The power system is becoming wide and complex. The aim of the power system is to keep the operating state of the power system to lie in the normal state. In the normal operating state all constraints like voltages at nodes, real and reactive power generation, real and reactive power flows are satisfied, all the system variables are within acceptable ranges and in that case, system is said to be secure. The power system at any time can never be totally secure. Any slight disturbance in the system initiates abnormality. The disturbances can be small or large, localised or widespread. Sequence of events may lead to a total or partial collapse of the system. At occasions some of the components operating limits are violated; some of the states wander outside the acceptable ranges, or the system frequency starts decreasing and the power system enters an emergency state. Restorative strategies are adopted to bring back the power system to normal operating state. In fact, the operating state of a power system determines the security of power system.
The power system should continue to operate “normally” even in the case of credible contingencies in such a manner that the system integrity and quality of power supply is maintained. The planning and design of the power system must achieve a certain level of security. The growth of large interconnected power systems demands a high degree of security for normal operation. System security involves practices designed to keep the system operating when components fail. The significant factor in the operation of a power system therefore craves to maintain system security. Planning of the power system, that is, the decision to add new generation, transmission or distribution has to consider security criteria.
Power system security connotation
Power system security is defined as the ability of the power system to withstand sudden disturbances and remain secure without serious consequences to any pre-selected list of credible contingencies by maintaining the system operation within defined boundaries. It is the degree of risk apropos to survive imminent disruption without interruption of customer service. The power system has operational propensity to maintain or to regain an acceptable state of operational condition after disturbances. Violation of any security related inequality constraints pushes the system to emergency or insecure state, thereby initiating corrective actions to be taken to bring the system back to secure state. Secure state implies that the load is satisfied and no limit violations will occur under present operating conditions and in the presence of unforeseen contingencies. Operational security is prevalently correlated to internal threats and medium-high probability events and its limits are the acceptable operating boundaries for secure grid operation, voltage limits, short circuits current limits, frequency and dynamic stability limits.
In the infrastructure dimension, power security is assessed in terms of electricity value chain as a capability to supply end users with minimum service criteria. As a source, it is the capability of power system to ensure the accessibility, in the various time-frames, to primary sources to be converted in the power plants to meet the required total demand of electricity. In the regulatory and market facet power security is evaluated in terms of market capability to adequately fulfil power delivery mission with a set of laws, rules and market arrangements at a transparent and cost-oriented price of electricity. In the geopolitical aspect power security is to assure the availability of primary sources and cross-border electricity exchanges within geopolitical constraints and stresses.
Power system security challenges
The security of power system is not a function of its single components but of their dynamic interactions. It is these interactions that determine the capacity of the system to tolerate disturbance and to continue to deliver affordable electricity to consumers. The fact is that electricity is not easily and economically storable; consequently, frequency and voltage, due to continuous changes both in demand and supply side, are constantly subject to variations. Frequency and voltage have to be kept as close as possible within the admissible variation ranges, to ensure that all the system components operate in the most appropriate way according to their technical design specifications. Keeping the transmission system secure and stable is a complex task. In order to avoid disruptions and wide area disturbances, power flows must be within the stability limits. The main technical challenges relating time-frame is to keep the system stable following a perturbation, balancing generation and demand across the whole system. The system has to rapidly react at very short notice for increasing or decreasing generation output to manage the imbalance.
During large disturbances, the speed governor has to restore the equilibrium between the input and output of electricity. Frequency restoration reserves are needed to get the system frequency back to the nominal reference value. For voltage regulation at transmission level the reactive power either capacitive or inductive needs to be supplied by generators, to maintain the voltage at the required level. Major faults in the system require trigger protection devices. Several generation units are connected to distribution systems if put on large frequency such threshold jeopardise the security of the entire interconnected system. To check the effects of an outage on the transmission and distribution system requires significant amount of computation. Usually unwanted protection operations result in a low level of security thus security problems are augmented. Electricity security performance is largely a legacy of investments dating back to several decades ago. Ageing capacity needs to be replaced within a competitive market framework along with decarbonising of the electricity sector. As such there are several challenges and strains affecting power system security.
Approaches for power system security
The approach to power system security has direct implications on the methodological requirements for a comprehensive quantitative assessment. It includes the functioning of power system in response to the wide range of different events affecting the system. An approach to power security assessment, basically, deals with evaluating the ability of the system to continue to provide service in the event of an unforeseen contingency. Conventional method of security evaluation involves solving full alternating current load flow equations and transient stability analysis of the current system state by time domain simulation program. The analysis is performed to determine whether, and to what extent, the system is reasonably safe from serious interference to its operation.
Ensuring power security in the new environment requires the use of advanced power system analysis tools capable of comprehensive security assessment with due consideration to practical operating criteria. These tools must be able to model the system appropriately, compute security limits in a fast and accurate manner to become a dominant weapon against system blackouts. A single tool or model is not likely to be sufficient for addressing the multidimensional nature of all relevant factors affecting the security of power system.
The Integrated Markal-EFOM System (Times) is a widely applied linear programming system modeling tool. Electricity supply and demand is considered simultaneously to help decision-makers in problems of identifying, quantifying, and controlling a system. Although considering multiple objectives, constraints, resources, it aims to specify possible courses of action, together with their risks, costs, and benefits. It generally captures seasonal changes in supply and use of power in addition to limited diurnal variations pertaining day, night and peak.
Plexos power systems model has a dedicated focus on the electricity and examines the power system using higher levels of technical and temporal resolution, by representing the key technical characteristics combining long-term planning. This power system modeling tool is carried out for using deterministic or stochastic programming techniques that aim to minimise an objective function or expected value subject to the modeled cost of electricity dispatch and to a number of constraints including availability and operational characteristics. The model solves using linear or mixed integer linear programming.
Spica is used to calculate the voltage collapse limits by load flow calculations. The load flow is calculated at operating point where balance in active and reactive power can be made operational.
Power system security control
To the extent practicable, the power system should be operated such that it is and will remain in a secure operating state. Following a contingency event whether or not a credible contingency event or a significant change in power system conditions, all reasonable actions should be taken to adjust, wherever possible, the operating conditions with a view to returning the power system to a secure operating state as soon as it is practical to do so. Adequate load shedding facilities initiated automatically by frequency conditions outside the normal operating frequency excursion band should be available and in service to restore the power system to a satisfactory operating state following significant multiple contingency events. Sufficient system restart ancillary services should be available in accordance with the system restart standard to allow the restoration of power system security and any necessary restarting of generating units following a major supply disruption.
It is essential to monitor and coordinate the operating status of the power system to ensure that high voltage switching procedures and arrangements are utilised to provide protection to the power system. It has to be ensured that all plant and equipment are under control and are operated within the appropriate operational or emergency limits. Constraint on the dispatch of generating units, loads, market network services and ancillary services has to be determined including availability, adequacy, appropriate level of reactive and contingency power reserves are available. There should be an arrangement to ensure rotation of widespread interruption of demand in the event of a major supply shortfall or disruption and manage an extensive disruption. There should be a backup plan to investigate and review all major power system operational incidents to initiate action plans and manage any abnormal situations or significant deficiencies which could reasonably threaten power system security.
The object to maintain power security system is that the power system must operate satisfactorily. The frequency at all energised bus bars of the power system must be within the normal operating frequency band, except for brief excursions and the voltage magnitudes at all energised bus bars at any switchyard or substation of the power system are within the relevant limits. The current flowing on all transmission lines is within the ratings. All plant forming parts that may impact the power system are being operated within the relevant operating ratings. The configuration of the power system must be such that the severity of any potential fault is within the capability of circuit breakers to disconnect the faulted circuit or equipment; and the conditions of the power system are stable in accordance with the designated requirements. This has to be ensured to keep the power system in the normal state.
Harsha Rajwanshi is the Assistant Professor of Law, Gujarat National Law University & Faculty Advisor to GUVNL-GNLU Research Fellowship on Energy Law and Policy.