hvdc challenges in grid operation byv.g.rao chief manager hvdc kolar

HVDC Challenges in Grid Operation

WELCOMEWELCOME

ByBy

V.G.RaoV.G.Rao

Chief ManagerChief Manager

HVDC KolarHVDC Kolar

Kolar

Chintamani

Cudappah

HoodyHosur

Salem

Udumalpet

Madras B’lore

+/- 500 KV DC line 1370 KM

ElectrodeStation

ElectrodeStation

TALCHER

400kv System

220kv system

KOLAR

TALCHER KOLAR SCHEMATIC

Points related to operation of HVDCPoints related to operation of HVDC

• RPC control– Filter switching seq.– Limitations by RPC

• Stability Controls– Power Limitations– Frequency limit controller– Run-backs / Run-ups– Power Swing damping control

• GRM operation & electrode limitation• Overload of HVDC• SPS scheme• Power / current limits due to protection• Power reversal

Reactive power control / RPC : 2 modes– Q- mode – Reactive Power control mode –

• The switching limit for the filter can be adjusted by entering the maximum set value of Reactive power (Q) by the operator.

• Possibility to select between Q-basic or Q-extended mode

• Max limit for Q-basic: Talcher - +100MVAr, Kolar - +500MVAr

• Max limit for Q-extended: Talcher - +500MVAr, Kolar - +500MVAr

Reactive power control / RPC : 2 modes• U-mode – Voltage control mode

– The switching limit for the filter can be adjusted by a maximum and minimum set values of AC bus Voltage.

– is maintained.

– If the voltage of the bus reaches the minimum limit, filter will be switched into service.

– If the voltage of the bus reaches the maximum limit, filter will be switched out of service

– Upper limit : Talcher / Kolar 440kV

– Lower limit : Talcher / Kolar 360kV

– Bandwidth of 20KV

Reactive Power ControlReactive Power Control

• Reactive Power Control is mainly achieved by switching individual reactive power sub banks

• Provided Reactive Power Sub Banks - KolarDouble tuned 12/24 harmonic (type A) – 8 no’s- 120MVAr each

Double tuned 3/36 harmonic (type B) – 4 no’s- 97MVAr each

Shunt capacitor sub-bank (type C) – 5 no’s – 138MVAr each

• Provided Reactive Power Sub Banks - TalcherDouble tuned 12/24 harmonic (type A) – 7 no’s- 120MVAr each

Double tuned 3/36 harmonic (type B) – 4 no’s- 97MVAr each

Shunt reactors (type L) – 2 no’s – 80MVAr each

Shunt capacitor sub-bank (type C) -1 no. -66MVAr


• Switching ON criteria of individual sub banks and their hierarchy:

– sub bank switching according to Harmonic Performance – given higher priority and depends on actual DC power flow

– AC bus bar voltage within operator reference values – if RPC is in U-mode – next priority

– total station reactive power within operator reference values – if RPC is in Q-mode – next priority

• Switching OFF criteria of individual sub banks and their hierarchy:

– Sub banks switches out based on the AC bar voltage only

Filter Switching settings for KolarFilter Switching settings for Kolar

Load / IdcLoad / Idc No. of filtersNo. of filters

>10% 1A+1B

>25% 2A+1B

>40% 3A+1B

>55% 4A+1B

>70% 4A+2B

>85% 5A+2B

>95% 6A+2B

>100% 7A+2B

>105% 7A+3B

>110% 8A+3B

OR

7A+4B>120%

>125%

>130% 8A+4B

Bipolar operation -100% DC voltageBipolar operation -100% DC voltage


>12.5% 1A+1B

>25% 2A+1B

>40% 3A+1B

>55%

>70% 4A+2B

>85%

>100%

Bipolar operation -80% DC voltageBipolar operation -80% DC voltage

Filter Switching settings for KolarFilter Switching settings for Kolar


>10% 1A+0B

>25% 1A+1B

>40% 2A+1B

>55%

>70%

>85% 3A+1B

>100% 4A+2B

>110% 5A+2B

>120% 6A+2B

>125%

>130%

Monopolar operation -100% DC Monopolar operation -100% DC voltagevoltage


>12.5% 1A+0B

>25% 1A+1B

>40% 2A+1B

>55%

>70% 3A+1B

>85%

>100%

Monopolar operation -80% DC Monopolar operation -80% DC voltagevoltage

• Manual control of sub banks is possible by the operator

• AC voltage limitation is permanently active irrespective of manual / automatic

switching of filters

• CONNECT INHIBIT level – filters/shunt-c cannot be connected in manaul /auto

– AC bus voltage is above 431kV ( reset at 424kV) or

– Reactive power export to the grid is high compared to active power (refer table)

• ISOLATE level - filter sub banks/ shunt C are switched OFF in 0.5 seconds interval

automatically at 440kV

• ISOLATE INHIBIT - Switching off of sub-banks is blocked if the AC voltage drops

below 380kV

• CONNECT limit - additional banks will be switched on (in 1 second interval)

automatically if the AC voltage reaches 360kV


RPC sub bank connect inhibit levels

Bipole Power at Kolar

Maximum no. of filters / shunt C

500 6

550 7

600 8

650 9

700 10

800 11

1000 12

1200 13

1400 14

1600 15

1800 16

2000 17

At Connect Inhibit level – Control system prevents switching ON of filters / shunt C in auto or manual irrespective of AC voltage to prevent export of excessive reactive power

STABILITY FUNCTIONSSTABILITY FUNCTIONS– Power Limitations

• Always enabled in the control system• Becomes active once the AC switchyard configuration

for NTPC at Talcher or 400kV S/y at Kolar changes- refer tables

• Introduced to improve stability in the regions, self excitation of generators, failure of control systems etc.

• Power capability depends upon the no. of generators / lines connected to HVDC

• Automatic limitation of power takes place

POWER LIMITATIONS-TALCHERPOWER LIMITATIONS-TALCHER

Signal (Bit code) DC Power Limit in MW No. of Generators

& pair of AC lines

0 000 loss of comm. between AC SC &PC

1 001 controlled shutdown or ESOF 0

1 010 500 MW 1

0 011 1000 MW 2

1 100 1500 MW 3

0 101 no limit 4

0 110 no limit 5

1 111 no limit 6

•Two lines / one pair of lines equivalent to 500MW

•If all the generators at Talcher trips / only lines are considered for power limitation

POWER LIMITATIONS-KOLARPOWER LIMITATIONS-KOLARSignal (Bit code) DC Power Limit in MW Number of pair of lines

0 000 loss of comm. between AC SC &PC

1 001 controlled shutdown or ESOF 0

1 010 controlled shutdown 1

0 011 500 MW 2

1 100 1000 MW 3

0 101 1500 MW 4

0 110 2000 MW 5

1 111 no limit 6

Ramp Rates Talcher Kolar

With telecontrol 1300 A / sec 66 A / sec

Without telecontrol 10A/sec Nil

– Frequency limit controller• Stability functions needs to be enabled by the operator• FLC comes into action if the frequency limits are set

within a band of current frequency• Enabled automatically during islanding or split bus

mode at Talcher• Enabled automatically during split bus mode at Kolar• Can be enabled individually at Talcher or Kolar• If telecom is faulty – FLC of Kolar is disabled

auotmatically

STABILITY FUNCTIONSSTABILITY FUNCTIONS

– Run-backs / Run-upsRun-backs / Run-ups• If stability functions are enabled, these functions are automatically

enabled• At present this functions are not programmed• Automatic ramping up of power is possible with certain conditions• 5 conditions can be programmed / hardware inputs• Automatic ramping down of power is possible with certain

conditions• 5 conditions can be programmed / hardware inputs• Individual run ups/run backs can be enabled or disabled for

Talcher/Kolar station


– Power Swing damping controlPower Swing damping control• Stability functions are to be enabled & power swing damping

function to be enabled• Power Swing Damping function provides positive damping to the

power flow in the parallel AC system• This function becomes active automatically during emergency

conditions or major disturbance of the AC system• Additional DC power is calculated based on the frequency variation

/ swing of the connected AC system• This function is provided for each pole at each station


Modes of Operation

DC OH Line

Converter Transformer

ThyristorValves

400 kV AC Bus

AC Filters,Reactors

Smoothing Reactor


ThyristorValves

400 kV AC Bus

AC Filters, shunt capacitors

Smoothing Reactor

Bipolar

Current

Current

Modes of Operation

DC OH Line


ThyristorValves

400 kV AC Bus

AC Filters,Reactors

Smoothing Reactor


ThyristorValves

400 kV AC Bus

AC Filters

Smoothing Reactor

Monopolar Ground Return

Current

Modes of Operation

DC OH Line


ThyristorValves

400 kV AC Bus

AC Filters,Reactors

Smoothing Reactor


ThyristorValves

400 kV AC Bus

AC Filters

Smoothing Reactor

Monopolar Metallic Return

Current

Automatic MR-GR changeoverAutomatic MR-GR changeover

• Normal operation – Balanced Bipolar operation• When one pole trips, healthy pole goes to Ground

Return mode• Limitation in Kolar electrode• Healthy pole goes to Metallic Return mode

automatically – power flow restricted to 1000MW• Operator can increase the power manually to the

overload capability of the healthy Pole after one Pole trips

Automatic MR-GR changeoverAutomatic MR-GR changeover

• If line fault / failure of Metallic return changeover healthy Pole remains in GR mode

• Changeover from GR mode to MR mode takes around 75secs• Failure of changeover may be due to problems in the DC

switches or tele-control failure • If all Blocking devices are healthy power flow settles at

500MW in GR mode• If any Blocking device faulty power flow settles at 150MW• Operator can set the 150MW limit / 500MW limit manually if

required• During the automatic seq. process power flow follows the

defined curve as shown• At present 150MW limit is set in GR mode

Electrode Current limitation characteristicsElectrode Current limitation characteristics

UPGRADATION OF HVDC PROJECT - PURPOSE

• Outage of 400 kV transmission lines from Talcher that requires transmission of maximum power over this link

• Outage of one Pole which requires maximum possible transmission of power on the other pole continuously in metallic return mode due to the restrictions in the GR mode at Kolar

Converter Xmers

Valve Halls

-Thyristors

-Firing ckts

-Cooling ckt

Smoothing Reactor

Basic Components of HVDC Terminal

400 kV

DC Line

Control Room

-Control & Protection

-Telecommunication

AC PLC

AC Filter

DC Filter

• Existing overload capacity of the equipment being used for long time loads – The overload characteristics of HVDC are modified in Upgrade

• All the equipment ratings were studied and critical equipment has been identified for modifications

• Smoothing Reactor, Converter Transformer, LVDC bushing and PLC reactors (at Kolar) requires additional modifications / replacement

• Relative ageing of the critical eqpt.- Converter Transformer and Smoothing reactor are being monitored in real time

UPGRADATION OF HVDC PROJECT - UPGRADATION OF HVDC PROJECT - HIGHLIGHTSHIGHLIGHTS

New Over load features of HVDC New Over load features of HVDC

Overload Uac Ambient Temperature Remarks

0-33ºC 40ºC 50ºC

Long time over load

Normal (380-420kV)

1250 1250 1150 New long time overload

Extreme (360/440)

1150 1110 1050

Half – Hour Normal (380-420kV)

1300 1250 1200 Continuous/ New long time overload

Extreme (360/440)

1300 1200 1200

The overload under upgradation is only long time loading of HVDC but not the continuous loading under which HVDC can operate at 1.25 p.u for max. of 10 hrs in a day while the rest of the day at 1.0p.u at ambient <40ºC

• It is permissible to apply the half-hour overload once in every 12 hour period

• The five second overload remains unchanged -1470MW• It is permissible to apply the five second overload power

once in a five minute period and up to at least 5 times during a two hour period

• The five second overload can be applied during operation at the long time overload or half an hour overload.

• With Telecom out of service above overloads are not applicable

Over load features of HVDC Over load features of HVDC

Long time limits with redundant coolingat normal & extreme AC bus voltages at Kolar

Half-hour limits with redundant cooling

at normal & extreme AC bus voltages at Kolar

Extended long time limitwith redundant Cooling

for extreme ac voltage range

Half hour limitwith redundant Cooling

for extreme ac voltage range

Smoothing Reactor

• Hot spot temperature of the insulation to be within limits at new extended overload

• The extended overload is achieved without sacrificing the designed life of the Smoothing reactor

• Forced air cooling ducts are installed to keep the hot spot temperature within limits

• Overload capacity is monitored by using Relative Ageing Indication (RAI) and Load Factor Limitation (LFL)

• The status of the SMR forced cooling system decides the overload limits of the system

• The SMR cooling will be automatically switched ON if the DC current is >1950A and the ambient temperature is >28 ºC

DEFENCE MECHANISM FOR SRDEFENCE MECHANISM FOR SR

Operational from March 2006

• Based on absolute power• Power loss being calculated as

• Loss = Power 2 Secs prior to trip – Power after trip

• Tripping due to line fault is considered – since during LF, healthy pole power is limited to 150MW in GR mode

• Signals transmitted through FO instead of PLCC

• Separate protection couplers installed

DEFENCE MECHANISM FOR SRDEFENCE MECHANISM FOR SR

Trip generation LOGIC• Condition 1:

• (500MW<Power loss ≤1000MW) & Pole Block = TRIP I

• Condition 2:• (1000MW<Power flow ≤1500MW) & Line fault & Pole Block =

TRIP I

• Condition 3:• (Power loss >1000MW) & Pole Block = TRIP II

• Condition 4:• (Power flow >1500MW) & Line fault & Pole Block = TRIP II

Whenever Trip II is generated, Trip I also generates

BLOCK DIAGRAM OF DEFENCE MECHANISM FOR SRBLOCK DIAGRAM OF DEFENCE MECHANISM FOR SR

TRIP ITRIP I

P 1P 1

PowerPower

Line faultLine fault

DeblockDeblock

BlockBlock

P 2P 2

PLCPLC

BlockBlock

PowerPower

DDeblockeblock


HVAC PLCCHVAC PLCC

Protection couplersProtection couplers

Fault RecorderFault Recorder

SERSER

Protection couplersProtection couplersTRIP IITRIP II

P 1P 1

PowerPower


DeblockDeblock

BlockBlock

P 2P 2

PLCPLC

BlockBlock

PowerPower

DDeblockeblock


HVAC PLCCHVAC PLCC



SERSER


P 1P 1

PowerPower


DeblockDeblock

BlockBlock

P 2P 2

PLCPLC

BlockBlock

PowerPower

DDeblockeblock


HVAC PLCCHVAC PLCC



SERSER


DEFENCE MECHANISM FOR SR

Load relief: TRIP I

Trip ITrip I

Chinakampalli

HosurSriperambudur

Selam

KolarChintamani

Hoody

Andhra PradeshAndhra Pradesh

150MW150MW

KarnatakaKarnataka

Tamil NaduTamil Nadu

250MW250MW

300MW300MW

DEFENCE MECHANISM FOR SR

Trip IITrip II

GootyAnantapur

SomayajulapalliKurnool

TrichurKozhikode

Kannur

Somanahalli

Andhra PradeshAndhra Pradesh

200MW200MW

KarnatakaKarnataka

KeralaKerala

200MW200MW

200MW200MW

Load relief: TRIP II

MaduraiKaraikudiThiruvarur

TrichyIngur

Tamil NaduTamil Nadu

200MW200MW

Recent cases of SPS non-operation• The system has worked perfectly in all cases and saved the SR grid

• Some improvements are being done in following cases• Pole 2 trip on 11.06.2010

– Problem in the Pole control system selection– DC power was around 700MW and Pole 1 has taken over the power

immediately after tripping– Hence inter trip signal need not be generated.

• Pole 2 trip on 19.08.2010– Problem in the Pole control system selection– The power loss was >500MW– Inter trip signal was not generated in this case– Since both Pole control system 1 &2 of Pole 2 had failed, the Pole

Block signal was not transmitted by the Pole control to the defence mechanism

– This in turn could not generate the inter trip signal though the power loss was sufficient for the signal generation.

Proposed modification• Pole control system generates ESOF (Emergency

Switch OFF) signal to DC protection & SER• This signal is available even during the complete

power supply failure in both the Pole controls• One more Binary input for the detection of the Pole

trip in the above cases from each Pole• This signal can be used with OR logic for the existing

Block /Deblock logic for Pole 1 & 2• Additionally 2 relays have to be installed in Pole

Control and logic modification has to be carriedout.

Proposed addition of I/O signalsProposed addition of I/O signals

P 1P 1

PowerPower


DeblockDeblock

BlockBlock

P 2P 2

PLCPLC

BlockBlock

PowerPower

DDeblockeblock


HVAC PLCCHVAC PLCC



SERSER

Protection couplersProtection couplers"ESOF""ESOF"

"ESOF""ESOF"

Trip signal -3Trip signal -3

• Addition of Trip signal-3 : Trip signal 3 will be initiated – Any one pole / both poles block AND Power loss compared with

power flow 2 secs prior to Pole block is >2000MW

• OR– If one of the pole block on line fault AND Power flow just prior to that

instant was >2000MW

• List of DTPCs to be wired for the load relief of 500MW for trip signal -3 has to be provided by SRLDC. The S/w and H/w modifications in the PLC & DTPC panels will be carriedout at HVDC Kolar.

Trip Signal-2Trip Signal-2

• Modified logic:• Any one pole / both poles block AND Power loss

compared with power flow 2 secs prior to Pole block is >1000MW and less than or equal to 2000MW

• OR• If one of the pole block on line fault AND Power

flow just prior to that instant was >1500MW and less than or equal to 2000MW.

Trip-1&2 during trip-3

Generation of Trip 1 & Trip 2 signals when Trip 3 is generated: Modifications will be carriedout at HVDC Kolar as per the desired logic.

Trip signal on Line fault

• Generation of trip signal on Line fault: SRLDC has suggested following solutions to overcome the problem with inter trip signal on line fault.

– Generate trip signals -2 after 3 re-tries if set point is >1500MW: The number of retries may vary depending upon the generator / line condition at Talcher. Hence, this logic may fail in some cases.

– Generate trip signal -2 if (original set point – Current power flow) exceeds 1500MW in a minute interval: For measuring the power flow 2 secs prior to the Block signal, at present 10 samples for every 200msec are being considered. For 2 minutes we have to take 300 samples and the PLC may hang.

Proposed scheme• Instead of above suggestions, it is proposed to modify the logic as below:• The existing logic for line fault is seeing the power flow at the instant the

Line fault signal AND Blocked signal are available. In most of the cases it is seen that only trip signal-1 on Line fault is generated. Since during line fault recovery (restart time) of one pole, the power flow on HVDC is reduced to the capability of the healthy pole (say 1200MW). This power flow may be less and the Inter trip signal- 1 on line fault is generated.

• To overcome this problem, it is suggested that the logic can be designed to monitor the power flow 2sec prior to the Line fault signal AND Blocked signal for both trip signal 1 & 2. The time taken for fault recovery sequence is <1sec (approx.) and hence 2 sec may be considered.

Power / Current limits due to protection

• One of the advantage of HVDC is controllability

• On operation of certain protections especially due to external AC system disturbances

• Current / power limitations are executed instead of tripping the system immediately

• Improves the transient stability of the system

Single phase faults on AC systemSingle phase faults on AC system

• Single phase faults lasting for >500msec at Inverter

• Causes severe commutation failures at Inverter

• After 500msec, power limited by 1/3rd pre fault power

• After 1200msec, Pole blocks

• A reduced short circuit level – Low SCR• Caused by disconnected lines or loss of generators • This produces transient stresses • Over voltages or repeated commutation failures occur during

recovery from external AC system faults or after a change of power

• The power is limited to a safe power transfer level by pole control to lead the stable steady state conditions of the HVDC transmission.

Reduced short Circuit ratioReduced short Circuit ratio

StageNo. of

commutation failure / minute

Reduced Current /power

levelBi pole operation

Stage 1 3 75% Balanced

Stage 2 6 50% Balanced

Stage 3

After one minute delay, If again a commutation failure occurred, the affected Pole will trip. The

time delay of Pole 1 and Pole 2 are set for 200 ms and 400 ms to avoid Bipole tripping.

Reduced Short Circuit Ratio

DC LINE FAULTSDC LINE FAULTS

• DC line faults detected by the DC protection based on Wave front / under voltage protection

• Line fault recovery seq. initiated• De-ionisation times

– 1st – 200msec– 2nd – 250msec– 3rd - 300msec– 4th – 300msec at RVO– After 300msec Pole block

• Line fault locator – distance accuracy upto one tower• On one pole trip – healthy Pole in GRM – 150MW

IdL

Idee1

Idee2

IdE

U dN

IdN

IdHU dL

D C -Line

E lectrode L ine E lectrode L ine

IdH

Idee1

Idee2

IdE IdN

U dLIdL

A B

Power Reversal on HVDCPower Reversal on HVDC• Power reversal can only be initiated by the operator

SR ER• Pole needs to be Blocked before going for reverse

power operation• Off-line power reversal can be performed in

monopolar or bipolar operation • In bipole power control mode the power direction is

changed on a bipolar basis• Power reversal on a pole basis is provided in current

control mode

hvdc challenges in grid operation byv.g.rao chief manager hvdc kolar

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