To understand the operational characteristics of a current transformer (CT), it is essential to determine whether it will saturate when exposed to large currents during an external fault. This can affect the accuracy of protective relay actions. One of the most direct methods to test this is by applying a real load on the secondary side and injecting current from the primary side, observing the secondary current to identify the saturation point. However, for CTs rated for high currents, the saturation point may occur at 15 to 20 times the rated current. As CTs become larger, performing such tests on-site becomes increasingly difficult.
Another method to measure the saturation point is through a volt-ampere characteristic test. Saturation in a CT occurs due to excessive magnetic flux density in the core, which can be reflected by the induced electromotive force. Therefore, the saturation current can be estimated from the saturation voltage on the volt-ampere curve. The procedure involves opening the primary circuit and applying current from the secondary side while measuring the voltage drop across the secondary winding. Since the primary is open, there is no demagnetization, and even a small current can cause the core to saturate. This makes the volt-ampere test more practical and easier to implement in the field.
Under normal operation, the core flux in a CT remains unsaturated, with low load impedance and excitation current, resulting in high excitation impedance. The magnetic potentials of the primary and secondary windings are balanced. However, if the core's magnetic flux density increases and reaches saturation, Zm decreases rapidly, breaking the linear relationship between excitation current and flux. Factors leading to CT saturation include excessively high current or heavy load. When the connected load is too large, the secondary voltage increases, raising the core’s magnetic flux density and causing saturation.
When a CT saturates, the secondary current decreases, and its waveform becomes distorted with significant harmonic components. The internal resistance drops, sometimes approaching zero. During a fault, the current waveform may appear near-zero, disrupting the linear transmission of current. Saturation typically occurs about 5 seconds after a fault. It is strictly prohibited to open the secondary circuit of a CT. If the secondary is opened during operation, the primary current becomes the excitation current, increasing the core’s flux density and causing rapid saturation. This can lead to dangerously high voltages, damaging insulation and posing safety risks.
**1. Impact of Transformer Protection and Countermeasures**
Transformers are generally small in capacity and reliable, often installed on 10kV or 35kV busbars. The high-voltage short-circuit current matches the system's level, while the low-voltage side has higher short-circuit current. If protection is inadequate, it could severely impact the transformer’s safe operation or the entire system. Traditional fuse protection is safe and reliable, but as system automation increases and short-circuit capacity grows, traditional methods fall short. Many new or renovated substations now use switchgear with protection similar to 10kV lines, but they often overlook CT saturation. Due to the transformer’s capacity, the primary current is small, and a standard CT is used to ensure measurement accuracy. However, during a failure, the CT may saturate, reducing the secondary current and causing the protection to fail. A high-voltage side fault triggers backup protection, but a low-voltage side fault may not reach the backup threshold, leading to unremovable faults and potential damage to the transformer.
To address these issues, proper CT configuration is crucial. When selecting CTs, the saturation caused by transformer failures should be considered. Different CTs should serve distinct purposes—metering CTs are placed on the low-voltage side for accuracy, while protection CTs are usually on the high-voltage side to ensure effective protection.
**2. Current Protection Effects and Countermeasures**
After CT saturation, the secondary equivalent current decreases, leading to protection refusal. When the fault is far from the power source or the impedance is high, the short-circuit current at the line entrance may be small. However, as the system expands, the short-circuit current can increase significantly, reaching hundreds of times the primary current and causing previously functional CTs to saturate. Additionally, the transient nature of short-circuit faults introduces various phase components, accelerating CT saturation. If a 10kV line experiences a short-circuit, the CT saturation reduces the secondary current, causing the protection device to refuse action. This can result in extended fault duration and wider fault areas, affecting power supply reliability. In severe cases, it may threaten equipment safety.
From the above analysis, when a CT saturates, the primary current is converted into excitation current. The secondary current becomes zero, and the relay current also drops to zero, causing the protection device to fail. To mitigate these issues, the transformer’s load impedance should be minimized to avoid overloading the CT. Increasing the cable cross-section and length can help. The CT ratio should not be too small, and attention must be given to the saturation risk during short circuits.
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