Station Service Voltage Transformer (SSVT)

General protection information

Technical Data

Generally, the station service voltage transformer as applied in electrical substations will be protected by the substation class arresters and overhead shield/ground wires within the substation. However, if they will not be installed into this environment, it is recommended that the transformers be protected by properly sized station class arresters mounted in close proximity to the unit. Along with arrester protection, listed below are additional options relative to SSVT coordinated protection that have been successfully applied on prior applications.

Transformer impedance The impedance of SSVT transformers is typically between 5% to 10% depending upon the design and other customer specified requirements. This means the bolted secondary fault current varies from 20x rated to 10x rated current based upon such an impedance range.

A typical 100 kV, 120/240 VAC SSVT with 5% impedance would source 8.34 kA current into a bolted secondary fault. Therefore, 10 kA short-circuit rated secondary equipment is adequate for most SSVT applications.

Nature of the loadThe nature of the load is a critical factor in determining how the circuit should be protected. Large inductive loads may require a time delay feature to cope with sizable inrush currents. Further, the connected load may be of such critical nature that loss of power must be avoided even approaching the thermal limit of the supply transformer (SSVT).

The full load rated current in the case of a 100 kVA SSVT with 120/240 VAC output is 100 kVA = 417A RMS.

In accordance with the National Electrical Code (NEC), Article 450-3, a transformer with less than 600 V rating shall be protected by 125% over-current protection. However, exercise caution when applying this, as mentioned above, as any motors may have in-rush concerns requiring time-delay. It is recommended to consult a fuse supplier or fused-disconnect supplier for their recommendations.

CAUTION - Information contained in this technical data sheet is intended as an aid to define what some users have done relative to establishing a system-coordinated primary protection scheme when installing a Station Service Voltage Transformer (SSVT). Not all methods shown are considered common practices when applying the SSVT, nor is this list all-inclusive, but is provided only for full disclosure information to assist specific users’ course of action when applying this transformer to their system. It is recommended to follow official end-user company policy for applying such devices on high voltage services.

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Customer specific standardsCommonly, utilities have standards that dictate circuit protection philosophy. This may include the type of equipment (breakers vs. fuses), their characteristics, and even preferred brands. For these reasons, it is difficult to make a firm fusing recommendation. However, consider these suggestions related to what is defined in the NEC.

The SSVT requires secondary over-current protection. Generally, this consists of a fused disconnect in the secondary circuit to prevent a secondary short circuit from damaging the SSVT as already described above. This should be located as close to the SSVT secondary box as possible to prevent the transformer from being damaged by secondary short-circuit or overload. Examples of this type of protection are shown on this page. However, in many applications, such as high voltage applications above 138 kV, users tend to direct connect to the primary conductor without a disconnecting or fusing means.

The reason for this is that the primary winding of the SSVT has sufficiently small copper wire size that will completely vaporize and open the primary circuit in the event of a line to ground fault. And just like the inductive VT counterpart, customers normally do not fuse them due to this fact. The unit is not capable of drawing down the system voltage and will quickly disconnect in the event of a fault due to its small size primary winding.

Station Service Voltage Transformer (SSVT) application - General protection information | 3

Some users elect to install fused disconnects on the primary circuit (when proper voltage rated equipment is available from the industry) to supply protection for the system, against an internal fault within the SSVT itself. Fused disconnect switches are generally available at 138 kV and below and have been applied to the SSVT installations shown in the adjacent photo.

Sudden Pressure Relay optionAlso available as a factory-installed option on the SSVT is a Sudden Pressure Relay (SPR) that can be mounted to the tank wall of the SSVT to provide early warning of a fault by detecting the pressure build up in the SSVT tank. The SSVT tank must be factory adapted by adding a 2” gate valve in the tank wall nearest the end of the HV winding. In this position, the valve is most effective at detecting any pressure wave coming from the transformer’s high voltage coil.

This option is suitable for monitoring the condition of the SSVT and taking this resulting output for appropriate relaying operation. To ensure the protection of the SPR during transit, the relay is shipped separately from the transformer, and the factory-installed gate valve is covered temporarily for shipment. Once the transformer is installed, the valve is attached by the end user to the transformer. Photos of the relay and the gate valve (as shipped) are shown below.

4 | Station Service Voltage Transformer (SSVT) application - General protection information

Protective circuits using ground fault CTsFor secondary to ground fault monitoring and protection, a window type GFCT can be placed on the H0 bushing, without negatively impacting the SSVT operation. The customer can route the H0 bushing leads through a CT to detect secondary to ground fault referred to the primary. It is highly recommended to select a ratio of 100:5 or higher, with a relay accuracy of C50 design that would maintain 4-5% accuracy or better at 5% of ratio (5 ampere minimum).

From a current perspective, a SSVT-900 100 kVA model has a normal operating current, 100,000 VA / 1.732 x 230,000 (at unity pf) = 0.251 A at rated kVA rating flowing from the primary winding to ground through the H0 bushing. Considering in-rush levels, worst case, that would be 10x in-rush x .251 A = 2.51 A normal operating current, as this design typically has 10% impedance. Using a 100:5, C50 CT will detect faults in the 5 A or higher range. ABB Type SCT-983E, 3” ID Window CT having a 100:5 ratio with C50 accuracy is recommended, equipped with conduit box. This rating assumes a secondary relay circuit connected to the CT with 0.5 ohms or less.

As to detecting line to ground fault, a H1 GFCT with 600:5 ratio with C200 relay accuracy can be used. This assumes the relay burden level is 2 ohm or less. Most utility users that monitor primary line to ground fault on the SSVT, do so on the tank ground circuit, as all line to ground faults end up going to ground through the tank wall. By placing a GFCT on the ground conductor to monitor for primary and secondary line to ground fault current, a smaller CT can be used and it can be mounted away from the SSVT and not impact seismic withstand on the SSVT. The ABB Type SCT-983, 600:5, C200 relay accuracy for primary line to ground faults is recommended.

This CT can be located off the SSVT support stand and monitor current flow to ground from the SSVT tank, through the stand and into the substation ground grid. Assuming that the SSVT stand anchor bolts are embedded in the concrete, connected to the rebar, but not solidly grounded to the substation ground, the primary path for any current flow from the tank through the stand would be via the copper ground conductor used to connect to the ground grid matt. This current flower from the SSVT tank to the substation grounding grid can be monitored using the SCT-983 small window CT. See the attached sketch.

SSVT FAULT PROTECTION

GFCT, 100:5, C50, 3” window type SCT-983E with conduit box, mounted on side of steel stand or on separate support supplied by others. Application – detect secondary faults to ground. Check ratio needed and verify the relay burden connection is less than 0.5 ohms

GFCT, 600:5, C100, 5” window SCT-983 with conduit box, mounted on foundation or on separate support supplied by others. Application – detect primary line to ground faults. Check ratio needed and verify relay burden connection is less than 1.0 ohm

Eyebolt connectors attached to steel support frame for copper ground conductor

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For more information please contact:

ABB KuhlmanHigh Voltage Instrument Transformers3101 Beaumont Centre CircleSuite 225Lexington, KY 40513Toll free: +1 800-950-6966Phone: +1 859-219-6056Fax: +1 859-223-2025

www.abb.com/highvoltage

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