This invention relates to a method and system of transformer protection and a method of detecting a defective winding, and particularly to a method of transformer protection and method of defective winding detection capable of fast and sensitive operation based on the digital processing in which the terminal voltages and currents of a transformer to be protected are sampled at a proper time interval and the sampled data are subjected to a prescribed computation.
The conventional transformer protection against such abnormalities as short-circuiting of winding is based on the detection of a differential current among the terminal currents of the transformer. Since a differential current arises at the inrush of excitation of the transformer as well as at the occurrence of a defect of a winding, conventional technique is designed to prevent the transformer protection relay from responding to the inrush excitation currents by adopting the "second harmonic suppression system" which utilizes the nature of the inrush excitation current which includes the second harmonic wave in a large proportion.
However, depending on such conditions as the earth capacity and the reactance of the power transmission system and the impedance of the transformer, the current in trouble can include low-order harmonic components near the second harmonic, and the conventional second harmonic suppression system can cause a retardative operation of the transformer protection relay and eventually incur a catastrophic event due to a faulty operation.
As a counter measure against the above impropriety, there is proposed a digital processing system disclosed in Japanese Patent Laid-Open No. 62-89424 (JP-A-62-89424) Specification. The process, as described in the following, is based on the fact that when a transformer is modeled as a multi-terminal network and expressed by the admittance equation, the transfer admittance at the excitation inrush or at the occurrence of an internal defect does not differ significantly from the normal state, whereas the drive-point admittance or the parallel admittance induced from it differs significantly between the excitation inrush and an internal defect. In this specification, the reciprocal of the inductance is termed "admittance" expediently since there is no proper wording thereof.
(1) The transfer admittance of the transformer to be protected is evaluated from the leakage inductance and memorized as a constant in the relay in advance.
(2) Terminal voltages and currents of the transformer are sampled at a proper time interval.
(3) A differential current is evaluated from the sampled terminal currents.
(4) Based on the implication that a differential current in excess of a certain detection level implies the state of internal defect or excitation inrush, the drive-point admittance or parallel admittance is evaluated from the terminal voltages and currents and the memorized transfer admittance and, depending on its magnitude, the discrimination between an internal fault and excitation inrush is made.
(5) If the judgement of internal defect repeats for a certain number of consecutive samples, the breaker opening command is issued so as to disconnect the transformer.
This transformer protection system has outstanding features as compared with the conventional second harmonic suppression system, in that the transfer admittance used as a coefficient of the operational equation is a known constant, allowing the high-accuracy computation and the application to transformers of various types of structure; the discrimination between an internal defect and the excitation inrush is made irrespective of the current waveform, allowing the application to transformers of any power system regardless of the magnitude of earth capacity; and the discrimination between an internal defect and excitation inrush is made on the first wave of the current, allowing the fast relay operation in the event of an internal fault.
However, the foregoing conventional technique involves the following problems.
(1) Computation of the parallel admittance includes the division operation, consuming a lot of time.
(2) When the equation has the denominator of a small value, a large operational error arises, resulting possibly in an erroneous judgement.
(3) The parallel admittance has different values depending on the transformer structure, requiring the setting of a threshold value based on the inference of the value for each transformer, which imposes a cause of erroneous setting.
(4) The parallel admittance at the occurrence of a fault has a constant value, compelling irrationally the event discrimination with the same weighting within a cycle regardless of the magnitude of the current in trouble.
The Japanese Patent Laid-Open No. 62-89424 discloses a method of fault determination without involving the division operation, but by reforming the judgement formulas so that the judgement is based on the polarities of the denominator and numerator. However, the principle of fault determination is identical to the determination based on the value of parallel admittance, and above problems of items (3) and (4) remain unsolved.