1. A metal-graphite brush comprising graphite, a metal, and a lubricant, which brush further contains bismuth or bismuth oxide in an amount of 1.0 wt. % to 3.0 wt. %.
2. A metal-graphite brush as described in claim 1, wherein the metal is copper.
3. A metal-graphite brush as described in claim 1, wherein the lubricant is a metal sulfide.
The claims below are in addition to those above.
All refrences to claim(s) which appear below refer to the numbering after this setence.
1. A method for disconnecting a generator from a power system when the generator and the thereto connected power system are non-synchronized, wherein the method comprises the steps of:
determining that there is a loss of synchronization between the generator and the power system, which loss of synchronization provides an amplitude variation of an electrical parameter associated with the generator, wherein a frequency of the amplitude variation is proportional to a rotor angle variation of the generator, which variation of the rotor angle arises due to the loss of synchronization between the generator and the power system,
determining when a minimum amplitude of the electrical parameter will occur, wherein when the occurrence of the minimum amplitude has been predicted,
providing a command signal for disconnecting the generator from the power system, wherein the command signal is provided prior to the minimum amplitude of the electrical parameter is attained, and
disconnecting the generator from the power system approximately when the determined minimum amplitude of the electrical parameter occurs.
2. The method as claimed in claim 1, wherein the step of determining when the minimum amplitude will occur involves determining a slope of the amplitude variation so as to allow predicting when the minimum amplitude will occur.
3. The method as claimed in claim 1, wherein the step of determining when the minimum amplitude will occur involves determining the frequency of the amplitude variation so as to allow predicting when the minimum amplitude will occur.
4. The method as claimed in claim 1, wherein the step of determining when the minimum amplitude will occur involves determining the occurrence of a first minimum amplitude of the electrical parameter.
5. The method as claimed in claim 4, wherein the disconnecting involves disconnecting the generator from the power system approximately when the first minimum amplitude of the electrical parameter is attained.
6. The method as claimed in claim 1, wherein the electrical parameter is an impedance associated with the generator.
7. The method as claimed in claim 1, wherein the command signal is provided to at least one circuit breaker for performing the step of disconnecting the generator from the power system.
8. The method as claimed in claim 1, wherein the amplitude variation of the electrical parameter is essentially periodic.
9. A protection system for a generator connected to a power system, the protection system comprising:
a measuring unit arranged to measure an electrical parameter associated with the generator,
a processing unit arranged to:
determine that there is a loss of synchronization between the generator and the power system based on the measured electrical parameter, which loss of synchronization provides an amplitude variation of the electrical parameter, wherein a frequency of the amplitude variation is proportional to a rotor angle variation of the generator, which variation of the rotor angle arises due to the loss of synchronization between the generator and power system, and
predict when a minimum amplitude of the electrical parameter will occur, and
at least one circuit breaker,
wherein the processing unit is arranged to provide a command signal to the at least one circuit breaker prior to the minimum amplitude is attained, to disconnect the generator from the power system so as to allow for the at least one circuit breaker to disconnect the generator from the power system approximately when the minimum amplitude occurs.
10. The protection system as claimed in claim 9, wherein the processing unit is arranged to determine a first attained minimum amplitude of the measured electrical parameter after the loss of synchronization is determined, wherein the command signal is generated so as to disconnect the generator approximately when the first minimum occurs.
11. The protection system claimed in claim 9, wherein the processing unit is arranged to determine a slope of the amplitude variation so as to predict when the minimum amplitude will occur.
12. The protection system as claimed in claim 9, wherein the processing unit is arranged to determine the frequency of the amplitude variation so as to predict when the minimum amplitude will occur.