APPLICATION OF NON PROTECTION FUNCTIONS

The non-protection features for the scheme are summarised below:

• Scheme is centralised.

• Local, zone and scheme measurements – various measurements are available locally via the relay LCD or remotely via the serial communication link.

• Event, fault and disturbance recording – Comprehensive post fault analysis available via event lists, disturbance records and fault records which can be accessed locally via the relay LCD or remotely via the serial communication link.

• Real time clock/time synchronisation – Time synchronisation available via IRIG-B input.

• Four settings groups – Independent remotely selectable setting groups to allow for customer specific applications.

• CB and isolator state monitoring – indication of the circuit breaker/isolator position via the auxiliary contacts, scheme acts accordingly should discrepancy conditions be detected.

• Commissioning test facilities.

• Continuous self monitoring – extensive self checking routines to ensure maximum reliability.

• Graphical programmable scheme logic – allowing user defined protection and control logic to be tailored to the specific application.

9.1 Function keys

The following default PSL logic illustrates the programming of function keys to enable/disable the commissioning mode functionality.

FIGURE 27: COMMISSIONING MODE DEFAULT PSL

Note: Energizing two inputs to an LED conditioner creates a YELLOW illumination.

Function Key 2 is set to ‘Toggle’ mode and on activation of the key, the commissioning mode will be in service as long as the function has been enabled in the “Configuration” menu. The associated LED will indicate the state of the protection function in service as GREEN and RED for the Overhaul mode.

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10. CT REQUIREMENTS

10.1 Notation

IF max fault maximum fault current (same for all feeders) in A IF max int

cont maximum contribution from a feeder to an internal fault (depends on the feeder) in A

Inp CT primary rated current

In nominal secondary current (1A or 5A) RCT CT secondary winding Resistance in Ohms R_B Total external load resistance in Ohms Vk CT knee point voltage in Volts

SVA Nominal output in VA

KSSC Short-circuit current coefficient (generally 20)

General recommendations for the specification of protection CTs use common rules of engineering which are not directly related to a particular protection.

10.2 87BB Phase CT Requirements

10.2.1 Feeders connected to sources of significant power (i.e. lines and generators)

The primary rated current is specified above a 1/20^th of the maximum contribution of the feeder to internal faults.

i.e. Inp = I_F max int/20

e.g. A power line likely to import electricity at 20 kA gives rated primary current Inp as 1000 A.

In any case the maximum peak current shall be less than 90 In (90A for 1A input and 450A for 5A Input) i.e. 32 In RMS fully offset.

This recommendation is used for the majority of line or transformer protection applications.

The CT must be sized so as not to saturate during internal faults:

For each CT, IFeederMax = maximum contribution of the feeder to an internal fault (could be different for each feeder):

Vk > IFeederMax * (RCT + RB)

Note: This specification is valid for internal faults.

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10.2.2 CT Specification according to IEC 185, 44-6 and BS 3938 (British Standard)

1. Class X according to British Standard: Minimum knee point voltage for saturation Vk min = secondary IF max x (RCT + RB)

With secondary IF max not less than 20 (if IF max < 20 In then IF max = 20) Note: This specification is valid for external faults.

This provides a sufficient margin of security for CT saturation immunity.

2. Class 5P to IEC 185. Conversion of class X (BS) with the 5P equivalent (IEC)

3. Class TPX and TPY according to IEC 44-6. IEC defines a composite error as a percentage of a multiple of the rated current (IN) on a definite load SVA.

e.g. CT 1000/5 A – 50VA 5P 20 [CT Inp / InA – SVA Accuracy P Kscc]

This definition indicates that the composite error must be lower than 5%, for a primary current of 20Inp when the external load is equal to 2 ohms (50VA to In). If secondary resistance, RCT, is known it is easy to calculate the magnetising EMF developed with the fault current (20In). Actually if the error is 5% (= 5A) with this EMF, the point of operation is beyond the knee point voltage for saturation. By convention one admits that the knee point voltage, Vk, is 80% of this value. For a conversion between a class 5P (IEC) and a class X (BS) CT one uses the relation:

Vk=0.8 X [(SVA x Kssc)/In + (RCT x Kssc x In) ] SVA = (In x Vk/0.8 Kssc) – RCT x In2

In particular cases, calculation could reveal values too low to correspond to industrial standards. In this case the minima will be: SVA min = 10 VA 5P 20 which correspond to a knee point voltage of approximately Vkmin = 70 V at 5A or 350V at 1A. Class TPY would permit lower values of power, (demagnetisation air-gap). Taking into account the weak requirements of class X or TPX one can keep specifications common.

For accuracy, class X or class 5P current transformers (CTs) are strongly recommended.

The knee point voltage of the CTs should comply with the minimum requirements of the formulae shown below.

k is a constant depending on:

If = Maximum value of through fault current for stability (multiple of In) X/R = Primary system X/R ratio (for the P746 system, X/R up to 120) The following CT requirement can be developed for the P746 scheme Vk > secondary If max x (RCT + RB)

With RB = 2 RL

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10.3 Support of IEEE C Class CTs

MiCOM Px40 series protection is compatible with ANSI/IEEE current transformers as specified in the IEEE C57.13 standard. The applicable class for protection is class “C”, which specifies a non air-gapped core. The CT design is identical to IEC class P, or British Standard class X, but the rating is specified differently. The following table allows C57.13 ratings to be translated into an IEC/BS knee point voltage.

IEEE C57.13 – “C” Classification (volts)

C50 C100 C200 C400 C800 TABLE 3: IEC/BS KNEE POINT VOLTAGE VK OFFERED BY “C” CLASS CTS

Assumptions:

1. For 5A CTs, the typical resistance is 0.0004 ohm secondary per primary turn (for 1A CTs, the typical resistance is 0.0025 ohm secondary per primary turn).

2. IEC/BS knee is typically 5% higher than ANSI/IEEE knee.

Given:

1. IEC/BS knee is specified as an internal EMF, whereas the “C” class voltage is specified at the CT output terminals. To convert from ANSI/IEEE to IEC/BS requires the voltage drop across the CTs secondary winding resistance to be added.

2. IEEE CTs are always rated at 5A secondary

3. The rated dynamic current output of a “C” class CT (Kssc) is always 20 x In

Vk = (C x 1.05) + (In. RCT. Kssc)

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