CHAPTER 4: SETPOINTS MONITORING869 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL 4–297FunctionsPower Factor The 869 is applied on a synchronous machine, it is desirable not to trip or alarm on powerfactor until the field has been applied. Therefore, this feature can be blocked until themachine comes up to speed and the field is applied. From that point forward, the powerfactor trip and alarm elements will be active. Once the power factor is less than either theLead or Lag level, for the specified delay, a trip or alarm will occur indicating a Lead or Lagcondition. The power factor alarm can be used to detect loss of excitation and out of step.The relay calculates the average Power Factor in the three phases as follows:Average Power Factor = Total 3-Phase Real Power / Total 3-Phase Apparent PowerFor delta-connected VTs, the Power Factor feature is inhibited from operating unless allthree voltages are above the selected voltage threshold and one or more currents areabove the selected current threshold. Power Factor element delay timers are only allowedto time when the voltage threshold is exceeded on all phases and current threshold isexceeded on one phase. In the same way, when a Power Factor condition starts the PowerFactor delay timer, if all three phase voltages fall below the threshold and one phasecurrent threshold falls below the timer has timed-out, the element resets withoutoperating. A loss of voltage during any state returns both Power Factor elements to theReset state.For wye-connected VTs, the power factor value is calculated from the valid phase(s) forwhich voltage and current are above the user selected thresholds. Power Factor elementdelay timers are only allowed to time when the supervision conditions are met. In thesame way, when a Power Factor condition starts the Power Factor delay timer, if one ormore valid phases no longer satisfy the supervision conditions, the power factor is re-calculated based on the still valid phase(s). If the element is continuously asserted with thenew power factor value, the timer would continue timing, otherwise, the element resetswithout operating.The minimum operating voltage and current are set as a threshold below which theelement is reset.The following figure illustrates the conventions established for use in the 869 relay, wherethe negative value means the lead power factor, and the positive value means the lagpower factor. For details on the convention used for measuring power, see Chapter 6:Metering/ Power.