Patent ID: 7222518

Claim:
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: θ oc ( t )=θ Ai ( t )+θ mi ( t ) (1) 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 Ai (t) is a virtual ambient temperature; and θmi (t) is an oil temperature rise converted value, wherein the second equation is: θ Ai ⁡ ( t ) = ∑ T = 0 T = n ⁢ ( { 1 - ⅇ - [ ( T ) × t T A ] } - { 1 - ⅇ - [ ( T + 1 ) × t T A ] } ) × θ A ⁡ ( - Tt ) ( 2 ) where T A [minutes] is a time constant of an oil temperature with respect to the ambient temperature; θ A (−nt) [° C.] is an ambient temperature n×t minutes earlier; t [minutes] is a monitoring interval; and n is a number of past measurements used for calculation, wherein the third equation is: θ ⁢ mi ⁢ ⁢ ( t ) = ∑ T = 0 T = n ⁢ ( { 1 - ⅇ - [ ( T ) × t T m ] } - { 1 - ⅇ - [ ( T + 1 ) × t Tm ] } ) × θ ⁢ mu ⁡ ( - Tt ) ( 3 ) 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: θ ⁢ mu ⁢ ⁢ ( - nt ) = θ ⁢ ou × [ Wi + Wc × ( m ⁢ ⁢ ( - nt ) 100 ) 2 Wi + Wc ] k ( 4 ) 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: m ⁢ ⁢ ( - nt ) = i ⁢ ⁢ ( - nt ) Is × 100 ⁢ ⁢ ( % ) ( 5 ) 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: Is = P Es × 1 F ( 6 ) where Es [V] is a secondary rated voltage; P [kVA] is a transformer rated capacity; and F [phase] is the number of transformer phases.