Electrical Distribution Architecture In Water Treatment Plants

Water treatment plants

For both drinking water and wastewater treatment, 4 different sizes of plants have been distinguished. The size of plants can be expressed in quantity of treated water per day, or in corresponding number of inhabitants.

Four different types of (waste) water treatment plants have been distinguished, depending on destination and size:

• T1 – Autonomous water treatment plant // See single-line diagram
• T2 – Small water or wastewater treatment plant // See single line diagram
• T3 – Medium sized water or wastewater treatment plant
  • See single line diagrams //
    – Double-radial architecture and
    – Open medium voltage loop
• T4 – Large water or wastewater treatment plant
  • See single line diagrams //
    – Radial-double feed
    – Open medium voltage loop

Electrical Distribution

Guidance is given for the selection of Electrical Distribution architecture in water treatment plants. This includes the selection between different possible configurations of MV and LV circuits and the implementation of back-up power sources.

The most relevant characteristics of the electrical installation are taken into account, such as typology, power demand, sensitivity to power interruptions, …

• Connection to the utility network
• MV circuit configuration
• Configurations of LV circuits
• Backup generators
• Presence of uninterruptible power supply (UPS)

Connection to the Utility network

4 most commonly used configurations for connection are as follows, in the order of increasing power demand and service reliability //

1. LV or MV single-line service
2. MV ring-main service
3. MV duplicate supply service
4. MV duplicate supply service with double busbar

Metering, protection and disconnection devices, located in the delivery substations are not represented on the following diagrams. They are often specific to each Utility company and do not have an influence on the choice of installation architecture. For each connection, one single transformer is shown for simplification purposes, but in practice, several transformers can be connected.

For the different possible configurations, the most probable and usual set of characteristics is given in the following table:

The connection configurations may differ from the above table due to particular constraints relative to water industry plants – spread-out configuration and suburban location.

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MV circuit configuration

The three main possible connection configurations are as follows:

1. Single feeder, one or several transformers,
2. Open ring, one MV substation,
3. Open ring, 2 MV substations.

The basic configuration is a radial single-feeder architecture, with one single transformer. In the case of using several transformers, an open ring is commonly realized for improved power availability.

High power motors may be directly supplied at MV for reduction of cable size. The preferred basic configuration comprises one single substation. However, a larger number of substations is possible in some circumstances:

• A large site,
• A site with several different process lines,
• A total power higher than 5 MVA,
• A need for redundancy, to cope with a natural disaster for example.

The preferred configuration of MV/LV transformers comprises a single transformer supplying the total power of the installed loads.

For the different possible configurations, the most probable and usual set of characteristics is given in the following table:

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Configuration of LV circuits

Here are the main possible configurations, to be selected according to the requested level of power availability //

Radial single feeder configuration

This is the reference configuration and the most simple. A load is connected to only one single source. This configuration provides a minimum level of availability, since there is no redundancy in case of power source failure.

Two-pole configuration

The power supply is provided by 2 transformers, connected to the same MV line. When the transformers are close, they are generally connected in parallel to the same Main Low Voltage Switchboard (MLVS).

Variant: Two-pole with two ½ MLVS and NO link

In order to increase the availability in case of failure of the busbars or authorize maintenance on one of the transformers, it is possible to split the MLVS into 2 parts, with a normally open link (NO). This configuration generally requires an Automatic Transfer Switch (ATS).

Sheddable switchboard (simple disconnectable attachment)

Sheddable circuits can be connected to a dedicated switchboard. The connection to the MLVS is interrupted when needed (overload, generator operation, etc…)

Interconnected switchboards

If transformers are physically not too far from one another, they may be connected by a busbar trunking. A critical load can be supplied by one or other of the transformers.

The redundancy can be //

• Total: each transformer being capable of supplying the whole installation,
• Partial: each transformer only being able to supply part of the installation. In this case, part of the load must be disconnected (load- shedding) in the case of one of the transformers failing.

Double-ended power supply

This configuration is implemented in cases where maximum availability is required. The principle involves having 2 independent power sources, e.g.:

• 2 transformers supplied by different MV lines
• 1 transformer and 1 generator
• 1 transformer and 1 UPS

An automatic transfer switch (ATS) is used to avoid the sources being parallel connected. This configuration allows preventive and curative maintenance to be carried out on all of the electrical distribution system upstream without interrupting the power supply.

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Back-up generators

The electrical power supplied by a back-up generator is produced by an alternator, driven by a thermal engine. No power can be produced until the generator has reached its rated speed. This type of device is therefore not suitable for an uninterrupted power supply.

The generator can function permanently or intermittently. Its back-up time depends on the quantity of available fuel. According to the generator’s capacity to supply power to all or only part of the installation, there is either total or partial redundancy.

The implementation of a back-up generator should take consideration of the sensitivity of circuits to power interruptions and the availability of the public distribution network.

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Presence of Uninterruptible Power Supply (UPS)

The electrical power from a UPS is supplied from batteries. This system eliminates any power failure. The back-up time of the system is limited: from several minutes to several hours. The simultaneous presence of a back-up generator and a UPS unit is used for permanently supply loads for which no failure is acceptable.

The back-up time of the batteries must be compatible with the maximum time for the generator to start up and be brought on-line.

A UPS unit is also used for supplying power to loads that are sensitive to disturbances (generating a “clean” voltage that is independent of the network).

Main characteristics to be considered for implementing a UPS //

• Sensitivity of loads to power interruptions
• Sensitivity of loads to disturbances.

The presence of a UPS unit is essential if and only if no power failure is acceptable. Generally, process lines in Water applications are not supplied by UPS because of the large power involved and the possibility to accept short interruptions. UPS are necessary only for process control supply.

Example of configuration for uninterruptible supply of critical circuit:

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Guide to Wastewater treatment plants

Recommended electrical network design for efficient plant and energy operations
// by SCHNEIDER ELECTRIC

Download handbook [8MB]

References //

• Wastewater treatment plants – Recommended electrical network design for efficient plant and energy operations by Schneider Electric
• Water and Wastewater Power Solution Handbook by Schneider Electric