from the 40 FF department...
Generator Protections
Application Questions and Problems topic
Posted by Aziz Al-Saffar on 5 July, 2007 - 11:41 pm
I am working on frame 5 gas turbine units. I have difficulty in understanding the field failure (40 FF). How does it work? Its parameters are –xa and xb, where (-xa = xd' * CT ratio/ VT ratio) and (xb = xd * CT ratio/ VT ratio). From where can you get these factors xd and xd'?

Posted by Anonymous on 8 July, 2007 - 11:08 pm
Please check genr datasheet.

1 out of 1 members thought this post was helpful...
Posted by Phil Corso, PE on 9 July, 2007 - 11:04 pm
Responding to Aziz Al-Saffar's 05-Jul (23:41) questions:

A) Background.
The field-failure protection relay (ANSI/IEEE Device # 40) primarily protects a generator from thermal damage occurring because of sudden loss of its field-excitation. Previous posts have shown that a generator produces only two electrical parameters, volts and amps. Together they provide a third measursble electrical parameter, power factor. Consider the case of a generator in parallel with others. Upon loss of excitation its terminal voltage drops (not necessarily precipitously) causing its output kW to fall resulting in an increase in generator speed so that it now operates asynchronously, i.e., out-of-synchronism, with the other generators. Simultaneously, a change in its pre-fault kVAr (some call it reactive power, but let's not quibble) occurs! Both the change in kW, and the kVAr change from lagging to leading, drastically alters the machine's output current and power factor. Unless corrected quickly damage can occur. Depending on generator design, the tolerable delay
could range from a few seconds to several minutes!

B) How the Field-Failure Relay Functions.
The relay can contain several functional components: a) a directional-unit that detects the ampere-change from lagging to leading (often referred to kVAr-out to kVAr-in); an impedance-unit which mimics the machine's impedance change; an undervoltage-unit that responds to terminal voltage; and a time-delay unit.

C) Definition of Xd and Xd'
Xd is the generator's steady-state synchronous reactance, while Xd' is the generator's direct-axis transient-reactance! These two parameters should have already been supplied by the machine builder.

If you require more detail, please provide specific information regarding your plant's parameters!

Regards, Phil Corso, PE (

Posted by Aziz Al-Saffar on 12 July, 2007 - 10:14 pm
Thank you very much and I appreciate your cooperation, It is clear to me now, also I read some subjects and topics that are relevant to this matter and I almost understand the principles of how this protection is working. It is depending on the generator terminal impedance and reactance through CT & VT readings, but I am asking now why they depend on the generator terminals measurements, or why they are detect this fault from the stator of the generator and not from the rotor (I believe that there is a good and reasonable reason). On the other hand, what is the wrong if we detect the field failure from the excitation system (DC cct), as an example by sensing the exciter voltage or current and build the field failure protection on it, is it easier?

With respect, Aziz Al-Saffar, (aim_mgtps @ msn. com)

1 out of 1 members thought this post was helpful...
Posted by Phil Corso on 15 July, 2007 - 1:02 pm
Responding to Aziz Al-Saffar's 12-Jul (22:14) question:

When rotor excitation was supplied through slip-rings loss-of-field protection was provided by undercurrent dc relays. But, such systems were subject to too wide a range of operating conditions. For example, they were susceptible to induced ac currents during synchronizing, and during external faults. Further complication was introduced when solid-state excitation units, having small slip-rings for field coil control, became part of the rotating structure, requiring additional slip-rings to carry the monitoring and tripping circuits. The introduction of stator volt and amp monitoring became the more reliable, less cumbersome, method!

Phil Corso (cepsicon@aolcom)

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