Currently, the 330kV capacitive voltage transformer (referred to as CVT hereafter), which serves as critical equipment for power system energy metering, relay protection, and carrier communication, is extensively utilized across the Northwest China power grid. Due to the inherent structure of the CVT, which includes saturated inductors and capacitor energy storage components, the transient performance of the isolation switch can impact the proper functioning of substation relay protection, potentially disrupting the safe and normal operation of the entire power system. For instance, in 2000, during the operation of the 330kV Taoqu substation's bus CVT isolation switch by the duty officer in Tongchuan, opening the C phase CVT secondary knife disconnect switch triggered the No. 1 main transformer’s microcomputer overexcitation protection, leading to a trip of all three sides of the transformer. The primary and secondary equipment were operating normally prior to the incident. A similar event occurred in Tongchuan’s 330kV Golden Lock Substation in 2005. My focus in this study is primarily on the secondary voltage of the 330kV CVT sub-isolation switch and how it influences the over-excitation protection action of the main transformer.
1. Simulation of CVT Secondary Knife Gate Operation Transients
Since the CVT loop contains a core transformer, under conditions of significant system overvoltage, the core of the intermediate transformer in the CVT may saturate. Once saturated, the excitation branch becomes equivalent to a nonlinear inductance, destroying the linear relationship between the primary and secondary voltages of the CVT. This results in the excitation of various harmonic components in the secondary voltage and waveform distortion. The secondary voltage cannot accurately represent the actual voltage, placing the transformer in an unstable state. This situation could lead to misoperations in system relay protection. The electromagnetic transient calculation program (EMTP) is an essential tool for examining power system transient performance. I utilized the visual version of ATP software to simulate the transient processes during the operation of the CVT isolation switch.
Given that the CVT sub-isolation switches used in 330kV substations are staged operations, the timing of each phase operation can vary significantly. Thus, only one phase CVT was simulated during the operation, while the other two phases remained non-operational.
1.1 Opening Operation of CVT Secondary Knife Gate
The instantaneous phase of the CVT secondary isolation switch is random relative to the bus voltage phase. For the specific substation equipment and CVT involved, the main influencing factor of the overvoltage during the breaking of the CVT sub-isolation switch is the voltage phase at the time of opening.
Below are the secondary voltage waveforms of the C-phase CVT secondary isolation switch when the C-phase bus voltage phase is 0°, 45°, and 90°. When the A and B two-phase CVTs are withdrawn, the C-phase CVT secondary isolation switch exhibits a secondary voltage waveform at the bus voltage phase of 90°. From the simulation results, we observe that during normal substation operations, the secondary voltage oscillates and diminishes to zero during the operation of the CVT secondary isolation switch. When breaking, the influence of the bus voltage phase on the secondary voltage is negligible and can be disregarded.
It is possible for the CVT secondary isolation switch to reignite during the breaking process. Considering the case of reignition during the breaking of the isolation switch, refer to the secondary winding voltage waveform. From the waveform diagram, it can be seen that if reignition occurs at the primary isolation switch during the CVT secondary isolation switch operation, the secondary voltage will experience severe overvoltage, with the highest amplitude reaching 7.4 times the normal value, followed by the disappearance of oscillations.
1.2 Closing Operation of CVT Secondary Isolation Switch
Considering the other parameters in the system unchanged, when the voltage phase angles are 0°, 45°, and 90°, the secondary voltage of the CVT sub-isolation switch during closing is examined. Refer to the secondary voltage waveform. From the simulation results, 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 after four cycles. At 60°, the distortion begins and lasts only four cycles. At 90°, the waveform distortion is most severe, with the amplitude reaching 1.5 times the normal value, but the duration is only five cycles.
1.3 Factors Influencing CVT Transient Performance
For a particular size of CVT product, the capacitance C remains constant. The factors affecting transient performance are mainly the damper parameters and the core magnetic density of the voltage transformer.
1.3.1 Impact of Damper Parameters
The fast-saturation damper primarily includes a fast-saturation reactor and a damping resistor. The function of the fast-saturation reactor is to saturate when resonance occurs, reducing the reactance value. At this point, the connected resistance is activated to achieve damping resonance. The selection 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 power frequency overvoltage plus fractional harmonics to maximize the damping power generated under the same overvoltage. Additionally, the current of the damper circuit should exceed the minimum current required for the reactor core to reach saturation; 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 19.06(3)5 ohms. In the 1.2 section, the damping resistance value of the 330kV CVT is 8 ohms. Now, the damping resistance value is changed to 20 ohms and 200 ohms for simulation calculations. From the simulated results, it can be observed that when the damping resistance value is small, the CVT experiences continuous ferromagnetic resonance, with the secondary voltage waveform having significant distortion, and the voltage amplitude reaching 1.55 times the rated value. When the damping resistance is increased to 200 ohms, the damping effect is significantly improved, and resonance is eliminated within four cycles. Within a reasonable range, using 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 experiences ferromagnetic resonance upon closing the primary knife, and the secondary voltage waveform is severely distorted, with the value reaching 3.36 times the rated value. However, due to the action of the fast saturation damper, resonance disappears within six cycles.
This can cause the main transformer over-excitation protection to act.
2 Analysis of Induced Voltage in CVT Secondary Winding
When the substation stops a bus or CVT, the mother and mother CVT must run side by side to prevent the running protection device from losing pressure, and then perform switching operations. During the switching process, the CVT secondary isolation switch operation of each phase also follows a strict sequence. When withdrawing a bus CVT, it is necessary to first break the isolation switch with the auxiliary contact, ensuring that based on the above simulation results, there will be a ferromagnetic resonance phenomenon during the CVT secondary isolation switch closing operation. However, under the action of the fast saturation damper, the resonance quickly disappears, and the voltage amplitude and duration do not reach the threshold 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 into operation, it must be strictly operated in accordance with the sequence to avoid the parallel triangular windings being connected during the operation of one isolation switch.
If the operation sequence is incorrect, causing the two open triangular windings to be paralleled during operation, an unbalanced voltage is generated on one of the open triangles, which is superimposed onto the other open triangular winding, altering the secondary winding voltage via coupling. If the mother phase CVT is withdrawn or two phase CVTs are withdrawn, the open delta winding will have a voltage of 100V. Moreover, since the six CVT residual windings of the two open triangular windings connected in parallel are 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. This 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 onto 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: In the substation, the I mother CVT is in normal operation, and the mother CVT is put into operation. The mother A phase CVT secondary isolation switch has auxiliary contacts. Assuming the mother A phase CVT secondary isolation switch is first closed, a voltage of 100V is generated 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 the voltage reaches 9.62V, 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 Phase B and Phase C CVTs are first withdrawn, leaving only Phase A operational, the same issue will occur.
Case 2: In the substation, the I mother CVT is in normal operation, and the mother CVT withdrawal operation is performed. The mother A phase CVT secondary isolation switch does not have auxiliary contacts. Assuming the mother A phase CVT secondary isolation switch 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 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 CVT energization process.
Based on 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 Jinshen substation should belong to the aforementioned situations. When the operator withdraws the 330kV mother CVT, the operation sequence is incorrect. First, the C-phase CVT secondary isolation switch is opened, then the A-phase CVT secondary isolation switch is opened, and the B-phase CVT secondary isolation switch with auxiliary contacts is opened. At the end of the break, the parallel connection of the I and the mother open delta windings is not broken. The secondary voltage of the I-B phase CVT rises to 72.1V, triggering the main transformer over-excitation protection and resulting in a protective action. The Jinshu substation incident occurred during the CVT commissioning process of the line. The operator's operation sequence was incorrect. First, the isolation switch with auxiliary contacts was closed, causing the secondary voltage of the A-phase CVT of the Jinhan line to rise and reach the main transformer over-excitation protection threshold, resulting in a protective action.
3 Conclusions
The simulation results indicate that there is a certain degree of ferromagnetic resonance during the CVT secondary isolation switch closing process, causing the secondary voltage waveform to distort and the amplitude to increase. However, since the voltage rise amplitude is small and the duration is brief, the threshold for the main over-excitation protection action is not reached, and the main over-excitation protection does not activate. If the operator performs the wrong sequence during the CVT commissioning or withdrawal, causing the open triangular windings of the two busbar CVTs to be paralleled, 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 used; 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 prolong their life. 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. Normal work, 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, that is, one switching, and the other has not yet started to supply energy, so it is necessary to realize 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 complicated energy supply systems and lower reliability.
4 Reliability Design
As mentioned earlier, electronic transformers have many advantages over traditional transformers, but because electronic transformers include sensing technology, electronic technology, high voltage technology, optoelectronic technology, computer network technology, etc., the knowledge system of the multi-disciplinary cross-field is technically challenging. Whether electronic transformers can ultimately replace traditional transformers depends on their long-term reliability. The reliability design of the electronic transformer 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 isolation switch. Relevant units can also improve the equipment. For example, when changing the action criterion of the main transformer over-excitation protection, when 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 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 about, and the fine research of key technologies is directly related to whether electronic transformers can finally meet the high safety, stability, and reliability requirements of power systems.
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