1.Capacitor Definition: A capacitor is a combination of two conductors separated by an insulating material. The two conductors are called plates, and the insulating material is called the dielectric. In power systems, capacitors improve the power factor; in electronics, they serve functions such as filtering, coupling, blocking DC, tuning, bypassing, and frequency selection. In machining processes, capacitors are used for electric discharge machining.

2.Classification by Structure: Capacitors are classified into fixed capacitors, variable capacitors, and trimmer capacitors. Fixed capacitors have a fixed capacitance and cannot be adjusted, while variable capacitors allow capacitance to be adjusted within a certain range. Common air variable capacitors change capacitance by rotating a movable plate to alter the relative area between the movable and fixed plates. Ceramic trimmer capacitors have a small range of capacitance variation and are used for fine-tuning frequencies in tuning circuits.

3.Classification by Dielectric Material: Types include mica capacitors, ceramic capacitors, metal film capacitors, and aluminum electrolytic capacitors. Mica capacitors are stable, compact, and have low leakage but small capacitance; ceramic capacitors are stable and compact with low leakage; metal film capacitors are small with larger capacitance and self-healing capability after breakdown; aluminum electrolytic capacitors have high capacitance but significant leakage and losses.

4.Polarity of Capacitors: Capacitors are categorized as polarized and non-polarized. Polarized capacitors cannot be used in AC circuits, as they may be damaged. Electrolytic capacitors have positive and negative terminals; the positive terminal must be connected to the higher potential, and the negative terminal to the lower potential. Non-polarized capacitors can be used in AC circuits without polarity concerns.

5.Meaning of "Capacitance": The term "capacitance" has two meanings: one refers to the electronic component "capacitor," and the other refers to the electrical quantity "capacitance." While both can be abbreviated as "C," a capacitor is a charge storage device, whereas capacitance measures the ability to store charge under a given voltage.

6.Distributed Capacitance: Not only do capacitors have capacitance, but there is also capacitance between power transmission lines, between transmission lines and the ground, between transistor leads, and between components. These are referred to as distributed capacitance. Although generally small and often negligible, distributed capacitance can significantly impact circuits and equipment in strict conditions.

7.Capacitance Formula: The charge (Q) stored on a plate of a capacitor divided by the voltage (U) between the plates defines the capacitance (C), expressed as C = Q/U. Common capacitors have capacitance ranging from tens of pF to thousands of μF.

8.Factors Affecting Capacitance: The capacitance of a capacitor is directly proportional to the plate area and inversely proportional to the distance between the plates, depending on the dielectric properties. It is independent of the applied voltage and charge. When the dielectric constant is constant, C remains a constant that represents the capacitor's charge storage ability, expressed as C = εs/d, where s is the effective plate area (m²), d is the distance between plates (m), and ε is the dielectric constant.

9.AC and DC Behavior: Capacitors allow AC to pass while blocking DC based on their reactance frequency characteristics, enabling high-frequency AC to pass while obstructing low-frequency DC.

10.Series Connection of Capacitors: In series, several capacitors are connected sequentially without branching. This arrangement effectively increases the distance between their plates. The reciprocal of the equivalent capacitance for series capacitors equals the sum of the reciprocals of each capacitor's capacitance. If n identical capacitors are connected in series, their equivalent capacitance is C s = C/n.

11.Charge Distribution in Series: Each capacitor in a series connection carries the same charge, equal to the charge on the equivalent capacitor. The equivalent capacitance is less than the smallest capacitance in the series, and the voltage across each capacitor is inversely proportional to its capacitance. Capacitors with larger capacitance will have lower voltage across them, while those with smaller capacitance will have higher voltage. When using capacitors in series, it's essential to consider each capacitor's voltage rating to prevent breakdown.

12.Parallel Connection of Capacitors: In parallel, several capacitors are connected across the same two points. The equivalent capacitance (C) for parallel capacitors equals the sum of their capacitances. If n identical capacitors with capacitance C are connected in parallel, the equivalent capacitance is C p = nC.

13.Increasing Capacitance in Parallel: Connecting capacitors in parallel increases the total capacitance. The more capacitors added in parallel, the larger the equivalent capacitance. Each capacitor directly connects to the applied voltage, so their voltage rating must exceed the external voltage.

14.Selecting Capacitors: When selecting capacitors, it’s essential to meet electrical performance requirements, particularly capacitance and voltage rating. Consideration must also be given to circuit requirements and operating environment; for instance, mica or ceramic capacitors are suitable for tuning circuits, while electrolytic capacitors are better for filtering. Additionally, factors such as assembly form, size, and cost should be considered.

15.Operating Conditions: Capacitors must operate under normal voltage to effectively provide reactive power compensation. Capacitor banks can operate at 1.1 times the rated voltage for an extended period. However, overvoltages may occur due to switching operations, voltage adjustments, or load changes. While temporary overvoltages are permissible, the frequency of exceeding 1.15 times the rated voltage should not exceed 200 times during the capacitor's lifetime.

16.Current Ratings: Capacitor banks can operate at a maximum of 1.3 times the rated current. Of the 30% allowed above the rated current, 10% results from permissible frequency overvoltages, while 20% is due to higher harmonic voltages. Efforts should be made to eliminate prolonged overvoltages and higher harmonics in the circuit.

17.Heat Generation: Capacitors do not experience copper or iron losses but only dielectric losses. The heat generated by capacitors depends on dielectric losses, which are influenced by capacitance, frequency, voltage, and the insulation properties of the dielectric. Excessive temperatures can shorten a capacitor's lifespan and potentially lead to dielectric breakdown, damaging the capacitor. Typical temperature ranges for capacitors are -40°C to 40°C, while self-healing capacitors can withstand -45°C to 50°C.

18.Inrush Current and Overvoltage: When connected to power, capacitors may experience inrush overvoltage and overcurrent. If the voltage at the moment of closure coincides with the peak value, a surge current (high-frequency, high-amplitude transient current) can occur, placing significant mechanical stress on the switchgear and endangering electrical equipment.

19.Switching Off Capacitor Banks: Disconnecting capacitor banks can cause oscillations in inductive-capacitive circuits, leading to operational overvoltage. If arcing occurs during circuit breaker disconnection, it may trigger intense electromagnetic oscillations, resulting in higher overvoltages. The amplitude of this overvoltage depends on the size of the disconnected capacitors and the bus-side capacitance, as well as the potential difference during arcing.

20.Resonance with Harmonics: When operating capacitors resonate with higher harmonics in the grid, significant resonant currents may develop. These currents can overload and overheat the capacitors, causing vibrations and abnormal noises.

21.Switching Procedures: After a total power outage in a substation, the capacitor bank switches must be turned off. Upon restoring power, the outgoing switch should be closed first with a certain load before engaging the capacitor bank.

22.Re-energizing After Tripping: After a capacitor switch has tripped, it is essential not to force a re-energization. The situation must be assessed, and it should only be re-energized after confirming there are no faults. Each time the capacitor bank is re-energized after a shutdown, the switch must be opened, and the capacitors must be discharged for 3 minutes before re-energizing.

23.Iron-Core Resonance: Energizing a transformer or parallel reactor under light load can lead to ferroresonance, causing overcurrent. To mitigate this effect, the capacitor should be disconnected before energizing the transformer or parallel reactor.

24.Maintenance: Maintenance of capacitors should include cleaning the surfaces of bushings, capacitor housings, related electrical components, and iron frames to remove dust or debris, typically done quarterly. The fuses protecting the capacitors should be inspected monthly to ensure they are functioning correctly. If any oil leakage is observed from the capacitor housing, it should be repaired with solder; severely leaking capacitors should be replaced.

25.Failure Patterns: Capacitors may fail shortly after being energized, often due to poor manufacturing or severe defects. High-voltage capacitors tend to fail more frequently than low-voltage capacitors. Failures are more common in hot summer months and outdoor installations compared to indoor setups. Capacitors operating under overload or overvoltage conditions are more likely to fail than those under normal or light load and low voltage.

26.Conditions for Disconnection: Capacitors must be taken offline if: (1) the bus voltage exceeds 1.1 times the rated voltage; (2) the current through the capacitor bank exceeds 1.3 times the rated current; (3) the surrounding temperature exceeds the allowable range (typically 40–50°C) or the hottest point of the capacitor housing exceeds 60–80°C; (4) visible swelling or rupture of the housing occurs; or (5) severe overheating of contacts, significant discharges or arcing occur, or unusual sounds are detected from internal components.

27.Causes of Swelling: The swelling of a capacitor housing often results from residual gases during manufacturing. If not removed, increased internal pressure during operation due to voltage can lead to breakdown of insulation materials, causing further gas production and swelling. If swelling occurs, the capacitor should not be operated under reduced voltage, as internal arcing can cause further expansion and potential explosion.

28.Indications of Insulation Breakdown: If a capacitor produces a "gurgling" sound, it indicates partial discharges caused by ionization of the insulation material, signaling impending insulation failure. Immediate operation cessation and inspection are required.

29.Residual Charge Handling: After deactivating a capacitor bank, residual charge may still exist between the terminals despite automatic discharging through resistors. Prior to handling, the charge must be completely discharged to prevent electric shock. When manually discharging, ensure that the grounding wire is securely attached to the grounding network, and use a discharge rod multiple times until no sparks or discharge sounds are present.

30.Discharging Faulty Capacitors: If there is an internal open circuit, blown fuse, or poor contact in the capacitor, residual charge may still be present across the terminals. Insulated gloves should be worn, and a shorting wire should be used to discharge the faulty capacitor terminals. For capacitors connected in series, individual discharging must be performed as well.