Sepam - HV/MV Protection and control units (ENG) - Trinet

Rate of change of frequency protection (cont’d)CommissioningCheck:c the connections,c the positions of the microswitches SW associatedwith the voltage inputs,c the general parameters in the status menu.Set the following:c dFs/dt set point:dFs/dt is set in Hertz per second.The 999 Hz/s setting disables the protection:all the outputs are set to zero.c T time delay.It is impossible to access the protection settings if theNumber setting in phase VT heading of the statusmenu is set to V or 1U. If that is the case, theprotection is disabled: all the outputs are set to zero.Application, settingThe rate of change of frequency protectionis a complement to the underfrequency andoverfrequency protections used to detect lossof mains protection or load shedding.The protection settings must be consistent with theother frequency protections.Loss of mains applicationThis function is used on the incomer of an installationwhich includes a generator that can be run in parallelwith the mains.Its role is to detect the loss of the mains, resulting inisolated operation of the generator. If the powerthrough the mains prior to isolated generatoroperation is not zero, the generator frequency varies.The rate of change of frequency protection detectsthis conditions more rapidly than a conventionalfrequency protection would.Other disturbances such as short-circuits, loadchanges and switching can cause frequency changes.The low set point may be temporarily reached duringsuch conditions so a time delay is necessary.In order to maintain the frequency derivativeprotection’s advantage of speed compared toconventional frequency protections, a second, higherset point with a short time delay may be added.Overfrequency and underfrequency protections in loss of mains applications areconventionally set to the fundamental frequency + 0.5 Hz or – 0.5 Hz. Based onthe assumption of a constant rate of change of frequency, the tripping times of therate of change of frequency protection and the underfrequency and overfrequencyprotections may be shown on the same graph:tripping time (s)21,81,61,41,210,80,60,40,200overfrequency protection: Fs = 50.5 Hz T = 0.1srate of change of frequency protection:dFs/dt = 0.15 Hz/s T = 0.5 srate of change of frequency protection:dFs/dt = 0.15 Hz/s T = 0.5 s0,5 1 1,5 2 2,5 3 3,5 4 4,5 5dF/dt (Hz/s)Comparison of an overfrequency protection and two rate of change of frequency protectionsset points. The rate of change of frequency protection detects isolated generator operationfaster than a frequency protection does.In fact, the rate of change of frequency is not constant. Often the rate of changeof frequency is the highest at the beginning of the disturbance and then decreases,which extends the frequency protection tripping time but does not affect thetripping time of the rate of change of frequency protection.c Low set pointv If rules have been established by the utility, they should be followed.v If no rules are given by the utility:- If the maximum rate of change of frequency on the mains in normal conditions isknown, dFs/dt should be set above it.- If no information on the mains is available, the low set point may be establishedbased on generator data. A good approximation of the rate of change of frequencyfollowing a loss of mains accompanied by a load change ∆P is:df ∆ P * Fn= -dt 2 * Sn * HwithSn: Rated loadFn: Rated frequencyH: inertia constant MW*s/MVATypical inertia constant value:0.5 i H i 1.5 for diesel and low rated generators (i 2 MVA)2 i H i 5 for gas turbine and medium rated generators (i 40 MVA)H = 1 2J x Ω 2SnExampleswithJ = inertia momentΩ = machine speedrated load 2 MVA 20 MVAinertia constant 0.5 s 2 sload variation 0.1 MVA 1 MVAdf/dt - 2.5 Hz/s - 0.6 Hz/s2/48 Protection functions