Power system protection

Power system protection is a branch of electrical power engineering that deals with the protection of electrical power systems from faults through the isolation of faulted parts from the rest of the electrical network. The objective of a protection scheme is to keep the power system stable by isolating only the components that are under fault, whilst leaving as much of the network as possible still in operation. Thus, protection schemes must apply a very pragmatic and pessimistic approach to clearing system faults. For this reason, the technology and philosophies utilized in protection schemes can often be old and well-established because they must be very reliable.

Components

Protection systems usually comprise five components:

• Current and voltage transformers to step down the high voltages and currents of the electrical power system to convenient levels for the relays to deal with;
• Protective relays to sense the fault and initiate a trip, or disconnection, order;
• Circuit breakers to open/close the system based on relay and autorecloser commands;
• Batteries to provide power in case of power disconnection in the system.
• Communication channels to allow analysis of current and voltage at remote terminals of a line and to allow remote tripping of equipment.

For parts of a distribution system, fuses are capable of both sensing and disconnecting faults.

Failures may occur in each part, such as insulation failure, fallen or broken transmission lines, incorrect operation of circuit breakers, short circuits and open circuits. Protection devices are installed with the aims of protection of assets, and ensure continued supply of energy. The three classes of protective devices are:

Protective devices

protective relay for distribution networks

• Protective relays control the tripping of the circuit breakers surrounding the faulted part of the network
• Automatic operation, such as auto-reclosing or system restart
• Monitoring equipment which collects data on the system for post event analysis

While the operating quality of these devices, and especially of protective relays, is always critical, different strategies are considered for protecting the different parts of the system. Very important equipment may have completely redundant and independent protective systems, while a minor branch distribution line may have very simple low-cost protection.

Types of protection

• Generator sets – In a power plant, the protective relays are intended to prevent damage to alternators or to the transformers in case of abnormal conditions of operation, due to internal failures, as well as insulating failures or regulation malfunctions. Such failures are unusual, so the protective relays have to operate very rarely. If a protective relay fails to detect a fault, the resulting damage to the alternator or to the transformer might require costly equipment repairs or replacement, as well as income loss from the inability to produce and sell energy.

• High voltage transmission network – Protection on the transmission and distribution serves two functions: Protection of plant and protection of the public (including employees). At a basic level, protection looks to disconnect equipment which experience an overload or a short to earth. Some items in substations such as transformers might require additional protection based on temperature or gassing, among others.

• Overload – Overload protection requires a current transformer which simply measures the current in a circuit. If this current exceeds a pre-determined level, a circuit breaker or fuse should operate.

• Earth fault – Earth fault protection again requires current transformers and senses an imbalance in a three-phase circuit. Normally the three phase currents are in balance, i.e. roughly equal in magnitude. If one or two phases become connected to earth via a low impedance path, their magnitudes will increase dramatically, as will current imbalance. If this imbalance exceeds a pre-determined value, a circuit breaker should operate.

• Distance – Distance protection detects both voltage and current. A fault on a circuit will generally create a sag in the voltage level. If this voltage falls below a pre-determined level and the current is above a certain level, the circuit breaker should operate. This is useful on long lines where if a fault was experienced at the end of the line the impedance of the line itself might inhibit the rise in current. Since a voltage sag is required to trigger the protection, the current level can actually be set below the normal load on the line.

• Back-up – At all times the objective of protection is to remove only the affected portion of plant and nothing else. Sometimes this does not occur for various reasons which can include:
  • Mechanical failure of a circuit breaker to operate
  • Incorrect protection setting
  • Relay failures

A failure of primary protection will usually result in the operation of back-up protection. Back-up protection will generally remove both the affected and unaffected items of plant to clear the fault.

• Low-voltage networks – The low voltage network generally relies upon fuses or low-voltage circuit breakers to remove both overload and earth faults.

Coordination

Protective device coordination is the process of determining the "best fit" timing of current interruption when abnormal electrical conditions occur. The goal is to minimize an outage to the greatest extent possible. Historically, protective device coordination was done on translucent log-log paper. Modern methods normally include detailed computer based analysis and reporting.

Disturbance monitoring equipment (DME)

Disturbance monitoring equipment monitors and records system data pertaining to a fault. DME accomplish three main purposes: 1) Model validation, 2) disturbance investigation, and 3) assessment of system protection performance^[1]. DME devices include ^[2]:

• Sequence of event recorders, which record equipment response to the event

• Fault recorders, which record actual waveform data of the system primary voltages and currents.

• Dynamic Disturbance Recorders (DDRs), which record incidents that portray power system behavior during dynamic events such as low frequency (0.1 Hz – 3 Hz) oscillations and abnormal frequency or voltage excursions

References

• Mason, C. Russell. "The Art and Science of Protective Relaying". General Electric. http://www.geindustrial.com/pm/notes/artsci/artsci.pdf. Retrieved 2009-01-26. 
• "Coordinated Power Systems Protection". Army Corps of Engineers. 1991-02-25. Archived from the original on 2008-01-13. http://web.archive.org/web/20080113071902/http://www.usace.army.mil/publications/armytm/tm5-811-14/. Retrieved 2009-01-26. 
• "How Do Protection Relays Work?". http://www.littelfuse.com/protection-relays/introduction/solving-electrical-problems.html. 
• "What is SCADA?". http://www.roseindia.net/technology/scada/what-is-SCADA.shtml. 

1. ^ http://www.nyiso.com/public/webdocs/documents/tech_bulletins/tb_172.pdf
2. ^ http://www.nerc.com/files/Glossary_12Feb08.pdf