light industry

Port protection technology of frequency conversion system
1 Introduction
Electrostatic discharge (ESD) and electric fast transient pulse group (EFT) have different degrees of harm to the frequency converter system. When the cables are exposed to 4 ~ 8Kv electrostatic discharge environment, information transmission cable terminal load can be measured by the induction voltage up to 600V. This voltage far beyond the value of the threshold voltage of the inverter input, output port and communication port and induction in a typical pulse duration is about 400ns.
Frequency converter in use often encounter unexpected voltage transient and surge, which leads to frequency converter internal electronic components damage phenomenon is the frequency converter in semiconductor devices including diodes, transistors, thyristors integrated circuit and so on were burned or breakdown. According to statistics inverter control part of the fault is 75% due to transients and surge caused by. The voltage transient and surge everywhere, grid, lightning, blasting, even on the carpet walk will have thousands of volts of static induction voltage. These are inverter control and communication ports of the invisible hazards. Therefore, in order to improve the reliability of the inverter, it is necessary to protect the voltage transients and surge.

2 lightning protection port
According to the engineering practice of the application of frequency converter, frequency converter by lightning can be divided into direct lightning, lightning and thunder conduction. But no matter which form of arrival of the equipment can be summarized from the following 4 parts of the intrusion of the lightning surge, in this part of the site known as the lightning protection, and the inverter for example.

2.1 shell port
For example, we can put any big or small frequency changer or system regarded as a shell as a whole, such as sensor, transmission line, signal relay, the scene of the instrument, DCS system, they have may be completely exposed in the environment by direct lightning strike, causing damage to the equipment. Standard regulations, when the equipment shell by 4KV lightning electrostatic discharge, it will affect the normal operation of the inverter or the system. For example, sensors placed in the outdoor input transducer are likely to be exposed to lightning, and the frequency converter cabinet in the room may be discharged by the discharge of the lightning arrester in the building.

2.2 signal line port
In the inverter control system, in order to realize the information signal or transfer to the parts connected with the outside world, then these from the external input signal or inverter output signal interface are likely to be affected by the impact of the lightning surge. Because the inverter cabinet external signal input inverter port surge often through long cable, so the use of s 10/700 waveform, the standard provisions of the line to line surge voltage between 0.5kV, line to ground surge voltage 1kv. And building transducer transmission signal ports by surge is in power line surge, the 1.2/50 (8 / 20) u s combination wave, line to line, line to ground voltage surge limit value remains unchanged. Once more than the limit value, the signal port and the port after the device is likely to suffer damage.

2.3 power port
Power supply port is the most widely distributed and most easily induced or conduction lightning surge parts, inverter power supply port for the power supply from the power distribution panel to the power input terminal and output from the inverter to the motor. Standard provisions in the 1.2/50 (8/20) s waveform line between the line and the line surge voltage limit of 0.5kV, line to ground surge voltage limit is 1kv. But here the surge voltage is specified voltage into AC 220V, if a low voltage will not use this as a standard, the power line by smaller surge may not immediately damage to the device, but at least to life.

2.4 earth port
In spite of the fact that there is no specific reference to the ground port in the standard, the ground port of the converter is very important. In the lightning occurs when the grounding port is likely to be affected by the potential impact of the ground potential rise, or because of poor grounding, grounding to the resistance too much, can not meet the requirements of the reference potential to cause damage to the equipment. Connect the port not only grounding resistance / ground very (length, diameter, material, etc.), ground, ground network setting etc. requirements, but also with equipment electrical characteristics, frequency, work environment have direct relationship. At the same time, it is possible to counter the DC operating power supply port in the frequency converter, and damage the unit circuit of DC voltage. In summary, the inverter's lightning protection can be considered from four key ports, as shown in figure 1.
Fig. four 1 key ports of the converter
3 converter port protection
3.1 shell port
Converter shell port protection is not only the building shell, but also should include the inverter shell or inverter cabinet shell, such as the inverter, inverter cabinet room, etc.. In accordance with IEC 1312 - 1 the lightning electromagnetic pulse protection "in the first part (general principles) applicable scope: building or building at the top of the inverter system and effective lightning protection system design, installation, inspection and maintenance. The protection method mainly has three kinds: grounding, shielding and equal potential connection.
(1) grounding
Iec1024 - 1 has been described in the building lightning protection grounding method, mainly through the building underground network grounding system to achieve the requirements. Lightning protection of the converter system also requires to between two adjacent buildings through the power line, signal transmission cable must be and grounding system of building connected (and can not form a loop), to take advantage of multi parallel paths to reduce the current in the cable.
The grounding of frequency converter system should pay more attention to the security of the system and prevent the interference of other systems. Generally speaking, the inverter system can not be directly connected with the lightning protection ground, otherwise there will be a stray current into the inverter system to cause signal interference. The correct connection mode should be in the ground to two different network, through the low voltage lightning arrester connection, so that it is in the lightning state automatically connected.

(3) a series of equal potential
The purpose of the equal electric potential connection is to reduce the electric potential difference between the frequency converter and the metal parts. In the lightning protection area at the interface of the equipotential connection to consider a transducer system in the building, in those of lightning electromagnetic pulse effect place minimal requirements, such as potential connected with best use of metal plate, and repeatedly with buildings of reinforced connected or connected to other shielding materials, the structural members. For the transducer system exposed conductive material should be established, and the principle of a potential connection network is not directly connected to the earth, but in fact all the potential connection network is connected to the earth.

3.2 signal line port
Signal line port protection is now available in many types of more mature protection devices, such as the frequency converter signal port protection, frequency converter communication port protection, etc.. In the selection of protection device, in addition to the performance of the protection device itself, should pay attention to the transmission rate of the protection equipment, insertion attenuation limits, voltage standing wave ratio (VSWR), operating voltage, operating current and other related indicators, if in the same system using multilevel protection should also consider coordinate.
The transient current in the signal port is most likely to damage the signal switching or switching unit and the control unit in the frequency converter, such as the main board, the parallel port, the signal interface card, etc.. In fact current or transient surge may by different ways were introduced to the signal transmission in the network, if the inverter control system and the host computer communication adopts the structure of Ethernet, IEEE 802 - 3 Ethernet standard lists four may pose a threat to the network:
(1) the local network element and the power supply circuit or the electric circuit which is affected by the movie to have the direct contact;
(2) the effect of static electricity on local area network cable and components;
(3) the high energy transient current is coupled with the local area network system (introduced by the cable in the vicinity of the network cable);
(4) there is a small difference between the ground voltage of the network element connected to each other (for example, the safety ground voltage of the two different buildings may be slightly different).
In inverter communication line, for example, in RS-232 serial and parallel port standard, for discharge high inrush and fault current of ground with the data signal return path sharing a line, and transient voltages ranging from tens of volts have may through the serial and parallel port and destroy machine and terminal etc. equipment, signal transmission line can also directly to the transient surge conduction of outdoor power line in, and signal interface can conduction by lightning and static electricity leakage caused by the surge voltage.
Users should be careful of the data line protector choice, some protector, although the "triage" role, but often it is a silicon avalanche photodiode (SAD) connected to the protected line and protector shell, clamping performance tests show that sad is very good, but it surge diversion capacity of the limit. At the same time, the pressure sensitive resistor (MOV) can not be used in the data line protector. Advanced process control system interface signal lightning protection device (both RS-232 series communication interface and computer coaxial network adapter interface) are currently using the transient overvoltage of semiconductor discharge tube, the residual voltage impacted parameters is very important. When the conditions are able to take multi-level protection design circuit better effect.
 
3.3 power port
On the principle of using multilevel SPD do power protection, but the inverter control system power protection due to its sensitivity to the low residual voltage protection device and the residual pressure should be less than what is required for the pressure resistance of the protection equipment. At the same time, it is necessary to take into account the impact of electromagnetic interference on the frequency converter system, so the shunt design with filter should be more ideal. So for the inverter system power supply protection of the two points are special attention: the first two uses a large current capacity of the protective device, the inverter terminal at the low residual voltage protection device. The last stage of the protection of the best filter circuit. The following problems should be paid attention to when installing Spd on power supply port of frequency converter system:
(1) the multi stage SPD should consider the energy cooperation, the time coordination, the distance coordination. If inappropriate, the effect will be counterproductive.
(2) the lead wire connecting the lightning protector shall be as thick and short as possible.
(3) to tie all the connecting wires together as far as possible when fully protected.

4 concluding remarks
Frequency variable speed drive system, the importance of the inverter port protection, has become a variable frequency drive system design, application must face the problem, but also one of the key issues in the application and promotion of frequency converter. Port protection of the inverter will continue to develop and improve.