ABB RELION REX640 Technical Manual page 437

Protection and control

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1MRS759142 F

The module calculates the start duration value

percentage ratio of the start time and the set operating time. The value is available

in the monitored data view.

Once activated, the alarm timer is activated. The time characteristics are according

to DT. When the alarm timer reaches the value set by

output is activated. If the fault disappears before the module generates an ALARM

signal, the reset timer is activated. If the reset timer reaches the value set by

delay time , the alarm timer resets. The activation of the BLOCK signal resets the

Timer 2 and deactivates the ALARM output.

In order to prevent the generator shaft from getting electrically charged the

shaft is grounded. Different types of generators have different ways for shaft

grounding. With hydro-generators, the water acts as a grounding point, and with

turbogenerators the shaft can be grounded with a slip ring. However, the leakage

current can still occur if the bearing pedestal, which is isolated from the ground,

becomes grounded. This imposes shaft voltage on the oil-film of the bearing. The

current can break the insulation that the oil-film provides and thus it can cause

pitting and abrasion on the bearing face.

The measured shaft current is fed to the protection relay. The function compares

the current signal to the set alarm and operate levels. Operate time characteristics

are according to definite time.

Measurement device options

There are two different applicable shaft current measurement devices, the

Rogowski coil and the current transformer (CT). Both devices are installed around

the generator shaft. For both devices it is necessary to amplify the voltage signal.

The recommended measurement principles are depicted in

Figure 250: Current transformer measurement arrangement

Technical Manual

Protection functions

START_DUR , which indicates the

Alarm delay time , the ALARM

Protection relay