Function group REF_1 provides a protection function, stabilized by a characteristic, for autotransformers; a typical example is displayed in the next figure. In this case, the phase currents in ends a and b as well as the neutral-point current must be taken into account.

For such applications with REF_1, the transformer end b may be selected, and end a is permanently included in the protection function.

(019 120) REF_1: Add.meas.inp. end b = Yes / No (default setting is “No”)

The neutral-point current is permanently assigned to the measuring input for end a (T14).

A further amplitude matching factor is calculated and displayed by the P632 for each end added. All currents involved must be referred to a common reference value as this application entails protection of an electric node in a zero-sequence system. This common reference current value is calculated based on the settable reference power and the nominal voltage for end a. The amplitude matching factors then result from the ratio of the primary transformer current values for the relevant end and the common reference current value.

Reference current:

Amplitude matching factor:

for x = End a, b

with

Sref, prim = (019 031) REF_1: Reference power Sref

Vnom, prim a = (019 017) MAIN: Vnom prim. end a PSx

Inom, CT, prim a = (019 020) MAIN: Inom C.T.prim.,end a

Inom, CT, prim b = (019 021) MAIN: Inom C.T.prim.,end b

The reference current and matching factors for the transformer ends are displayed at the P632.

As before, the P632 checks that the reference current and matching factors remain within the permitted ranges. The permitted range for the reference current can be looked up in the operating system. The following rule applies to the matching factors:

Should the P632 calculate a common reference current value or matching factors not satisfying the above conditions then an alarm will be issued and the P632 will automatically be blocked.

The measured values are multiplied by the matching factors and they are then available for further processing. Consequently, all threshold values and measured values always refer back to the relevant reference current rather than to the transformer nominal current or the nominal current of the device.

As shown below, the differential current is formed from the sum of all phase currents involved and the neutral-point current:

Idiff,N = |kam,a ·I̲N,a + kam,b·I̲N,b + kam,Y·I̲a,Y|

= |kam,a·∑{I̲A,a,I̲B,a,I̲C,a} + kam,b·∑{I̲A,b,I̲B,b,I̲C,b} + kam,Y·I̲a,Y|

The calculation of the restraining current depends on the set operating mode.

If the operating mode is set to “Low imped. / sum(IP)”, the restraining current is calculated from the sum of all ends involved:

IR,N = |kam,a ·I̲N,a + kam,b·I̲N,b|

= |kam,a·∑{I̲A,a,I̲B,a,I̲C,a} + kam,b·∑{I̲A,b,I̲B,b,I̲C,b}|

If the operating mode is set to “Low imped. / IP,max”, the formula to calculate the restraining current remains unchanged from the application for a single end. But now the highest phase current is calculated from the amplitude-matched sum of the currents on the relevant phase of all ends involved.

IR,N = 0.5·(max{|I̲A|,|I̲B|,|I̲C|} + kam,Y·|I̲a,Y|) (unchanged)

with I̲x = kam,a·I̲x,a + kam,b·I̲x,b for x = A, B or C

The value pairs ( Id,N / IR,N ) calculated with the above formula are then compared with the already identified tripping characteristics of the ground differential protection function.