A conventional protection system has relays with fixed setting parameters. With the growing complexity in operating power systems, the increasing shares of power electronic connected generating units, a lack of short circuit current injection to correctly detect faults, and increased harmonics that can falsely trigger protection relays, various challenges arise to fulfil the protection requirements in variable operation conditions. Adaptive protection schemes result from the application of microprocessors in the area of protective relays and are growing in importance in the electrical power systems. They enable grid operators to have flexible protection schemes in response to changes in the power system.
Protection relays are devices deployed in the field that trip off under certain conditions. They can be categorised as:
- Analogue relays: the conventional electromechanical or static relays operating offline, with low maintenance, reliable but unable to determine the exact location of the fault. They establish a zone of protection in which the relay will operate or trip if they measure an overcurrent. They have high maturity but allow only for a fixed protection scheme.
- Digital relays: recent microprocessor / numerical relays which operate online and can adapt their settings in real time in response to power system conditions.
Adaptive protection schemes can be categorised by the adaptive protection device and adaptive protection system. Developed since the 1980s, they are based on the underlying idea of the ability of the protection system to adapt to the current operating condition of the power system.
Various definitions of the adaptative protection function can be found in the literature, such as the one proposed by the IEEE Power System Relaying Committee: ‘automatically adjusts the operating characteristics of the relay system in response the changing power system conditions’. This definition clearly reflects the two main features of an adaptative protection scheme: (i) the adjustment of the protection functions or configurations, and (ii) the automatic nature of this adjustment.
Protection functions deployed in transmission and distribution systems differ. This results mainly from the more stringent stability requirements for transmission with more complex schemes in addition to the necessary redundancy. Distribution networks require more cost-effective protection solutions due to the volume of assets that require protection.
Protection principles include generally unit-based protection, for which the zone of the protection boundary is limited, and the non-unit protection, which relies on local measurements to inform about protection features (e.g. overcurrent, distance protection):
- unit-based protections are mostly applied to transmission networks where the cost of required communications is justified, such schemes are highly selective in their operation
- non-unit protection schemes can be found in both transmission and distribution.
Due to its simplicity, overcurrent protection is mostly applied in distribution, but also in transmission as a backup protection function.
Distance protection is a mature protection mainly used in transmission systems . It is based on the observation that the protected line impedance is proportional to its length: monitoring changes in the impedance (e.g. by measuring locally voltage and current) thus enables the identification of a fault without any communication device.
System integrity protection schemes (SIPS) are used to protect the overall integrity of the power system against events leading potentially to unstable transients, overloads or blackouts. The advent of wide area measurements promises more flexibility in available protection actions.
Components & enablers
When studying adaptative protection literature over the last three decades and the applications in system operators, a progression is thus observed from basic concepts such as subsystems (e. g. adaptive distance relay), and localisation studies or rapid coordination calculation, to technologies and experiments of wide geographic area protection that could be considered as extensions of the protection system.
Consequently, the adaptive protection scheme aims to monitor the power system to determine its state and adjust its configuration accordingly. In particular, adaptive relaying means changing relay settings and picking up relay currents in online mode as the operating conditions of the system changes.
Adaptive protection schemes require typically the following components: hardware (referring to the digital relay itself and intelligent electronic devices – IEDs), computational and communication systems that model and monitor the relays and coordinate the adaptation of parameters, the communication protocol for intelligent electronic devices at electrical substations and the algorithms ruling the settings, as well as the protocols to interface to human factors.
Wide Area Monitoring System (WAMS) is an enabling technology based on an information facility with monitoring purposes to improve situational awareness and visibility within power systems. Based on Phasor Measurements Units (PMUs), WAMS allow monitoring transmission system conditions over large areas in view of detecting and further counteracting grid instabilities. As mentioned above, such an early warning system contributes to increasing system reliability and can be considered as an extension and enabler of an adaptive protection system:
- PMUs sensors - measures bus angles and frequencies at high sampling rate
- WAMS – monitoring device that time synchronise via geolocalised PMU measurements.
Advantages & field of application
An ex post study of the National Electric Reliability Council on black-outs in the US showed that the maloperation of relays has contributed to 70% of US black-outs. Adaptive protection technologies therefore a potential facilitator of the reliability, resilience and security of the future power system. Indeed, real-time adaptation of the system protection actions to the true system state enable the prevention of cascade failures and wide area disturbances to power system blackouts, ensure the security of back-up relays and limit the impact of hidden failures that are revealed under stressed conditions.
In addition, the increasing use of grid assets constitutes a considerable benefit: as protection setting and thermal limits are calculated for worst case conditions, power system assets are underutilised throughout most of their lifetimes.
Technology Readiness Level
Wide area, adaptative protection addresses a large variety of technologies of various levels of maturity and TRL ranges from TRL 2 – Technology formulation (concept and application) to TRL 9 – System ready for full scale deployment.
Research & Development
Current fields of research include topics that could be grouped.
- Multi-agent system based protection
- Distributed adaptive protection schemes
- Resilience to system splits
- Smart coordination of overall protection devices, improved speed for detection and clearing
- Algorithms for robust operation to all network configurations and network conditions and wide area backup protection scheme, possibly based on ma-chine learning techniques, fuzzy logic (-neuro, -wavelet based)
- Blockchain technology for protection relay configuration.
HVDC protection and web architecture:
- Hybrid HVDC/HVAC fault clearing
- HVDC protection devices/breakers
- Numerical directional overcurrent relays.
Protection relays and new equipment:
- Adaptation of protection components and systems to new market requirements, development of new material (e.g. fault current limiters with super-conductive materials).
WAMS / PMUS:
- PMU-based concepts for transmission line protection
- Signal accuracy and reliability, communication architectures and data processing, as well as on standards for data processing and large-scale demonstrations, possibly in combination with other active equipment.
- Regional WAMS and PMUs applications operational in TSOs to enable the operation of the transmission system closer to its physical limits with high reliability.
- Distributed observability of the transmission system through steady state and dynamic state estimation of transmission systems, using intelligent monitoring devices such as PMUs.
Best practice performance
Performance improvements are very case-specific and are subject to a pattern of changes: distance wise, from local measurement-based to remote measurement while, function wise, adaptation algorithms will combine advanced techniques such as multi-variable, multi-objectives or adaptative algorithms, able to operate stand-alone, even in cases of the loss of communication of any other unexpected event.
Best practice application
One of the four German TSOs upgraded the main control center using a new state of the art grid system. This TSO monitors approximately 11,000 km HV grid in Germany.
A new adaptive protection function will be integrated in the control system.
Combined with overhead line monitoring, this function is used to dynamically adapt the system limits to climatic conditions and to adapt protection device settings respective to the varying short-circuit level.
The ELECTRA Integrated Research Program on Smart Grids envisions for the future power system a novel grid architecture, called the Web-of-Cells with a wholescale deployment of distributed energy resources. The adaptive scheme has been applied to a representative cell grid and it addresses appropriate actions by protection devices to cope with the flexibility required by the Web-of-Cells concept.
The arrangement of this concept includes power system cells bounded to a geographical area which are interconnected via tie lines and where each of these cells is managed by a Cell Control Operator (CCC). The feature of the Web-of-Cell concept combines flexible resources, the use of advanced information and communication technology as well as novel frequency and voltage control schemes.
The case study simulation with the designed adaptive functionality for IED models showed that the proposed protection scheme responded appropriately at disturbed grid conditions coping with different cell operating states. The results show that the CCC enables a proper automatic shifting of IED tripping curves according to the short-circuit level during the islanding condition of the cell.
 ENTSOE. Best Protection Practices for HV and EHV AC-Transmission Systems of ENTSO-E Electrical Grids. Version 2 June 2018.
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