Currently, the 330kV capacitive voltage transformer (referred to as CVT), which serves as the primary equipment for energy metering, relay protection, and carrier communication in power systems, is extensively utilized across the Northwest region of China. Given the CVT's inherent design, which includes saturable inductors and capacitor energy storage components, the transient behavior of its isolation switches can significantly impact the correct functioning of substation relay protection, thereby potentially compromising the safe and reliable operation of the entire power system. For instance, in 2000, when the operator at the 330kV Taoqu substation in Tongchuan attempted to open the isolation switch of the 330kV busbar CVT, specifically the C-phase secondary knife disconnect switch, the No. 1 main transformer's microcomputer overexcitation protection was triggered, resulting in the tripping of all three sides of the transformer. Consequently, the primary equipment in the substation had to cease operations. A similar incident occurred at the Tongchuan 330kV Jinshuo substation in 2005. The focus of this study is primarily on the secondary voltage of the 330kV CVT during sub-isolation switch operations and its influence on the over-excitation protection of the main transformer.
1. Simulation of CVT Secondary Knife Gate Operation Transients
Due to the presence of an iron-core transformer within the CVT circuit, when the system experiences significant overvoltages, the core of the intermediate transformer in the CVT tends to saturate. Once saturated, the excitation branch behaves like a nonlinear inductance, disrupting the usual linear relationship between the primary and secondary voltages of the CVT. This leads to the excitation of harmonic components of various frequencies in the secondary voltage, resulting in waveform distortion. Under these circumstances, the secondary voltage fails to accurately represent the actual voltage, leaving the transformer in an unstable state. This condition often triggers misoperations in the system’s relay protection. The electromagnetic transient calculation program (EMTP) is a critical tool for examining the transient performance of power systems. The author employs the visual version of ATP software to simulate the transient processes involved during CVT isolation switch operations.
Since the CVT sub-isolation switches used in 330kV substations operate sequentially, there is typically a noticeable time difference between the operations of each phase. Therefore, only one phase of the CVT is simulated during calculations, while the other two phases remain static.
1.1 CVT – Secondary Isolator Opening Operation
The timing of the CVT-secondary isolator operation relative to the bus voltage phase is random. For the specific substation equipment and CVT under consideration, the primary influencing factor during the overvoltage operation of the breaking CVT-secondary isolator is the voltage phase at the moment of opening.
This graph represents the secondary voltage waveform of the C-phase CVT-secondary isolator when the busbar C-phase voltage phase is 0°, 45°, and 90°. When the A and B-phase CVTs are withdrawn, the C-phase CVT-secondary isolator exhibits a secondary voltage waveform at a bus voltage phase of 90°. From the simulation results, we observe that during regular substation operations, the secondary voltage oscillates and diminishes to zero during the CVT-secondary isolator operation. When breaking, the influence of the bus voltage phase on the secondary voltage is negligible and can be disregarded.
It is plausible for the CVT-secondary isolator to reignite during the breaking process. Considering the possibility of reignition during the isolator breaking, examine the secondary winding voltage waveform. From the waveform diagram, it is evident that if reignition occurs at the primary isolator during the CVT-secondary isolator operation, the secondary voltage will experience a severe overvoltage, reaching up to 7.4 times the normal value, followed by the disappearance of oscillations.
1.2 CVT – Secondary Isolator Closing Operation
Considering other system parameters unchanged, when the voltage phase angles are 0°, 45°, and 90°, respectively, the secondary voltage of the CVT-secondary isolator during closure is examined. See the secondary voltage waveform. From the simulation results, it is apparent that when the voltage phase angle is 0°, the CVT secondary voltage waveform is slightly distorted, but the secondary voltage amplitude is relatively low, and the distortion disappears within four cycles. At a closing phase angle of 60°, the distortion begins and lasts only four cycles. At a closing phase angle of 90°, the waveform distortion is most severe, with the amplitude reaching 1.5 times the normal value, but lasting only five cycles.
1.3 Factors Affecting CVT Transient Performance
For a given size of CVT product, the capacitance value C remains constant. The factors impacting the transient performance are primarily the damper parameters and the core magnetic density of the voltage transformer.
1.3.1 Influence of Damping Parameters
The fast saturation damper mainly comprises a fast saturation reactor and a damping resistor. The role of the fast-saturated reactor is to saturate when resonance occurs, reducing the reactance value. At this point, the series-connected resistance turns on, achieving damping resonance. The choice of the damping resistance value directly affects the damping effect. The size of the CVT damping resistor is matched to the reactance value of the reactor under overvoltage conditions, including power frequency and fractional harmonics, to maximize the damping power generated under the same overvoltage. Additionally, the current of the damper circuit should exceed the minimum core saturation current. Otherwise, the reactor design should be adjusted. The selection of the damping resistor should fall within a specific range. According to calculations, the damping resistance used in the studied 330kV CVT should be less than 19.06Ω. In section 1.2, the damping resistance value of the 330kV CVT is 8Ω. Now, the damping resistance value is changed to 20Ω and 200Ω for simulation calculations. It is observed that when the damping resistance value is small, the CVT experiences continuous ferromagnetic resonance, causing significant distortion in the secondary voltage waveform, with the voltage amplitude reaching 1.55 times the rated value. When the damping resistance is increased to 200Ω, the damping effect is notably enhanced, and resonance is eliminated within four cycles. Within a reasonable range, selecting a larger damping resistor is beneficial for suppressing resonance.
1.3.2 Influence of Core Transformer Magnetic Density on Ferromagnetic Resonance
In the simulation calculations, the magnetic density of the CVT voltage transformer is relatively low. The magnetic density curve is shown in the middle curve. Now, a core with higher magnetic density is selected for simulation calculations. The magnetic density curve is shown in the middle curve 2. It can be seen that when the core magnetic density is high, the CVT undergoes ferromagnetic resonance when the primary knife is closed, causing severe distortion in the secondary voltage waveform. The value reached 3.36 times the rated value, but due to the action of the fast saturation damper, the resonance disappeared within six cycles.
This could lead to the main transformer's over-excitation protection action.
2 Analysis of Induced Voltage in CVT Secondary Windings
When a substation stops a bus or CVT, both the mother and mother CVT must run side by side to prevent the operational protection devices from losing pressure, followed by switching operations. During the switching process, the operation of the CVT-secondary isolators must follow a strict sequence. When withdrawing a bus CVT, the isolator with auxiliary contacts must be opened first, ensuring that according to the above simulation results, there will be a ferromagnetic resonance phenomenon during the CVT-secondary isolator closing operation. However, under the action of the fast saturation damper, the resonance phenomenon quickly disappears, and the voltage amplitude and duration do not reach the threshold value for the main over-excitation protection action. The triangular winding of the open CVT is not disconnected, avoiding the parallel connection of the two open triangular windings. When the CVT is put back into service, it must be operated strictly in accordance with the operation sequence to avoid the parallel connection of triangular windings during the operation of one isolator. See the principle of parallel delta winding connection.
If the operation sequence is incorrect, causing the two open triangular windings to be connected in parallel during operation, an unbalanced voltage is generated on one of the open triangles, superimposed on the other open triangular winding, and the secondary winding voltage changes due to coupling. If the mother phase CVT is withdrawn or the two phase CVT is withdrawn, the open delta winding will have a voltage of 100V. Furthermore, since the six CVT residual windings of the two open triangular windings connected in parallel are connected in series, the I mother and the mother open triangular windings are each divided by 50V, and the remaining windings of each phase CVT will be divided into a 50/3V voltage. The 50/3V voltage is transmitted through the voltage transformer to generate a certain voltage on the secondary winding of the I mother CVT, which is superimposed on the operating voltage of each phase of the I mother.
The secondary to tertiary transformation ratio of CVT is (100/√3)/100 = 0.577, and the voltage coupled to the secondary winding at 50/3V is approximately 9.62V.
Two cases are analyzed below:
Case 1: The I mother CVT in the substation is in normal operation, and the mother CVT is put into operation. The mother A phase CVT-secondary isolator has auxiliary contacts. Assume that the mother A phase CVT secondary knife gate is first closed, generating a voltage of 100V on the open triangular winding of the mother CVT. Since the I mother open delta winding is connected in series with the mother open delta winding, the remaining windings of the I mother phase CVT are superimposed with a voltage of 50/3V. At this time, the I-phase CVT secondary winding voltage vector diagram is shown. After calculation, it is known that U=9.62V, V=57.7V, and the rated value is 1.17 times. Similarly, in the normal operation of the I mother CVT, when the mother CVT is withdrawn, if the B and C phase CVTs are first evacuated, leaving only the A phase in operation, the same problem will occur.
Case 2: The I mother CVT in the substation is in normal operation, and the mother CVT withdrawal operation is performed. The mother A phase CVT-secondary isolator does not have auxiliary contacts. Assume that the mother A phase CVT-secondary isolator is first turned off. At this time, a voltage of 100V is generated on the open triangular winding of the mother CVT. The CVT voltage vectors of each phase of the I mother are seen as 0. The I mother B phase and C phase CVT secondary voltage rises significantly, reaching up to 63.1V, which is 1.09 times the rated voltage, and the secondary voltage of the I mother A phase CVT drops to 48.1V. The same situation occurs during the process of putting the mother CVT back into service.
According to the above analysis and the accident records provided by the substation, the main transformer over-excitation protection malfunction of the 330kV Taoqu substation and the 330kV Jinshuo substation should belong to the above scenarios. When the operator withdraws the 330kV mother CVT, the operation sequence is incorrect. First, the C-phase CVT-secondary isolator is opened, then the A-phase CVT-secondary isolator is opened, and the B-phase CVT-secondary isolator with auxiliary contacts is opened. At the end of the operation, the parallel connection of the I and mother open delta windings is not broken. The secondary voltage of the I-B phase CVT rises to 72.1V, reaching the action threshold of the main transformer over-excitation protection and triggering the protection action. The Jinshuo substation accident occurred during the CVT operation of the commissioning line. The operator's operation sequence was incorrect. First, the isolator with auxiliary contacts was closed, causing the secondary voltage of the A-phase CVT of the Jinhan line to rise and reach the action threshold of the main transformer over-excitation protection, triggering the protection action.
3 Conclusions
The simulation results indicate that there is a certain degree of ferromagnetic resonance during the CVT-secondary isolator closing process, causing distortion in the secondary-side voltage waveform and an increase in amplitude. However, since the voltage rise amplitude is small and the duration is brief, it does not reach the threshold for the main over-excitation protection action, and thus the main over-excitation protection does not operate. If operators perform operations out of sequence during the commissioning or withdrawal of the CVT, causing the open triangular windings of the two busbar CVTs to be connected in parallel, the unbalanced voltage of one open triangular winding will be evenly distributed to six (down to page 118) (30 years), at the same time its cost is very expensive, which is another key factor hindering the industrialization of electronic current transformers. Therefore, the current high-energy side energy supply method generally adopts a composite power supply mode: when the primary current is large, the CT power supply mode is adopted; when the secondary current is small, the laser energy supply mode is adopted. This method can minimize the working time of high-power lasers and extend their lifespan. However, there are also two problems: 1) When the line is closed after inspection, the CT energy supply requires a long settling time. At this time, it can only rely on the laser to supply energy. However, if the laser fails at this time, it will directly cause the transformer to fail to work normally, so it is generally required to use two lasers: one for one, but this further increases the cost. CT and laser switching control must have a reasonable control strategy, and there should be no "vacuum" for power supply, i.e., one switching, and the other has not yet started to supply energy, so it is necessary to achieve the pre-judgment of switching between the two modes. Moreover, when the CT power supply mode needs to consider the short circuit of the system, the impact of the short circuit current may cause damage to the CT. The existence of these factors will directly lead to more complex energy supply systems and lower reliability.
4 Reliability Design
As mentioned earlier, electronic transformers have many advantages over traditional transformers, but because electronic transformers involve sensing technology, electronic technology, high voltage technology, optoelectronic technology, computer network technology, etc., they belong to the interdisciplinary field of knowledge systems. The technical difficulty is high. Whether electronic transformers can ultimately replace traditional transformers depends on their long-term reliability. The reliability design of electronic transformers includes the following main contents.
4.1 Redundant Design
Redundancy design is a common method to improve equipment reliability. In the electronic current transformer, the air-core coil and the A/D converter constituting the protection channel must adopt a dual redundant design.
4.2 Self-test Function Design
For key devices, such as power modules, A/D converters (upper page 115), the remaining windings of the CVT, and coupled to the secondary winding of the CVT are superimposed with the original voltage. The voltage of the secondary winding of the one-phase CVT with the highest voltage will reach 1.17 times the normal value. In addition, the manual grading operation time is relatively long, and the action threshold value of the main transformer over-excitation protection is reached, and the protection malfunctions.
To avoid such accidents, operators should strictly follow the operation sequence when operating the CVT-secondary isolator. Relevant units can also make improvements to the equipment. For example, when changing the action criterion of the main transformer over-excitation protection, if the secondary voltage of the bus CVT and the line CVT are simultaneously raised, the main transformer over-excitation is determined, and the main variation over-excitation protection action is initiated. If only the line or bus CVT secondary voltage rises, it is not necessary to have a basic self-test function.
4.3 Electromagnetic Compatibility Design
Electronic transformers using CT power supply or CT composite power supply need to take protective measures, so that the large current impact during a short circuit will not damage the power supply CT. At the same time, it is necessary to consider the impact of a short-circuit current on the subsequent sampling circuit of the low-power iron-core coil and the impact of the larger di/dt on the subsequent protection circuit electronic circuit of the air-core coil.
4.4 Safety Design
Because high-power lasers are used for energy supply, protective measures must be taken to prevent damage to the operation and maintenance personnel caused by high-power lasers during operation and maintenance. Recommended method: When the high-power laser is working normally, once the power module is detected to be powered down, stop the laser immediately, in case the power supply fiber loop has problems, which will endanger the safety of personnel.
5 Conclusions
Electronic transformers are new types of power equipment that are currently widely concerned, and the detailed research of key technologies is directly related to whether electronic transformers can ultimately meet the high safety, stability, and reliability requirements of power systems.
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