Ch-5 Preventive, Emergency & Restorative Control.pdf

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180906Advanced Power System-II

Chapter-5pPreventive, Emergency and Restorative Control

Prof.ChintanPatel

AssistantProfessorDepartmentofElectricalEngineeringG.H.PatelCollegeofEngineeringandTechnology VVNagar(Gujarat)Email:[email protected]

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Introduction Nature of Control Actions in a Power System

1) Frequency, voltage and power flow control

2) Real and Reactive power scheduling2) Real and Reactive power scheduling

These are "routine" control actions. manual controller ( a system/plant operator) an automatic controller (a generator voltage regulator) Th ti d lit l t l t

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These actions ensure a good quality supply at a low cost.

However, an important class of control actions areappropriate when a system is not in a "normal" state.

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Operating states and nature of control actions The state (or condition) of a power system can be judgedfrom the answers to the following questions:

Is the demanded load being met ? (i.e. is there a real andti b l ?)reactive power balance?)

Are all equipments within their current and voltage limits?

Can the system withstand stresses due to a possiblecontingency (leading to a loss of equipment)?


Definition of states and control actions System operation in steady state is governed by equationswhich express:

Real and Reactive power balance at each node(Equality Constraints)( q y ) Limitations of physical equipment, such as currentsand voltages must not exceed maximum limits(Inequality Constraints)

(1) Normal (Secure) State

All equality (E) & inequality (I) constraints are satisfied.


q y ( ) q y ( )

Generation is adequate to supply the existing loaddemand and no equipment is overloaded.

The reserve margins are sufficient.

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(2) Alert (Insecure) State

Definition of states and control actions

The security level is below some threshold of adequacy.

There is a danger of violating some of the inequality (I)constraints when subjected to disturbances (stresses).

Security constraints are not met.


Preventive control enables the transition from an alertstate to a secure state.

Definition of states and control actions(3) Emergency State

Due to a severe disturbance, the system can enter theemergency state.

Here Inequality (I) constraints are violated.

The system, would still be intact and emergency controlaction can be initiated to restore the system to an alertstate.


The system may breakdown & enter the In Extremis state if measures are not taken in time or are ineffective the initiating disturbance or a subsequent one is severeenough to overstress the system

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Definition of states and control actions(4) In Extremis State

Both (E) and (I) constraints are violated. The violation of equality constraints implies that partsof the system load are lost.of the system load are lost. Emergency control action should be directed foravoiding total collapse.

(5) Restorative State This is a transitional state in which I constraints aremet from emergency control actions taken but the E


g yconstraints are yet to be satisfied. From this state, the system can transmit to either thealert or the normal state depending on the circumstances.

NORMAL E,I SECURELoad tracking, Economic dispatch

Definition of states and control actions

ALERTE,I

INSECURE

E IE I

Ev,I RESTORATIVE

System litti

Preventive ControlResynchronization


EMERGENCYE,IvEv,Iv

IN-EXTREMIS

E: Equality ConstraintsI : Inequality ConstraintsEv, Iv: Constraints not satisfied

splitting

Load Loss

Heroic Actionprotect equipments

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The power system emergency is defined as due to either Viability crisis: resulting from an imbalance between generation,load and transmission whether local or system-wise.

Stability crisis: lti f l t d t ffi i t l l resulting from energy accumulated at sufficient level

in swings of the system to disrupt its integrity. In-Extremis state corresponds to a system failurecharacterized by

the loss of system integrity involving uncontrolledislanding (fragmentation) of the system


islanding (fragmentation) of the system

uncontrolled loss of large blocks of load. The objective of the emergency control action is to avoidtransition from emergency state to a failure state (In-Extremis).

Load Dispatch Centre (LDC)

A load dispatch center (or more appropriately, an EnergyManagement Centre) enables operators & other supportingengineers

to monitor a power system in real time

to capture the current operating state of the system

to instruct a generating plant or any other controllable


to instruct a generating plant or any other controllablesystem components so that a system operates withgood quality and security.

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The major components of a load dispatch centre are:1) Engineers for carrying out scheduling and monitoring2) Software programs to carry out monitoring and

scheduling functions3) Displays for adequate visualization


3) Displays for adequate visualization

There is a hierarchy of controls in a power system.National Load Dispatch Centre

Regional Load Dispatch Centre


g p

State Load Dispatch Centre

Area Load Dispatch Centre

A Central Load Dispatch centre oversees the operation ofthe entire grid using a SCADA system.


A SCADA system obtains data from various levels in thecontrol hierarchy and displays it in a meaningful way (like aone line mimic diagram on a large LCD screen)one line mimic diagram on a large LCD screen).

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The software tools in an LDC process data obtained fromfield measurements to identify the topology of the networkin real time.


Al ith lt d t t thi d t i Along with voltage and current measurements, this data isused to estimate the "state" of the system.

Since the number of components need to be monitored arevery large, a sophisticated digital processing of the data isrequired.


If data is simply displayed, then an operator has to use hispast experience to co-relate the displayed data and thesystem state and even take remedial actions.

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Normal and Alert State in a Power SystemA Power System in the Normal State:

After collecting all the data, the operator checks the stateof the system.

The state may be The state may be Normal StateAlert State Emergency State

When the dynamic state information is also available, theoperator may not be able to utilize it due to the limited time


operator may not be able to utilize it due to the limited timeframe. (e.g. loss of synchronism takes place in few seconds)

Therefore dynamic measurements can be made use ofmainly by automatic control or protection strategies.

Normal and Alert State in a Power System If all the equipments in the system are within theirrespective limits, then a system could be in the normal oralert state. If a system can withstand potential contingencies withoutequipment limits being violated or without losing stability,then we say that the system is in a normal or "secure state.

A network configuration or loading state which canwithstand an element outage without loss off supply to anyload is called "n-1" secure.


Otherwise we classify the system as being "insecure", i.e.,in the alert state.

The classification of secure and insecure is done bysimulating contingencies on a computer.

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Normal and Alert State in a Power SystemNormal and Alert state:

To distinguish between a normal state and an alert state, asystem operator carries out the following studies

(1) Static Security analysis

(2) Dynamic Security analysis

For the purpose, the operator essentially needs


the network configuration load generation

Normal and Alert State in a Power System

(1) Static Security analysis

This involves checking for equipment limit violations ifany of the equipments is tripped due to contingency.

Normal and Alert state:

any of the equipments is tripped due to contingency.

This element is not actually tripped by an operator, butonly simulated using a computer program.

(2) Dynamic Security analysis

This involves checking the stability of the system if


g y yany of the equipments is tripped due to contingency.

This element is not actually tripped by an operator, butonly simulated using a computer transient analysisprogram.

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It is important to carefully choose the element whoseoutage is to be simulated.


A set of critical elements are chosen by some roughi b d t ' iscreening based on an operator's experience.

If the security analysis shows that the system is secure, itis classified as a normal state.

If the state is normal, then a system operator may wish todo some minor changes in real and reactive scheduling


do some minor changes in real and reactive scheduling(from an economic perspective).

However any such change should not bring the system outof the secure state.

If the system is not secure (alert), then the operator has totry to steer it into the secure state by real or reactivepower re-scheduling (Preventive Control - rescheduling).


This re-scheduling is done to improve security and mayresult in higher cost.

So, even if preventive control is to be done, it should bedone in a way which will minimize any cost increase whilei lt l i it


simultaneously ensuring security.

This is done using a security constrained optimal powerflow program.

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Schematic of Security Assessment Procedure


An Example

It is assumed that voltages at all buses are equal to thenominal value (1.0 pu).

Also, we assume that sin(ddiff) = ddiff and cos(ddiff) = 1,


where ddiff is the phase angle difference between thevoltages at any 2 buses.

We assume that the thermal limits of all the lines are equaland should not exceed 1500 MW.

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An Example: Case 1


An Example: Case 1


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An Example: Case 2


An Example: Case 2


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Preventive Re-scheduling of generation


Preventive Re-scheduling of generation


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System security cannot be assessed by only consideringpost-contingency steady state power flows (as is done inthe example presented).

A system could be unstable for a disturbance even if A system could be unstable for a disturbance even if a post-disturbance steady state exists

power-flows and voltages for that steady state arewithin equipment limits


If a system is unstable, it will not settle down to that steadystate.

Emergency ControlTransition from an alert state to an emergency state

It is possible that the system operator is unable to act intime before a contingency actually occurs.

A grid may even be operated insecurely (in an alert state)due to a high cost of preventive control or due to inadequatereserve margins.

This situation is undesirable since it may lead to blackouts(if emergency control actions fail) which can cause great


( g y ) geconomic loss.

Even though the system has been classified as being in anormal state, several improbable disturbances take place.

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Transition from an alert state to an emergency state

Therefore the system can transit from a perceived alertstate to an emergency state.

The system into an emergency state may cause acomplete blackoutcomplete blackout.

Emergency control actions (manual or automatic) arerequired to retrieve the situation.

If there is a thermal overload of an equipment then there isti t t & i k "h i ti ld b d d


some time to act & quick "heroic action would be needed.

However in most cases one has to rely on automaticcontrols to quickly respond to such a situation.

Some emergency control actions are :


Generator / Load tripping or fast change of generationor load.

Control of voltage (Tap changers, PST) Power flow control devices (FACTS Devices)


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Emergency Control A system in an alert state may cascade into an emergencyand subsequently into a total blackout if no control actionsare taken. Emergency control measures can try to arrest this. Si t i t ith t d h t ti th l Since most equipment can withstand a short-time thermaloverload, there is a small window of time in which somemanual emergency measures can be executed.

For other emergency situations (like instability), time maybe too short and predesigned automatic emergencymeasures are necessary.


measures are necessary. One may consider the following alternatives:

a) Control of generationb) Tripping of generation or loadc) Re-routing of power flows

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Emergency Control


Emergency Control


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Emergency Control


Emergency Control

We have restricted our discussion of alert and emergencystates arising from line thermal overload.

However, it should be recognized that many disturbancesl d t th i t li it b i i l t dmay lead to other equipment limits being violated.

A sudden loss of generation or load due to some fault.

Large disturbances may cause Angular Instability.


Weakening of transmission system along with heavyreactive power demand and low reactive powergeneration margin may cause voltage instability

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Emergency Control : An example

Consider the two machine system shown below:



This fault is cleared by tripping the lines using CircuitBreakers which are triggered by protective relays


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What are the possible consequences of such disturbance ? Possible Consequences are


a) The system settles to a new acceptable equilibrium after) y p qsome initial transients die down.

b) The system settles to a new equilibrium, but theequilibrium is violative of some steady state equipmentlimit (leading to tripping out of that equipment).


c) The system does not attain a new equilibrium due toangular or voltage instability.


Voltage instability leads to unacceptably low voltages.

Angular instability (loss of synchronism) leads to violentexcursions in current, voltage and power leading toequipment damageequipment damage.

Therefore, the generators which have lost synchronismhave to be disconnected from each other.

This situation


is shown.

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A BlackoutWhen can a blackout occur ?

Preventive Control actions ensure that any contingency (ifit occurs) does not lead to equipment limit violation orinstability.y

Emergency control actions come into play if an actualdisturbance is occurred.

These actions try to prevent an emergency situation totransit into a near-complete loss of generation and load (a


blackout !).

In spite of security analysis and preventive actions (doneduring actual operation), and emergency control actions(usually pre-designed offline), blackouts do occur.

When can a blackout occur ?

Following are some of the reasons of blackout:

Cascade tripping Unnoticed disturbance or contingency scenario

pp g

Malfunctioning of protective equipments and relays


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BLACKOUT-2012 There were major grid disturbances in Northern Region at 02.33 hrs on 30/07/2012 and 13.00 hrs on 31/07/2012. Due to the first disturbance which led to the collapse of NR Due to the first disturbance which led to the collapse of NRElectricity grid, following states were suffered:

Uttar Pradesh,Uttarakhand, Rajasthan,Punjab, Haryana


Haryana, Himachal Pradesh, Jammu & Kashmir, Delhi ,Union Territory of Chandigarh.

BLACKOUT-2012

The reasons of the blackout on 30/07/2012 are as follow:

Northern Regional Grids load was about 38,000 MW at thetime of disturbance.

Extremely heavy over-drawal by the constituents of NRgrid.

Some thermal/gas generating units in the NR wereunder forced outage either due to technical reasons ordue to unavailability of coal.


due to unavailability of coal.

Forced outage of few hydro-generating units in NR dueto high silt.

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BLACKOUT-2012


Plot of Frequency in WR and NR

BLACKOUT-2012


Plot of Frequency in WR and NR

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BLACKOUT-2012 Small pockets of generation and loads in the NorthernRegion survived the blackout.

3 generating units at Badarpur thermal power stationwith approximately 250 MW load in Delhiwith approximately 250 MW load in Delhi

Narora Atomic Power Station in UP Some parts of Rajasthan system (around Bhinmal) that

remained connected to the Western Grid Some parts of Uttar Pradesh system (around Sahupuri)

that remained connected with Eastern region


that remained connected with Eastern region

BLACKOUT-2012

The second incident which was more severe than theprevious one occurred at 13.00 hours on 31/07/2012, leadingto loss of power supply in

NR

ER

NER

The total load of about 48,000 MW of 18 states was affectedin this black out.


in this black out.

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BLACKOUT-2012

53Prof.ChintanPatel(EE GCET)Plot of Frequency in WR and NR

BLACKOUT-2012

India on 31st July 2012


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BLACKOUT-2012: Effects Over 600 million people & all community power-dependent

systems affected.

Power grids in 18 of Indias States stretching from Assamto the Himalayas and the northwestern deserts ofto the Himalayas and the northwestern deserts ofRajasthan, shut down.

On Monday (July 30th), India was forced to buy power fromtiny Bhutan.

The scale of the blackouts caused India acuteembarrassment on the international stage


embarrassment on the international stage.

Two hundred miners were stranded in three deep coalshafts in the state of West Bengal when their electricelevators stopped working.

BLACKOUT-2012: Effects


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BLACKOUT-2012


Animated view of Blackout-2012.

Power System RestorationAfter a blackout

If a blackout takes place, efforts have to be taken to bringback the system to a normal state at the earliest.

This (black starting) is not an easy task. This (black starting) is not an easy task. Once a generator is tripped, it needs a significant amountof power to restart it.

Power is required for 2 types of activities: Survival Power: For emergency lighting, battery chargers, etc.


( 0.3% of generator capacity)

Startup Power: For starting unit auxiliaries.( for nuclear & thermal : 8% of unit capacity)( for hydro & gas: 0.5-2% of unit capacity)

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The major steps required for restoration are:Power System Restoration

(1) Islands which have survived need to be stabilized forfrequency and need to be used for starting other units.

(2) Hydro/Gas units which require less startup power need tobe started using in-house DG sets.

(3) Larger thermal units need to be fed "startup power" from: Islands which have survived Black-started generators Other synchronous grids (temporarily)


(4) Started units are synchronized with one another.

(5) Loads and Generation have to be matched as much aspossible to avoid large frequency variations (UsingGovernors).

Power System RestorationProblems in Restoration

a) Securing Islands

b) E t di P t L d f G t hi hb) Extending Power to Loads from Generators which areblack-started

c) Re-integrating the grid


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a) Securing Islands

Problems in Restoration

After a blackout a few islands may survive due toseparation of the system in time.

A few hydro or gas generators could be black-started usingin-house D-G sets.

So, some small pockets will be there in the blacked out gridwherein generators are supplying some loads.


The situation in these islands is usually precarious due tothe small number of generators within the island.

Problems in Restorationa) Securing Islands

So if the load in the island is fluctuating (traction loads),the rate of change of frequency within the island may bequite large.

Due to this, the island will be collapsed because ofexcessive frequency variations.

Therefore control of generated power (by governors) andfrequency based tripping or energisation of load isimportant


important.

Black-starting of large generators is done by availingstartup power from other started generators or islands.

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Startup power may also be availed from neighboringsynchronous grids if an AC transmission link exists.

Problems in Restorationa) Securing Islands

Startup power cant be availed via DC links as ACvoltages are not available in the blacked out system.



b) Extending Power to Loads from Generators which areblack-started

The next step in power system restoration is to supplyloads from black-started generators.g

Some of these loads may be in the form of the startuploads of other larger generating plants which need to beblack-started.

These loads are supplied via transmission lines.


Energizing a transmission line initially without any loadcan cause over-voltages.

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b) Extending Power to Loads from Generators which areblack-started This is avoided by:

1) Energizing fewer high voltage lines

2) Operating generators at minimum voltage levels

3) Deactivating switchable capacitors

4) Connecting shunt reactors and tertiary reactors

5) Pick up loads with lagging power factor

6) Charging more transformers


6) Charging more transformers

7) Charging shorter lines

8) Operating synchronous condensers / SVCs where available

9) Avoiding charging lines with series capacitors



Some islands, which have been secured, should beconnected with each other so that a better generation-loadbalance can be achieved.

An important step in reconnecting islands to one another is"synchronization".

The basic requirements for successful synchronization oftwo systems are the same as those for an individualgenerator connected to a large grid.


generator connected to a large grid.

The frequencies should be practically the same and phaseangular difference at the instant of connection should besmall.

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Problems in Restorationc) Re-integrating the grid

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Reference: Web course of Power System Operation and Control byProf. A.M.Kulkarni (IITB-Mumbai)

Documents

Ch-5 Preventive, Emergency & Restorative Control.pdf