Protection System of Grid

Substation Transformer

Assignment – 1

Power System Protection and Communication

Prepared By:

Habibullah Nahid Roll: 1015160004

Military Institute of Science and Technology

Date: 29 July, 2015

Electric Substation:

A station in the power transmission system at which electric power is transformed to a

conveniently used form. Its main function is to receive energy transmitted at high voltage from the

generating station, by either step-up or step-down the voltage to a value appropriate for local use

and provide facilities for switching. Substations have some additional functions. Its provide points

where safety devices may be installed to disconnect circuits or equipment in the event of trouble.

Major Tasks of a Substation

1. Protection of transmission system.

2. Controlling the Exchange of Energy.

3. Ensure steady State & Transient stability.

4. Load shedding and prevention of loss of synchronism. Maintaining the system frequency

within targeted limits.

5. Voltage Control; reducing the reactive power flow by compensation of reactive power, tap-

changing.

6. Securing the supply by proving adequate line capacity.

7. Data transmission via power line carrier for the purpose of network monitoring; control

and protection.

8. Fault analysis and pin-pointing the cause and subsequent improvement in that area of field.

9. Determining the energy transfer through transmission lines.

10. Reliable supply by feeding the network at various points.

11. Establishment of economic load distribution and several associated functions.

Based on configuration substation are usually classified in two ways.

Air-insulated switchgear (AIS) used to be the most common design, but this requires a lot

of space and for higher voltages is only feasible outdoors. Even then, AIS may be

unsuitable or undesirable in certain locations, such as residential areas.

Gas-insulated switchgear (GIS) uses a superior dielectric gas, SF6, at moderate pressure

for phase-to-phase and phase-to-ground insulation. The high voltage conductors, circuit

breaker interrupters, switches, current transformers, and voltage transformers are in SF6

gas inside grounded metal enclosures. Gas-insulated switchgear (GIS) may be more

expensive if only the unit cost is compared, but is safer and needs less maintenance. The

fact that GIS units are five times smaller than AIS means cost savings and smaller, less

intrusive buildings

Components in a Substation:

Power transformers, switching devices such as circuit breakers and disconnectors to cut power in

case of a problem, and measurement, protection and control devices needed to ensure its safe and

efficient operation.

Electrical Grids:

An electric grid is a network of synchronized power providers and consumers that are connected

by transmission and distribution lines and operated by one or more control centers. When most

people talk about the power "grid," they're referring to the transmission system for electricity.

The electricity generation, transmission, distribution and control networks make up the electrical

grid. The simplest grids link a local generator to homes, but grids can cover whole continents too.

Smaller grids have a radial structure with supply lines branching out from a large centralized

electricity supplier. This is relatively simple to operate, but if a line goes down, users are cut off.

Smaller grids have a radial structure with supply lines branching out from a large

centralized electricity supplier. This is relatively simple to operate, but if a line goes down, users

are cut off.

To ensure reliable supply, most grids use a mesh structure. In this configuration, the power lines

of any given electricity supply source are interconnected with those of other sources. If one line

has a problem, power can be rerouted from elsewhere while the damaged line is repaired.

To plan, operate and manage large interlinked systems, we need control centers where operators

monitor grid status. They will adjust to electrical demand variations in real time, using

sophisticated network management systems.

Increasingly, intelligence is being built into electric grids. Smart grid initiatives seek to improve

operations, maintenance and planning by automating operations and ensuring that components of

the grid can communicate with each other as required.

The power goes from the transformer to the distribution bus. The bus distributes power to local

distribution lines. The bus has its own transformers that can also step down or step up voltage

according to local energy needs. At the bus, there may be two separate sets of distribution lines at

two different voltages. Smaller transformers attached to the bus step the power down to standard

line voltage (usually 7,200 volts) for one set of lines, while power leaves in the other direction at

the higher voltage of the main transformer.

Substations may be indoor or outdoor, with a selection of high-voltage incoming sections, a choice

of transformer types, and an arrangement of Switchgear to suit the application. Most switchgear

assemblies are configured as unit substations. Unit substations follow the system concept of

locating transformers as close as practicable to areas of load concentration at utilization voltages,

thus minimizing the lengths of secondary distribution cables and buses.

TRANSFORMER PROTECTION AND TRANSFORMER FAULT

Under Electrical Protection

Different transformers demand different schemes of transformer protection depending upon their

importance, winding connections, earthing methods and mode of operation etc. It is common

practice to provide Buchholz relay protection to all 0.5 MVA and above transformers. While for

all small size distribution transformers, only high voltage fuses are used as main protective device.

For all larger rated and important distribution transformers, over current protection along with

restricted earth fault protection is applied. Differential protection should be provided in the

transformers rated above 5 MVA. Depending upon the normal service condition, nature of

transformer faults, degree of sustained over load, scheme of tap changing, and many other factors,

the suitable transformer protection schemes are chosen.

All kinds of faults and protection in transformer

Transformer is a static device plus it is protected by the main circuit breaker thus there is almost

no possibility of external fault to it other than internal faults like open circuit fault, over heating

fault and winding short circuit fault.

Open circuit fault occurs when one phase of transformer become open which is relatively harmless

just because temperature rise that can be detected by temperature alarm and disconnect the

transformer. Short circuit fault is relatively dangerous and need extra caution.

The complete protection schemes of transformer is not a single protection system unlike alternator

which can be protected by only Mertz Price differential circulating current protection system.

A combination of protection system is needed for the complete protection of transformer. The

deciding factors of requiring protection system are (1)Size of transformer,(2) type of cooling

system, (3)transformer location in the network (4)Load type & nature, (5) Importance of

transformer.

Nature of Transformer Faults:

Although an electrical power transformer is a static device, but internal stresses arising from

abnormal system conditions, must be taken into consideration.

A transformer generally suffers from following types of transformer fault-

1. Over current due to overloads and external short circuits,

2. Terminal faults,

3. Winding faults,

4. Incipient faults.

All the above mentioned transformer faults cause mechanical and thermal stresses inside the

transformer winding and its connecting terminals. Thermal stresses lead to overheating which

ultimately affect the insulation system of transformer. Deterioration of insulation leads to

winding faults. Sometime failure of transformer cooling system, leads to overheating of

transformer. So the transformer protection schemes are very much required.

The short circuit current of an electrical transformer is normally limited by its reactance and for

low reactance, the value of short circuit current may be excessively high. The duration of

external short circuits which a transformer can sustain without damage as given in BSS

171:1936.

Transformer % reactance Permitted fault duration in seconds

4 % 2

5 % 3

6 % 4

7 % and over 5

The general winding faults in transformer are either earth faults or inter-turns faults. Phase to phase

winding faults in a transformer is rare. The phase faults in an electrical transformer may be

occurred due to bushing flash over and faults in tap changer equipment. Whatever may be the

faults, the transformer must be isolated instantly during fault otherwise major breakdown may

occur in the electrical power system.

Incipient faults are internal faults which constitute no immediate hazard. But it these faults are

over looked and not taken care of, these may lead to major faults. The faults in this group are

mainly inter-lamination short circuit due to insulation failure between core lamination, lowering

the oil level due to oil leakage, blockage of oil flow paths. All these faults lead to overheating. So

transformer protection scheme is required for incipient transformer faults also. The earth fault,

very nearer to neutral point of transformer star winding may also be considered as an incipient

fault.

Influence of winding connections and earthing on earth fault current magnitude.

There are mainly two conditions for earth fault current to flow during winding to earth faults:

I. A current exists for the current to flow into and out of the winding.

II. Ampere-turns balance is maintained between the windings.

The value of winding earth fault current depends upon position of the fault on the winding, method

of winding connection and method of earthing. The star point of the windings may be earthed

either solidly or via a resistor. On delta side of the transformer the system is earthed through an

earthing transformer. Grounding or earthing transformer provides low impedance path to the zero

sequence current and high impedance to the positive and negative sequence currents.

Star Winding with Neutral Resistance Earthed

In this case the neutral point of the transformer is earthed via a resistor and the value of impedance

of it, is much higher than that of winding impedance of the transformer. That means the value of

transformer winding impedance is negligible compared to impedance of earthing resistor. The

value of earth current is, therefore, proportional to the position of the fault in the winding. As the

fault current in the primary winding of the transformer is proportional to the ratio of the short

circuited secondary turns to the total turns on the primary winding, the primary fault current will

be proportional to the square of the percentage of winding short circuited. The variation of fault

current both in the primary and secondary winding is shown below.

Star Winding with Neutral Solidly Earthed

In this case the earth fault current magnitude is limited solely by the winding impedance and the

fault is no longer proportional to the position of the fault. The reason for this non linearity is

unbalanced flux linkage.

Protection of Transformer in Grid Substation:

BUCHHOLZ RELAY

Double element relays can be used in detecting minor or major faults in a transformer. The alarm

element will operate, after a specified volume of gas has collected to give an alarm indication.

Examples of incipient faults are:

(a) Broken-down core bolt insulation

(b) Shorted laminations

(c) Bad contacts

(d) Overheating of part of windings

The alarm element will also operate in the event of oil leakage, or if air gets into the oil system.

The trip element will be operated by an oil surge in the event of more serious faults such as

(a) Earth faults

(b) Winding short circuits

(c) Puncture of bushings

(d) Short circuit between phases

Function of Buchholz relay: In the following the operation of a Buchholz relay is explained using

the example of a double-float Buchholz relay. The relay is built in the connecting pipe between

the transformer tank and the conservator. During normal operation it is filled completely with

insulating liquid.

Due to buoyancy the floats are at their top position. If a fault occurs inside the transformer, the

Buchholz relay responds as follows:

Fault Type: Free gas is available in the insulating liquid.

Response of the Relay: The gas in the liquid moves upwards,

accumulates in the Buchholz relay and displaces the insulating

liquid level. The moving float actuates a switch contact (magnet

contact tube). An alarm signal is tripped. The lower float is not

affected as from a certain gas volume the gas flows through a

piping to the conservator.

Fault Type: Insulating liquid loss due to leakage.

Response of the Relay: As the liquid level falls the top float moves

downward. An alarm is tripped. If the liquid loss continues,

conservator and piping as well as the Buchholz relay will be emptied.

As the liquid level falls, the lower float moves downward. The

moving float actuates a switch contact so that the transformer is

disconnected.

Fault Type: A spontaneous incident generates a pressure wave

moving in the direction of the conservator.

Response of the Relay: The liquid flow reaches a damper arranged

in the liquid flow. If the flow rate exceeds the operating threshold

of the damper, the latter moves in flow direction. Due to this

movement a switch contact is actuated so that the transformer is

disconnected.

Advantages of using Buchholz Relay:

1. Buchholz relay indicates inter turn faults and faults due to heating of core and helps in the

avoidance of severe faults.

2. Nature and severity of fault can be determined without dismantling the transformer by

testing the air samples.

Limitations:

It can sense the faults occurring below the oil level only. The relay is slow and has a minimum

operating range of 0.1second and an average operating range of 0.2 seconds.