Reles-spanish

GER-3966

MUL TILIN
GE Power Management

Distance Relays Fundamentals

DISTANCE RELAYS FUNDAMENTALS
J. G. Andrichak, G.E. Alexander General Electric Co Malvern, PA

INTRODUCTION
Distance functions have been in use for many years and have progressed from the original electromechanical types through analog types and now up to digital types of functions. The purpose of this paper is todiscuss fundamental features of the three types of functions and possible problems that may be encountered in their design and application.

MHO FUNCTIONS
SIMPLE MHO FUNCTION A simple mho distance function, with a reach of Z ohms, is shown in Figure 1. This diagram is exactly equal to an R-X diagram except that all of the impedance vectors have been operated on by the current I. The mhofunction uses the current and voltage measured at the relay to determine if the apparent impedance plots within the mho characteristic. The determination is made by comparing the angle between the operating quantity (IZ - V) and the polarizing quantity (V, where V = IZf). If the angle is less than or equal to 90°, then the fault impedance Zf plots within the characteristic, and the function willproduce an output. If the angle is greater than 90°, then Zf falls outside of the characteristic and no output will be produced. Assume that the angle of maximum reach (θ) and the angle of ZL (φ) are equal. On that basis, the conditions shown in Figure 2 will be obtained. The key point to note in this phasor analysis (a convenient way to view relay performance) is the magnitude of the IZ - V (Vop)phasor and its relationship to the V (Vpol) phasor. Operation will occur whenever Vop and Vpol phasors are within 90° of each other and provided both Vop and Vpol are greater than the minimum values established by the sensitivity of the relay design. For the balance point fault, IZ-V is zero, therefore no operation occurs, which is expected. For an internal fault, IZ V and V are in phase, thereforethe function operates as expected. For the external fault, operation does not occur because IZ V and V are 180° out of phase. Observe that for the balance point fault, the V is exactly equal to IZ. This is true for the three-phase fault shown (also for a phase-to-phase fault) and for a phase distance function only. For a ground distance function, this will only be true if the function includeszero sequence current compensation as discussed later in this paper.

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The polarizing quantity for this simple mho distance function is simply equal to the fault voltage V, therefore the function is said to be self-polarized and has the simple characteristic shown in Figure 1. In general, a voltage different than the fault voltage is used to polarize the function and this will have an effecton the characteristic. POLARIZING QUANTITY A number of polarizing quantities have been used in developing phase and ground mho distance functions. Following are some of the more commonly used: • • • • • Self-polarized (Va for Phase A function, Vab for the Phase AB function, etc.) Positive Sequence Voltage (Va1 for Phase A function, Vab1 for Phase AB function, etc.) Quadrature Voltage (Vbcshifted leading 90° for Phase A function) Median (midpoint of Vbc to Va for Phase A function) Leading phase (Vc shifted leading 240° for Phase A function)

An mho function that is other than self-polarized is often described as being cross-polarized. No attempt will be made here to describe the effect of all types of cross-polarization. Suffice it to say that cross-polarization will still result in acircular characteristic, but one that may also swivel and vary in size dependent on system conditions. For example, consider the case of a distance function that uses positive sequence voltage as the polarizing signal. The characteristics for a phase distance function and a ground distance function that use positive sequence voltage polarization are shown in Figure 3 (1) and are drawn for a...