abcCABHigh ZREFRLRCTRLRLRLRCTTransformer:90 MVA33/132 kVDyn11, X = 5%Buderns:RCT = 0.5 WRL = 0.98 W400:1V00687Figure 86: High Impedance REF for the LV winding4.3.2.1 STABILITY VOLTAGE CALCULATIONThe transformer full load current, IFLC, is:IFLC = (90 x 106) / (132 x 103 xÖ3) = 394 ATo calculate the stability voltage the maximum through fault level should be considered. The maximum throughfault level, ignoring the source impedance, IF, is:IF = IFLC / XTX = 394 / 0.05 = 7873 AThe required stability voltage, VS, and assuming one CT saturated is:Vs = KIF(RCT + 2RL)The following figure can be used to determine the K factor and the operating time. The K factor is valid when:● 5 ≤ X/R ≤ 120and● 0.5In ≤ I f ≤ 40InWe recommend a value of VK/VS = 4.With the transformer at full load current and percentage impedance voltage of 394A and 5% respectively, theprospective fault current is 7873 A and the required stability voltage Vs (assuming that one CT is saturated) is:Vs = 0.9 x 7873 x (0.5 + 2 x 0.98) / 400 = 45.5 VThe CTs knee point voltage should be at least 4 times Vs so that an average operating time of 40 ms is achieved.4.3.2.2 PRIMARY CURRENT CALCULATIONThe primary operating current should be between 10 and 60 % of the winding rated current. Assuming that therelay effective setting or primary operating current is approximately 30% of the full load current, the calculationbelow shows that a setting of less than 0.3 A is required.Effective setting = 0.3IFLC / CT Ratio = 30.3 x 394 / 400 = approximately 0.3 A4.3.2.3 STABILISING RESISTOR CALCULATIONAssuming that a setting of 0.1A is selected the value of the stabilizing resistor, RST, required isChapter 7 - Restricted Earth Fault Protection P14D158 P14D-TM-EN-8
