Lessons learnt, the Northern grid collapse

Collapse of the Northern Regional Grid of India
K Ramanathan, Senior Fellow, TERI, New Delhi, India


The Northern Region Grid of India witnessed a massive disturbance on 2 January 2001. Over 1500 MW of generation was lost and the entire region
plunged into darkness, subjecting public at large to immense inconvenience and a loss to the tune of 7 billion rupees. It took 16–20 hours for the system to limp back to near normalcy. The last major grid disturbance in the region was in January 1997.


The CEA (Central Electricity Authority) carried out a comprehensive enquiry
into the incident following directions from the Ministry of Power, and the
results have been made public (www.powerwin.nic.in). The CERC (Central
Electricity Regulatory Commission) initiated suo moto proceedings and
passed an order based on the public hearings held 15–16 January 2001
(http://www.cercind.org/ ).


Northern Regional Grid
The Northern Regional Grid covers Uttar Pradesh, Delhi, Punjab, Haryana,
Rajasthan, Himachal Pradesh, and Jammu and Kashmir. It is the second
largest interconnected network in the country with an installed capacity of
over 27 000 MW and a plant mix of 31% hydro, 65% thermal, and 4%
nuclear.

A 400-kV network and one + 500 kV, 1500 MW HVDC (high voltage
direct current) bi-pole link constitute the backbone of the grid. The bulk of
this network is owned by the CTU (central transmission utility), Powergrid.
Nearly 6500 MW generation is located in the south-eastern tip of the grid
and the bulk of this power is to be transported to the western side.


Therefore, there is generally a heavy flow of power along the east–west axis
of the grid. This transmission corridor has five 400 kV links (two owned by
the STU or state transmission utility) besides the HVDC bi-pole link, and its
performance is crucial to grid security.

The region also has a 500-MW HVDC interconnection with the western region. The real time operation of the grid is organized in a hierarchical structure and achieved through the regional load dispatch centre (operated by the CTU) and the state load dispatch centres (operated by the STU).


Build up to collapse
In the early hours of 2 January 2001, the demand was much less than the availability, following widespread rains, and a few thermal machines had been closed down to contain the frequency. However, there was a constraint in the transmission capability in the east–west corridor due to outage of one of the HVDC bi-poles and a reduction of the power capability of the other pole to 500 MW. This reduced further when parts of the two 400-kV links owned by the STU tripped in quick succession due to insulator flash -overs. This in turn led to heavy loading of the remaining links, with power flow touching 800 MW in one of the 400-kV lines, and cascade tripping of the remaining lin ks.

Eventually the grid split into two. In the western part, the frequency dropped due to shortfall in generation and the subsystem collapsed, while in the eastern part the frequency shot up to about 53 Hz, tripping all the running machines and leading to the eventual collapse of the subsystem.

Only a few gas turbine units in Delhi and one nuclear unit survived on local load. All other nuclear units tripped and were poisoned out.


System restoration
The system was restored in two parts: the eastern part by taking power
through the AC bypass of the inter-regional HVDC link and the western part
after starting the hydel units at Bhakra. The two parts were synchronized
after 8.5 hours. There were no doubt a number of hiccups due to tripping of
lines, problems in controlling voltage, improper load generation balance, etc.
A large part of Delhi remained without power for over 12 hours.


Analysis
The enquiry carried out by the CEA committee and the submission made
during the public hearing held by the CERC revea l the following.
There was a general lack of grid discipline. For example, (1) the generators
had not been on ‘free-governor’ mode operation although this was
required by the IEGC (Indian Electricity Grid Code), (2) instructions given
from the RLDC (regional load dispatch centre) were not promptly acted
upon, and (3) underfrequency relay schemes were not fully operational.


The role played by the RLDC was not adequate or effective. Its orders for backing down, for example, did not reflect the urgency or the gravity of the situation.


Also, it does not appear to have considered other options for congestion
management like increase in hydro generation in the western part,
coupled with load shedding. This might have helped avert the incident as
it was not triggered by a sudden event and the system depletion was
gradual.


The maintenance and performance of the transmission system was not
adequate. The insulator flash -overs and the resultant tripping of the two
important links in the east–west corridor occurred due to the poor
maintenance of the 400-kV lines owned by the STU, which pass through
the polluted areas near Panki.

Similarly, the performance of the HVDC link, especially of the convertor transformers manufactured by BHEL (Bharat Heavy Electricals Ltd), was not satisfactory and affected the transmission capability in the corridor. Such lapses could jeopardize the security of the grid since the system is planned to an n-1 contingency criterion.

There were also inadequacies in the switchgear and reactive power compensation devices at some locations, as the restoration process
revealed.


The load dispatch and communication facilities are not adequate. Lack of time synchronization between recording instruments installed at different locations makes it difficult to correctly establish the sequence of events.
The restoration procedure was slow. Extending supply and build up of
load was difficult due to equipment problems, delays in providing start-up
power, lack of coordination, etc. The black start procedure and the grid
protection scheme must be reviewed.


There is need for better training and capacity building for grid operators.

The CERC order
The CERC has looked into the incident primarily as part of its function to
regulate interstate transmission. The ambit of its enquiry was restricted to
the role of various functionaries in ensuring compliance of the provisions of
IEGC.


In its order, the CERC noted that there was blatant violation in operating the
grid as per the IEGC. While it has not taken action against the erring bodies,
it has warned the concerned functionaries of penalties in case of future noncompliance.


It has severely criticized the CTU/RLDC for not implementing the grid code with full commitment and for not managing the grid efficiently.


It has directed the CTU to see if it is necessary for the RLDC to control the state
level generation as well if it has to have adequate control over the grid and maintain its security.


Action plan of ministry
The Ministry of Power has decided to adopt a time-bound action plan for
various organizations to take remedia l measures to prevent the recurrence of
such events. This covers tuning of governors, provision of self-start facilities
at gas turbine stations, implementation of revised underfrequency relay
scheme, review of schemes for protection, islanding, and black start,
simulator training for dispatch engineers, etc. A close follow up of the action
plan is proposed.


Reform and regulatory issues
The collapse of the Northern Grid and the similar incident in the Eastern
Region recently have brought to fore certain reform and regulatory issues.
These include the delineation of the interstate transmission system and the
responsibility of the CTU for STU-owned lines; command hierarchy in grid
operation; desirability of CTU/STU looking after the load dispatch functions;
overlaps in roles, if any, of CERC, CEA, regional electricity board, and the
state regulatory commissions in enforcing grid code; and urgency for putting
in place suitable commercial mechanisms to promote grid discipline.


The collapse of the grid highlighted once again that integrated operation of
large grid systems is a complex task requiring orchestrated functioning of all
agencies connected to the grid (generators, transmission companies, and
generation companies), a well-equipped hierarchical load dispatch system
manned by trained professionals, and a comprehensive grid code.