Industrial Network Control

Recently, I read an article in a technical publication about electromagnetic interference and its prevention. After reading it, I was deeply inspired and it sparked extensive discussions among our readers. The author, drawing from years of practical experience, shares some insights in the hope of raising awareness among readers about the safe and stable operation of electrical equipment.

Power plants and substations, due to their complex and lengthy production processes, are inevitably subject to various types of interference. Among all interference sources, electric and magnetic fields have a significant impact on measuring instruments, relay protection, and safety control devices.

Hazards of Interference

It is well known that measuring instruments are the “eyes” of electrical workers. Under normal circumstances, relay protection and safety automation devices are the “guardians” of the power system. When interference does not cause significant impacts, it is often overlooked. The general underestimation of interference signals is a common occurrence in production sites, and there is insufficient recognition of the extent of their harm. Firstly, interference signals can cause inaccuracies in measuring instruments, especially digital display instruments, leading to deviations in our measurement results from actual values. This can lightly affect equipment monitoring and supervision (e.g., high-frequency interference from 220 kV disconnector operations affecting instrument readings) and severely impact safety production and economic efficiency (e.g., active energy metering, pre-commissioning tests for booster station equipment). Secondly, interference signals can cause switching circuits to flip, leading to erroneous data or address transmission in digital circuits, resulting in logical chaos, calculation program errors, or data loss. In severe cases, this can cause protection delays, false operations, refusal to operate, or device crashes. Strong interference signals can also degrade the performance of power electronic equipment and even cause damage. Additionally, high-frequency oscillations induced by circuit breaker operations on unloaded lines can cause resonance in the line voltage transformer circuit, leading to equipment damage.

Industrial Network Control

Sources and Modes of Interference

Complex and harsh working environments are the source of electromagnetic interference. Electrical equipment is directly and indirectly affected by external factors such as electric sparks from welding operations, overvoltage during equipment operation, atmospheric overvoltage, high-frequency radio waves from wireless communication devices, large-capacity motors, and switchgear operations (e.g., 35 kV and above booster station disconnector operations and direct feed line stop/start operations), and grounding faults in the power system where the fault current flows into the grounding network, resulting in significant potential differences between two points (with a maximum value of up to 10 volts per kiloampere of fault current). Harsh weather conditions, such as lightning, also contribute to external interference. Internal interference can also arise from the electrical equipment itself, such as high-order harmonics in the output of excitation or silicon rectifier excitation systems affecting the rotor protection of the machine, voltage fluctuations, potential differences due to multiple grounding points, and internal interference from poor filtering or floating charge power supply quality in substation relay protection power supplies without batteries.

Electromagnetic interference can be classified into different modes, such as radiation interference, where electromagnetic fields are directly generated around electrical equipment during welding, wireless communication, or high-voltage testing. Coupling interference occurs due to improper equipment layout or wiring, where adjacent or interconnected devices have capacitive, inductive, or insulation weak points leading to leakage. Common-mode interference, where multiple sources affect a single measurement device, is generally less impactful. Differential-mode interference, where interference signals overlap with measurement signals, causing significant deviations from actual values, requires special attention.

Measures to Prevent and Reduce Interference

1. Isolation: For example, using optocouplers to completely isolate electrical measurement switch signals and achieve ground potential isolation, which is particularly effective in suppressing common-mode interference. Using isolation transformers, such as voltage, current, DC inverter power supplies, and pilot line protection, to avoid placing weak signal lines with power lines in the same cable, and separating signal cables, control cables, and power cables in different layers. Avoiding using the same grounding wire for measurement and high-power circuits.
2. Shielding:
   • Electric Field Shielding: Using a Faraday cage made of good conductors and ensuring proper grounding to maintain zero potential and prevent electric fields outside the shield from entering.
   • Magnetic Field Shielding: At low frequencies, using materials with good magnetic properties like silicon steel as the shield to guide the magnetic field lines through the shield with lower magnetic resistance, reducing the penetration of the interference magnetic field. At high frequencies, using both electric and magnetic field shielding methods to prevent the propagation of high-frequency electromagnetic fields in space: using metal conductors to reflect and absorb electromagnetic waves, where part of the wave is reflected due to impedance differences, and the other part forms eddy currents in the metal shield, causing absorption losses.

   For example, using control cables with armored lead shielding, ensuring reliable and good grounding at both ends of the booster station and control room, can effectively reduce ground potential rise interference on instruments and relay protection. Maintaining the continuity of the shielding layer during intermediate transitions or relay connections in cables is crucial. Do not overlook the insulation status of signal cables, ensuring they have good insulation or a dry environment. The shielding layer of secondary plugs in measurement circuits should be reliably grounded at the protection screen. Avoid using two-end grounding of cable cores as an anti-interference measure.

3. Grounding: For example, grounding the neutral point of the voltage transformer secondary and the current transformer secondary current circuit at the control room; ensuring a distance of 3 to 5 meters between the grounding points of the secondary voltage cable circuit of high-frequency protection and the primary grounding wire; sometimes using multiple grounding wires; eliminating high-frequency interference by grounding capacitors before introducing AC voltage, AC current, and DC input into the measurement device; keeping the wiring away from DC operation power lines and high-frequency circuit wires; avoiding bundling input wires in the same direction.
4. Other Measures: Conducting 1 MHz pulse group interference tests, electrostatic discharge tests (typically with an 8 kV test voltage), radiated electromagnetic field tests, and fast transient interference tests on measurement relays for relay protection;

   Conducting radio signal interference tests in areas where wireless communication devices may be used near relay protection devices; otherwise, prohibiting the use of wireless communication devices or improving shielding measures; selecting appropriate locations for secondary cable laying, keeping them away from high-voltage busbars and avoiding parallel placement, staying away from capacitive devices like capacitive voltage transformers, and avoiding placing power cables and control cables in the same cable tray; keeping signal voltage-weak cables away from power or signal voltage-strong cables; arranging and arranging cables to reduce and eliminate parasitic voltage interference;

   Improving the anti-interference performance of equipment itself, using reliable filtering devices to ensure the ripple factor of the rectified output voltage is less than 5%; maintaining and maintaining grounding copper bars to eliminate oxidation and corrosion’s impact on grounding performance; regularly checking whether the grounding of protection screens or devices is reliable (should have more than two grounding connection points) and whether the grounding resistance meets anti-error requirements.