuits have their neutral terminals earthed at the VT marshalling box. The connection is made between the VT side of the link and the earth bar. - Single phase VT’s earthed at the “a1” ends of their secondary windings at the VT marshalling

box.

- Open delta connected VT’s earthed at the terminal corresponding to the C end of the A-C winding. The connection is made between the VT side of the link and the earth bar in the relay panel.

 A new VT marshalling box in accordance with the current ZSS Drawings must be installed. The exceptions to this replacement are:

- if the existing marshalling box is used for the marshalling of other related bay circuitry and considerable design and construction would be required.

 Control cables between the voltage transformer and the voltage transformer marshalling box must be replaced in accordance with the ZSS Template Drawings.

 Control cables between the voltage transformer marshalling box and the substation protection and control panels should be replaced in accordance with the ZSS Template Drawings. The exception to this is if existing control cables are in accordance with the current Control Cable Standard and have sufficient length to terminate in new control box without intermediate junction boxes, or substantial construction work would be required to replace the control cables.

(b) Replacing Two Phases of a VT Installation

If the project scope is requiring only two phases of a VT Installation to be replaced, then the Designer should question the validity of this scope with the Project Manager, Project Sponsor and Asset Manager as it can be more economical and prudent to replace all three phases.

 Footing and structure must be assessed in accordance with Section 11.2.5 of this manual  The existing marshalling box may be re-used. There must be adequate space in the

existing marshalling box to ensure the termination of all new VT secondary cores. There is no requirement to upgrade the existing VT cores, including the installation of secondary VT fuses to current Standards.

 Control cables between the voltage transformer and the voltage transformer marshalling box must be replaced in accordance with the ZSS Template Drawings.

Brownfield Site Considerations

When replacing voltage transformers in existing substations, the Designer must take into account the following design considerations:

(a) Replacing Three Phases of a VT Installation

 It is often more economical to replace the support structure with a new standard structure, rather than carrying out modifications to the existing structure.

 Footing and structure must be assessed in accordance with Section 11.2.5 of this manual

In document Substation Design -Manual (Page 51-53)