ELECTRICAL POWER SUPPLY SYSTEMS

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At present it is hard to imagine our life without electricity. Electrical power supply system is a complex of items used for generation, distribution and transfer of electrical power.

Electrical power supply system is a part of electrical unit of the organization or company and it is a subsystem to a larger electrical power system.

The purpose of electrical power networks is the receipt and distribution of electrical power supplied from an external source of electrical power supply (power system). The network comprises substations, power transfer lines, conductors, connecting devices, control and protection instruments.

An important technical parameter is reliability of power supply. It is determined by the number of independent sources providing redundancy for each other. There are four categories of reliability:

category III - one source
category II - two independent sources with a manual switch to a second source in case of voltage failure at the first one (second sources may use a diesel generator set or uninterruptible power supply etc.)
category I - two independent sources with an automatic switch to a second source in case of voltage failure at the first one (second sources may use a diesel generator set or uninterruptible power supply etc.)
Special group of category I is three independent sources (the third ones may use a diesel generator set or uninterruptible power supply etc.)

To enable category I and special category I an automatic load transfer unit is used.

An automatic load transfer unit is made with application of programmable controllers enabling to provide the following logic and functions:

The operation of the automatic load transfer unit is allowed only in case there is voltage at all phases at another terminal.
One-time operation of the automatic load transfer unit is allowed
The automatic load transfer unit operation is not allowed in case of an emergency disconnection of the main or bus section switch.
Possibility of multiple time actuation of the automatic load transfer unit in case of a short circuit with any single failure in the circuit.
Operation of the automatic load transfer unit is blocked in case automatic switches in voltage control relay circuits are disconnected.
There is an alarm in the manual mode of the automatic load transfer unit and an alarm of the automatic load transfer unit actuation.

Power supply automation is an integral part of the building automation and dispatching system. Automation and dispatching of electrical power supply systems is required to optimize operational costs and to reduce the probability of failures. In the electrical power supply system of the company or other large facility the necessity of changing requirements to the reliability category is quite real and in case of a traditional power supply system will require upgrading of the current system. In case there is a controlled distribution panel that has a guaranteed power supply section changing of the power supply reliability category at each connection is achieved by programming of controllers.

Electrical power supply system automation provides additional possibilities to optimize the operation modes, enables to reduce the time of unplanned downtime of equipment and power loss.

An example of the current electrical power supply system of a critical facility is shown in figure below:

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1, 2 are independent power supply sources

3 is a power transformer

4 is a diesel generator unit

5 is an automatic input breaker of the diesel generator set

6,7 are sections of buses

8 is a power-factor correction unit

9 is an automatic input breaker of an independent source

10 is an automatic bus section breaker

11 is an automatic distribution switch

12 is outgoing lines

Load types:

A – information and calculation systems, telecommunication systems, voice alarm and dial telephone office, security and fire alarm systems, monitoring and control system, video surveillance system, emergency and security lighting and dispatching control system. Electrical power supply interruption is not allowed
B – fire pumps, air-supply and smoke removal system, process premises conditioning system, refrigerating chambers in the canteen, signal lights. Electrical power supply interruption is allowed for the time of a standby power source actuation.
C – other process and utility systems not included in groups A and B. Electrical power supply interruption is allowed for the time of the accident response.

ST Solution specialists have an experience in designing, mounting and delivery of power supply system equipment up to 35kV, as well as automation of power supply systems for different industries: