GROUND FAULT PROTECTION
A robust ground-fault protection system is disclosed which can possess a reduced sensitivity to system noise. In a ground-fault protection system for an electrical machine such as a generator, an injection signal with an injection frequency f i is applied to the electrical machine in order to generate a periodic bias voltage on a conductor of the electrical machine, and a response signal thereto is evaluated. The injection frequency can be adapted such that a flexible approach respective of a most recent value of a system quantity of the electrical machine, can provide increased stability and reliability of the ground-fault protection system.
1 . A method of adapting a ground-fault protection system for an electrical machine, wherein the system injects an injection signal u i — with an injection frequency fi into a conductor of the electrical machine and evaluates a response signal u RE thereto to identify a ground-fault in the electrical machine, the method comprising:
measuring a system quantity of the electrical machine indicative of system noise; and
adapting the injection frequency according to the system quantity.
2 . The method according to claim 1 , wherein the system quantity is a line frequency fg of an electrical power grid connected to the electrical machine, the method comprising:
adapting the injection frequency fi according to a deviation of an actual line frequency from a nominal line frequency.
3 . The method according to claim 1 , comprising:
repeatedly adapting the injection frequency.
4 . A method according to claim 2 , comprising:
repeatedly adjusting the injection frequency proportionally to a deviation of the actual line frequency from the nominal line frequency.
5 . The method according to claim 1 , wherein the system quantity is a noise spectrum n (f) , the method comprising:
measuring the noise spectrum over a frequency range including a plurality of candidate injection frequencies; and
selecting a base injection frequency with lowest noise from among the candidate injection frequencies.
6 . The method according to claim 1 , wherein the system quantity is a generator speed threshold n t the method comprising:
measuring, during generator start-up, a first system noise spectrum for a candidate start-up injection frequency and a second system noise spectrum for a candidate base injection frequency;
identifying a generator speed threshold below which the first noise spectrum is acceptable and above which the second noise spectrum is acceptable; and
selecting the injection frequency to be the start-up injection frequency below the generator speed threshold and the base injection frequency above the generator speed threshold.
7 . The method according to claim 1 , comprising:
recording a first noise spectrum indicative of noise in a band around a first frequency as a function of generator speed during a start-up of a generator;
recording a second noise spectrum indicative of noise in band around a second frequency as a function of generator speed during start-up the generator;
identifying a generator speed threshold n t below which the first noise spectrum is acceptable and above which the second noise spectrum is acceptable; and
selecting the injection frequency to be the first frequency for generator speeds below the threshold n t and the second frequency for generator speeds above the threshold n t .
8 . The method according to claim 1 , comprising:
selecting a first candidate injection frequency and a second candidate injection frequency;
measuring, during generator start-up, a first system noise spectrum representing system noise around the first candidate injection frequency as a function of generator speed;
measuring, during generator start-up, a second system noise spectrum representing system noise around the second candidate injection frequency as a function of generator speed;
identifying a generator speed threshold n t below which a first noise spectrum is uncritical and above which the second noise spectrum is uncritical; and
changing the injection frequency from the first to the second candidate frequency at the generator speed threshold n t during subsequent generator start-ups.
9 . The method according to claim 5 , wherein a further electrical machine is conductively connected to the first electrical machine, the method comprising:
selecting a further base injection frequency for the further machine that is different from the base injection frequency chosen for the electrical machine.
10 . The method according to claim 1 , comprising:
adapting the injection frequency in relation to different generator load conditions.
11 . The method according to claim 1 , wherein the ground fault protection system is configured for rotor and/or stator ground fault protection in a generator with a power in excess of 20 MVA.
12 . A ground-fault protection system for an electrical machine, comprising:
a signal generator for injecting a frequency signal u i with an injection frequency f i into a conductor of the electrical machine; and
an earth fault protection relay for evaluating a response signal u RE thereto, to identify a ground-fault in the electrical machine,
wherein the signal generator is communicatively connected to the protection relay via a communication link, and configured for receiving a value of the injection frequency f i adapted to a measured system quantity of the electrical machine indicative of system noise.
13 . The method according to claim 2 , comprising:
repeatedly adapting the injection frequency.
14 . The method according to claim 4 , comprising:
recording a first noise spectrum indicative of noise in a band around a first frequency as a function of generator speed during a start-up of a generator;
recording a second noise spectrum indicative of noise in band around a second frequency as a function of generator speed during start-up the generator;
identifying a generator speed threshold n t below which the first noise spectrum is acceptable and above which the second noise spectrum is acceptable; and
selecting the injection frequency to be the first frequency for generator speeds below the threshold n t and the second frequency for generator speeds above the threshold n t .
15 . The method according to claim 4 , comprising:
selecting a first candidate injection frequency and a second candidate injection frequency;
measuring, during generator start-up, a first system noise spectrum representing system noise around the first candidate injection frequency as a function of generator speed;
measuring, during generator start-up, a second system noise spectrum representing system noise around the second candidate injection frequency as a function of generator speed;
identifying a generator speed threshold n t below which a first noise spectrum is uncritical and above which the second noise spectrum is uncritical; and
changing the injection frequency from the first to the second candidate frequency at the generator speed threshold n t during subsequent generator start-ups.
16 . The method according to claim 4 , wherein a further electrical machine is conductively connected to the first electrical machine, the method comprising:
selecting a further base injection frequency for the further machine that is different from the base injection frequency chosen for the electrical machine.
17 . The method according to claim 10 , comprising adapting the injection frequency in relation to different generator load conditions.
18 . The method according to claim 11 , wherein the ground fault protection system is configured for rotor and/or stator ground fault protection in a generator with a power in excess of 20 MVA.