Instrument transformers

An instrument transformer gives the meters and the protection relays a copy of the primary current or voltage at a level they can take. A current transformer (CT) gives a secondary current of 1 A or 5 A. A voltage transformer gives a secondary voltage of 100 V, 110 V or 115 V between lines. The design specifies the ratio, the number of secondary cores, the accuracy class of each core and the rated burden, and prints them on the single-line diagram (SLD) and in the bill of materials.

Read first: Bays and their equipment · Circuit breakers and isolators

1Current transformersCopy link to Current transformers

The HV CT is a single-phase unit, three per bay. In a line or transformer bay it sits between the circuit breaker and the line or transformer isolator (Bays and their equipment, Line bay). Its primary follows the bay's rated current. The design takes the bay rated current of the level as the higher primary, unless the user enters another value.

The CT has a dual ratio, from a primary winding reconnected in series or in parallel. The lower primary is the current nearest half of the higher one in the design's series of CT primaries, 100, 125, 160, 200, 250, 315, 400, 500, 630, 800, 1000, 1250, 1600, 2000, 2500, 3150, 4000, 5000 and 6300 A. A higher primary below 200 A has a half under that series, and the CT then has one ratio. The ratio prints as the higher primary, the lower primary and the secondary, for example 1600-800/1 A at 220 kV.

The design's CT has 5 cores, connected in the order 0.2S, 0.2S, 5P20, 5P20, PS. The two measuring cores of class 0.2S feed the meters of the grid connection (Bays and their equipment, Line bay). The two protective cores of class 5P20 and the class PS core feed the protection relays. The rated burden is 15 VA, one of the rated outputs of IEC 61869-2, and the secondary is 1 A by default.

The MV switchgear's CTs take the panel's rated current, rounded up on the same series. An incomer CT follows the incomer's rating and a feeder CT the feeder's. The MV CTs have the same cores as the HV CTs.

2Voltage transformersCopy link to Voltage transformers

The design places a capacitor voltage transformer (CVT) at the entry of each line, and one set on each main bus or bus section for the bus voltage. At a 33 kV HV level the voltage transformers are inductive, a potential transformer (PT) to IEC 61869-3, in place of each CVT.

The ratio is the level's voltage over root three to the secondary, 220/√3 : 110/√3 V at 220 kV. The secondary is 110/√3 V by default, and 100/√3 V is the other choice. The voltage transformer has 3 secondaries: 0.2, 3P, open delta, that is a measuring secondary of class 0.2, a protective secondary of class 3P and an open delta winding. The rated burden is 50 VA, one of the rated outputs of IEC 61869-5.

A ring bus has no main buses and no bus voltage transformer. Each circuit has its own potential source (RUS Bulletin 1724E-300 §4.6.4), so each line and each transformer has its voltage transformer. In a gas-insulated switchyard (GIS), the bus voltage transformers are inductive units inside the GIS, and the CVT stays outdoors at each line exit (Gas-insulated switchgear (GIS), Exits and links). The MV bus has one PT panel per section.

3The ANSI / IEEE setCopy link to The ANSI / IEEE set

The ANSI / IEEE set takes its instrument transformers from IEEE C57.13-1993 as RUS Bulletin 1724E-300 reprints it. A new project's CT secondary is 5 A. The HV CT is a multi-ratio (MR) CT whose full ratio is the bay current, from the standard multi-ratio CTs 600:5, 1200:5, 2000:5, 3000:5, 4000:5 and 5000:5 A. A primary outside that list prints as one ratio.

The CT has 5 cores: 0.15B-1.8, 0.15B-1.8, C800, C800, C800. The two 0.15 class metering cores feed the main and the check revenue meters at the B-1.8 burden, 45 VA at 5 A. The three C800 cores feed the relays. A CT with a 1 A secondary keeps the burden of the IEC + CEA / CBIP set and prints no MR, because the standard multi-ratio CTs are 5 A ratings.

The MV CT is the standard multi-ratio CT at or above the panel's current, and 600:5 A MR is the smallest the design takes. That is the design's value for switchgear CTs, which carry the relaying cores and the bus's short-time current. The voltage transformers are 230/√3 : 115/√3 V, 115 V between lines, with secondaries 0.15, 0.3, 1.2 for revenue metering, metering and relaying, at 50 VA. The bill of materials calls the CVT a coupling capacitor voltage transformer (CCVT).

4The inputsCopy link to The inputs

The design's values are defaults, each an input with Auto on the Define page. The CT inputs are the primary, the secondary, the cores, their classes and the burden. The voltage transformer inputs are the secondary voltage, the secondaries, their classes and the burden. A value entered there prints on the SLD and in the bill of materials in place of the default. Enter the core classes in the order the cores are connected.