S102 Roles Inertia Fault Level Reactive Power

8/6/2019 S102 Roles Inertia Fault Level Reactive Power

1/29

System Role of Generation

Leslie Bryans & Alan Kennedy


2/29

Generators contribute to

System stability and power quality

Inertia and fault level

Reactive power balance SONI will discuss

Active power balance

SONI will discuss


3/29

What is Inertia

It is the energy stored in generators

because they are rotating.

They continue to rotate after

the energy source is removed


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Inertia

Large spinning machines provide stored energydue to the rotating mass of their rotor, drivingturbine shafts etc. Engineers model this as a

number of rotating masses.

When a disturbance occurs e.g. theunanticipated loss of a generator, the stored

energy is released into the system and arreststhe rate at which the system speed or frequencychanges.


5/29

Restorative power

Without wind With wind

smaller

smaller

System Frequency Issue

Solutions wind farms to contribute to restorative power

and system inertia?


6/29

182MW loss 852MW system load

0

50

100

150

200

250

300

-10 -5 0 5 10 15 20 25 30

Time (secs)

MW

48.4

48.6

48.8

49

49.2

49.4

49.6

49.8

50

50.2

Hz

frequency

unit1

unit2

Frequency trace

NI system only - low inertia


7/29

165MW loss 970MW system load

0

50

100

150

200

250

-10 -5 0 5 10 15 20 25 30

Time (secs)

MW

49

49.1

49.2

49.3

49.4

49.5

49.6

49.749.8

49.9

50

Hz

unit1

unit 2

freq

Frequency trace for the combined NI

& RoI System - higher inertia


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Why is the rate of change of

system frequency important? We need to understand this better as it

likely to be a limiting factor in

accommodating higher levels of wind. ROCOF protection

Question could this become backup Loss of

Mains protection? (DG&SEE with Prof. Jim

McDonald)

System effects e.g. traditional generation

auxiliary plant trips; CCGT flame outs.

Raised with DG&SEE as requiring investigation


9/29

Wind turbines

Fixed speed turbines seem to deliver an

inertia of around 4.3MWs/MVA which is

similar to traditional plant; so do not

reduce system inertia.

DFIGs and Fully converted wind farms are

presently thought to deliver close to

0MWs/MVA; so at present reduce systeminertia.


10/29

Work done

In order to understand the problem, wehave explored what happens to inertia andrate of change of frequency with very high

levels of penetration of wind power on theisland of Ireland.

This is to understand what we need totackle rather than alarm the presentposition.


11/29

Inertia, Rate of change of

frequency and frequency nadir

80% wind operating

(40% energy target)


12/29

Inertia summer min (~ 80% wind)


13/29

Inertia - winter (~ 80% wind)


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48.60

48.80

49.00

49.20

49.40

49.60

49.80

50.00

50.20

0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0

Hz

No Wind

WTGs lift 10% in 3 secs




WTGs don't lift

0.14Hz/s

400MW trip in Winter80% wind


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49.20

49.30

49.40

49.50

49.60

49.70

49.80

49.90

50.00

50.10

0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0

Hz No Wind





WTGs don't lift

400MW trip in Summer80% wind


16/29

49.20

49.30

49.40

49.50

49.60

49.70

49.80

49.90

50.00

50.10

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Hz No Wind





WTGs don't lift

0.5Hz/sec

Enlarged Graph section 400MW trip in summer80% wind


17/29

Work by UMIST for DG&SEE

DFIGs could deliver inertia as required but

require control modifications

Hardware transient loading is not anobstacle. ( Prof. Jenkins says that the

transient loading on gearboxes, drive

trains etc. falls well within that needed for

dynamic braking of the turbines.)


18/29

Our Proposal

To seek a modification to the NI Grid Code

to require wind turbines to provide inertia

at 4 6MWs/MVA.

This would need to be harmonised with an

equivalent provision in the EirGrid Grid

Code and Distribution Codes.

This will be a world first and waiting does

not seem an option.


19/29

Fault level

With wind farms


20/29

What is Fault level?

Fault level is measured in current or MVA.

When a short circuit occurs a large current flows

this causes the operation of protective devices

(fuses and circuit breakers open) to isolate thefault.

Protection systems are designed to isolate only

the faulty section of network.

Fault level is also a measure of system stiffness

which determines power quality issues.


21/29

High and Low fault level

Too high fault currents can be hard tointerrupt.

Too low and the protective devices maynot operate.

The problem with high penetration of wind

farms is too low fault levels.(Work is on-going with DG and SEE for the

UK Grid. They are going to study NI aswell).


22/29

Power quality

Very low fault level results in excessive

voltage dips when equipment is switched

on.


23/29


24/29

Our proposal

To ensure that the fault level tools developed byDG&SEE (which assess fault level throughoutthe range of generation dispatches) are applied

to the NI network and to consider what furtheraction may be needed. At the extremewe mayneed to convert the distance protection schemesto unit schemes which is very expensive.

Also, DG&SEE have not considered the knock-on effect of low fault level on the distributionprotection systems. This may be more seriousand potentially intolerable. Further work isrequired.


25/29

Reactive power


26/29

Reactive Power BalanceStatic Balance

To ensure an adequate voltage everywhere

on the system and provide reactive power

requirements to load More heavily loaded transmission system requires

relatively more reactive power due to IX losses

Reactive power doesnt travel well on a transmission

system due to the high X/R ratio, but NI transmission

system is small.

Studying the reactive power balance is more

complex when embedded generation is

included


27/29


28/29

Problem with Embedded Gen

G

P

Q

R XStep-downTransformer Gen Bus

loadR X

P & Q

Voltage

Max limit at EG

Min limit at load

Back-bonetransmissionnetwork


29/29

Static Reactive Power Balance

- Within reason this could be achieved with suitably sited

capacitors but there are technical difficulties with manycapacitors installed

Dynamic Reactive Power Balance

- Needed during and after system disturbances

- Quantity depends on many factors but rule of thumb isdynamic = static

- Synchronous generators can supply high levels ofdynamic reactive power

- Reactive power output of DFIGs and fully-converted windturbines is limited to ~ rated MVA, but even this is of limiteduse on the transmission system

- Need for alternative forms of dynamic reactive support on

transmission system eg. SVC, STATCOM, synch-comp

Documents

S102 Roles Inertia Fault Level Reactive Power