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Distance Relay Fundamentals

# Distance Relay Fundamentals - Distance Relay Fundamentals J...

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63 Distance Relay Fundamentals 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 includes zero sequence current compensation as discussed later in this paper. 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 effect on 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 (V a for Phase A function, V ab for the Phase AB function, etc.) positive Sequence Voltage (V a1 for Phase A function, V ab1 for Phase AB function, etc.) quadrature Voltage (V bc shifted leading 90° for Phase A function) median (midpoint of V bc to V a for Phase A function) leading phase (V c shifted leading 240° for Phase A function) 1. 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 to discuss fundamental features of the three types of functions and possible problems that may be encountered in their design and application. 2. 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 mho function 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 = IZ f ). If the angle is less than or equal to 90°, then the fault impedance Z f plots within the characteristic, and the function will produce an output. If the angle is greater than 90°, then Z f falls outside of the characteristic and no output will be produced. Assume that the angle of maximum reach (Θ) and the angle of Z L (Φ) are equal. On that basis, the conditions shown in 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 (V op ) phasor and its relationship to the V (V pol ) phasor. Operation will occur whenever V op and V pol 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

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