Apparatus and method for predicting an overload trip for an electrical power transformer

An apparatus for predicting an overload trip condition for an electrical power transformer supplying a load current comprising:

• a current sensing means for providing a first signal indicative of the load current;
• a temperature sensing mean for providing a second signal indicative of the hot spot temperature ΘH, wherein said hot spot temperature is an estimate of the hottest spatial location within the transformer;
• and
• a signal processor for deriving from said first and second signals an estimated a time value, T, representative of the estimated time before an occurring overload trip condition of the transformer at the present load current.

An apparatus for predicting an overload trip condition for an electrical power transformer supplying a load current, the apparatus comprising:

• a current sensing means for providing a first signal indicative of the load current;
• a temperature sensing means for providing a second signal selectively indicative of ambient temperature external to the transformer or transformer temperature at a selected location;
• and
• a signal processor for deriving from said first and second signals an estimated time value, T, representative of the estimated time before an occurring overload trip condition of the transformer at the present load current, wherein said signal processor includes,
• computational means for estimating the hot spot temperature ΘH of the transformer as a function of said first and second signals, wherein said hot spot temperature is an estimate of the hottest spatial location within the transformer,
• computational means for deriving said time value, T, as a function of said estimated hot spot temperature and a preselected loss of life value, Lset, and
• wherein said time value, T, satisfies the time integral of the rate of change of transformer loss of life factor, L, mathematically described by:
• L set = ∫ 0 T ⁢ ⅆ L ⅆ t ⁢ ⁢ ⅆ t ⁢ ⁢ and ⁢ ⁢ L = a ⁢ ⁢ function ⁢ ⁢ of ⁢ ⁢ Θ H .

An apparatus for predicting an overload trip condition for an electrical power transformer supplying a load current, the apparatus comprising:

• a current sensing means for providing a first signal indicative of the load current;
• a temperature sensing means for providing a second signal selectively indicative of ambient temperature external to the transformer or transformer temperature at a selected location;
• a signal processor for deriving from said first and second signals an estimated time value, T, representative of the estimated time before an occurring overload trip condition of the transformer at the present load current;
• and
• a time display coupled to said signal processor for indicating the remaining portion of time from the present time to the time at which one of said first or second preselected time values is exceeded.

A method for predicting an overload trip condition for an electrical power transformer supplying a load current, the method comprising the steps of:

• sensing the load current and providing a first signal indicative thereof;
• selectively sensing ambient temperature external to the transformer or transformer temperature at a selected location and providing a second signal indicative thereof;
• deriving an estimated time value, T, representative of the estimated time before an occurring overload trip condition of the transformer at the present load current, wherein said time value, T, is derived from values of said first and second signals;
• estimating the hot spot temperature ΘH of the transformer as a function of said first and second signals, wherein said hot spot temperature is an estimate of the hottest spatial location within the transformer;
• and
• deriving said time value, T, as a function of said estimated hot spot temperature and a preselected loss of life value, Lset, wherein said time value, T, satisfies the time integral of the rate of change of transformer loss of life factor, L, mathematically described by:
• L set = ∫ 0 T ⁢ ⅆ L ⅆ t ⁢ ⁢ ⅆ t ⁢ ⁢ and ⁢ ⁢ L = a ⁢ ⁢ function ⁢ ⁢ of ⁢ ⁢ Θ H .

An apparatus for predicting an overload trip condition for an electrical power transformer supplying a load current comprising:

• a current sensing means for providing a first signal indicative of the load current;
• a temperature sensing mean for providing a second signal indicative of the hot spot temperature ΘH, wherein said hot spot temperature is an estimate of the hottest spatial location within the transformer;
• a temperature sensing means for providing a second signal selectively indicative of ambient temperature external to the transformer or transformer temperature at a selected location;
• a signal processor for deriving from said first and second signals an estimated time value, T, representative of the estimated time before an occurring overload trip condition of the transformer at the present load current;
• wherein said signal processor includes,
• computational means for estimating the hot spot temperature ΘH of the transformer as a function of said first and second signals, and
• computational means for deriving said estimated time value, T, as a function of said estimated hot spot temperature and a preselected loss of life value, Lset, and wherein said time value, T, satisfies the time integral of the rate of change of transformer loss of life factor, L, mathematically described by:
• L set = ∫ 0 T ⁢ ⅆ L ⅆ t ⁢ ⁢ ⅆ t ⁢ ⁢ and ⁢ ⁢ L = a ⁢ ⁢ function ⁢ ⁢ of ⁢ ⁢ Θ H
• and L=a function of ΘH.