This document is intended to be read completely through. To assist in navigating, or if you don't want to read it all, it has been broken into a series of numbered paragraphs. If you return from a paragraph to the "top", you will come here, and can then either select another paragraph, or leave.

Go to paragraph 1

Go to paragraph 2

Go to paragraph 3

SCADA and the Water industry

Zone Concept of Operation

Definition of Zones

The zone concept mirrors the hydraulic design of a water supply in that it allows the identification of self contained hydraulic units, which operate autonomously. This ability to operate autonomously is fundamental to designing SCADA systems for remote water supply systems, which cannot be dependant on lengthy and often unreliable communications links. The zone concept is really a methodology to describe the physical system in terms that make the system easy to automate.

The identification of zones also allows a complex system to be automated by breaking it down into a series of simple systems. A zone consists of

a source of water (optional as the source is often a tank that was the destination of a previous zone.

a destination (tank/reservoir)

a single object of control such as tank or reservoir volume at the end of the zone , or pressure.

a number of controllable assets eg pumps, bores, valves

a number of uncontrolled (within this zone) offtakes.

a "Switch" (optional - see the definition of sections)

Constraints (such as minimum pressure levels to be maintained.

a number of parameters to be monitored (such as pressure).

It must also be possible to define (in a Sequencing Table - ST) a sequence of modes of operation (eg one pump, two pumps, valve open, three pumps) which successively increase the flow through the hydraulic zone.

Example

Example with two zones, no switches, no offtakes. Sequencing in Zone 1 is the order the bores are to turn on. Sequencing in Zone 2 is simple - Pump off, Pump on (one pump is a standby).

The above example is straightforward, as it is quite obvious that when the collector tank is empty, you must start some more bores. When the high level tank is empty, you start the transfer pump station.

To see a simulated water supply system being controlled by SCADA, click here (preferably if you have a 800x600 screen and a relatively recent version of IE4 or Netscape (4.07 say)).

A zone can comprise of a number of sections. Sections are only really required when the mode of operation can change, and the system must dynamically reconfigure. For example a zone may pump to a break pressure tank, and from there to the storage at the end of the zone. The zone comprises two sections. If the break pressure tank fills, the valve closes and the tank becomes isolated from the pipeline. The zone is now only one section. The SCADA system monitors the status of the valve (This is the "Switch" referred to earlier) and automatically reconfigures from a two section zone to a single (combined) section zone without the tank. (For convenience when a zone has one section, then the terms zone and section are used interchangeably.

The next example shows a zone which has an intermediate tank. It starts pumping to the intermediatery tank, but when the tank fills, it becomes isolated from the main. The hydraulics of the situation change.

In this example the entire system acts as one zone when the intermediate tank is full and the altitude valve closed. The zone is shown in red. An example sequencing could be "Start bores 1, 2, 3 then start the transfer pump, and then bores 4 and 5. (Presumably 5 bores at once would be too much or inefficient).

When the altitude valve opens, the zone becomes split into two "sections". The bores pump to the intermediate tank. The transfer pump takes water from this tank and pumps to the end tank. The position of the altitude valve acts as a "switch", switching the zone from one scenario to the other. The sequencing reverts back to the earlier example.

Click here to go back to the top
Time Varying Operating Constraints

The Sequencing Table defines the physical constraints in a Zone. It is used for Start-up, Shutdown, and for Stepup/Stepdown from one tariff condition to another. To completely automate a zone, a constraints matrix (CM) is used. The Constraints Matrix allows other (usually economic) constraints to be imposed on the operation. These constraints (such as maximum mode in the sequence table, target tank levels etc) should be specified on a frequent (2 hourly?) basis for each controllable asset (eg pump, or valve). This allows operators to implement various operating strategies. eg pump off peak to save power costs.

Click here to go back to the top

Complete systems can be built up from these sections and zones. The following is a more complex example buillt up from these simple modules.

This approach uses the zone concept to allow SCADA systems to be designed in which the local RTU systems in each zone run autonomously. The only interconnection between zones is hydraulic . (If a tank fills, then the zone will shut down, thereby reducing its intake from the previous zone. This will cause that zone to fill, and begin to shut down. And so on back through the system.) These local RTU systems continue to operate even if communications to the central SCADA system are lost. They will use the last constraints matrix downloaded, allowing them to continue operation when communications fail, even if it is not optimal.

Having broken down the water supply scheme into small modules, it is then a matter of putting RTU's at the major assets with the RTU's at each tank controlling its zone, designing the appropriate communications network, and installing a SCADA master system in the Operations Centre. The system will run automatically, and is able to be optimised.