To understand the working characteristics of a current transformer, it is essential to determine whether it will saturate when subjected to large currents during an external fault. This can impact the accuracy of protective relay actions. One of the most direct methods to test this is by applying an actual load on the secondary side and injecting current from the primary side. By observing the secondary current, the saturation point can be identified. However, for current transformers rated at higher levels, the saturation point may exceed 15 to 20 times the rated current. As the size of the transformer increases, conducting such tests on-site becomes increasingly challenging.
Another method to measure the saturation point is through a volt-ampere characteristic test. Saturation in a current transformer occurs due to excessive magnetic flux density in the core, which can be indirectly measured by the induced electromotive force. Therefore, the saturation current can be estimated based on the voltage value from the volt-ampere curve. The procedure involves opening the secondary circuit and applying current from the secondary side while measuring the voltage drop across the secondary winding. Since the primary circuit remains open, there is no demagnetization, and the core can easily saturate with even a small current. This makes the volt-ampere test more practical and easier to perform in the field.
Under normal operating conditions, the core of the current transformer remains unsaturated, with low load impedance and excitation current, resulting in high excitation impedance. The magnetic potentials of the primary and secondary windings remain balanced. However, if the core’s magnetic flux density increases and reaches saturation, the excitation impedance (Zm) drops rapidly, breaking the linear relationship between excitation current and voltage. Factors that cause saturation include excessively high current or heavy load. When the connected load is too large, the secondary voltage increases, leading to higher magnetic flux density and eventual saturation.
When saturation occurs, the secondary current decreases significantly, and the waveform becomes distorted with increased harmonic components. The internal resistance also drops, sometimes approaching zero. During a fault, the secondary current waveform may appear near zero, disrupting the linear transmission of the primary current. Saturation typically begins after about 5 seconds of a fault. It is strictly prohibited to open the secondary circuit of a current transformer during operation, as this converts the primary current into excitation current, causing rapid saturation. This can lead to dangerously high voltages, damaging insulation and posing safety risks.
**1. Impact of Transformer Protection and Countermeasures**
Transformers generally have small capacities and high reliability, often installed on 10kV or 35kV busbars. The short-circuit current on the high-voltage side is similar to the system’s level, while the low-voltage side has relatively larger short-circuit currents. If protection is inadequate, it can seriously affect the safe operation of the transformer or the entire system. Traditional fuse protection is reliable, but with increasing automation and higher short-circuit capacities, traditional methods are no longer sufficient. Many new or upgraded substations now use switchgear with protection devices similar to those used for 10kV lines. However, they often overlook the issue of current transformer saturation. Due to the transformer's capacity, the primary current is small, and a standard transformer is used. To ensure measurement accuracy, the current transformer ratio is reduced. When a fault occurs, the current transformer may saturate, reducing the secondary current and causing the protection system to fail to operate. If a fault occurs on the high-voltage side, the backup protection should act automatically. However, if the fault is on the low-voltage side, the short-circuit current may not reach the backup protection threshold, leading to unremovable faults and potential damage to the transformer, affecting the overall system’s safety.
To address these issues, proper configuration of the transformer is crucial. When selecting a current transformer, the risk of saturation due to failure must be considered. Current transformers for different purposes should be clearly distinguished. For example, metering transformers are usually placed on the low-voltage side to ensure accurate readings, while protection transformers are typically located on the high-voltage side to ensure effective protection.
**2. Effects of Current Protection and Countermeasures**
When a current transformer becomes saturated, the secondary equivalent current decreases, leading to protection failure. In areas far from the power source or where 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 dramatically, reaching hundreds of times the primary current of the transformer, causing previously functioning transformers to saturate. Additionally, short-circuit faults are transient events with complex phase components, which can accelerate the saturation process. If a short-circuit occurs on a 10kV line, the current transformer may become saturated, reducing the secondary current and causing the protection device to fail. This can result in delayed removal of the low-voltage side switch, expanding the fault area and prolonging the outage, thereby affecting power supply reliability. In severe cases, it can threaten equipment safety.
From the above analysis, it is clear that when a current transformer saturates, the primary current is converted into excitation current. This results in zero secondary current, and consequently, zero relay current, leading to protection rejection. To mitigate these issues, the load impedance of the transformer should be minimized as much as possible to avoid overloading the current transformer. Increasing the cable cross-sectional area and reducing its length can help. Additionally, the current transformer ratio should not be too small, and special attention must be given to the saturation problem caused by short circuits.
Bluetooth Headphones & Speakers
Bluetooth headphones & speakers
DongGuan BoFan Technology Co.,Ltd. , https://www.ufriendcc.com