Stabilizing Resistor in Motor Earth-Fault Protection Application Note

Transcription

1 Stabilizig Resistor i Motor Earth-Fault Applicatio Note kasikuva_bw

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3 1MRS Issued: Versio: A/ Stabilizig Resistor i Motor Earth-Fault Applicatio Note Cotets: 1. Scope Itroductio Techical implemetatio Coectio diagram Calculatio of stabilisig voltage Calculatio of the relay settig Calculatio of the stabilisig resistor Calculatio of the voltage-depedet resistor Calculatio example Summary Refereces List of symbols...11 Copyrights The iformatio i this documet is subject to chage without otice ad should ot be costrued as a commitmet by ABB Oy. ABB Oy assumes o resposibility for ay errors that may appear i this documet. I o evet shall ABB Oy be liable for direct, idirect, special, icidetal or cosequetial damages of ay ature or kid arisig from the use of this documet, or shall ABB Oy be liable for icidetal or cosequetial damages arisig from use of ay software or hardware described i this documet. This documet ad parts thereof must ot be reproduced or copied without writte permissio from ABB Oy, ad the cotets thereof must ot be imparted to a third party or used for ay uauthorized purpose. The software or hardware described i this documet is furished uder a licese ad may be used, copied, or disclosed oly i accordace with the terms of such licese. Copyright 2005 ABB Oy All rights reserved. Copyright 2005 ABB Oy, Distributio Automatio, Vaasa, FINLAND 3

4 Stabilizig Resistor i Motor Earth-Fault 1MRS Scope This documet explais how to calculate the value of the stabilisig resistor used i motor earth-fault protectio. Typically, a stabilisig resistor is eeded whe residually coected curret trasformers are used to measure the earth-fault (residual) curret. The purpose of the stabilisig resistor is to prevet the relay from operatig due to saturatio of the curret trasformers durig motor start-up. This documet itroduces the formulas eeded for the calculatio of the stabilisig resistor ad cotais a calculatio example. The priciples described ca be geerally used for motor earth-fault protectio ad for calculatig, for example, the protectio settigs for SPACOM ad RED 500 series protectio relays. KEYWORDS: earth-fault protectio, motor protectio, curret trasformer saturatio, stabilisig resistor 4

5 1MRS Stabilizig Resistor i Motor Earth-Fault 2. Itroductio The earth-fault curret ca be measured i differet ways. Oe way is to use a core balace curret trasformer. The core balace curret trasformer is preferred whe high sesitivity is eeded. A stabilisig resistor is ot used with the core balace curret trasformer. Aother method is to measure the earth-fault curret through a residual coectio of the three phase CTs. This coectio is also called the Holmgree coectio. The residual coectio ca be used whe the sesitivity requiremet is about 10% of the CT omial curret, or higher. For very sesitive protectio a core balace curret trasformer has to be used. Whe the residual coectio of the three phase CTs is used, a uwated operatio of the earth-fault protectio relay sometimes occurs durig motor start-up. The reaso for the uwated relay operatio may be that the start-up curret of the motor occasioally causes oe or more of the three phase CTs i residual coectio to saturate. The CT saturatio is caused by the DC compoet of the usymmetrical start-up curret. To avoid this problem a suitable stabilisig resistor ca be coected i series with the earth-fault curret iput of the protectio relay. The stabilisig resistor forces the curret fed by a o-saturated curret trasformer to flow through the secodary circuit of a saturated curret trasformer. 5

6 Stabilizig Resistor i Motor Earth-Fault 1MRS Techical implemetatio 3.1. Coectio diagram Fig illustrates the coectio priciple whe the earth-fault curret is measured through the residual coectio of the three phase CTs. The stabilisig resistor is coected i series with the earth-fault curret iput of the protectio relay. The purpose of the voltage-depedet resistor is to limit the voltage of the secodary circuit to a safe level. The eed for a VDR is case-specific ad ca be checked by calculatio. VDR Rs L1 L2 L3 Io Fig M fig Residual coectio of three phase CTs ad applicatio with a stabilisig resistor (Rs) ad a voltage-depedet resistor (VDR) Calculatio of stabilisig voltage The stabilisig voltage U s is calculated assumig that oe of the residually coected curret trasformers will be fully saturated at motor start-up: U s = Where: I start R ct R m I start ( R ct + R m ) (1) start-up curret (primary value) of the motor CT ratio iteral resistace of the CT resistace of the logest loop of the CT secodary circuit The curret trasformer kee-poit voltage U k has to be at least twice as high as the stabilisig voltage U s. 6

7 1MRS Stabilizig Resistor i Motor Earth-Fault 3.3. Calculatio of the relay settig The sesitivity of the protectio ca be calculated as: I prim = ( I s + 3 I e + I u ) (2) where: I prim I s I e I u primary curret at which the relay starts CT ratio start curret (secodary value) of the earth-fault protectio magetizig curret of the CT at the voltage U s curret through the VDR at the voltage U s The relay start curret ca be calculated usig formula (2), whe the primary sesitivity requiremet for the protectio is kow: I s = I prim ( 3I e I u ) The relay settig is the simply: I r = where: ---- I s I 2 (3) (4) I 2 rated curret of the relay (typically 1 A or 5 A) 3.4. Calculatio of the stabilisig resistor The stabilisig resistor R s is calculated as: (5) R s U = s = I s I start ( R ct + R m ) I s where: I start R ct R m I s start-up curret (primary value) of the motor iteral resistace of the CT resistace of the logest loop of the secodary circuit CT ratio start curret (secodary value) of the earth-fault protectio The power ratig of the stabilisig resistor R s is calculated as: ( Uk) (6) P R s where: 7

8 Stabilizig Resistor i Motor Earth-Fault 1MRS U k kee-poit voltage of the CT A wire woud resistor type is recommeded. Whe selectig the resistor it should be oted that the measurig wires ca be reliably coected to the resistor, that a suitable istallatio kit is icluded ad the voltage withstad is high eough for the applicatio cocered Calculatio of the voltage-depedet resistor Durig faults iside the zoe of protectio the voltage of the measurig circuit may grow high eough to exceed the isolatio level the relay, curret trasformers ad the wires. This ca be avoided by istallig a voltage-depedet resistor ito the circuit. The voltage-depedet resistor is typically eeded if the peak voltage durig a fault is 3 kv or higher. The voltage U f, which igores the CT saturatio, durig the fault ca be calculated as follows: U f = where: I max I max ( R s + R ct + R m + R r ) maximum earth-fault curret (7) The peak voltage, u peak, which icludes the CT saturatio, ca be estimated from the followig formula (give by P.Mathews 1955): u peak = 2 2U k ( U f U k ) (8) 3.6. Calculatio example The power system is low-impedace earthed. The maximum earth-fault curret is limited to 400 A. The required motor earth-fault protectio sesitivity is assumed to be 20% of the full load curret. The startig curret of the motor is assumed to be 4.8 x I m. The resistace of the logest loop of the secodary circuit is 0.2 Ω ad the relay iput impedace is 0.02 Ω. Calculate a suitable stabilisig resistor value as follows: Motor data Rated power, P m 2800 kw Rated voltage, U m 3300 V Rated curret, I m 549 A The CT specificatios from the CT maufacturer's catalogue are assumed to be: 600/5 A, 5P10, 15 VA, R ct = 0.28 ohm, U k = 34.0 V, I m = 100 ma (at U k ) First the stabilisig voltage is calculated usig formula (1): U A s = ( 0.82Ω + 0.2Ω) 10.5V 120 I this case the requiremet for the curret trasformer kee-poit voltage is fulfilled because U k > 2U s. 8

9 1MRS Stabilizig Resistor i Motor Earth-Fault The magetisig curve of the CT is assumed to be liear. The magetisig curret at the stabilisig voltage will the be: I 10.5V e = mA 31mA 34.0V The secodary start curret of the relay is calculated usig formula (3). The curret through the voltage-depedet resistor is assumed to be zero A I s = ma 822mA 120 The relay settig is the calculated usig formula (4): 0.82A I r = A The resistace of the stabilisig resistor is calculated usig formula (5): 10.5V R s = Ω 0.82A The power of the stabilisig resistor is calculated usig formula (6): P V2 89W 13Ω Based o formula (7), the eed for the voltage-depedet resistor is checked: U f = 400A ( 13Ω Ω + 0.2Ω Ω) 44V 120 The voltage-depedet resistor is ot eeded i this case, because the voltage durig the fault is much lower tha 3 kv. Formula (8) does ot have to be used i this case, because formula (7) already gives a low voltage value. 9

10 Stabilizig Resistor i Motor Earth-Fault 1MRS Summary 5. Refereces This documet describes how to select a suitable stabilisig resistor to be itroduced i a motor earth-fault protectio scheme. The iformatio give is valid for ABB's SPACOM ad RED 500 protectio relays. The required calculatio formulas are preseted ad a calculatio example is icluded to show how to use the formulas. The correctly selected stabilisig resistor prevets udesired operatio of the earthfault protectio due to CT saturatio. If the voltage durig a i-zoe fault ca be expected to grow so high that it will exceed the isulatio level of the circuit, a voltage-depedet resistor should be iserted i the circuit. IEC Istrumet trasformers Part 1: Curret trasformers. 10

11 1MRS Stabilizig Resistor i Motor Earth-Fault 6. List of symbols I 2 I e I max I m I prim I s I start I r I u P P m R ct R m R r R s U f U k U m U s u peak VDR rated curret of the relay magetizig curret of the CT at the voltage U s maximum earth-fault curret rated curret of the motor primary curret at which the relay starts start curret (secodary value) of the earth-fault protectio start-up curret (primary value) of the motor relay settig curret through the VDR at the voltage U s CT ratio rated power of the stabilisig resistor rated power of the motor iteral resistace of the CT resistace of the logest loop of the CT secodary circuit relay iput impedace stabilisig resistor voltage which igores the CT saturatio kee-poit voltage of the CT rated voltage of the motor stabilisig voltage peak voltage which icludes the CT saturatio voltage-depedet resistor 11

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