Grid Island

8/11/2019 Grid Island

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Power Research & Development Consultants Pvt. Ltd.1

Application of Electrical Transient

Studies to Enhance the Success Rateof Grid Islanding

R. Nagaraja

Power Research & Development Consultants Pvt. Ltd.

No. 4, Ist B Main, Ist N Block,

Rajajinagar

Bangalore560012


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Major mind boggling problem in the industrial system -

How quickly the faults can be isolated without affecting the

production.

Grid synchronization is essential for process industries with CPP

What is the success rate of getting islanded from the grid for

major disturbances in the grid

Addresses the issues in grid islanding

How to incorporate state of the art numerical relays to enhance

the success rate of grid islanding.

Introduction


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Liberalization and de-regulation in the power sector

more and more industries setting up the captive power plants.

Why grid connection?

Operation becomes impossible without the grid support for

varying and bulk loads

One more source of power supply for system emergencies

Third party power transfer

Introduction


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Application of state of the art numerical relays in the

industrial system - Grid islanding has been more simplified

Setting calculations of the relays are very important to have

maximum success rate of islanding.

The success rate of grid islanding -

Following the grid disturbance the islanding should occur only if

the grid could not survive.

After the islanding all the generators within the plant should

survive

There should not be any un-called tripping in the plant

Introduction


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The presentation addresses the following issues

System behaviour during different types of disturbances

The application of numerical relays to track the system

disturbance

Arriving at proper relay settings based on simulation studies

Post islanding measures to minimize the production loss

Maximization of the pyramid like functions of the numerical

relay i.e. protection, monitoring and control.

Introduction


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Grid Islanding

Grid islanding scheme

Single or a set of protective relays

Connected at the islanding bus

Will sense the disturbance in the grid

Trip command is issued to the islanding breaker whenever the

grid disturbance exceeds a set limit.

By opening the islanding breaker, the CPP and CPP side loads

are isolated from the grid.


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Grid Islanding

The load on the grid side will survive if the grid survives

If the grid is not surviving following the disturbance, only part

of the plant load on the grid side gets affected.

The process of disconnecting the CPP from the utility system iscalled islanding of the CPP.

Grid islanding scheme helps both the utility as well as the CPP.


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Grid IslandingBenefits to Utility

The utility, which has the responsibility to maintain the grid,normally do not want an additional independent power source

(CPP) in their grid network, when the grid is disturbed.

Since the power source is not in their control, it will complicate

utilitys methods of dealing with the disturbed grid.

While the utility is in the process of solving the grid

disturbance, they do not like to have another power source

which they do not control and which may add to thedisturbance.


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Grid IslandingBenefits to Utility

Once the CPP is disconnected from the grid, it becomes easier

for the utility to locate the source of disturbance and rectify the

same.

The main idea is that the utility eliminates the possibility of theCPP feeding the disturbance.

Hence the utility will insist on installing the Grid islanding

scheme in the CPPs premises, before allowing the CPP to beconnected to grid.


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Grid IslandingBenefits to CPP

It is strongly advisable to disconnect the CPP from the grid, when the grid is

disturbed.

The main reason is that the DG or the TG of CPP may get damaged due to

grid disturbances resulting in heavy repair costs and shut downs.

It is necessary to protect the CPP generators from grid disturbances.

It is also better to disconnect the CPP units from utility whenever the utility

fails, by opening the islanding breaker.

If not, the CPP which is running will experience a severe jolt when theutility power comes back. This will cause extreme damage to CPP

generators.


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What are grid disturbances?

A grid is said to be disturbed when

there is an under voltage (U/V)there is over voltage (O/V)

there is under frequency (U/F)

there is over frequency (O/F)

there is a rapid fall or rise in frequency

( +dF/dT ordF/dT )there is a power failure in the grid

there is a fault in the grid

there is vectorial shift in the voltage


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-1.5

0Volts

T

Voltage Vector Jump:

Advantages

Can operate more quickly than voltage/frequency protection

Secure for single phase faults

Applicable to conventional DGs

Disadvantages

Can be difficult to set

Fails when P+jQ = 0


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What are grid disturbances effect?

It may happen that the CPP may experience a large variation in

the power at the islanding breaker which is not within the

acceptable limits.

Continued low voltage of the grid will result in the operation ofCPP generators at low power factor.

If compelled to work with low PF, the CPP generators will

experience a reverse reactive power flow and this can damagethe generators.


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How the relay operates?

An over loaded grid will give rise to fall in frequency.

A fault in the grid will give rise to fall in voltage.

Loss of grid will result in sudden change in power at theislanding breaker.

Loss of grid will also result in sudden change or vectorial shift

in the voltage

It is possible to detect each of the above symptoms by single

numerical relay.


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What else required at CPP?

Pref

Droop

Wref

+

+

+

-W

Chang over facility from constant power mode

to constant frequency mode on grid islanding


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Modes of operation : 2 MW deficit

TG 2

Grid

Transformer

Islanding

Breaker

CPP side load 13 MW

12 MW

import

DGTG 1

2 MW 4 MW5 MW

Grid side load 10 MW

2 MW


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Load shedding logic

< Set value

CPP side sudden generation

loading margin

+

-

Grid to CPP

side flow

AND

1

Islanding breaker trip

1

Output of load

management software

Trip non essential loads


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Modes of operation : 2 MW excess

TG 2

Grid

GridTransformers ( 3 numbers)

Islanding

Breaker

CPP side load

13 MW

12 MW

import

DGTG 1

3 MW 5 MW7 MW

Grid side load10 MW

2 MW


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System Considered for Simulation

Sl no

Description Values

1. Grid/Source data 70 kV, 50 Hz system, 1000 MVA fault level,

Inertia constant of 100 sec. on 100 MVA base

2. Grid transformers 3 numbers of 70/6 kV, 7.5 MVA, 9% impedance

3. Grid side load 10 MW at 0.85 pf

4. CPP side load 13 MW at 0.85 pf

5. Diesel generators 5/6 numbers, 6 kV, 1200 kW, 0.8 pf.

6. Turbo generator 1 1 number, 6 kV, 4000 kW, 0.8 pf, back pressure

7. Turbo generator 2 1 number, 6 kV, 6000 kW, 0.8 pf, back pressure

8. CPP side load

priority

11 MW essential, 2 MW non essential

9. Grid side load

priority

6 MW essential, 4 MW non essential.
http://localhost/var/www/apps/conversion/tmp/instructor/island_paper.guihttp://localhost/var/www/apps/conversion/tmp/instructor/island_paper.gui


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Loss of grid

Loss of grid results in sudden shift in the voltage vector and hencea vector shift relay best suits the logic to find the loss of grid.

The relay operation is instantaneous in nature and hence should

not operate for momentary grid faults.

Phase angle shift obtained at 6 kV bus of grid side for

A. Loss of grid5 degrees

B. Three phase fault in 70 kV2 degreesC. Three phase fault in 6 kV system - 0.2 degree

Recommended voltage vector shift relay setting is 5 degrees.


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Loss of grid

In the absence of the vector shift relay, df/dt relay in conjunctionwith under frequency relay can be used.

As frequency relay time setting has to be with a time delay,

chances of successful islanding will not be 100%, as some of the

drive auxiliaries and power plant auxiliaries may trip.

Following the islanding, the non essential load of 2 MW on the

CPP side is tripped immediately using the load manager software

based on the logic that further cushion is not available.

In the absence of the load manager software, the load will be

tripped with further time delay.


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Grid Island

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Transcript of Grid Island