Method and system for optical calculation in relay protection based on the faraday magneto-optical rotation effect

An optical calculation system for relay protection based on Faraday magneto-optical rotation effect, comprising:

• a light source, a first optical current transformer, a second optical current transformer, a third optical current transformer, and a processing unit;
• a light signal emitted from the light source passes through the first optical current transformer, the second optical current transformer, and the third optical current transformer in sequence, causing the light signal to carry a Faraday rotation angle φ, wherein the Faraday rotation angle φ comprises differential current information;
• an outgoing light from the third optical current transformer enters the processing unit;
• light signals between the light source and the first optical current transformer, the first optical current transformer and the second optical current transformer, the second optical current transformer and the third optical current transformer, as well as the third optical current transformer and the processing unit, are transmitted through multimode fibers;
• Faraday magneto-optical elements of the first optical current transformer and the third optical current transformer are respectively located on a high-voltage side and low-voltage side of a protected phase of transformer;
• a Faraday magneto-optical element of the second optical current transformer is located on a high-voltage side of adjacent phase to the protected phase of the transformer;
• under normal operating conditions, a phase difference between the protected phase and its adjacent phase on the high-voltage side of transformer share the same phase with the protected phase on the low-voltage side;
• the Faraday magneto-optical elements of the first optical current transformer, the second optical current transformer, and the third optical current transformer are allowed to sense a magnetic field generated by use of a current on their respective sides;
• angles between optical paths of the first optical current transformer, the second optical current transformer, and the third optical current transformer and a current direction on their respective sides satisfy following relationship:
• sin
• ⁢
• θ
• YA
• :
• sin
• ⁢
• θ
• YB
• :
• sin
• ⁢
• θ
• dA
• =
• 1
• :
• 1
• :
• 3
• n
• T
• ,
• wherein,
• θYA represents the angle between the optical path of the first optical current transformer and the current direction on its respective side;
• θYB represents the angle between the optical path of the second optical current transformer and the current direction on its respective side;
• and
• θdA represents the angle between the optical path of the third optical current transformer and the current direction on its respective side;
• nT represents a transformation ratio of the transformer;
• the processing unit comprises following modules:
• a filtering unit, wherein the filtering unit is configured to detect a carrier signal JDC and a modulation wave signal JAC of an optical signal output from the third optical current transformer using a filtering circuit;
• a modulation ratio calculation unit, wherein the modulation ratio calculation unit is configured to calculate a modulation ratio m based on the following equation:
• m
• =
• J
• AC
• J
• DC
• ,
• and
• a differential current calculation unit, wherein the differential current calculation unit is configured to calculate a differential current id with the following equation:
• i
• d
• =
• m
• 2
• ⁢
• V
• ⁢
• L
• ,
• wherein, V is a Verdet constant of a Faraday magneto-optical element in the first, second and third optical current transformers, L is a coefficient of the multiple relationship between the magnetic field integral on the Faraday magneto-optical element in the first, second and third optical current transformers to its measured current.