Relay protection CT disconnection detailed explanation
What is a CT disconnection?
CT disconnection refers to the occurrence of circuit disconnection in the secondary circuit of the current transformer (CT), which will make the secondary side unable to transmit current signals normally to the relay protection device or measuring instrument. The main function of the current transformer is to reduce the large current in the high-voltage line to a safe level in a certain proportion to facilitate the measurement and protection of the equipment to work.
CT disconnection cause
Mechanical damage: external vibration, impact, pulling, or mechanical damage caused by secondary cables during the construction process may cause a wire break or loosening.
The connection is not firm: A disconnection problem may occur if the current transformer (CT) on the secondary side of the wiring terminal is not tight or if the wiring is loosening due to long-term operating vibration.
Line aging: After the secondary circuit runs for a long time, the insulating layer of the wire may be aging, and the joint may also oxidize, resulting in poor contact and further disconnection.
Environmental factors: Environmental conditions such as humidity, high temperature, and corrosive gases may accelerate the corrosion of the wiring terminal, cause poor contact, and increase the risk of disconnecting.
Artificial operation error: When maintaining the equipment, if the staff accidentally disconnects the CT secondary side circuit, it may cause a disconnection problem.
The harm of CT disconnection
The misunderstanding or rejection action of the relay protection device: The main responsibility of CT is to provide accurate, current signals for the relay protection system. When disconnection occurs, the current signal received by the relay protection device may become inaccurate or completely lost. If the relay protection device makes an error judgment without an accurate, current input, it may cause improper motion or non-action of the equipment. For example, in the actual failure, if the protective equipment cannot detect the accurate fault current, the movement may be rejected, and the fault cannot be isolated timely to reduce the system's stability further.
The risk of overheating or damage to the device: In the case of disconnection of the secondary side of the CT, because there is no load, the flowing current may suddenly decrease or become zero. This may cause the one-sided current to be unable to get appropriate feedback, which causes the overcurrent of the device and may eventually cause device damage. In addition, CT's iron core magnetic flux density may exceed the normal range and generate excessive voltage, which may not only endanger personal safety but also affect the stability of the equipment.
Loss of measurement accuracy: After the line is disconnected, the current value provided by the secondary side to the measurement equipment may deviate or be interrupted. This results in errors in measuring current and electrical energy and cannot accurately reflect the actual state of the system. This situation harms energy consumption management and fault diagnosis and may even threaten the safe and stable operation of the power grid.
The logical logic of the successor
To properly cope with the problem of disconnection of the current transformer (CT), the relay protection system usually integrates a series of special logical strategies. The following are some common processing mechanisms:
(1) The detection mechanism of the CT disconnection
1) Electricity detection: In the three-phase circuit, if the current of a certain phase is significantly low or zero, the protection device will determine that there may be a CT disconnection.
2) Voltage and current matching detection: Some protection devices will compare the relationship between the phase difference and amplitude between the current and the voltage. If the matching is not good, the CT may be disconnected.
(2) Protection response logic during CT disconnection
1）The logic of preventing error: To avoid misunderstanding caused by the CT break, some protection devices will choose not to choose when the CT is detected. This design prevents the protection from incorrectly triggering the protection when the current signal is missing or distorted. For example, after confirming that a side CT is confirmed, the status will be judged as non-fault to prevent misunderstanding.
2）Delayed action or reduction of sensitivity: Once the CT break is detected, the protection device may extend the action time or reduce the sensitivity to reduce the risk of misunderstanding. For example, when the overcurrent protection recognizes the current abnormality, it can extend the action time or adjust the protection by logical adjustment to ensure the stable operation of the grid.
3）Activation of spare protection: If the main protection fails due to the CT disconnection, the protection device can be switched to the spare protection mode. This spare protection is usually based on a different current or voltage signal from the Lord's protection as an effective supplement to the failure of the Lord's protection.
CT breaks from the standard
Differential protection should meet the following principles:
a) It has the function of preventing the error caused by preventing the rush of excitation surge. It does not need to choose a setting. The manufacturer should adopt the principles and methods of adaptive
Implement this function.
b) The braking characteristics of preventing misunderstanding of external failure in the area.
c) Have the functions of poor movement.
d) The current adopts the "Y -Y-shaped wiring" access protection device, and the protection device software realizes its phase and current compensation.
e) Each group of inner bridge wiring is connected to the protection device separately.
f) It has a CT disconnection alarm function. The control word allows you to select whether the CT break-out ratio is lock-up rate differential protection. When selecting lock-up differential protection, it is a conditional atresia. Jump.
Lord protection should meet the following technical principles:
a) The parent line protection has a reliable CT saturation discrimination function, and the out-of-district failure CT is not moving when saturated.
b) Bus protection can quickly remove failures outside the area.
c) The bus protection allows CTs with different variable ratios to be automatically corrected by the software.
d) With the CT disconnection alarm function, there is a disconnection times CT disconnection alarm for the disconnection between the CT and MU. Except), in addition to the protection of the CT break-off difference, the rest of the remaining circuit is closed after the CT is disconnected, and the difference between the parent line. Later, related bus breaks occurred in the area. The mother-in-law and segments of the CT break-up line first jump. (Except for the segmentation of the dual -mother dual -point wiring) After the CT breaks are disconnected, the related bus is not broken in the area in the area in the area and directly jumps the faulty bus
