GE L90 Instruction Manual

Also see for L90: Communications manual Communications guide Instruction manual Instruction manual Instruction manual

Contents

8-6 L90 Line Differential Relay GE Multilin8.1 OVERVIEW 8 THEORY OF OPERATION8The rotational rate of phasors is equal to the difference between the power system frequency and the ratio of the samplingfrequency divided by the number of samples per cycle. The correction is computed once per power system cycle at eachrelay. For conciseness, we use a phasor notation:(EQ 8.12)Each terminal computes positive sequence current:(EQ 8.13)Each relay computes a quantity derived from the positive sequence current that is indicative of the amount of rotation fromone cycle to the next, by computing the product of the positive sequence current times the complex conjugate of the posi-tive sequence current from the previous cycle:(EQ 8.14)The angle of the deviation phasor for each relay is proportional to the frequency deviation at that terminal. Since the clocksynchronization method maintains frequency synchronism, the frequency deviation is approximately the same for eachrelay. The clock deviation frequency is computed from the deviation phasor:(EQ 8.15)Note that a four quadrant arctangent can be computed by taking the imaginary and the real part of the deviation separatelyfor the two arguments of the four quadrant arctangent. Also note that the input to the loop filter is in radian frequency whichis two pi times the frequency in cycles per second; that is, .So the radian frequency deviation can be calculated simply as:(EQ 8.16)8.1.10 PHASE DETECTIONThere are two separate sources of clock phase information; exchange of time stamps over the communications channelsand the current measurements themselves (although voltage measurements can be used to provide frequency information,they cannot be used for phase detection). Current measurements can generally provide the most accurate information, butare not always available and may contain large errors during faults or switching transients. Time stamped messages arethe most reliable source of phase information but suffer from a phase offset due to a difference in the channel delays ineach direction between a pair of relays. In some cases, one or both directions may be switched to a different physical path,leading to gross phase error.For two or three terminal systems, the approach is:ï The primary source of phase information is current measurements (when available) and the secondary source is thetime-tagged messages. The filter uses a single input that is switched back and forth between the two sources of phaseangle information. This makes the system immune to changes in communications delays as long as current informa-tion is available. The rules for switching between the sources are:1. Phase angle deviations from both current information and ping-long information are always computed. The ping-pong algorithm has a wider range of validity, and is used to help decide which source of phase angle information isto be used by the filter.2. Phase angle deviation computed from currents is used whenever it is valid. Otherwise, phase angle informationfrom the ping-pong algorithm is used.3. Phase angle deviation computed from currents is deemed valid whenever the currents are large enough, andwhen the deviation computed from the ping-pong information is below a fixed threshold (± half-cycle.)I n( ) Re Phasorn( ) j Im Phasorn( )⋅+=Ia k, n( ) I n( )= for phase a from the kth terminal at time step nIb k, n( ) I n( )= for phase b from the kth terminal at time step nIc k, n( ) I n( )= for phase c from the kth terminal at time step nIpos k, n( ) 13--- Ia k, n( ) Ib k, n( ) e j2π 3⁄⋅ Ic k, n( ) e j2π 3⁄⋅+ +( )=Deviationk n( ) Ipos k, n( ) Ipos k, n N–( )∗×=FrequencyDeviation f∆f----- tan 1– Im Deviation( ) Re Deviation( )⁄( )2π-------------------------------------------------------------------------------------------------= =ω∆ 2π f∆⋅=ω∆ f∆ tan 1– Im Deviation( ) Re Deviation( )⁄( )⋅=