Steve Turner

Redundancy considerations for protective relaying systems

The basic concept of redundancy is simple. Instead of relying on a single piece of equipment, there are duplicate or triplicate sets that perform the same function. Consequently, if one piece of equipment fails, the function will still be performed by a redundant device. Redundancy of components plays a major role in elevating the reliability of protection systems. The impact on the power system when a protection device is not functioning when required is much less severe when there is a redundant device that takes over the job. If the redundant devices are of equal performance, there should be no detrimental effect at all on power system operations, and a non-functioning device would just need to be repaired or replaced. While local redundancy is generally applied, it is not the only mitigation that can be used to improve reliability. Remote protection systems may provide adequate protection system reliability in some situations, provided that remote protection can detect faults and provide clearing times that meet performance requirements. Different users have different terminology for referring to the redundant protection systems. They may be called “System 1” and “System 2” or “System A” and “System B” or sometimes “Primary” and “Backup.” This latter terminology, “Primary” and “Backup”, implies, although unintentionally, that one of the two systems serves the main function of protection and the other serves to assist in the case of failure of the first system, analogous to carrying an undersized spare tire in the trunk of a car in case of a flat. In actual practice, the redundant systems are each fully capable, each system is able to detect and clear faults on its own, and each system serves as a backup to the other. Note that this paper is a summary of the full report. The report will be available on www.pes-psrc.org – Published Reports – in May-June of 2010.

Using COMTRADE records to test protective relays

This purpose of this paper is to demonstrate how to use COMTRADE records to commission protective relays. COMTRADE records that have been captured by numerical relays and digital fault records during actual system events are of particular interest since these provide the ability to test protection for critical faults or disturbances such as an out of-step condition that are difficult to create using off the shelf test set software. Utilities and other customers can build a library of test cases. This paper shows how to use COMTRADE records to test transformer differential protection, loss of field protection and out-of step protection.

Black start generator protection considerations

Application of sound relaying principles helps prevent unwanted tripping during black starts. An unwanted trip during a black start is extremely detrimental since the utility is attempting to reconnect its high voltage grid and restore service throughout its operating region. A black start is the process of restoring a power station to operation without relying on external power sources. Normally, the electric power used within the plant is provided from the station generators. The highvoltage grid provides station service power through the transmission lines if all the main generators are shut down. This off-site power source is not available during a wide-area outage. Therefore, a black start is required in the absence of grid power. Generating plants using steam turbines require station service power up to ten percent of their total load capacity (to bring on-line: boiler-feed water pumps, boiler-forced draft combustion air blowers, and fuel preparation equipment, for example). It is uneconomical to provide such a large standby capacity at each station, so black start power is only provided over the electrical transmission network from specific stations. Often hydroelectric power plants are designated as the black start sources to restore network interconnections since these stations need very little initial power to start and can put a large block of power online very quickly to allow start-up of fossil fuel and nuclear stations. Careful detailed analysis is required to properly apply generator protection at stations with black start capability. Generator relay current inputs are subjected to high levels of dc offset and harmonic current due to energizing transformers during a black start. Proper selection of current transformers for the generator differential protection is necessary to avoid a mismatch. Both the system side and neutral side sets of current transformers should have the same saturation voltage characteristics because if there is a large DC offset present the current transformers can saturate with restraint current significantly less than two times the nominal relay current. The case of a generator differential protection misoperation during a black start at a large hydro station is examined in detail to illustrate what steps to take to prevent these types of nuisance trips without sacrificing relay sensitivity. An unwanted trip occurred because of the particular differential protection operating characteristic. One solution presented monitors the harmonic content of the relay current to momentarily desensitize the differential protection while the transformer draws heavy inrush current. Use of a second generator differential element in tandem with the first to minimize the chance of an unwanted trip from occurring during a black start is also discussed.

End-to-end testing transmission line protection schemes and double-ended fault locators

This paper explains how to test double-ended fault locators for high voltage overhead transmission lines. There are many similarities to testing high speed communication assisted tripping schemes (HSCATS) however there are some important differences which are covered here in detail. Note that double-ended fault location is coming into vogue now and these tests can be performed in unison while testing the HSCATS.

Improving reliability and security of system protection

This paper is based upon a NERC report released in 2013 that claimed a dramatic rise in the annual number of misoperations-due in large part to the complexity of programming and testing numerical protection relays. This paper illustrates results discussed in the NERC report, as well as provides several interesting examples of actual misoperations and how to mitigate them.

Optimally testing numerical multi-function generator protection elements

A numerical generator protection relay provides multiple functions and typically uses algorithms that do not work on the same principles as older electromechanical relays. Often relay test personnel still want to use old test methods developed long ago for electromechanical relays. Test results obtained from these old test methods may not be a good indication of whether or not the numerical relay protection functions properly. One such obsolete technique still often used is referred to as a static test. Fault voltage and/or fault current are injected to see if the selected protection function operates at the setpoint and is in tolerance. Many protection functions have dynamic characteristics and/or hidden features that cannot be properly tested using a static test. This paper demonstrates various techniques how to test numerical protection relays such as dynamic tests.

High-speed communication-assisted tripping and sectionalizing for distribution systems

The recent advent of Smart Grid has given rise to advances in communication systems for distribution systems. Modern numerical overcurrent relays have the technology available to utilize these communication channels for both high speed-assisted tripping and sectionalizing. Assisted tripping and sectionalizing allows the utility to operate their distribution system in a network as opposed to radial feeders. A networked system is much more reliable and customers experience fewer outages since there are multiple sources readily available. Assisted tripping and sectionalizing quickly isolates the fault and eliminates the need for long clearing times and complex coordination typically associated with classical time overcurrent protection. 1This paper demonstrates how to apply communication-assisted tripping and sectionalizing, taking into account specific considerations for distribution systems not necessary for transmission applications-for example, a feeder with distributed generation connected. Several detailed examples are presented as to how to implement these schemes for typical distribution systems. This paper also gives specific settings and operational details of these schemes. Additionally, a review of the operational history highlights the impact that these schemes have on the reliability of the utility distribution network.

Testing numerical transformer differential relays

Numerical transformer differential relays require careful consideration regarding how to test them properly. These relays provide different types of protection such as restrained phase differential, high set phase differential, restrained ground differential and overcurrent protection. All protection elements that are enabled should be adequately tested. A common commissioning practice is to test all the numerical relay settings to verify they were properly entered. Automated testing using computer software to run the test set has made this possible since the overall commissioning for a numerical relay could consist of several hundred tests. While this is a good check, it is still important to ensure that the transformer is thoroughly protected for the particular application.