Neutral Voltage Displacement - Application Guide

APPLICATION GUIDE ©2012AG013 Page 1 of 19 Issue C 21 May 2012 APPLICATION GUIDE AG013 - NEUTRAL VOLTAGE DISPLACEMENT (NVD) PROTECTION APPLIED TO CONDENSER BUSHINGS (CAPACITOR CONES) Location: L:\UK Sales\5 APPLICATIONS\2 SECTION REPORTS\Application Guides\Neutral Voltage Displacement (NVD) Protection applied to Condenser Bushings (Capacitor Cones)doc SUMMARY Voltage Transformers don’t tend to be fitted at distribution levels, due to their expense. Instead, capacitor cones, or condenser bushings, may be used at 11kV and 33kV substations to provide a neutral voltage displacement output to a suitable relay. Often, these bushings are starred together, and the star point is used to provide the displacement voltage to the relay. This application is not new, and indeed has been provided by Areva previously in the form of MVTU13. See application diagram 10MVTU1302 (Figure 11). Title Senior Applications Engineer Name A.W. Hassall Signature Date 21/05/12 T&D Automation & Information Systems - St Leonards Avenue – Stafford – ST17 4LX – England Tel: +44 (0)1785 223251 – Fax: +44 (0)1785 212232 Alstom Grid UK LTD. Registered Office: St Leonards Avenue – Stafford – ST17 4LX Registered in England: 4955841 ...................................................... 12 VOLTAGE SETTING ..............5.................................................................. 3............................................ 3...................................... 3.....................................................................3...................... 10 RELAY SELECTION ............. APPLICATION GUIDE INTRODUCTION ........ 5 DESCRIPTION ..........................................................4....................................................................... 2..... 3...............6................ 6 APPLICATION .....................2...................... 3.......................................................................... 15 APPLICATION DIAGRAM 10MVTU1302 .................................. 6 THEORY.......................................................... 3 REFERENCES ................................1................................. 13 CONNECTIONS .................................... 3....................AG013 Page 2 of 19 Issue C 21 May 2012 Table of Contents 1....... 3........................................................................................... 18 ................ Figure 1: . due to their expense. or condenser bushings. INTRODUCTION Voltage Transformers don’t tend to be fitted at distribution levels. capacitor cones. Instead. may be used at 11kV and 33kV substations to provide a neutral voltage displacement output to a suitable relay.APPLICATION GUIDE AG013 Page 3 of 19 Issue C 21 May 2012 1. . and the star point is used to provide the displacement voltage to the relay. and indeed has been provided by Areva previously in the form of MVTU13. Figure 2: Neutral Voltage Displacement Relay (NVD) This application is not new.AG013 Page 4 of 19 Issue C 21 May 2012 APPLICATION GUIDE Often. these bushings are starred together. See Figure 2. See application diagram 10MVTU1302 (Figure 11). APPLICATION GUIDE AG013 Page 5 of 19 Issue C 21 May 2012 2. TIME DELAYED NEUTRAL DISPLACEMENT VOLTAGE RELAY TYPE MVTU13 . Name APPLICATION DIAGRAM:-STATIC MODULAR DEF. REFERENCES Reference A Doc. Number 10MVTU1302 Sheet 3 Doc. DESCRIPTION 3.AG013 Page 6 of 19 Issue C 21 May 2012 APPLICATION GUIDE 3. THEORY Consider a single-phase fault to ground on B-Phase: Figure 3: IA Va N Vc Vb IC IF -jXc -jXc -jXc .1. = 3 x IA. giving correspondingly altered capacitor currents Ia and Ic. . three capacitor currents that lead their respective voltages by 90º and sum to zero. Figure 5: VCf Ia = √3IA VAf =√3VA The vector sum of the A & C phase capacitor currents is: If = √3 x Ia. the total fault current If equals three times a single capacitor healthy condition current IA. Thus. A and C voltages are √3 times their healthy magnitude & at 60º to each other. = √3 x √3 x IA.APPLICATION GUIDE AG013 Page 7 of 19 Issue C 21 May 2012 Figure 4 shows three healthy voltages. Figure 4: IA VA Figure 5 shows B phase earthed. Figure 6: Thus.359 = 1. For example. If is the current which will flow in the Neutral Displacement Relay under fault conditions (neglecting the impedance of the relay itself). the single capacitor healthy condition current IA is given by: IA = VA / XC IA = VA / (1/2πfC) = 33 x 103 / (1/(2 x π x 50 x 60 x 10-12)) √3 = 0. for a 60pF capacitor on a 33kV system.359mA Therefore.08mA .AG013 Page 8 of 19 Issue C 21 May 2012 APPLICATION GUIDE Figure 6 shows the vector sum of the fault condition If. the total fault current which would flow in an NVD relay (neglecting the impedance of the relay itself): If = 3 x 0. 69 If If is the total fault current which would flow in an NVD relay (neglecting the impedance of the relay itself).539 1.39 19. and also If for a 90pF capacitor.00 53. then knowing this current (If) and the input impedance of the relay (Rr) we can calculate the voltage which will be produced across it (Vr) during a fault condition: Vr = If x Rr Thus.62 150.359 1.08 90.00 35.00 0.00 0.APPLICATION GUIDE AG013 Page 9 of 19 Issue C 21 May 2012 Table 1 shows the total fault current If for a 60pF capacitor.23 19.00 0. Table 1: C (pF) Xc (MΩ) VA (kV) IA (mA) If (mA) 60. we would recommend setting the relay to less than half this voltage: Vs < Vr/2 . and for a 150pF capacitor.08 19.00 21.898 2. Note that there is the risk of high voltage developing on removal of the relay or PCB from its case. to fix the impedance.1W resistors are adequate. Thus. APPLICATION In practice. If continuously energised a pair of 0. MiCOM relay input impedance varies somewhat with voltage. Please see figure 7. A shorting contact is available on each relay.AG013 Page 10 of 19 Issue C 21 May 2012 APPLICATION GUIDE 3. and will have some effect on actual setting. but we would also recommend use of the available externally connected shorting contact in each case. This value is realised by two 47kΩ resistors in parallel. The fixed resistors on the input will prevent this risk. Utilising two in parallel also gives increased security.2. Figure 7: 47kΩ 47kΩ Neutral Voltage Displacement Relay (NVD) . we recommend use of a 23½kΩ resistor combination in parallel with this input. APPLICATION GUIDE AG013 Page 11 of 19 Issue C 21 May 2012 WARNING: As the proposal is to situate the relay in a primary circuit location (as per previous similar applications). . the relevant utility must satisfy itself that any necessary safety measures are in place. residual voltage measurement is derived from phase inputs.has a separate neutral voltage input. and these are: 1. but neutral voltage may be connected into one of the phase inputs. This assumes that voltage inputs are not used for other purposes.3. P94V .AG013 Page 12 of 19 Issue C 21 May 2012 3. 2. RELAY SELECTION APPLICATION GUIDE There are various possibilities in the MiCOM range for Neutral Voltage Displacement protection fed from capacitor cones. P141/2/3/5 . . P144 . 3.has a separate neutral voltage input. Table 2: P94V C (pF) Xc (MΩ) VA (kV) If (mA) Rr (kΩ)* Vr (V) Vs (V) 60. The operating voltage to be applied can be calculated for various capacitor ratings as demonstrated earlier.23 19. assuming 23½kΩ resistance applied in conjunction with the relay. which should be adequate for most applications (see table 3 below). which again should be adequate for most applications (see table 2 below).08 19 20 10.69 19 51 25.00 1.4.39 19.00 150.APPLICATION GUIDE AG013 Page 13 of 19 Issue C 21 May 2012 3.00 *Relay and Resistor Combination . The tables below detail maximum settings for each type of relay for various rating capacitors.00 21.00 1. P141/2/3/5 has a minimum setting on the residual OV/NVD protection of 1V. it is important to obtain a sensitive enough voltage setting. P94V has a minimum setting of 1V.00 2.62 19 30 15. P144 has a minimum setting on the residual OV/NVD protection of 4V. VOLTAGE SETTING For this application.00 90. which should also be adequate for most applications (see table 3 below).00 35.00 53.08 19. 00 59.08 22.81 90.23 19.39 19.53 *Relay and Resistor Combination .00 1.00 53.00 21.00 35.72 150.AG013 Page 14 of 19 Issue C 21 May 2012 APPLICATION GUIDE Table 3: P14x C (pF) Xc (MΩ) VA (kV) If (mA) Rr (kΩ)* Vr (V) Vs (V) 60.44 17.00 35.00 2.08 19.62 22.06 29.00 1.00 23.69 22.63 11. APPLICATION GUIDE AG013 Page 15 of 19 Issue C 21 May 2012 3.5. CONNECTIONS P94V: Figure 8: . AG013 Page 16 of 19 Issue C 21 May 2012 P14x: APPLICATION GUIDE Figure 9: C17 C18 . APPLICATION GUIDE AG013 Page 17 of 19 Issue C 21 May 2012 P144: Figure 10: C17 C18 . APPLICATION DIAGRAM 10MVTU1302 APPLICATION GUIDE Figure 11: .AG013 Page 18 of 19 Issue C 21 May 2012 3.6. Hassall Reason for change Original Complete Review Convert to ALSTOM Format/Add AGILE Date 30/01/08 18/05/09 21/05/12 . Hassall A.W.W.W.APPLICATION GUIDE ©2012 Page 19 of 19 Issue C 21 May 2012 REVIEW HISTORY Issue C Name A.W. Hassall Position Senior Applications Engineer VERSION CONTROL Issue A B C Author(s) A. Hassall A.