HVDC Challenges in Grid Operation WELCOME HVDC Challenges in Grid Operation By V.G.Rao Chief Manager HVDC Kolar

TALCHER KOLAR SCHEMATIC Electrode Station Electrode Station KOLAR +/- 500 KV DC line 1370 KM Kolar 400kv System 220kv system Hosur Hoody B’lore Chintamani Salem Madras Cudappah Udumalpet

Points 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 Control Reactive Power Control is mainly achieved by switching individual reactive power sub banks Provided Reactive Power Sub Banks - Kolar Double 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 - Talcher Double tuned 12/24 harmonic (type A) – 7 no’s- 120MVAr each Shunt reactors (type L) – 2 no’s – 80MVAr each Shunt capacitor sub-bank (type C) -1 no. -66MVAr

Reactive Power Control 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 Kolar Bipolar operation -100% DC voltage Load / Idc No. 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 -80% DC voltage Load / Idc No. of filters >12.5% 1A+1B >25% 2A+1B >40% 3A+1B >55% >70% 4A+2B >85% >100%

Filter Switching settings for Kolar Monopolar operation -100% DC voltage Load / Idc No. of filters >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 -80% DC voltage Load / Idc No. of filters >12.5% 1A+0B >25% 1A+1B >40% 2A+1B >55% >70% 3A+1B >85% >100%

Reactive Power Control 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 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-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 1 010 500 MW 1 0 011 1000 MW 2 1 100 1500 MW 3 0 101 no limit 4 0 110 5 1 111 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-KOLAR Signal (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 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

STABILITY FUNCTIONS 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 FUNCTIONS Run-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 Individual run ups/run backs can be enabled or disabled for Talcher/Kolar station

STABILITY FUNCTIONS Power 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 Bipolar Smoothing Reactor DC OH Line Thyristor Valves Thyristor Valves Current Converter Transformer Converter Transformer Current 400 kV AC Bus 400 kV AC Bus AC Filters, Reactors AC Filters, shunt capacitors

Modes of Operation Monopolar Ground Return Smoothing Reactor DC OH Line Smoothing Reactor Thyristor Valves Thyristor Valves Current Converter Transformer Converter Transformer 400 kV AC Bus 400 kV AC Bus AC Filters, Reactors AC Filters

Modes of Operation Monopolar Metallic Return Smoothing Reactor DC OH Line Smoothing Reactor Thyristor Valves Thyristor Valves Current Converter Transformer Converter Transformer 400 kV AC Bus 400 kV AC Bus AC Filters, Reactors AC Filters

Automatic 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 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 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

Basic Components of HVDC Terminal Converter Xmers DC Line Smoothing Reactor 400 kV AC PLC DC Filter AC Filter Valve Halls -Thyristors -Firing ckts -Cooling ckt -Control & Protection -Telecommunication Control Room

UPGRADATION OF HVDC PROJECT - HIGHLIGHTS 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

New Over load features of HVDC Uac Ambient Temperature Remarks 0-33ºC 40ºC 50ºC Long time over load Normal (380-420kV) 1250 1150 New long time overload Extreme (360/440) 1110 1050 Half – Hour 1300 1200 Continuous/ New long time overload 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

Over load features of HVDC 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

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

Half-hour limits with redundant cooling at normal & extreme AC bus voltages at Kolar

Extended long time limit with redundant Cooling for extreme ac voltage range

Half hour limit with 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 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 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 SR P 1 Power Line fault Deblock Block P 2 PLC HVAC PLCC Protection couplers Fault Recorder SER P 1 Power Line fault Deblock Block P 2 PLC HVAC PLCC Protection couplers Fault Recorder SER P 1 Power Line fault Deblock Block P 2 PLC HVAC PLCC Protection couplers Fault Recorder SER TRIP I TRIP II

DEFENCE MECHANISM FOR SR Load relief: TRIP I Trip I Andhra Pradesh Chinakampalli 150MW Kolar Chintamani Hoody Karnataka 250MW Hosur Sriperambudur Selam Tamil Nadu 300MW

DEFENCE MECHANISM FOR SR Load relief: TRIP II Trip II Gooty Anantapur Somayajulapalli Kurnool Andhra Pradesh 200MW Karnataka Somanahalli 200MW Madurai Karaikudi Thiruvarur Trichy Ingur Tamil Nadu 200MW Kerala Trichur Kozhikode Kannur 200MW

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 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 signals Power Line fault Deblock Block P 2 PLC HVAC PLCC Protection couplers Fault Recorder SER "ESOF" "ESOF"

Trip 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-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 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

Reduced short Circuit ratio 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 Ratio Stage No. of commutation failure / minute Reduced Current /power level Bi pole operation Stage 1 3 75% Balanced Stage 2 6 50% 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.

DC 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

Power 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

THANK YOU