M.Nageswar Rao,Sr.Manager (Engg.)NESCL, Noida

Date: 16.08.13

Presentation Layout

PROTECTION SCHEMES

CURRENT TRANSFORMER

CT DESIGN REQUIREMENTS

FOR VARIOUS PROTECTION

Protection schemesOver current protectionUnit Protection

Differential protectionREF protectionLine differential (Pilot wire)

Distance Protection

Diff. protectionMonitors an area limited by CTs which

measure incoming & outgoing currentsTypes

High impedanceLow impedance (Biased diff.)

High impedance Diff. protection Scheme used for

Bus bars, generator windings and Y-connected or auto

transformer windings.CTs must be selected with

Same ratioSame magnetizing curve

(same Vkmin & same Ie at Vk/2)

Same Rctmax.

High impedance Diff. protection

High impd. Busbar diff.

REF protection of T/f

High impedance Diff. protection

Line or cable diff. protection with pilot wires

Low impedance Diff. protection

For double bus bar protection

Used for busbar diff. protectionEHV lines

CTs can have different ratiosBias is used to correct

small ratio mismatchLarger ratios can be

matched using Aux. CTs

Low impedance Diff. – slope characteristicsHave operating characteristics with pickup increasing with

higher through fault currents. This is defined by a slope of the bias characteristics.

The higher the slope, the larger is the tolerance of the relay to errors and CT saturation.

Modern numerical relays using special saturation detectors or special through fault detectors.

Automatic slope adjustment is achieved with the help of modern numerical relays using special saturation detectors or special through fault detectors. low slope is maintained (sensitive differential protection) when

When there is no saturation or when no through fault is detected,

High slope is maintained (for good stability) when, severe saturation or through fault detection.

High & Low impedance diff. protectionHigh impd. diff. Low impd. Diff.

Application • Bus bars, • Generator windings and • Y-connected or auto transformer

windings

• Bus bars• EHV lines

CT Ratio Matching CT ratio to avoid spill current during healthy state

CTs can have different ratios

CT saturation voltage

Knee point voltage is of concern. Saturation can be tolerated, hence Vk is not of much concern.

Routing of CT connection

All CT connections are looped in the yard and single cable taken to the relay

CT wires directly to the relay

CT ckt. supervision

Detected by using a 3 phase rectifier relay to effect the summation of the bus wire voltages and short the pilot wire from the affected phase

A current operated auxiliary relay is used to detect any unbalance sec current for supervision of the CT ckts. Current setting of the supvn relay must be less than that of main diff relay

Cost & space req.

Less cost & space. Very costly and space consuming, as it requires large no. of modules & matching CTs.

Current TransformerCurrent

Transformer is an instrument transformer which transforms current from one level to another level.

e.g. 1000/1A, 200/5A

Terminal Box

PP

SS

Insulator

Secondary winding

Primary winding

Core

CBBus Feeder

CTs – windings & coresCTs have

1 or more primary windings (with 1 or more taps), and

1 or more secondary windings on different cores.

• Types of CT cores• Measuring cores• Protection cores• Protection cores for

special applications

CT secondary current rating5A Secondary 1A Secondary

Applications 1. Indoor switchgear cubicles

2. Higher primary current ratings.

Outdoor

When secondary gets open

low peak voltage high peak

voltageFine turns ratio adjustment

not possible when primary rating is low

Always possible

Saturation factor

•Ips/Ipn is called• Instrument Security Factor (FS) for the measuring CTs, and• Accuracy Limit Factor (ALF) for the protective CTs.

•These two saturation factors are practically the same, •FS or ALF = (Vsat/Vrated)*Inom.

CT - Knee Point Voltage CT excitation curve

is the magnetizing characteristic (plot between secondary applied voltage and the corresponding magnetizing current)

Knee point voltage Corresponds to the point on excitation curve beyond which an increase of 10% in

exciting e.m.f. produces an increase of 50% in the exciting current is defined as the point on the excitation curve where the tangent is at 45 degree

to the abscissa. represents the point beyond which the CT becomes non-linear.

Metering class Protection class Protection special class

Application Measuring Protection Unit ProtectionCT Selection Ratio Ratio Ratio

Accuracy class(0.1,0.2,0.3, 0.5,1,3,5)

Accuracy class (5P, 10P, 15P)

Knee Point Voltage (Vk)

Burden (15,20,30VA)

ALF (5, 10, 15, 20, 25, 30)

CT Secondary winding resistance (RCT) corrected to75OC

ISF (3.5.7) Burden (15,20,30VA)

Ie (Excitation current) at Vk or a stated % of Vk.

CT Selection example:

e.g.: 2000/1, Class 0.2, 20VA, ISF – 5

e.g. : 5P20, 40VA, ALF-5

e.g. : 200/1, PS Class, Vk > 200V, RCT < 2.0 ohms, Ie < 30mA at Vk/4

Application

IEC 60044-1

IEC 60044-6

IEEE C57.13 / ANSI

Metering 0.1,0.2,0.3, 0.5,1,3,5

0.3, 0.6, 1.2 (burden @ p.f. 0.9)

Protection 5P, 10P, 15P

C100, T100, C200, T200, C400, T400, C800, T800 (burden@ p.f. 0.5)

Protection special

PX TPS, TPX, TPY, TPZ

CT - RemananceRemanance flux is the value of flux, that would

remain in the core, 3 mins after interruption of exciting current of sufficient magnitude to induce the saturation flux.

CT Class

Air gap Remanance Application

TPS No High upto 85%

high impedance circulating current protection

TPX No High upto 85%

line protection.

TPY small Low <10% line protection with auto-reclose.

TPZ Large Negligible 0% special applications such as differential protection of large generators

CT specification – ANSI (IEEE Std C57.13- 1993)CT classes as per ANSI 

C

CT is furnished with excitation characteristics which can be used to “Calculate” the CT performance.

K

same as C rating but the knee-point voltage must be at least 70% of the secondary terminal voltage rating.

Tthe ratio error must be determined by ‘Test’.

ANSIVolt at 100A

Burden (ohm)

C100 100 1

C200 200 2

C400 400 4

C800 800 8

•The standard current transformer secondary winding is rated at 5A as per ANSI standards. (20times of 5A is max. recommended CT secondary current).

CT Saturation

In case of Rl (lead resistance)

1Φ to ground faults

Two-way

3Φ faults One-way

AC saturationTo avoid saturation, the CT

shall develop adequate voltage such that Vx > If (Rct+Rl+Rb)where, If = Fault current on CT secondary (Amps) Rct = CT Secondary resistance (Ohms) Rl = CT Secondary total lead resistance

(Ohms) Rb =CT secondary connected burden

(Ohms)

CT SaturationDC saturation

Decaying dc current introduces during a fault.

CT Saturation - Excursion of flux waveform Φ

Is well within the saturation limits with AC current waveforms

shoots past the saturation limits quickly with DC transients

CT SaturationCT shall have enough

capacity to develop the following voltage not to saturate at all for a combination of AC and DC transient. Vx > If (1+X/R) (Rct+Rl+Rb)

Saturation due to DC transient distorts the AC waveform output as well

CT saturation – how to avoidCT saturation can be avoided

By increasing the CT ratio (thereby reducing actual secondary current during fault to less than 100A)

By reducing the secondary connected burden by reducing the connected relay burden, reducing the lead resistance (by either

reducing the distance between the relay to the CT, multiple parallel runs of CT leads, thicker wire size etc.)

Most of the faults are ground faults which tend to have lesser DC offset and associated saturation issues. The ground faults tend to have more resistance (lower X/R ratio)

Protection Current demand

Operating time

Transient saturation

AC saturation

Remarks

Time OC 20-30 In NO YESHigh-set Phase or Ground OC

1 cycle YES YES high speed ofOperation is to be ensured.

Distance Protection

1.5 In YES NO Saturation is accepted after the operation of the Zone-1 operation.

Differential protection (Biased)

YES Saturation voltage is of concern

Differential protection (High impedance)

1 cycle knee point voltage rather is of concern

High Impedance Diff. Protection

setting VR >K x If x (RL + RCT ) (Volts) If = Secondary Fault current (Amps) RL = CT secondary lead resistance (Ohms) RCT = CT secondary resistance (Ohms) K = Margin Factor (=1 for full saturation)

CT requirements -for various Protection applications High Impedance Differential scheme

Vk≥2.If.(Rct+2.Rl) RCT= CT secondary winding resistance Rlead = lead resistance of the farthest CT in parallel group If = Maximum through fault current up to which relay should remain stable (referred to CT

secondary) Biased Differential scheme

Vk≥ K.2.IR.(Rct+2.Rl) IR= Relay rated current K = Constant specified by the manufacturer usually based on conjunction test (the constant is

usually chosen to ensure positive operation of highest differential unit on severe internal fault with extreme CT saturation)

Distance Protection schemeVk≥ (1+X/R).If.(Rr+Rct+n.Rl) X/R = Primary system reactance/resistance ratio (to account for the DC component

of the fault current) If= Maximum CT secondary current for fault at zone1 reach point Zrelay = Relay ohmic burden RCT= CT secondary winding resistance; nRlead= Lead resistance

If limited byTransformer Maximum through fault

currentZ1%

Busbar Maximum through fault current

switchgear breaking capacity

Generator Maximum through fault current

Xd”

Motor Maximum starting current

6x load current for DOL Motors

Shunt reactors

Maximum charging current

X

Short feeders

Maximum through fault current

for fault at busbar