Out-of-Synchronism occurs relatively rare in a large and robust power system, yet it may end up in an extensive and serious system disturbances, should it happen and persist for a long duration. To avoid such unexpected occurrences and in order to assure a good system security, a thorough and comprehensive study on generator protection is necessary.
In a system, which involves different users, the responsibility on system security should be borne by the grid participants and the system operator; and it ought to be based on a reference agreed by all.
The reference may concretely be realized in form of standard or agreement, or in form of rules agreed by the system users (grid code).
In many cases, not all matters in the code, especially those related to protection system coordination of the relays installed at the system operator’ side and that at the plant owner’s side, are well defined and fully understood.
The study here presented shall describe general condition of out-of-synchronism in a system and discuss the basic criteria that ought to be followed in defining the protection against out-of-synchronism.
In defining the criteria for the protection, some points should be considered: 1) asynchronous operation condition due to generator loss of field and 2) asynchronous operation due to system instability (out-of-synchronism).
Generator impedance varies as slip of generator relative to the system synchronous speed changes, and it will shift the electrical mid-point of the system-generator impedance. So, it will consequently influence the performance of generator protection.
Under-excitation operation of the generator will have, to some extents, no influence on the generator operation, unless it goes beyond the steady state stability limit.
Generally, the limits are given in the manufacturer’s manual.
In our study, the representation of generator’s current and voltage during the field current decrease (in parallel operation) is simulated in two conditions, as follows:
* decrease of generator’s e.m.f is not followed by rotor slip variation,
* decrease of generator’s e.m.f is followed by generator slip
Yet, in the real case, the field current decreases not linearly and the generator slip will also vary slightly during those interval. We should however bear in mind that the impedance locus moves towards the generator point.
The current decrease, that produces consequently new slip value, will take place when:
* decrease of the field current can not be sufficiently compensated by the under excitation limiter (UEL), or
* limit of the steady state stability is exceeded.
Those limits are generally determined based on the generator capacity to bear capacitive load, which is given by manufacturers.
In our study, the generator capacity has been taken from the loading curve of generator, determined by P (active power) and Q(reactive power) of the generator.
The reference may concretely be realized in form of standard or agreement, or in form of rules agreed by the system users (grid code).
In many cases, not all matters in the code, especially those related to protection system coordination of the relays installed at the system operator’ side and that at the plant owner’s side, are well defined and fully understood.
The study here presented shall describe general condition of out-of-synchronism in a system and discuss the basic criteria that ought to be followed in defining the protection against out-of-synchronism.
Generator impedance varies as slip of generator relative to the system synchronous speed changes, and it will shift the electrical mid-point of the system-generator impedance. So, it will consequently influence the performance of generator protection.
Under-excitation operation of the generator will have, to some extents, no influence on the generator operation, unless it goes beyond the steady state stability limit.
Generally, the limits are given in the manufacturer’s manual.
In our study, the representation of generator’s current and voltage during the field current decrease (in parallel operation) is simulated in two conditions, as follows:
* decrease of generator’s e.m.f is not followed by rotor slip variation,
* decrease of generator’s e.m.f is followed by generator slip
Yet, in the real case, the field current decreases not linearly and the generator slip will also vary slightly during those interval. We should however bear in mind that the impedance locus moves towards the generator point.
The current decrease, that produces consequently new slip value, will take place when:
* decrease of the field current can not be sufficiently compensated by the under excitation limiter (UEL), or
* limit of the steady state stability is exceeded.
Those limits are generally determined based on the generator capacity to bear capacitive load, which is given by manufacturers.
In our study, the generator capacity has been taken from the loading curve of generator, determined by P (active power) and Q(reactive power) of the generator.
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