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Tuesday, 7 October 2014

Design parameters and Standard layout of a 33/11 kV substation

An electrical substation:

An electrical substation is a combination of a number of major electrical equipments like power transformers, circuit breakers, instruments transformers, bus bars, lightning arresters, control panels, interconnecting cables, capacitor banks, battery set, and other related equipments designed as per the requirements, size, capacity and class of the substation. 
Control room is provided to house the various control panels and battery backup and to station the employ/operator.    
Power transmission and distribution in densely populated urban areas is a great challenge because of land availability and low noise and electromagnetic limitations. With the development in technology and enough improvement in substation design, Gas Insulated Substations are finding increasing use because of their safety, reliability, less space requirement and enhanced life.

Factors affecting the Layout and Design of a substation:

The physical layout and the design of the substation are governed by:

1.      The number of incoming and outgoing feeders (existing and proposed)
2.      Existing and expected load demand on the substation,
3.      Soil resistivity,
4.      Expected fault levels at 33 kV and 11 kV level ,
5.      Climatic conditions, such as temperature, altitude, rainfall etc.

Governing Rules & Regulations:

All electrical and civil works related to the installation and commissioning of an electric substation shall be carried out in accordance to the provisions given in Indian Electricity Act 2003 and the Indian Electricity Rule 1956 amended upto date. 
The standard instruction for construction of 33/11 kV substation is given in REC standards which specifies the layout along with the switchgear. The standard layout guides for the general arrangement of the equipments, structures, bus-bars etc. of outdoor substations. The general specifications of a substation cover the details of the major equipments and other items to be supplied. 
The tender for a particular substation clearly indicate these specifications. The tender specification also has the reference drawings, which indicate the schematic of major connections and form the part of the specification. Suitable provisions can be made in the switchyard depending upon the local condition and locations so as to facilitate transportation of heavy equipments, especially power transformers. While carrying out the substation construction all the labour regulations, safety codes and measures must be followed.

Number of Transformers and Feeders in a substation:

The REC standard mentions that an outdoor type 33/11 kV substation should have 2 power transformers and 3 outgoing feeders. There must be provision for 2 additional 11 kV outgoing feeder for load enhancements in future. The number of 33 kV incoming feeder may be one or two depending upon the requirement. The various arrangements for the 33 kV incoming and outgoing lines may be according to the standard practice adopted by the electricity board. 

Fig.1: The power transformer, circuit breakers, gantry of an under construction 33/11 kV substation in rural area. 

Control Panels: 

All the control and equipments needed to monitor and remote control the vital equipments such as transformers, circuit breakers, feeders etc. are housed in control panels. In these control panels, relays, meters, and instruments are mounted as per the drawing arrangements. Separately earthed and effectively segregated compartments/ panels (earthed panels) should be provided for circuit breaker, bus-bars, relays, CTs ,PTs etc so that faults in any one panel do not cause damage to other equipments or panels.

Relays:

Relays are the sensing device which sense the abnormal condition in any power system and accordingly give alarm and trip signal to circuit breaker. The different relays used normally in a 33/11 kV sub-station are earth fault relay, and over-current relay. Earlier electro-mechanical relays were used which are now being replaced. 


Fig.2: Control Panel with old Electro-mechanical relays.
  
The relays used in modern substation are of microprocessor based numerical relays with overload, earth fault and short circuit protection. 24/30 V DC supply with battery pack and battery charger is used for the protection as well as indication circuits. 

Current Transformers (CTs) and Potential Transformers (PTs):

Current Transformers (CTs) and Potential Transformers (PTs) used for metering and protection purpose. They should be of the burden more than the minimum burden mentioned and of accuracy class 0.5.

Earthing of a Sub-station:


The earthing of a substation is of prime importance for the satisfactory operation of protective devices and for the safety of equipments and personnel. All equipments must have separate earthing and in the numbers given in the manual. These earths should be well connected to each other and should form a grid. The resistance of the earthing should be maintained at the required minimum value.

There should be satisfactory drainage facility in the yard so that accumulation of rain water should not take place. Yard should be covered with granite metal to prevent the growth of grass and weeds, formation of mud etc. the substation may also have capacitor banks of appropriate capacity to improve the power factor and to control the harmonics.

Inspection is necessary before dispatch, delivery at site, installation, commissioning, putting into operation and handing over the equipments.