Case Analysis of CVT Capacitor Divider Terminal Discharge Failure

作者： GOZ Electric 时间：2024-06-21 09:36:06 阅读：17

      Capacitor voltage transformer (CVT) has the advantages of large insulation margin and high anti-resonance strength. It is widely used in power systems. Maintaining its normal operation is one of the necessary conditions to ensure the reliability of power systems. CVT is composed of a capacitor voltage divider and an electromagnetic unit. Due to its relatively complex structure, various problems will occur during operation, among which the abnormal discharge of the CVT body and oil leakage are more prominent. This paper disassembles and analyzes a product with discharge at the end of the capacitor voltage divider of 110kVCVT and causes oil leakage, and finds the cause of the failure and proposes improvement measures. When the product is running on site, the end of the capacitor voltage divider is not grounded, causing it to discharge to the secondary terminal board and produce cracks, causing the electromagnetic unit to leak oil. Starting from the design principle, this paper eliminates the causes of product discharge one by one, and conducts test verification through disassembly analysis combined with test methods, and improves the process design of the product from the design perspective.

1. Product design principle

CVT is a voltage transformer composed of a capacitor voltage divider and an electromagnetic unit, in which the electromagnetic unit is mainly composed of an intermediate transformer, a compensating reactor, a damper, a lightning arrester and other parts. CVT uses series capacitor voltage division. When voltage is applied to the capacitor voltage divider composed of C1 and C2, it can be regarded as a two-port network, with the input end being the high voltage end and the ground end, and the output end being the medium voltage end and the ground end. According to the Thevenin theorem, the medium voltage end voltage can be represented by an equivalent voltage.

The CVT capacitor voltage divider can also be used as a coupling capacitor for high-frequency carrier communication. When used for carrier communication, the end N of the capacitor voltage divider needs to be grounded through a combined filter. When not used for carrier communication, the N end must be directly grounded. Since information transmission is very developed nowadays, the domestic use of the transmission line carrier communication method to transmit information is rarely used. Therefore, the N terminal at the end of the CVT capacitor voltage divider and the E terminal at the end of the intermediate transformer of the electromagnetic unit are usually short-circuited in the secondary terminal box of the product through a copper shorting piece and then directly grounded.

2. Description of the fault

After the 110kVCVT product was put into operation, it was found that the electromagnetic unit had a discharge sound, the secondary voltage monitoring value had no obvious abnormality, and the electromagnetic unit secondary terminal box had oil leakage. After the power outage, the secondary box door was opened and it was found that the secondary terminal board had serious discharge and burning marks and cracks. No shorting piece was found between the wiring terminals N and E. The product could not operate normally. After the customer urgently replaced the product, the faulty product was disassembled and analyzed.

3. Possible causes of failure and judgment methods

3.1 Electromagnetic unit part

1) First, the no-load test of the electromagnetic unit intermediate transformer is required to determine whether there is a secondary inter-turn short circuit that causes the current to rise rapidly and cause the terminal board to burn.

2) Secondly, it is suspected that the product body has ferromagnetic resonance and cannot be blocked, resulting in the discharge sound of the electromagnetic unit on site. The product needs to be tested for ferromagnetic resonance. The product's own ferromagnetic resonance is stimulated by suddenly eliminating the short circuit after the secondary short circuit, and it is verified whether the damper can block the ferromagnetic resonance within the specified time. If the damper is damaged, the continuous resonant overvoltage and overcurrent may also damage the components of the electromagnetic unit, causing damage to the secondary terminal board and oil leakage.

3.2 Capacitor voltage divider

1) The main factory design parameters of the 110kVCVT capacitor voltage divider are shown in Table 1. It can be seen from the product parameters that the high-voltage part C1 consists of 66 capacitor elements, and the medium-voltage part C2 consists of 17 capacitor elements. When a C1 high-voltage element is broken, the capacitance change of C1 is 1/66=1.5%. When one or more C1 elements are broken, its capacitance value will increase. Assuming that C1 is damaged by 50% within a certain period of time under a certain overvoltage, the C capacitance value will become about twice the rated value, and the output voltage will increase significantly, causing the electromagnetic unit to overheat, and the secondary terminal board may be burned and leak oil.

2) If the capacitor voltage divider is not operated for carrier communication, the end N of the capacitor voltage divider must be reliably grounded. If the N terminal is not grounded, it is equivalent to connecting a capacitor C with a smaller capacitance in series between the N terminal and the ground, and its capacitance value may be much smaller than the total capacitance value of the product. Obviously, once the N terminal of the capacitor voltage divider and the ground are open, assuming that the capacitance value of C3 (N terminal to ground capacitance, pF) is small enough, it will generate a high floating voltage to the ground, causing the N terminal to discharge to the ground, causing the secondary terminal board to burn out under the high temperature arc, resulting in oil leakage in the electromagnetic unit, and thus threatening the safety of the entire transformer.