Power System Operation Corporation Limited
(a wholly owned subsidiary of POWERGRID)
National Load Despatch Centre
New Delhi
1st August 2012
Sub: Loading of Extra High Voltage (EHV) transmission elements on the
network and its impact on the grid security
1.0
Background:
Two grid disturbances occurred on two consecutive days on 30th and 31st
July 2012. In the first disturbance at 0233 hours on 30th July 2012, nearly the
entire Northern Grid was affected while in the second disturbance, the Northern,
Eastern and North Eastern grids were affected to a large extent.
In both the disturbances, in the antecedent grid conditions there was
heavy power flow on the 400 kV Bina-Gwalior-Agra single circuit section crossing
1000 MW on the single circuit available. (The second circuit was under outage
since 28th July 2012 for up-gradation to 765 kV level). A similar incident at 1510
hours on 29th July 2012 had also led to a near miss situation. This section has
Quad Bersimis conductor which has a Surge Impedance Loading (SIL) of 691
MW but loaded much above the SIL. Likewise, the loading on many circuits in the
Eastern region was also of the order of 550 MW and above against the SIL of
515 MW for a twin Moose configuration line.
1.0
Safe loading limits for the transmission system:
The Planning Criteria issued by Central Electricity Authority in June 1994
stipulates that the permissible loading on transmission lines would either be the
stability limit evaluated from St Clair’s curve or thermal limit depending on the line
length. It states that while Surge Impedance Loading (SIL) gives a general idea
of the loading capability of the line, it is usual to load the short lines above SIL
and long lines below SIL (because of stability considerations).
The thermal limit of a Quad Bersimis line at 40 degree centigrade ambient
temperature is of the order of 3200 amps which correspond close to 2200 MVA
at 400 kV voltage level in contrast to the SIL of 691 MW.
NLDC, POSOCO
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In several incidents over the past few years, the following concerns have
come to the fore in several disturbances involving high antecedent line loadings
in the system.
1.
Surprises in the form of protective relaying mis-operations either due to
incorrect settings, load encroachment or use of distance relays for power
swing blocking. These have led to cascading failures in the past.
2.
Sustained high loading for a period of ten minutes or more in daytime high
ambient temperature conditions have led to line tripping on transient faults
possibly due to increase in sag.
3.
High reactive power consumption by the transmission line under heavy
loading conditions which lead to voltage dips in the system. Lack of
dynamic reactive power consumption adds to the constraint.
Till the incidents of 30th and 31st July 2012 is analyzed in detail and any
shortcomings in different regions identified, the following action plan is being
implemented with immediate effect.
•
NLDC/RLDCs in consultation with CEA and CTU shall review transfer
capability of inter-regional and any other critical links keeping in view
CERC regulations and the experience during the two grid collapses.
•
The Short Term Open Access (STOA) approvals by RLDCs/NLDC would
be considered based on the transfer limits worked out above. However if
at any time due to an emergency in the system, RLDCs would take all
actions necessary to ensure system security including curtailment of
transactions.
•
NLDC/RLDCs to review and impose congestion charges as per CERC
regulation for inter-regional links for any drawal beyond the limits to be
specified as mentioned above.
•
The transfer limits considering SIL limits (indicated at Annexe-1) on each
trunk transmission lines in the inter-regional corridor and intra-regional
systems have been worked out by NLDC and the same is enclosed at
Annexe-II.
x----x----x
NLDC, POSOCO
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Annexe-1
NLDC, POSOCO
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Annexe-2
Power System Operation Corporation Limited
National Load Despatch Centre
Dated: 1st August 2012
Interregional
Corridor
Total Transfer
Capability
(TTC)
MW
Reliability
Margin
(RM)
MW
Available
Transfer
Capability (ATC)
MW
LTA/ MTOA
MW
Available
Margin for
STOA
MW
West to
North*
1250
200
1050
260
790
400 kV Bina-Gwalior
East to
North
3850
200
3650
1158
2492
400 kV KahalgaonBiharsharif
West to
East
850
300
550
0
550
220 kV BudhipadarTarkera
East-North
East
440
35
405
135
270
220 kV BTPS-Agia
East-South
900
0
900
170
730
400 kV Rourkela-Talcher
West to
South
1000
0
1000
992
8
Limiting Element
HVDC Bhadrawati link
capacity
*only one circuit is available presently on 400 kV Bina-Gwalior-Agra. The other line is under upgradation to 765 kV
level. 400 kV Zerda-Kankroli and 400 kV Kankroli-Bhinmal lines are under forced outage.
x----x----x
NLDC, POSOCO
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