Insulation Design and Temperature Rise Control of High Voltage Switchgear in 35kV Substation

2025-09-25 14:02:57

1、 Insulation Design: Building the "Immune System" of the System

The goal of an insulation system is to withstand various overvoltages (power frequency, lightning, operational impact) throughout the entire life cycle of the equipment without failure. Its design follows two core principles: "field strength uniformity" and "insulation margin".

1.1 Selection and combination of insulation media

Modern 35kV switchgear mainly adopts three types of insulation medium combinations:

Conclusion: The current mainstream design is based on the model of "air as the main component, solid as the auxiliary component, and composite insulation key parts". KYN61-40.5 and other models of mid mounted switchgear are typical representatives of this design.

1.2 Determination of Insulation Distance

According to GB/T 11022 "Common Technical Requirements for High Voltage Switchgear and Control Equipment Standards", the rated insulation level of a 35kV system is:

Rated voltage (Ur): 40.5kV

1-minute power frequency withstand voltage (Ud): 95kV (118kV between breaks)

Lightning impulse withstand voltage (Up): 185kV (215kV between fractures)

Based on this, the minimum air clearance (phase to phase, relative to ground) is usually designed to be ≥ 300mm. Considering manufacturing tolerances and safety margins, actual products are generally controlled within 360mm~400mm.

High altitude correction: For areas with an altitude above 1000m, correction is required. The formula is: 'Actual distance=Standard distance x Altitude correction factor Ka'. For example, if the correction factor for an altitude of 2000m is about 1.13, the minimum clear distance needs to be increased to about 340mm.

1.3 Electric field optimization design: voltage equalization and shielding

This is the essence of insulation design, aimed at eliminating local electric field concentration.

Pressure equalization measures:

Main electrical circuit: All right and sharp corners shall be rounded (R ≥ 5mm).

Circuit breaker fracture: Install a voltage equalizing ring (shielding cover) at the end of the moving and stationary contacts to evenly distribute the electric field between the fracture surfaces and improve insulation strength.

Shielding measures:

High voltage components: Grounding shields are installed around Current Transformers (CTs), voltage transformers (PTs), supporting insulators, and other components to prevent them from generating strong electric fields on the cabinet.

Modern design tool: Using finite element simulation (FEM) of electric field, the distribution of electric field can be simulated in the design stage, optimizing the shape and position of the shielding cover, and achieving "precise design".

2、 Temperature rise control: the "metabolism" of management systems

The temperature rise (Δ T) is the balance result of equipment heating and heat dissipation. According to the national standard (GB/T 11022), the temperature rise limits for each part of a 35kV switchgear at rated current are as follows:

Main electrical circuit (contacts, wiring Terminals): ≤ 65K (maximum temperature ≤ 105 ℃ at ambient temperature of 40 ℃)

Contactable metal casing: ≤ 30K

Insulation material surface: ≤ 70K

2.1 Analysis of Heat Source

1. Main electrical circuit resistance loss (I 2 R): This is the most important heat source, including the contact resistance of circuit breaker contacts, isolation switch contacts, static contacts, cable connection points, and the body resistance of the conductor itself.

2. Eddy current loss: The alternating magnetic field induces eddy currents on the steel plate shell, generating heat. This is the main cause of cabinet heating.

3. Hysteresis loss: It exists in core components such as CT and PT.

2.2 Heat dissipation pathways and strengthening measures

The core of temperature rise control is "open source throttling": reducing heat generation (lowering resistance) and enhancing heat dissipation.

3、 The mutual influence of insulation and temperature rise: vicious cycle and solutions

Insulation and temperature rise do not exist independently, but are closely coupled, which may form a vicious cycle:

>Poor contact → Increased contact resistance → Local overheating (temperature rise exceeding the standard) → High temperature aging of insulation materials (such as contact boxes), performance degradation → Insulation performance deterioration, forming partial discharge → Partial discharge further corrodes conductive parts, exacerbating poor contact →... ultimately leading to insulation breakdown or arc short circuit.

Solution:

1. Online monitoring: Install wireless temperature measurement systems (sensors placed at contacts, cable joints, etc.) on important switchgear to monitor temperature rise in real time.

2. Condition maintenance: Combined with regular inspections using infrared thermal imaging cameras, hidden hotspots were discovered.

3. Quality control: Before the equipment leaves the factory, it must pass a temperature rise test to verify whether its temperature rise at 1.1 times the rated current meets the design expectations.

4、 Summary: The Golden Rule of Excellent Design

1. Insulation is fundamental: optimize the structure through electric field simulation to ensure sufficient insulation distance and insulation margin, and ensure that the withstand voltage level meets the standard.

2. Temperature rise is key: reduce heat from the source by reducing contact resistance and suppressing eddy currents, and enhance heat dissipation capacity by optimizing heat dissipation channels.

3. Monitoring is the guarantee: transforming "regular maintenance" into "predictive maintenance", actively detecting hidden dangers through online temperature measurement and other means.

The ultimate goal is to create a high reliability 35kV switchgear with sufficient insulation strength, stable temperature rise, and maintenance free or minimal maintenance throughout its entire lifecycle, providing a solid guarantee for the safe and stable operation of the power grid.