Nowadays, the reliability and resilience of equipment in emergency situations is gaining more and more attention. This article focuses on construction of power supply systems that ensure trouble-free performance of electric equipment and outlines the modern methods of design and construction of automatic transfer switches developed by the ENELT GROUP team.

 

Viacheslav MARASHKIN, Head of LVES Department, ENELT Group Co. Ltd.

 

A modern world without electricity is no longer imaginable. The proper functioning of plants, hospitals, educational institutions, agricultural and housing facilities depends on reliable power supply. Today, power supply became one of the key elements of human civilization.

Many people here are no strangers to having power outages in their homes. Such power interruptions in medical setting or production facilities utilizing complex processes are likely to cost human lives and damage the expensive equipment. Therefore, to increase the reliability of power supply, automatic transfer switches are used, that automatically switch a consumer from its primary source to a backup source when it senses a failure or outage in the primary source. In most cases, ATS (automatic transfer switches) provide a means of transferring essential load connections between two independent supply lines or between the main power line and a diesel generator set, as the local backup power source. Some ATS arrangements utilize three and four power sources, to provide electricity supply to critical facilities.

Depending on the security of supply provided, current-using equipments are divided into three categories (I, II, and III), as established by Requirements for Electrical Installations. An outage affecting category I might cause loss of life, extensive damage to the national economy, failure of expensive capital equipment, a large number of rejected products, disruption of the complex technological process, and malfunction of essential elements of public utilities. Moreover, the first category also includes a special group of current-using equipments. Uninterrupted operation of the latter ensures safe stoppage of production so as to prevent danger to life, explosions, fire accidents and failure of capital equipment. Group II comprises another class of current-using equipments. A power cut affecting category II might cause a massive undersupply of products, employee downtime, machines and vehicle idle time, disruption to normal activities of many urban and rural residents. Group III includes all other current-using equipments that don't fit into categories I and II (Requirements for Electrical Installations 1.2.17).

The highest demands are placed on Category I current-using equipments. They should draw power from two independent mutually redundant power sources, and the supply of power to these equipments can only be interrupted while automatic power recovery is under way. Additional power supply from a third independent mutually redundant source should be provided for a special group of category I current-using equipments.

AUTOMATIC TRANSFER SWITCHES

Automatic transfer switches used in 0.4 kV networks are usually based on the following switching devices:

• Contactors: the most simple and widespread system. As a rule, circuits with two inputs and one output or two inputs and two outputs are based on contactors. In the former case, a circuit can be implemented in which primary input would have priority, or a circuit with no priorities;
• Motorized change-over switches. Similar to contactors, you can implement a circuit with two inputs and one output. Or you can use two change-over switches and implement a circuit having two inputs and two outputs, widely applied in lead-in distributors;
• Motorized automatic circuit breakers. You can implement a standard ATS circuit or an ATS circuit having an algorithm of any complexity, for instance, a circuit with two inputs and one output, two inputs and two outputs, as well as a combination of several inputs, including inputs from diesel-generator units, and several outputs.

Secondary control circuits of automatic transfer switches can be based on relays or on programmable logic controllers. Advantages offered by controllers lie in significantly reduced number of circuits in the layout and thus, in reduced number of make/break contacts (the latter detract from reliability of automatic transfer switches). In a complex algorithm with many secondary control circuits, a controller will help free up space in the cabinet, because it substitutes for up to several dozen different relays. In a complex algorithm, the use of controller is much less costly than the combination of auxiliary and time relays, as well as other elements. When it comes to labor intensity, installing the automatic transfer switches on controller takes you a lot less time than installing it on the relay. And if you have to modify the algorithm of the automatic transfer switch, add time delays or additional blocking, all this can be done by reprogramming the controller, without doing more installations or dismantling secondary control circuits of automatic transfer switch. However, using control systems based on a microcontroller... isn't always the right thing to do. For instance, when it comes to contactor based ATS design with two inputs and one output, the use of controller would be not economically feasible, because of the simplicity of relay based circuit. Despite all the advantages of microprocessor-based controller and its popularity with the manufacturers of low voltage electrical switchboard, the electrical installation maintenance staff of electric plants often prefers relay and contactor based ATS designs, since they are more easy to-see and straightforward.

Automatic transfer switches panels do not always run in automatic mode, the staff sometimes has to operate outputs manually during commissioning or other works. Manual operations can be performed both at the panel location, and remotely. Thus, some automatic transfer switches systems may have three control modes — automatic, local, and remote. Moreover, it may be wise to use automatic switches or reverse switches with motor drives, which, unlike contactors, can remain activated without external power supply.

In designing detached automatic transfer switch cabinets, attention should be given to the outgoing lines protection. Load switches are often installed at the input to the automatic transfer switches cabinet, and protection devices are located in an upstream lead-in distributor or main distribution board, while operating personnel does not always have access to them. If a short circuit arises on the outgoing line, the primary input is switched off first. The voltage-check relay will sense that the input is missing, and send a signal to activate the spare input, and then the spare line protection device will be triggered. You can solve this issue as follows. If the circuit has two inputs and one output, then you can install a common protection device at the output; if the circuit has two inputs and two outputs, then replace the load switches with automatic ones paying attention to selectivity with upstream devices, or connect the voltage-check relay to the protective device in the upstream panel, so that in an emergency shutdown, the voltage check relay could detect the voltage and does not give signals to activate spare input.

It is a good practice to include auxiliary contacts of protective devices into the control system of automatic transfer switches, to block the activation of spare supply line in the event a short circuit arises on the outgoing line.

To conclude, the design of automatic transfer switches that may influence operational performance of functional facilities in emergency situations is a serious challenge. These devices are used in uninterruptible power supply systems in hospitals, clinics, etc. healthcare centers, and help to save human lives in critical situations.

 

This article was published in «ELECTRIC POWER.
Transmission and Distribution», issue no.1 (4) January-February 2011