IEEE Power & Energy Magazine - September/October 2015 - 50

The Indian grid has been able to
maintain a reasonable degree of stability
over the years.

2006. The national load dispatch center (NLDC), which
coordinates operations of the five RLDCs, started operations in 2009. Interconnections among the five Indian
regional grids started in the early 1990s, and all regional
grids have been synchronized since 31 December 2013.
Currently, the five RLDCs and the NLDC are managed
by the Power System Operation Corporation (POSOCO),
a subsidiary of PGCIL. The Indian grid has been able to
maintain a reasonable degree of stability over the years,
despite the exponential expansion of loads and renewables integration.
The transmission licensees, NLDC, RLDCs, and state
load despatch centers (SLDCs) were mandated to facilitate
trade through open access to the transmission system. To
do this, they had to determine the operational margins in
the existing transmission system in line with the Electricity
Act 2003 and subsequent national electricity and tariff
policies by the Indian government. Thereafter, RLDCs and
the NLDC started assessing and declaring the interregional
import/export transfer capability for facilitating open access
as well as managing system security.
Maintaining angular separation between coherent groups
of generators within the acceptable limits is a fundamental
need for maintaining system stability in a synchronous inter-

Conventional SCADA/EMS
In the conventional SCADA/EMS system, information
was available at the control center through the direct measurements of voltage, frequency, megawatt, Mvar, and the
circuit breaker status and by the state estimator (SE) computations of the load angle of the buses. In the absence of
a properly converged SE solution, the angular difference
between adjacent buses was computed through SCADA
using the power angle formula from 2002 onward. In the
case of the HVdc back-to-back station interconnecting the
northern and western grids (in addition to a parallel ac network), the vector difference of either side bus voltages was
telemetered at the RLDC, and the angular separation was
locally computed using the cosine formula. Subsequently,

Load Angle of Ramagundum-Neyveli for 13, 14, and 17 May 2002

80
70
Angular Separation (°)

connection under various system conditions. The power flow
(megawatt/Mvar) on any transmission line or interconnecting
transformers can also be derived if the time-synchronized
voltage phasors of the two ends are known (along with the line
impedance). During the synchronization of energized ac systems, it is crucial to match the voltage magnitude, frequency,
and phase-angle difference to prevent equipment damage or
grid disturbance in the process. Thus, load angle is an important variable in power system operation.

17 May 2002

60
50
13 May 2002

40
30

14 May 2002

20
10
0
1

10 11 12 13 14
Time of the day

16 17

18 19 20

21 22

figure 1. The load angle between Ramagundam and Neyveli for 13, 14, and 17 May 2002.
50

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