On improving the power supply of CSB devices
Due to the regularly recurring cases of ignition of EC posts on the road network and according to the instructions of the Ministry of Internal Affairs on the North Caucasian Road, the grounding of EC posts and the adjustment of current protection parameters were carried out. To identify the general situation in the alarm system, centralization, and blocking, "Technical data sheets for the power supply of SSB facilities" have been developed. This made it possible to see more clearly the whole picture of the power supply of the SCB devices and to detect some dangerous technical solutions.
The inspection revealed that the existing projects of grounding and commissioning devices of EC posts did not comply with the new GOST standards adopted during 1993-1996. The situation is aggravated by the fact that the GOST R 50571 standards and some provisions of the PUE were not taken into account when drafting power supply and grounding projects for new EC posts with tone rail circuits in newly electrified areas. Also, the experience of using old equipment, which showed a low degree of reliability, is not sufficiently taken into account.
Even after eliminating the comments of joint inspections by employees of the electrification and power supply and alarm services, centralization and blocking in the power supply of EC posts, there were still issues requiring further study and improvement of devices. Many of these issues require project changes and the intervention of industry science.
According to old and new projects, the connection of the main grounding bus (PEN) of the input device (VU) of the EC posts (SHCHVP or SHCHVPU) to the grounding conductor (PE) is made by a conductor made of steel wire PSO-5 with a cross section of 19.5 mm2. But in the TN-C electrical network, this conductor is part of the protective conductor system and, according to GOST R50571.10-96 p. 543.1, must have a conductivity not less than that of the zero conductor of the supply line. The specified conductor enters the stray current circuit that occurs in some operating modes of electrical installations. In this regard, it, like the grounding line of the post, must be made with a steel strip with a cross section of 25x4 mm. There have been repeated cases of heating of these conductors from PSO-5 red hot on the road, which could lead to a fire.
The RE-conductor system of the EC post is part of a series circuit through the ground for zero conductors and a transfer link for all stray currents of the two substations. The better the grounding, the lower its leakage resistance, the more current flows through the conductors in the incomplete phase mode of the network (for example, when one of the fuses at a complete transformer substation burns out). EC posts having a protective ground resistance of less than 1 ohm have the highest probability of a significant amount of zero-sequence stray current appearing on the grounding device. On the North Caucasian Road, such a dangerous situation is observed at 9% of EC posts. The reconstruction of the conductor system on them must be carried out first.
When designing and installing the RE-conductor system, the requirement of GOST R 50571.10-96 (appendix "B") that the connection between the grounding conductor and the VU must be made over the shortest distance is not taken into account. In real electrical installations, this connection between the "three earths shield" and the SHCHVP-SHCHVPU is made via a grounding line and a protective conductor, which randomly pass through the premises of the post. Stray current, bypassing the zone of action of the current protections of the EC and KTP posts, can flow indefinitely to the conductors, cause them to heat up and ignite in various rooms. It is also dangerous that these conductors pass through cable channels, which are sometimes dusty.
At the EC post, after combining the zero (PEN) and grounding (PE) conductors on the VU (SHCHVP-SHVPU), they are re-combined on the input panel (SHCHPO, SHCHPO–1 conductors), which is strictly prohibited by GOST R 50571.10-96, since it creates a path for emergency and stray current to enter any statics and panels.
The design of the old-type VU SHCHVP-73 urgently needs to be equipped with a full-fledged PEN bus, since it is allowed to connect the zero cores of the supply cables and the grounding conductors of the post to a single bolted connection (terminal 02). This contradicts the requirements of GOST R 50571.3–94 (clause 413-1.3.2) and reduces electrical safety. It is also not allowed to connect two zero cores of power outgoing cables to a single bolted connection (terminal 01), which creates the possibility of severe phase distortion and an emergency situation on both power feeders at once in case of contact failure. It should be noted that the requirement of VNTP/MPS-84 for the unconditional connection of each conductor of the grounding and grounding system to a separate bolt was not included in NTP SCB/MPS-99 and
pr. 32 TSH 10.02-96.
The location of current protections and switching devices switched off by them on the input panel, and not on the control panel, contradicts the requirements of GOST R 50571.10-96 (clause 544.2.3), which states that the protective conductor must be connected by housings only of those electrical equipment that must be switched off if the protective device is triggered.
Due to the lack of current protection on the high-voltage switchboard and high-voltage switchboard, the power cables in the premises of the EC post and the equipment metallically connected to the entire system of grounding and zero conductors of the post are located in the protection zone of the KTP. In the event of an accident, emergency overcurrents of up to hundreds of amperes flow through the entire PEN-PE conductor system before the power is turned off by the second stage of protection at the checkpoint, the path, duration and consequences of which are unpredictable. All equipment may be under mains voltage. The section of the network from the SCHVP–SHCHVPU to the entrance panel, which is not switched off by the guards of the checkpoint, creates a zone of potential electrical injury in an accident.
When drawing up power supply passports for CST facilities, it turned out that very many of them have a resistance of grounding devices much less than the CTP. In this case, there is a slightly lower touch voltage at the post during an accident, but all other users of the KTP are in an unfavorable situation. The grounding of the post turns out to be in a serial circuit with the grounding of the substation and the high contact voltage turns out to be on all the zeroed equipment of other consumers. There is a significant shift in the neutral of the substation and a strong phase distortion throughout the network. The consequence of this is massive damage to consumers' electronic equipment due to overvoltage.
The practice of widespread re-earthing in the TN-C network creates an additional risk of damage from stray currents. There is an urgent need to use a TN - S five-wire network or a TT network without a metal N bond and PE conductors for security facilities.
In order to really increase the fire safety of existing and during the construction of new EC posts, it is necessary to reconstruct the system of RE-PEN conductors and bring it to the requirements of new regulations. This requires:
perform a protective conductor from the grounding to the SCHVP-SCHVPU with a 25x4 mm bus and lay it along the shortest distance between the grounding conductor and the VU in compliance with fire safety standards;
eliminate the re-connection of PEN and RE-conductors on the VU and on the input panel;
equip the SHCHVP-73 with a full-fledged PEN bus;
to clarify the regulatory documents on the connection of PEN conductors and the permissible limits of the difference in the values of the grounding resistance of the power source and the consumer.
There are features in the design of the input devices of the CSB facilities that are not found in similar devices of other departments and create additional difficulties during maintenance. Many factors have been identified that reduce electrical safety, reliability of devices and create a fire hazard throughout the building of the EC post.
Currently, the functions of the input and distribution device of the posts are performed by the control panel or control panel and the input panel, the design of which, the installation location of current protections and the purpose itself do not meet many requirements of the PUE and GOST standards, as well as the generally accepted practice of arranging such devices in industrial enterprises and civil buildings. Even in appearance, the ballscrews are more similar to one of the statics of the EC than to a power input device. The input panel must be a distribution panel for the post loads, and the input device of the main power circuit must be equipped with current and voltage protections.
The absence of feeder control knobs on the outer surface of the control panel and control panel makes it impossible to quickly and safely manually turn off the power in the service and technical building in case of an accident inside the control panel. Switches with short handles in the control panel cannot be called full-fledged control devices, the presence of which is due to PUE 7.1.19. The absence of protective covers and arc extinguishing chambers contradicts the requirements of PUE 4.1.10. The very attempt to turn off the electrical installation with these switches can lead to an accident even in normal operation.
The presence of a remote shutdown circuit in the control panel is an insufficient reliability factor due to the potential threat of failures of mechanisms or "sticking" of contacts. The absence of slots on the control panel door for the control knobs of circuit breakers forces operations with them in all modes near the open installation of feeders, which creates a risk for personnel in case of an accident and poor visibility.
The design of the control panel does not allow fulfilling the requirement of PUE 4.1.12, which states that it must be possible to remove voltage from each circuit breaker during its repair. Turning off the power with the remote shutdown button does not create a visible circuit break necessary to guarantee the safety of operation in the electrical installation.
04.08.2023, 14:09
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