DIFF>) and considers constant error currents such as magnetizing currents

Branch b considers current-proportional errors which may result from transformation errors of the main CTs or the input CTs of the relay, or which may be caused by mismatching or by the influence of tap changers in transformers with voltage control.

In the range of high currents which may give rise to current transformer saturation, branch c provides for additional stabilization.

In the presence of differential currents above branch d a trip command is issued regardless of the stabilizing current and the harmonic stabilization. This is the operating range of the ”High-Speed Trip Stage I_Diff>>”.

Figure 2-36 Operating Characteristic of the Differential Protection

The area of add-on stabilization is determined by the saturation indicator (see side title ”Add-on Stabilization During Current Transformer Saturation”.

The currents I_diffand I_stabare compared by the differential protection with the operating characteristic according to Figure 2-36. If the quantities result into a locus in the tripping area, a trip signal is given.

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Differential Protection (ANSI 87G/87M/87T)

High-Speed Trip Stage I_Diff>>

The high-speed trip stage I_Diff>> clears high-current internal faults instantaneously.

As soon as the differential current rises above the threshold I_Diff>> (branch d), a trip signal is issued regardless of the magnitude of the stabilizing current.

This stage can operate even when, for example, a considerable second harmonic is present in the differential current, which is caused by current transformer saturation by a DC component in the short-circuit current, and which could be interpreted by the inrush stabilization function as an inrush current.

This high-current stage evaluates the fundamental component of the differential current as well as the instantaneous values. Instantaneous value processing ensures fast tripping even in case the fundamental component of the current is strongly reduced by current transformer saturation.

High-current faults in the protected transformer may be cleared instantaneously without regard of the magnitude of the stabilizing currents when the amplitude of the differential currents can exclude that it is an external fault. This is always the case when the short-circuit current is higher than 1/u_sc⋅I_{N Transf}.

Add-on

Stabilization During Current

Transformer Saturation

During an external fault which produces a high through flowing short-circuit current causing current transformer saturation, a considerable differential current can be simulated, especially when the degree of saturation is different at the two measuring points. If the quantities I_diff/I_stabresult in an operating point which lies in the trip area of the operating characteristic (Figure 2-37), a trip signal would be the consequence if no special measures were taken.

Figure 2-37 Operation Characteristics of the Differential Protection with Fault Characteristic

The 7UM62 provides a saturation indicator which detects such phenomena and initiates add-on stabilization measures. The saturation indicator evaluates the dynamic behaviour of the differential and stabilizing current.

The dotted line in Figure 2-37 shows the instantaneous development of currents in case of a external fault with transformer saturation on one side.

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Immediately after the fault (A), the short-circuit currents rise strongly, causing a equally high stabilizing current (2xthrough-flowing current). Saturation occurring on one side (B) now causes a differential current and reduces the stabilizing current, so that the operating pointI_diff/I_stabmay move into the tripping area (C).

In contrast, the operating point moves immediately along the fault characteristic (D) when an internal fault occurs since the stabilization current will barely be higher than the differential current. Therefore, an internal fault is assumed as soon as the ratioI_diff/ I_stabhas exceeded an internal threshold for a fixed minimum time.

Current transformer saturation in case of an external fault is thus characterized by a high stabilizing current flowing at the beginning, i.e. by the operating point (diagram see Figure 2-37) moving into an area that is typical for a high-current external fault (”add-on stabilization). The add-on stabilization area is limited by the parameter I-ADD ON STAB.and the first straight line of the characteristic (withBASE POINT 1 andSLOPE 1) (see Figure 2-38). The saturation indicator makes its decision within the first quarter of a period after fault inception. When an external fault is detected, the differential protection is blocked for a selectable time. The blocking is cancelled as soon as the operating pointI_diff/I_stabmoves steadily (i.e. over 2 periods) within the tripping area. This allows to detect evolving faults in the protected area reliably even during an external fault with current transformer saturation.

Figure 2-38 Add-on Stabilization During Current Transformer Saturation

Harmonic Stabilization

In transformers in particular, high short-time magnetizing currents may be present during power-up (inrush currents). These currents enter the protected zone but do not leave it again, so that they act like fault currents entering from one side (Figure 2-39).

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Differential Protection (ANSI 87G/87M/87T)

Unwanted differential currents may also be cause by parallel connection of transformers or by transformer overexcitation due to excessive voltage.

The inrush current can amount to a multiple of the rated current and is characterized by a considerable 2nd harmonic content (double rated frequency) which is practically absent in the case of a short-circuit. If the second harmonic content exceeds a select-able threshold, tripping is blocked.

Figure 2-39 Inrush Current – Example

Recording of the Three Higher-Voltage Currents

Besides the second harmonic, another harmonic can be selected in the 7UM62 to cause blocking. A choice can be made between the third and fifth harmonic as harmonic stabilization.

Steady-state overexcitation of the transformer is characterized by odd harmonic content. The third or fifth harmonic is suitable to provide stabilization. But, as the third harmonic is often eliminated in power transformers (e.g. by the delta winding), the use of the fifth is more common.

Converter transformers also produce odd harmonic content which is practically absent in the case of internal short-circuits.

The differential currents are analyzed with regard to their harmonics content.

Numerical filters are used to perform a Fourier analysis of the differential current. As soon as the harmonics content exceeds the set thresholds, a stabilization of the respective phase evaluation is started. The filter algorithms are optimized with regard to their transient behaviour such that additional measures for stabilization during dynamic conditions are not necessary.

The harmonic stabilization is maintained for two periods after decrease of the differential current. This prevents an unwanted under-stabilization when external faults are cleared and the higher-order harmonics disappear.

Since the inrush stabilization operates individually per phase, the protection is fully operative even when the transformer is switched onto a single-phase fault, while inrush currents may possibly be present in one of the healthy phases.

In ”modern type” transformers in particular, the 2nd harmonics content may not exceed the threshold value in all three phases on switch-on. To avoid spurious tripping, the so-called “crossblock” function must be activated. As soon as an inrush current is detected in one phase, the other phases of the differential protection stage IDIFF> are blocked.

The cross-block function can be limited to a selectable duration. After this cross-block time has elapsed, no more cross-block is possible for as long as a running fault

condition lasts, i.e. cross-blocking is possible only once after a fault has occurred, and only for the set cross-block time.

The further harmonic stabilizations operate individually per phase. However, it is also possible – as it is for the inrush stabilization – to set the protection such that not only the phase with harmonics content in excess to the permissible value is stabilized but also the other phases of the differential stageI-DIFF>are blocked. The cross-block feature with 3rd or 5th harmonics works in the same way as with 2nd harmonics.

Increase of Pickup Value on Startup

An increase of the pickup value on startup provides additional security against overfunctioning when a non-energized protection object is switched in. As soon as the stabilizing current of one phase has dropped below a settable valueI-REST.

STARTUP, the increase of the pickup value is activated for theI-DIFF>stage. As the stabilizing current is twice the through-flowing current in normal operation, its dropping below that threshold is a criterion for detecting that the protected object is not energized. The pickup valueI–DIFFis now increased by a settable factor (see Figure 2-40); the other branches of the I_diff> stage are shifted proportionally.

This is done by dividing the DIFF current of the respective phase by the factor START-FACTORbefore the characteristic monitoring. The differential current for fault

recording, tripping current etc. is not affected by this.

The return of the stabilizing current indicates the startup. After a settable timeT START MAXthe increase of the characteristic is cancelled.

Figure 2-40 Increase of Pickup Value for Stage I_DIFF>on Startup

Pickup, Dropout The differential protection does not normally use a ”pickup”, since the detection of a fault is identical with the tripping condition. Like all SIPROTEC^®devices, however, the differential protection feature of the 7UM62 has a pickup that is the starting point for a number of ulterior activities. The pickup marks the inception of a fault. This is necessary e.g. for creating fault events and fault records. The pickup also controls internal functions for both internal and external faults (such as necessary actions of the saturation indicator).

A pickup is detected as soon as the fundamental wave of the differential current has attained 85 % of the setting valueI-DIFF>or more than 85 % of the stabilizing cur-rent are in the add-on stabilization area (see Figure 2-41). A pickup signal is also is-sued when the high-speed trip stage for high-current faults picks up.

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2021I DIFF>