Power system security is defined as the ability of the power system to withstand the occurrence of
credible disturbances as defined by security criteria or standards. Stability control, which is one of the
pillars to system security and the subject of this technical brochure, aims at maintaining the security
of supply according to cost-effective criteria. It has always been a top priority in both industrial
practice and academic research to ensure the reliable operation of power systems. To this end,
extensive activities have already been undertaken by e.g. CIGRE in the field of power system stability
control and Dynamic Security Assessment (DSA).
Analysis from past blackouts and major system disturbances has pointed out some potential
deficiencies in current stability control techniques. These deficiencies are related to various aspects of
design and maintenance of control systems, being a direct result of the insufficient systematic design
and adaptability of, and coordination among, the conventional stability controls. According to a
questionnaire survey conducted by this JWG, the full benefits from a systematic framework approach
for power system stability were recognized by many of the respondents. All these key elements are
integrated in the framework proposed by this JWG and reflect the existing experience.
This TB proposes a framework consisting of the four established control types described below and
coordinating them to achieve enhanced performance. As fundamental requirement to achieve this
goal, the proposed framework associates these stability control types with their respective system
states. The first control type is named preventive control. It is activated in the normal or alert state,
and is carried out to maintain a sufficient stability margin. Once a predefined contingency occurs the
system could rapidly evolve towards an emergency state, where the second stability control type,
called event-based control, is triggered. The third stability control type, called response-based control,
is usually initiated following the violation of key variable limits in the emergency state. In cases where
the operation of all the previous controls proves insufficient, the system will degrade to a blackout
state. Restorative control, the fourth control type, is activated following a blackout or after an
emergency state and remains active over the whole restoration process.
This TB emphasises that these control types gain added value if they are adaptive and coordinated as
recommended in the proposed framework. When adaptive, they are able to adjust their control
decision set to the current operating condition and identify the contingencies. The coordination of
these adaptive controls yields a more cost-effective set of planned control decisions
This TB describes the functional structure of the proposed framework whose design shares the same
architecture of an online DSA system as well as the same configuration and hardware of existing
automatic control devices at substations.
This functional structure is comprised of four high-levels modules:
Wide area data acquisition and information processing,
Real-time monitoring, online estimation and online stability analysis,
Adaptive and coordinated decision planning of stability control, and
Automatic activation of event-based and response-based controls.
The first and second high-level modules are already well established in the industry. However, the
third module is not yet at the same maturity level. To this end, this TB recommends the following
elements in order to design and develop software with required functionalities in the proposed
framework: Quantitative stability analysis, Determining in advance the optimal stability control decision for each relevant control type,
and
Coordinating the previously determined stability control decisions across all control types.
This framework and its underlying software strongly rely on its integration with online DSA and other
tools used at the control centre. Besides, these systems are fed with field data from measurement
devices (e.g. PMUs) whose number and location should be effectively selected. Data exchange
protocols between all these elements are critical for their integration into the framework. Bad data
detection is a prerequisite for online monitoring and analysis, and absolutely critical for the smooth
functioning of the framework so that proper stability control decisions are always taken.
This TB also provides some key considerations and recommendations in the design and
implementation of the proposed framework for 1) specification designers, 2) manufacturers and 3)
system operators.
Before implementing such a framework, the grid owner (and also the specification designer) should
conduct a cost benefit analysis to compare its effectiveness with alternatives.
After the grid owner decides to implement such framework, demonstration projects and trial
operations have to be conducted with a particular focus on validation of
control decision planning. It is
highly recommended to develop laboratory or field validation tests for key devices and systems. These
tests need to be conducted before as well as after commissioning. Especially for event-based and
response-based controls, which operate infrequently, post-commissioning testing remains important.
It has to be mentioned that there are still some remaining issues. One of these is the execution of the
optimisation process that is quite complex and consists of an iterative search based on simulations.
Currently this optimisation does not guarantee a global optimum or even convergence. This issue
should be the focus of research and development by both manufacturers and academia. Another
major issue to be tackled by specification designers relates to cyber security aspects, which have only
been briefly touched upon in this TB. Thirdly, the remote modification of control settings is not yet a
widely accepted practice, and the proper design of operator’s intervention and validation mechanisms
is still lacking. To address these issues more effort is needed mainly from the system operator’s perspectives.
The proposed framework is expected to overcome most of the deficiencies of the current stability
controls. Yet, some challenges do remain and the following suggestions for future work are provided.
Firstly, the applicability of the proposed framework in a multi-TSO environment with common grid
model, mainly in the emergency state, needs further investigation. Secondly, power oscillations that
occasionally occur and might cause the triggering of incorrect control decisions, should be fully
understood and have their adverse consequences mitigated. Further development would be directed
towards the improvement of control decision planning by combining system-wide response
measurements with pre-disturbance simulation results. Thirdly, the stability characteristics of modern
power systems are changing due to the increasing level of power electronics devices. Its impact on
stability control needs to be properly determined in order to ensure the correct operation of the proposed framework.
2018. , p. 147