Patent ID: 7222518
Filing Date: 2007-05-29
Classification: G01N,H01F

Abstract:
1. A transformer monitoring system comprising: a current measuring means to measure a current flowing in an oil-filled transformer; an ambient temperature measuring means to measure an ambient temperature; an oil temperature calculation means to calculate an oil temperature in the oil-filled transformer from measurements of the current measuring means and the ambient temperature measuring means; and an oil temperature abnormal decision means to obtain an oil temperature converted value by calculations on a sixth equation, a fifth equation, a fourth equation, a third equation, a second equation, and a first equation based on a voltage and specifications of the transformer, and to compare the oil temperature converted value with an output value from a measured oil temperature conversion means to decide that there is an abnormal condition if a preset oil temperature is exceeded, wherein the first equation is: where θoc (t) [° C.] is the oil temperature converted value t minutes later which is an output value from the oil temperature calculation means; θ θmi (t) is an oil temperature rise converted value, wherein the second equation is: where T A [minutes] is a time constant of an oil temperature with respect to the ambient temperature; θ t [minutes] is a monitoring interval; and n is a number of past measurements used for calculation, wherein the third equation is: where Tm [minutes] is a time constant of an oil temperature with respect to a total loss; and θmu(−nt) [K] is an oil temperature rise n×t minutes earlier in a steady state at a load factor of m %, wherein the fourth equation is: where θou [K] is an oil temperature rise in a steady state at a rated (100%) load; m(−nt.) [%] is an instantaneous load factor n×t minutes earlier; Wi [W] is a no-load loss; Wc [W] is a load loss at the rated (100%) load; and k is a constant, wherein the fifth equation is: where i (−nt) [A] is a secondary instantaneous current n×t minutes earlier; and Is [A] is a secondary rated current, and wherein the sixth equation is: where Es [V] is a secondary rated voltage; P [kVA] is a transformer rated capacity; and F [phase] is the number of transformer phases.