Why DC is used in substation ?

Concept of using DC supply in substation or power stations is to provide a continuous power supply to the control circuit. DC is a reliable source of power supply because it can obtain from batteries. ... So during power failure also, we have to provide the continuous power supply to the control circuit.

If we use AC supply which can not be obtained from the battery then we cannot provide the power supply to the control circuit during the power failure. If a power failure occurs, all devices will stop working then we cannot check the fault status, status of the circuit breaker and relay.

So during power failure also, we have to provide the continuous power supply to the control circuit. As we can provide continuous power supply only using battery and battery can provide only DC supply, so all control circuit devices are designed for DC Supply.

In a substation or power station, DC supply is used for Opening and Closing of Circuit breakers, to operate protective relays, to operate isolators, indication lamps, alarm circuits, PLCC(Power Line Carrier Communication) Panels, Emergency Lights in Control Rooms, etc.

Using of DC supply instead of AC Supply in Substation also gives some extra facilities such as,

1. When protective devices are operating with DC supply they take very less reaction time than when they are operating with AC Supply.

2. Static Devices or which need DC supply can be directly connected with batteries. But when AC supply is used, the rectifier circuit is needed.

3. Control Circuits are consist of many digital devices also such as Digital Displays, Sensors, LEDs, etc. These devices always need DC supply. So when using DC supply in the control circuit fewer circuits are needed.

What are the two types of incoming substation ?

Step-up Transmission Substation.

Step-down Transmission Substation.

Distribution Substation.

Underground Distribution Substation.

Substation Functions.

Substation Equipment.

What is electrical substation ?

 An electrical substation is a subsidiary station of an electricity generation, transmission, and distribution system where voltage is transformed from high to low or the reverse using transformers.

Substations consist of high voltage electrical equipment like transformers, switchgear, circuit breakers, and associated devices. Transformers step down the high voltage electricity coming in on the transmission lines, to a much lower voltage suitable to send out on distribution wires.

It is used to switch generators, equipment, and circuits or lines in and out of a system. It also is used to change AC voltages from one level to another, and/or change alternating current to direct current or direct current to alternating current. Some substations are small with little more than a transformer and associated switches. Others are very large with several transformers and dozens of switches and other equipment. There are three aspects to substations

Types of Substation

What is current Transformer ?

 A current transformer is a device that is used for the transformation of current from a higher value into a proportionate current to a lower value. It transforms the high voltage current into the low voltage current due to which the heavy current flows through the transmission lines are safely monitored by the ammeter.

What is BDV Test for Transformer ?

BDV test means the Breakdown Voltage Test. It is done for checking the dielectric strength of the oil of the Transformer. Dielectric strength means the maximum capacity to withstand the voltage of insulating oil. ... For the purpose of BDV test, oil sample from the Transformer is taken in a Sample Bottle

How relays are connected in the power system ?

The relays are connected to the power system through the current transformer (CT) or potential transformer (PT).

electrical power system protection is divided ?

 Generator protection, 

Transformer protection, 

Bus bar protection, 

Transmission line protection

 Feeder protection

What are the different relays that employed for protection of apparatus and transmission lines ?

  The relays that are usually employed for the protection of transmission lines include:  

Over-current relay, 

Directional relay, 

Distance relay, 

Under Voltage relay, 

Under frequency relay 

 Thermal relay, 

Differential relay, 

Phase sequence relays, 

pilot relays

What is protective relay ?

  It is an electrical device designed to initiate the isolation of a part of the electrical installation or to operate an alarm signal, in the event of abnormal condition or a fault. In simple words relay is an electrical device that gives the signal to isolation device (eg: Circuit Breaker) after sensing the fault and helps to isolate the fault system from the healthy electrical system

What is a feedback in Control System ?

 The Feedback in Control System in one in which the output is sampled and the proportional signal is fed back to the input for automatic correction of the error (any change in desired output) for further processing to get back the desired output.

What are different types of Control Systems ?

 Two major types of Control Systems are 

1) Open-loop Control System 

2) Closed-Loop Control Systems

 Open-loop Control Systems: The Open-loop Control System is one in which the Output Quantity has no effect on the Input Quantity. No feedback is present from the output quantity to the input quantity for correction. 

Closed-Loop Control System: The Closed-loop Control System is one in which the feedback is provided from the Output quantity to the input quantity for the correction so as to maintain the desired output of the system.

What is Control System ?

 In a System the output and inputs are interrelated in such a manner that the output quantity or variable is controlled by input quantity, then such a system is called Control System. The output quantity is called a controlled variable or response and the input quantity is called command signal or excitation.

What is a System ?

 When a number of elements or components are connected in a sequence to perform a specific function, the group of elements that all constitute a System

Why temperature rise is conducted in bus bars and isolators ?

 Bus bars and isolators are rated for continuous power flow that means they carry heavy currents which rises their temperature. So it is necessary to test these devices for temperature rise.

What is the full form of KVAR ?

 there are three types of power in Electricals as Active, apparent & reactive. So KVAR is stand for `Kilo Volt Amps with Reactive component.

What is BOM?

 BOM stands for the Bill of Materials; it is a list of items or parts that make up a product assembly. For example, a lawnmower requires a handle assembly, metal deck assembly, a control assembly, motor and blade assembly.

What is electric traction ?

Traction means using the electric power for traction system i.e. for railways, trams, trolleys, etc. electric traction means the use of the electricity for all these. Nowadays, magnetic traction is also used for bullet trains. Basically, dc motors are used for electric traction systems.

What is ACSR cable and where we use it ?

 ACSR means Aluminum conductor steel reinforced, this conductor is used in transmission & distribution.

What is vacuum circuit breaker ? Define with cause and where we use it.

  A breaker is normally used to break a circuit. While breaking the circuit, the contact terminals will be separated. At the time of separation, an air gap is formed between the terminals. Due to the existing current flow, the air in the gap is ionized and results in the arc. Various mediums are used to quench this arc in respective CB’s. But in VCB the medium is vacuum gas. Since the air in the CB is having vacuum pressure the arc formation is interrupted. VCB’s can be used up to 11kv.

Which motor has high Starting Torque and Staring current DC motor, Induction motor or Synchronous motor ?

 DC Series motor has high starting torque. We cannot start the Induction motor and Synchronous motors on load, but cannot start the DC series motor without load.