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

2. 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

4. 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

5. 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 MVAr, Kolar MVAr
Max limit for Q-extended: Talcher MVAr, Kolar MVAr

7. 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

8. 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

9. 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

10. 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%

11. 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%

12. 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

13. 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

15. 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

16. 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

17. 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

18. 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

20. 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

21. 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

23. 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

24. 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

25. 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

26. 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

27. 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

28. Electrode Current limitation characteristics

29. 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

30. 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

31. 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

32. New Over load features of HVDC
Uac
Ambient Temperature
Remarks
0-33ºC
40ºC
50ºC
Long time over load
Normal ( kV)
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

33. 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

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

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

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

37. Half hour limit with redundant Cooling for extreme ac voltage range

38. 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

39. 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

40. 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

41. 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

42. 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

43. 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

44. 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
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
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.

45. 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.

46. Proposed addition of I/O signals
Power
Line fault
Deblock
Block
P 2
PLC
HVAC PLCC
Protection couplers
Fault Recorder
SER
"ESOF"
"ESOF"

47. 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.

48. 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.

49. 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.

50. 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.

51. 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.

52. 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

53. 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

54. 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.

55. 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.

56. 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

58. 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

59. THANK YOU