7.27 What is online testing of solar cable routes? How to proceed?
1. The significance of online testing
Online detection refers to the measurement of technical data such as insulation resistance, dielectric loss, or micro direct current components flowing through the insulation of running cable lines without power outage, and based on this, the insulation condition of the running cable lines is determined.
The water tree aging phenomenon of cross-linked polyethylene insulated cables poses a threat to the safe and reliable power supply of the cables. Therefore, it is very important to timely grasp the aging status of water tree branches in cable lines. Measuring the insulation resistance of a cable after a power outage cannot detect the generation and development of water tree branches, and measuring the dielectric loss angle can only indicate the average aging degree of the cable. However, online detection is an effective detection method for the insulation condition of cables in operation that has been studied and tried out in recent years.
2. One of the online detection methods — superimposed DC voltage method
Add a DC voltage of approximately 50V to the running cable line, and measure the insulation resistance of the cable by reading the leakage current flowing through the cable insulation layer to determine the aging condition of the cable. According to the different ways of stacking DC voltage, it can be divided into the following three types.
(1) Using three-phase star reactors: Set a set of column reactors at any location on the high-voltage distribution bus (as shown in Figure 7–41), and apply a DC voltage (25–50V) to the neutral point of the reactor through an additional DC power source to measure the DC micro current flowing out from the cable grounding wire.
The specific testing method is:
① Before testing, the grounding wires at both ends of the metal sheath of the cable should be removed or changed to grounding through a large capacitor (50F); ② Add the output voltage of the DC power supply (25–50V) to the busbar of the distribution line through a reactor;
③ Connect an insulation resistance tester between the grounding wire led out from the shielding of the tested cable and the grounding grid;
④ Read the leakage current value passing through the cable insulation layer and calculate the insulation resistance value.
By using a three-phase reactor and serializing an LC parallel resonator in the grounding circuit of the reactor, it has a high impedance to power frequency AC, while a low impedance to DC. It can detect both the main insulation and sheath insulation of cables, without affecting the safe power supply of the power system.
(2) Utilizing high-voltage voltage transformers in substations: For substations with electromagnetic voltage transformers, the center point N1 of the voltage transformer (TV) is directly grounded, and the metal sheath N2 of the cable is also grounded, as shown in Figure 7–42.
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When measuring, switch N1 and N2 to grounding through capacitors C1 and C2 respectively, as C1 and C2 are insulated from the ground for DC and grounded through low impedance for AC. When the DC voltage U is connected from N1, leakage current flows through the busbar and leakage current flows through the cable insulation layer. Therefore, the insulation resistance of the busbar and cable insulation layer can be calculated separately, or the insulation resistance value can be directly displayed using a microcontroller.
When measuring the insulation resistance of the protective layer, switch the cable metal sheath N2 from direct grounding to grounding through capacitor C2. Due to the DC voltage U, leakage current flows through the cable protective layer to calculate the insulation resistance of the protective layer or directly display the insulation resistance value.
(3) For 35kV and below neutral points through arc suppression coil or small resistance earthing system, after adding large capacitance to the neutral points of power transformer and generator, series resonance is easy to occur. Connect a low resistance to prevent a decrease in grounding current, while delivering current from a constant current source to generate the required DC voltage at both ends of the low resistance. For example, when 5 outputs a 10A DC current, it is equivalent to adding 50V DC voltage to the high-voltage system. Therefore, when S1 is closed, the neutral point N is still connected to the ground with low resistance, and the neutral point will not float and resonance will not occur.
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3. Method 2 of Online Detection — Micro DC Component Method
During cable operation, changes in electric field and current occur between the conductor of cross-linked polyethylene insulated cable and the shielding copper strip due to the generation of water tree branches in the insulation. By utilizing the changing current and the micro DC component induced by the cable grounding circuit, measurements are made using a low-pass filter and a DC tester. Based on the size and polarity of the micro DC component method, the type of water tree branches in the cable insulation and the degree of insulation aging are determined.
The characteristics of the micro DC component method are: ① the test is only between the shielding tape and the Earth electrode, without touching the high voltage; ② No DC power supply is required. However, there is a high requirement for the insulation resistance of the outer protective layer of the cable.