What causes packets to be lost on the LAN? There are different options: redundancy is incorrectly configured, the network cannot cope with the load, or the LAN βstormsβ. But the reason is not always in the network layer.
Arktek LLC made automated process control systems and video surveillance systems for the Rasvumchorrsky mine of Apatit JSC based on .
There was a problem on one part of the network. Between FL SWITCH 3012E-2FX switches β and FL SWITCH 3006T-2FX - the communication channel was extremely unstable.
The devices were connected by a copper cable laid in one channel to a 6 kV power cable. The power cable creates a powerful electromagnetic field, which has caused interference. Conventional industrial switches do not have sufficient noise immunity, so some data was lost.
When FL SWITCH 3012E-2FX switches have been installed at both ends - , the connection has stabilized. These switches comply with IEC 61850-3. Among other things, part 3 of this standard describes the requirements for electromagnetic compatibility (EMC) of devices that are installed in power stations and substations.
Why did EMC-enhanced switches perform better?
EMC - General
It turns out that the stability of data transfer in a LAN is affected not only by the correct settings of the equipment and the amount of data transferred. Dropped packets or a broken switch can be caused by electromagnetic interference: a walkie-talkie that was used near network equipment, a power cable laid nearby, or a power switch that opened the circuit during a short circuit.
Walkie-talkie, cable and switch are sources of electromagnetic interference. Enhanced EMC switches are designed to operate normally when exposed to these interferences.
There are two types of electromagnetic interference: inductive and conductive.
Inductive interference is transmitted through the electromagnetic field "through the air". This interference is also called radiated or radiated.
Conducted interference is transmitted through conductors: wires, ground, etc.
Inductive interference appears when exposed to a powerful electromagnetic or magnetic field. Conducted interference can be caused by switching current circuits, lightning strikes, impulses, etc.
Switches, like all equipment, can be affected by both inductive and conducted interference.
Let's look at different sources of interference in an industrial facility, and what kind of interference they create.
Interference sources
Radio emitting devices (walkie talkies, mobile phones, welding equipment, induction furnaces, etc.)
Any device emits an electromagnetic field. This electromagnetic field affects the equipment both inductively and conductively.
If the field is generated strong enough, then it can create a current in the conductor, which will disrupt the signal transmission process. Very strong interference can also lead to equipment shutdown. Thus, an inductive effect is manifested.
Operating personnel and security services use mobile phones, walkie-talkies to communicate with each other. Stationary radio and TV transmitters operate at the facilities, Bluetooth and WiFi devices are installed on mobile units.
All these devices are powerful generators of the electromagnetic field. Therefore, for normal operation in industrial environments, switches must be able to tolerate electromagnetic interference.
The electromagnetic environment is determined by the strength of the electromagnetic field.
When testing the switch for resistance to inductive effects of electromagnetic fields, a field of 10 V/m is induced on the switch. In this case, the switch must be fully functional.
Any conductors inside the switch, as well as all cables, are passive receiving antennas. Radio emitting devices can generate conducted electromagnetic interference in the frequency range from 150 Hz to 80 MHz. The electromagnetic field induces voltage in these conductors. These voltages, in turn, cause currents, which create interference in the switch.
To test the switch for immunity to conducted electromagnetic interference, voltage is applied to the data ports and power ports. GOST R 51317.4.6-99 establishes a voltage value of 10 V for a high level of electromagnetic radiation. In this case, the switch must be fully functional.
Current in power cables, power lines, ground circuits
The current in power cables, power lines, ground circuits creates a magnetic field of industrial frequency (50 Hz). The influence of a magnetic field creates a current in a closed conductor, which is a hindrance.
The magnetic field of industrial frequency is divided into:
- magnetic field of constant and relatively low strength, caused by currents under normal operating conditions;
- a magnetic field of relatively high intensity, caused by currents under emergency conditions, acting for a short time until the devices are triggered.
When testing switches for the stability of the impact of a magnetic field of industrial frequency, a field of 100 A/m for a long period and 1000 A/m for a period of 3 s is applied to it. When tested, the switches should be fully functional.
For comparison, a conventional household microwave oven generates a magnetic field strength of up to 10 A/m.
Lightning strikes, emergency conditions in electrical networks
A lightning strike also causes interference in network equipment. They do not last long, but their magnitude can reach several thousand volts. Such interference is called impulsive.
Surge noise can be applied to both the power ports of the switch and the data ports. Due to high overvoltage values, they can both disrupt the functioning of the equipment and completely burn it out.
A lightning strike is a special case of impulse noise. It can be attributed to microsecond impulse noise of high energy.
A lightning strike can be of different types: a lightning strike into an external voltage circuit, an indirect strike, a strike into the ground.
When a lightning strikes an external voltage circuit, interference occurs due to the flow of a large discharge current through the external circuit and the ground circuit.
An indirect lightning strike is considered to be a lightning discharge between clouds. During such impacts, electromagnetic fields are formed. They induce voltages or currents in the conductors of an electrical system. This is what causes interference.
When lightning strikes the ground, current flows through the ground. It can create a potential difference in the grounding system of the vehicle.
Exactly the same interference is created by switching capacitor banks. Such switching is a switching transient process. All switching transients cause high energy microsecond impulse noise.
Rapid changes in voltage or current when protective devices operate can also generate microsecond transients in internal circuits.
To test the switch for immunity to impulse noise, special test pulse generators are used. For example, UCS 500N5. This generator supplies pulses of various parameters to the tested ports of the switch. The parameters of the pulses depend on the tests being carried out. They can differ in the shape of the pulse, output resistance, voltage, exposure time.
During microsecond transient immunity tests, 2 kV pulses are applied to the power ports. For data ports - 4 kV. With this test, it is assumed that the operation may be interrupted, but after the disappearance of the interference, it can be restored independently.
Switching of reactive loads, "bounce" of relay contacts, switching with AC rectification
Various switching processes can occur in an electrical system: interruptions of inductive loads, opening of relay contacts, etc.
Such switching processes also create impulse noise. Their duration is from one nanosecond to one microsecond. Such impulse noise is called nanosecond impulse noise.
To carry out tests, bursts of pulses of nanosecond duration are fed to the switches. Pulses are applied to the power ports and to the data ports.
The power ports are pulsed at 2 kV and the data ports at 4 kV.
During testing for exposure to nanosecond impulse noise, the switches must be fully functional.
Pickups from industrial electronic equipment, filters and cables
When the switch is installed near power distribution systems or power electronic equipment, unbalanced voltages can be induced in them. Such interference is called conductive electromagnetic interference.
The main sources of conducted interference are:
- power distribution systems, including direct current and a frequency of 50 Hz;
- power electronic equipment.
Depending on the source of interference, they are divided into two types:
- constant voltage and voltage with a frequency of 50 Hz. Short circuits and other disturbances in distribution systems generate interference on the fundamental frequency;
- voltage in the frequency band from 15 Hz to 150 kHz. Such interference is usually generated by power electronic installations.
To test the switches, the power and data ports are supplied with an effective voltage of 30V continuously and an effective voltage of 300V for 1 s. These voltage values ββcorrespond to the highest degree of severity of the GOST tests.
The equipment must be able to withstand these effects when installed in a harsh electromagnetic environment. It is characterized by:
- the devices under test will be connected to low-voltage electrical networks and medium-voltage lines;
- devices will be connected to the grounding system of high-voltage equipment;
- power converters are used that inject significant currents into the grounding system.
Similar conditions can be found in stations or substations.
Rectifying AC voltage while charging batteries
After rectification, the output voltage always pulsates. That is, the voltage values ββchange randomly or periodically.
If the switches are powered by DC voltage, then large voltage ripples can disrupt the operation of the devices.
As a rule, all modern systems use special smoothing filters and the level of ripple is not high. But the situation changes when batteries are installed in the power supply system. When the batteries are charged, the amount of ripple increases.
Therefore, it is also necessary to take into account the possibility of such interference.
Conclusion
Enhanced EMC switches allow data transmission in harsh electromagnetic environments. In the example of the Rasvumchorr mine at the beginning of the article, the data transmission cable was exposed to a powerful magnetic field of industrial frequency and conducted interference in the frequency band from 0 to 150 kHz. Conventional industrial switches were unable to handle data transmission under such conditions and packets were lost.
Switches with improved electromagnetic compatibility can fully operate when exposed to the following interference:
- radio frequency electromagnetic fields;
- magnetic fields of industrial frequency;
- nanosecond impulse noise;
- microsecond impulse interference of high energy;
- conducted interference induced by radio frequency electromagnetic field;
- conducted interference in the frequency band from 0 to 150 kHz;
- DC power supply voltage ripple.
Source: habr.com