Figure 1 shows a typical differential relay connection diagram. Protective devices that are described in this course can be used in multiple generator protection configurations. Differential protection principle differential overcurrent protection. This protective system, the principle of which is shown in figure 3. Quite a number of protection systems have been designed for busbars. Directional blocking protection of these, 1 is suitable for small substations only, while 4 and 5 have become obsolete. Introduction to system protection protection basics and terminology.
Current differential line protection setting consideration. Learn the basics of differential protection of transformers. A sensitive current element in series with the highimpedance element is calibrated in volts based on the voltage drop across the internal impedance. Biaseddifferential protection theory and application eep. Within the zone of differential protection as demarked by ct location. Thus by comparing the two currents either in magnitude or in phase or both fault is detected. The fundamental operating principle of transformer differential protection is based on comparison of. Differential protection of transformer differential. In short, products with the highest levels of quality and cuttingedge. Faults instrument transformers correlation to drawings.
Percentage restraint differential protective relays have been in service for many years. The comprehensive transformer protection provided by combination of protection elements, with biased differential protection. The highimpedance input is created by an internal impedance, typically resistive, of 2000 ohms or higher. Differential protection of yy and connected power transformer mayurdhvajsinh p. Transformer differential protection electrical concepts. Differential relays are very sensitive to the faults occurred within the zone of protection but they are least sensitive to the faults. It is demonstrated that it is possible to make numerical differential protection relays which can properly calculate differential currents for any power transformer, regardless of. As discussed in the post transformer differential protection, differential protection is supposed to operate for the internal faults or for the zone of protection it is intended for. Modern line current differential protection solutions. Differential protection is the best technique in protection. A differential relay is defined as the relay that operates when the phase difference of two or more identical electrical quantities exceeds a predetermined amount. Principle and operation of differential relay what is differential relays. Differential protection for standard power transformers has been used for decades. Protection devices for electrical systems minimize fault damages, assist in maintaining power system stability and help to limit supply interruptions to consumers.
If the net difference equal zero, it means no fault exist. Your source for universal protective relay testing training. Medium voltage products technical guide protection. Generator protection introduction this course covers generator protection concepts and theory. Percentage restrained differential, percentage of what. There are various protection relays and those are used for protection against a wide variety of conditions. In this simple scheme, we can assume that under normal operating conditions, the current entering into the piece of equipment under protection is equal.
Principles of differential relaying the restraint characteristic what needs to be realised is that the first one is properly termed the restraint characteristic rc whilst the latter is an operating characteristic. Due to the long lead time for repair of and replacement of transformers, a major goal of transformer protection is limiting the damage to a faulted transformer. The need to act quickly to protect circuits and equipment often requires protective relays to respond and trip a breaker within a few thousandths of a second. Alexander ge corporate ge power management research and development malvern, pa schenectady, ny updated and presented by subhash c. An appraisal of differential association theory sy2003 introduction to criminology many have criticized sutherlands differential association theory on a number of grounds. The term biased differential or percentage differential is based on the fact that the operating threshold increases with the through current.
The relay which is used to checks the difference between the output and input currents for power system current in known as differential relay. Differential protection compares the magnitude and phase angle of the currents i1 and i2 i. Introduction the allenbradley 865 differential protection system includes all the essential protection functions needed to protect transformers for distribution networks of utilities, industry, power plants and offshore. Abstractline current differential protection creates challenges for relay design and application. Protection functions bias differential protection 87 harmonic restraint protection over fluxing i. Current magnitude and phase angle difference compensation. Thompson, schweitzer engineering laboratories, inc. The user will find an overview of nearly all of the popular protection strategies as well as a. A new approach to current differential protection for transmission lines m. Protection relays differential relays b high impedance differential protection. Pdf high impedance restricted earth fault protection.
The operating principle of the differential protection scheme is. Differential protection 87 generator differential step up transformer differential overall differential generator and transformer. It is based on ampereturnbalance of all windings mounted on the same magnetic core lag. Differential protection is not supposed to operate for a through fault. Differential protection may be considered the first line of protection for internal phase to phase or phase to ground faults. Busbar protection busbar differential protection scheme. Unequal ct performance challenges to transformer differential protection. The suite of overcurrent elements 51, section 8 is generally considered backup transformer protection, or for protection of the bus and backup protection for the feeder relays.
A new approach to current differential protection for. System protection is the art and science of detecting. Line differential protection substation automation. Differential protection is applicable to all parts of the power system. Overall differentail protection for thermal power plant. Principles of differential relaying my protection guide. The introduction of digital technology has led to further improvements in bus differential. An alternative principle for line protection that is quickly becoming the norm is differential protection. Most importantly is the inability to empirically verify the theory, as noted by cressey and.
Principle and operation of differential relay my tech info. Through fault means a fault outside the zone of protection. Differential protection is based on kirchhoffs laws, stating that all current into a network node shall add up to 0 in an ideal system. Backup relay x y load l2 load l1 t1 s bus over current backup relay differential relay. Differential protection what goes in must come out. The relays used in power system protection are of different types. 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. Pdf on dec 1, 2016, nikolina arapoglou and others published differential. A typical dc circuit for busbar differential protection scheme is given below. Pdf differential protection schemes and techniques for power.
Powersystem protection is a branch of electrical power engineering that deals with the protection of electrical power systems from faults through the disconnection of faulted parts from the rest of the electrical network. Differential protection is based on the fact that any fault within electrical equipment will result into the difference in current entering it and leaving it. Transformer differential protection scheme with internal faults detection algorithm using second harmonics restrain and fifth harmonics blocking logic ouahdi dris farag. If overall differential protection is set to 20% the function will pickup when 20% of the base secondary current is injected as a symmetrical threephase current from each respective side see section 4 of this document for more details about testing. The differential relay works on the principle of comparison between the phase angle and magnitude of two or. The phase and ground differential 87p and 87n, section 4 and sudden pressure relay 63, section 6 provide the primary transformer fault protection. Because of that, comparison of upstream and downstream currents in the same phases is not convenient for transformer differential protection. Line differential pilot wire, charge comparison, traditional differential, and the alpha plane. The differential protection of power transformer is a unit protection scheme. The faults occur in the transformer inside the insulating oil can be detected by buchholz relay.
Strictly speaking the rc tells us how much current a relay will use to restrain based on the currents. Selection of the protection system for machines and plants 28 9. Differential protection for arbitrary 3ph power transformer. A differential protection relay is defined as the relay that operates when the phase difference of two or more identical electrical quantities exceeds a predetermined amount. Siprotec protection relays figure 1 shows a simple differential protection scheme, also known as a merzprice scheme. Comparing current flowing into a line to the current flowing out of the same line allows for a simple protection scheme. In electric current differential protection of busbar, if ct secondary circuits, or bus wires is open the relay may be operated to isolate the bus from live system.
Differential protection has excellent operation in most fault cases. Pdf easy method for testing transformer differential relays. Electrical faults l phase to phase l phase to ground l phase to phase to phase l phase to phase to phase to ground. The differential protection of transformer has many advantages over other schemes of protection. The theory and application of these protective devices is an important part of the education of a power engineer who specializes in power system protection. Pdf a new method for differential protection in power transformer.
Generally differential protection is provided in the electrical power transformer rated more than 5mva. The principles of differential protection you must understand on photo. Hence, operation of the different protection scheme would be restrained in this region. Among them differential relay is very commonly used relay for protecting transformers and generators from localised faults. Two different protection solutions will be given depending on which winding of the step up. The differential relay works on the principle of comparison between the phase angle. Percentage differential protection slope in differential. Line differential protection is a strict selective object protection. The difference amongst the currents may also be in phase angle or in magnitude or in each. In the event of such faults, the quick response of the differential element may limit the damage that may have otherwise occurred to the motor. It is a static device, totally enclosed and usually oil immersed, and. In this type of protection the electrical quantities entering and leaving the protected zone or area are compared by current transformers c. Differential protection in power transformer duration.
The principles of differential protection you must. Introduction to system protection if you are still awake, nudge your sleeping neighbor and tell himher that the lecture is over and it is almost time for the next lecture, which may actually prove to be an interesting and informative lecture unlike this one. For generator differential protection, one set of current transformers ct are located on generator neutral side, whereas second set of ct is located on generator phase side. This paper carries the idea of selection of dual slope differential relay parameters for various faulty conditions on the system. Patel ge power management malvern, pa presented at the electric council of new england. Introduction transformers are a critical and expensive component of the power system.