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1. (WO2017064148) IMPROVEMENTS IN OR RELATING TO THE PROTECTION OF POWER TRANSFORMERS
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CLAIMS:

1. A method of protecting a power transformer comprising the steps of:

(a) monitoring an external fault indication signal (12A,12B,12C);

(b) carrying out locally a fault determination (46);

(c) carrying out locally an inrush current determination; and

(d) issuing a final trip signal (48) to protect the power transformer if:

(i) the external fault indication signal ( 12A, 12B, 12C) identifies that a fault has occurred and the locally carried out fault determination (22) confirms that a fault has occurred; or

(ii) the external fault indication signal ( 12A, 12B, 12C) identifies that a fault has occurred and the locally carried out fault determination (22) identifies that no fault has occurred but after a predetermined delay the locally carried out inrush current determination (46) confirms the absence of an inrush current.

2. A method of protecting a power transformer according to Claim 1 , wherein step (b) of carrying out locally a fault determination (22) includes comparing different frequency components of a measured current of the power transformer.

3. A method of protecting a power transformer according to Claim 2, wherein comparing different frequency components of a measured current of the power transformer includes:

extracting (16) components of first and second frequency bands of the measured current;

determining (18) the amplitude of the first and second frequency band components;

comparing (20) a ratio of the amplitudes to a fault determination value; and confirming a fault has occurred when the ratio differs from the fault determination value (22).

4. A method of protecting a power transformer according to any preceding claim, wherein step (c) of carrying out locally an inrush current determination (46) includes comparing a harmonic current of the power transformer with a fundamental current of the power transformer.

5. A method of protecting a power transformer according to Claim 4, wherein comparing a harmonic current of the power transformer with a fundamental current of the power transformer includes:

obtaining a 2nd harmonic frequency component and a fundamental frequency component of a measured current of the power transformer;

determining the magnitude of the 2nd harmonic and fundamental frequency components;

comparing a ratio of the magnitudes to a inrush current determination value; and identifying the presence of an inrush current when the ratio differs from the inrush current determination value (46).

6. A method of protecting a power transformer according to Claim 5, wherein obtaining a 2nd harmonic frequency component of a measured current of the power transformer and determining the magnitude of the 2nd harmonic frequency component includes the convolution of the measured current with complex coefficients, wherein the complex coefficients are established by a predetermined parameter matrix based on a sampling regime used to measure the current of the power transformer.

7. A method of protecting a power transformer according to Claim 5 or Claim 6, wherein the magnitude of the fundamental frequency component is determined using a half-cycle Fourier method.