135692909-Dobble

India

Knowledge Is Power SM DATE 1/10/2005

Apparatus Maintenance and Power Management For Energy Delivery

DOBLE ENGINEERING PVT LTD, 305-SAKAR, OLD PADRA ROAD, VADODARA PH: (+91) (265) 555 77 15, FAX: (+91) (265) 235 62 85

DOBLE ENGINEERING COMPANY, WATER TOWN, MA, USA www.doble.com

1

India





2

1) Mr. Sameer Gaikwad (Regional sales manager) 2) Mr. Kamin Dave (Relay Application Engineer) 3) Mr. B.Sundaram (Relay Application Engineer)

1) Mr. Mike ODowd (Regional sales manager)

4) Mr. Jay Gosalia (Vice-President-Engineering & Marketing) 5) Mr. Denis Tierney (Product Manager-F6 & F2 series) 6) Mr. Yakov knobel (Sr. Application Engineer-Protective relaying) 7) Mr. Jun Verzosa (Sr. Application Engineer-Protective relaying) 8) Mr. He fang (Application Engineer-Protective relaying)

India





3

Important considerations when design protection system:

1. Types of fault and abnormal Conditions to be protected against 2. Quantities available for measurement 3. Types of protection available 4. Speed 5. Fault position discrimination 6. Dependability / reliability 7. Security / stability 8. Overlap of protections 9. Phase discrimination / selectivity 10. CTs and VTs ratio required 11. Auxiliary supplies 12. Back-up protection 13. Cost 14. Duplication of protection

Types of protection A - Fuses For LV Systems, Distribution Feeders and Transformers, VTs, Auxiliary Supplies B - Over current and earth fault Widely used in All Power Systems

1. Non-Directional 2. Directional

C - DIFFERENTIAL For feeders, Bus-bars, Transformers, Generators etc

1. High Impedance 2. Low Impedance 3. Restricted E/F 4. Biased 5. Pilot Wire

D - Distance For transmission and sub-transmission lines and distribution feeders, also used as back-up protection for transformers and generators without signaling with signaling to provide unit protection e.g.:

1. Time-stepped distance protection 2. Permissive under-reach protection (PUP) 3. Permissive overreach protection (POP)

India





4

4. Unblocking overreach protection (UOP) 5. Blocking overreach protection (BOP) 6. Power swing blocking 7. Phase comparison for transmission lines 8. Directional comparison for transmission lines

E - Miscellaneous: 1. Under and over voltage 2. Under and over frequency 3. A special relay for generators, transformers, motors etc. 4. Control relays: auto-reclose, tap change control, etc. 5. tripping and auxiliary relays

Speed Fast operation: minimizes damage and danger Very fast operation: minimizes system instability discrimination and security can be costly to achieve. Examples:

1. differential protection 2. differential protection with digital signaling 3. distance protection with signaling 4. directional comparison with signaling

Fault position discrimination Power system divided into protected zones must isolate only the faulty equipment or section Dependability / reliability Protection must operate when required to Failure to operate can be extremely damaging and disruptive Faults are rare. Protection must operate even after years of inactivity Improved by use of:

1. Back-up Protection and 2. duplicate Protection

Security / Stability Protection must not operate when not required to e.g. due to:

1. Load Switching 2. Faults on other parts of the system 3. Recoverable Power Swings

Overlap of protections

1. No blind spots 2. Where possible use overlapping CTs

India





5

Phase discrimination / selectivity Correct indication of phases involved in the fault Important for Single Phase Tripping and auto-Reclosing applications Auxiliary supplies Required for:

1. Tripping circuit breakers 2. Closing circuit breakers 3. Protection and trip relays

AC. auxiliary supplies are only used on LV and MV systems. DC. auxiliary supplies are more secure than ac supplies. Separately fused supplies used for each protection. Duplicate batteries are occasionally provided for extra security. Modern protection relays need a continuous auxiliary supply. During operation, they draw a large current which increases due to

operation of output elements. Relays are given a rated auxiliary voltage and an operative auxiliary voltage range. The rated value is marked on the relay. Refer to relay documentation for details of operative range. it is important to make sure that the range of voltages which can appear at the relay auxiliary supply terminals is within the operative range. IEC recommended values (IEC 255-6): Rated battery voltages: 12, 24, 48, 60, 11 0, 125, 220, 250, 440 Preferred operative range of relays: 80 to 10% of voltage rated AC. component ripple in the dc supply:

India





6

4. Reliability Total cost should take account of:

1. Relays, schemes and associated panels and panel wiring 2. Setting studies 3. Commissioning 4. CTs and VTs 5. Maintenance and repairs to relays 6. Damage repair if protection fails to operate

Lost revenue if protection operates unnecessarily

India





7

Cautions for Use: Check List Check list Coil Drive Input

1) Is the correct rated voltage applied? 2) Is the applied coil voltage within the allowable continuous voltage limit? 3) Is the ripple in the coil voltage within the allowable level? 4) For voltage applied to a polarized coil, is polarity observed? 5) When hot start is required, is the increase in coil resistance resulting from coil temperature rise taken into account in setting coil voltage? 6) Is the coil voltage free from momentary drop caused by load current? (Special attention for self-holding relays.) 7) Is supply voltage fluctuation taken into account when setting the rated coil voltage? 8) The relay status may become unstable if the coil voltage (current) is gradually increased or decreased. Was the relay tested in a real circuit or with a real load?

Load (Relay contacts)

1) Is the load rated within the contact ratings? 2) Does the load exceed the contacts minimum switching capacity? 3) Was the relay tested with a real load? A DC load may cause contact lock-up due to large contact transfer. Was the relay tested with a real load? 4) For an inductive load, is a surge absorber used across the contacts? 5) When an inductive load causes heavy arc discharge across

the relay contacts, the contacts may be corroded by chemical reaction with nitrogen in the atmosphere. Was the relay tested

with a real load? 6) Is the contact switching frequency below the specification? 7) When there are more than two sets of contacts (2T) in a relay, metallic powder shed from one set of contacts may cause a contact failure on the other set (particularly for light loads). Was the relay tested in a real circuit? 8) A delay capacitor used across relay contacts may cause contact welding. Was the relay tested with a real load? 9) For an AC relay, a large contact bounce may cause contact welding. Was the relay tested in a real circuit or with a real load?

India





8

Operating Environment

1) Is the ambient temperature in the allowable operating temperature range? 2) Is relative humidity below 85 percent? 3) Is the operating atmosphere free from silicon gas? Depending on the load type, silicon gas may cause a black substance to from on the contacts, leading to contact failure. 4) Is the operating atmosphere free from excessive airborne dust? 5) Is the relay protected from oil and water splashes? 6) Is the relay protected from vibration and impact which may cause poor contact with the socket? 7) Is the relay free from mechanical shocks after it is installed in position? 10) Is insulation coating applied to the relay along with the PC

board? Depending on the load type, a black substance may form to cause contact failure.

Storage and Transport

1) Is the relay subject to freezing or condensation (especially when shipping)? 2) Is the temperature in the allowable temperature range? 3) Is the humidity in the allowable humidity range? 4) Is the storing atmosphere free from excessive airborne dust? 5) Is the relay protected from oil and water splashes? 6) When shipping does vibration and impact exceed the allowable range?

India





9

CHAPTER-1 DISTRIBUTION FEEDER PROTECTION

CHAPTER-2 POWER SYSTEM MODEL: SHORT CIRCUIT STUDY, O/C & E/F RELAY CO-ORDINATION

CHAPTER-3 TRANSFORMER PROTECTION

CHAPTER-4 SHUNT CAPACITOR BANK PROTECTION

CHAPTER-5 BASIC FUNDAMENTAL OF POWER FACTOR IMPROVEMENT

CHAPTER-6 BASIC FUNDAMENTAL OF HARMONICS

India





10

MULTIPLICATION OF THIS DOCUMENT IS STRICTLY PROHIBITED

India





11

TABLE OF CONTENTS:

TITLE Sr.No. PAGE NUMBER Introduction 1.1 12 IDMT OC & E/F Prot. 1.2 12 DMT & IDMTL characteristics variations & their Applications

1.2.1 13

Functional characteristics 1.3 19 Non-directional phase & earth fault OC relays

1.4 21

Advantage of Three OC & E/F scheme against two OC & E/F scheme

1.4.1. 22

Non-directional IDMTL relay with highest Application in TR feeder

1.4.2 26

Directional Phase & earth fault OC relays

1.5 27

Connection for Directional Phase OC relays

1.5.1 31

Directional E/F OC elements

1.5.2 36

Non-directional standby E/F Protection

1.6 38

Sensitive E/F Protection using CBCT

1.7 43

E/F Protection in 3-phase & 4-wire system

1.8 44

2-O/C & 1-E/F scheme in same parallel feeder operation

1.9 45

Unit Protection 2.0 46 Circulating Current system 2.1 46 Balanced voltage system 2.2 47 Summation arrangement 2.3 48 Supervision of pilots 2.4 49

India





12

1.1 INTRODUCTION: Industrial power distribution systems make extensive use of cable feeder for example, between captive generation bus or grid supply bus to load center / power control center. These feeders are too often radial or sometimes form part of ring main system. While IDMTL OC / E/F protection is mostly used for radial distribution feeders particularly in the tail end, unit type protection such as a pilot wire protection, are also sometimes used on critical feeders. The unit protections are highly selective, sensitive & fast in operation but dont have any backup capabilities. The IDMTL protections on the contrary, are simple & economical but slower in operation to necessities time co-ordination between adjacent sections for selective tripping. IDMTL relays, however provide excellent backup protection to the downstream system. 1.2 IDMT OVER CURRENT & EARTH FAULT PROTECTION: Application of relay is used as a primary protection as well as backup protection of transformer/motor/capacitor/cable feeders. Primary Protection & Backup Protection: - Primary Protection: Device Closest to the Fault or Main Protection - Backup Protection: Device next in the line, if main protection fails Definition : A device which operates, usually after slight delay, if the normal relay does not operate to trip its circuit breaker - Backup Protection should be operate, if the primary protection fails. - Reason for providing Backup Protection; 1) Failure of Primary Protection due to; - No operation of relay - Incorrect system design - Wrong selection of the relay - Improper installation & maintenance 2) Circuit Breaker failure (Stuck breaker) -Mainly Three kinds of back up relays: 1) Relay which trip the same breaker if the main relay fails (Relay back up) 2) Relay which open the next nearest circuit breakers on the same bus in case one of the local breakers fails to open (Breaker back up) 3) Relay which operate from a neighboring station so as to back-up both relays and breakers and their supplies (Remotes Back-up) in case of

India





13

the failure of any local supply including the battery, or in case a circuit breaker or relay fails to function. While at the lower end of the distribution system (particularly at low voltage levels), fuses or series connected trip coils operating on switching devices, are used for short circuit protection, IDMT over phase/earth current relays find wide application at medium voltage levels. As the name implies, IDMTL relays have an inverse time OC characteristic (i.e. the operating time is inversely proportional to the current) and a definite minimum time (DMT) for high multiples of setting current. The time /current curve is usually represented on a logarithmic scale and gives the operating time at different multiples of setting current, for the maximum Time Multiple Setting (TMS). The TMS is continuously adjustable giving a range of time/current characteristic. 1.2.1 DMT & IDMTL CHARACTERISTIC VARIATIONS AND THEIR APPLICATIONS: There are different variation of IDMTL characteristics. These are

i) IEC Standard Inverse t = (0.14*TMS)/((PSM)0.02 - 1)

ii) IEC Very Inverse t = (13.5*TMS)/((PSM) 1)

iii) IEC Extremely Inverse t = (80*TMS)/((PSM)2 1)

iv) UK Long Inverse t = (120*TMS)/((PSM) 1)

v) IEEE Moderately Inverse t = (TD/7)*{((0.0515)/(PSM0.02 1)) + 0.114}

vi) IEEE Very Inverse t = (TD/7)*{((19.61)/(PSM2 1)) + 0.491}

vii) IEEE Extremely Inverse t = (TD/7)*{((28.2)/(PSM2 1)) + 0.1217}

viii) IEEE US C08 Inverse t = (TD/7)*{((5.95)/(PSM2 1)) + 0.18}

India





14

ix) US C02 Short Time Inverse

t = (TD/7)*{((0.02394)/(PSM0.02 1)) + 0.01694}

Where, PSM = Plug setting multiplier = If/Is TMS = Time multiplier setting TD = Time dial setting

India





15

Below figure shows the above characteristics at the maximum time multiplier setting of 1.0

Figure (1A):- IEC60255 CURVES at TMS1.0

India





16

Figure (1B):- IEEE CURVES at TD=7.0

India





17

Definite Time Current Relays (DMT): In radial or loop circuits, where there are several line sections in series, there is no difference in current magnitude between a fault at the end of one section and a fault at the beginning of the next section as shown below;

Where there are many section in series the tripping time for a fault near the power source may be dangerously high. This is obviously un-desirable because such faults involve large currents and are destructive if not removed quickly. In fact, the disadvantage of time-graded over current relays is the heaviest faults are cleared slowest. Standard Inverse Definite Minimum Time lag (IDMTL) over current relays: Its a combination curve of Inverse time + Definite minimum time. This relay covers majority of the application in power system Very Inverse Time Current relays: Application of this relay is suitable in cases where there is a substantial reduction of fault current as the distance from the power source increases.

0.2s0.5s

0.75s1.0s

G1 TRF1 F2 F3

R1 R2

Disadvantage: Relay R1 will operate after 1.0s on F1, F2 & F3

India





18

Extremely Inverse Time Current relays: Application of this relay is suitable in grading with fuses & at the same time to remain inoperative on the switching current. As shown in formula the operating time of relay is inversely proportional to the square of the applied current. Long Inverse Time Current relays: Application of this relay is mainly for over load protection of NGR (Neutral grounding resistor). The IDMTL relays provide both time & current grading to achieve discrimination between successive stage in the distribution system.

India





19

1.3 Functional Characteristics: Three essential characteristics of the protective relaying: 1) Sensitivity 2) Selectivity 3) Speed 1) Sensitivity: - Pickup Setting must be greater than maximum load current in circuit. - Lower the pickup of the relay more will be the sensitivity. - Relay should be able to detect minimum fault current. Example: ---- Fault current magnitude = 35kA ---- CTR = 3000/1 ---- Relay Pickup, set at 75% Relay Operating current on Primary side = (75/100)*(3000) = 2250 Amp Relay Operating current on Secondary side = (75/100)*(1) = 0.75 Amp Sensitivity of relay during a fault = (2250/35000)*100 = 6.42% ---- Relay Pickup, set at 100% Relay Operating current on Primary side = (100/100)*(3000) = 3000 Amp Relay Operating current on Secondary side = (100/100)*(1) = 1.0 Amp Sensitivity of relay during a fault = (3000/35000)*100 = 8.57% Thus, lower the setting, higher will be the sensitivity for fault detection. 2) Selectivity: Three methods to achieve the discrimination

1) Discrimination by Time 2) Discrimination by current 3) Discrimination by both Time & current 4) Direction of the fault current

1) Discrimination by Time:

Basically used with definite time relays Time of operation is independent of current magnitude Discrimination time between successive relays say 0.3 sec

India





20

EXAMPLE: For fault on one outgoing feeder3 Fuse Operates in 0.01s Next feeder2 relay operates in 0.25s Next feeder1 relay operates in 0.50s All upstream relays are graded accordingly

DISADVANTAGE:

Operating time of the upstream relays will be very high The fault closest to the source takes longest time to clear

ADVANTAGE:

Defined operating time for variable source operating condition.

2) Discrimination by Current: Applicable only when substantial difference between the fault

current magnitudes exist for the faults on the two ends of the equipment.

The impedance of the equipment shall be substantial that will create the above difference.

EXAMPLE:

For the fault on the L.T. side of transformer TR Fault current = 40,000 A @ 415V = 1509.09 A @ 11kV (Reflected fault on 11kV side) For the fault on the H.T. side of Transformer TR Fault current = 13.5 kA @ 11kV IDMTL unit of Transformer Primary side (11kV) relay should be

operate as a backup protection on L.T. fault HIGHSET unit of Transformer Primary side (11kV) relay should be

operate on H.T. fault

DISADVANTAGE: The discrimination is obtained but no backup ensured

3) Discrimination by both Time & Current:

Obtained using Inverse time relays (IDMTL) IDMTL Inverse definite minimum time lag Relay operating time varied by adjusting time dial & current setting

India





21

Relay operating time depends on fault current. Higher fault, less operating time. Operating time inversely proportional to current magnitude

Discrimination achieved by both current & time

EXAMPLE: For a given fault current magnitude, discrimination time is 0.3s for

all the successive relays Pickup value for all the relays set below the fault current level For the fault on the outgoing feeder-1 Fault current = 40,000 A @ 415V = 1509.2 A @ 11kV (Reflected fault current on H.T. side) 250 A fuse operates in 0.01s Next feeder-2 relay operates in 0.25s on L.T. side fault Next feeder-3 relay operates in 0.5s on L.T. side fault Next feeder-4 relay operates in 0.75s on L.T. side fault For H.T. side of the Transformer TR Fault current = 13.5 kA @ 11kV Same Feeder-4 relay operates in 0.25s on H.T. faults

ADVANTAGE:

With the same Pickup & Time dial settings, lower Tome of operating for near end faults and higher operating times for near end faults inherently achieved.

In case of the difference in the fault current magnitude along system, IDMTLL relays are superior to the DMT relays

In case of same fault current magnitude along system, desired operating time can be achieved by adjusting pickup & time dial

4) SPEED:

Speed: the clearance time of the fault Fault clearing time < 100ms, high speed tripping Necessity of high speed tripping:

Minimizing the damage of the equipment Increasing stability margin for synchronous machines Avoiding unwanted tripping of voltage sensitive loads

Speed without sensitivity: Unsatisfactory co-ordination

India





22

Methods to achieve high speed tripping: Unit protection:

1) Protection provide to trip instantaneously for faults only within unit

under protection 2) No co-ordination with external protections

Example: Bus differential protection Feeder pilot wire protection Transformer or motor protection Directional protection Restricted earth fault protection

1.4 Mainly Two types of Phase & Earth fault Over current relays: [A] Non-directional Phase & Earth fault Over current relays:

Non-directional means when the fault current can flows on both direction through relay & fault current magnitude is more than threshold level of relay, it gives trip command to Circuit breaker.

Where, R1 = Non-directional OC relay F1 = In zone fault F2 = Out zone fault

F1 F2

R1

G1

F2 F1

India





23

During fault F1 , the fault current can flows through the relay towards the source G1 & fault F2, the fault current can flows through the relay away from source G1. So, relay R1 seen the magnitude of fault current which flows in both directions. 1.4.1 Non-directional Three Over current and One Earth Fault Scheme of Protection of a DY1 & DY11 Transformers feeder If the relays are protecting a transformer feeder, two over current and one earth fault

scheme of protection will give inadequate protection.

Why two over current & one earth fault scheme of Protection is inadequate? For the Y-B fault as shown in figure-2, the directions and magnitudes of fault currents are shown. If Y-B fault occurs on the secondary side of a transformer having vector group DY1, the magnitude of the current will be Iy, 2Iy & Iy in the R, Y & B lines of primary side respectively. The magnitude of the fault current is maximum in Y-ph & in Y-ph there is no over current element in this scheme as shown in figure-3. The tripping of the circuit breaker will be delayed because of low current (Iy) flows in R-ph & B-ph. Thus two over current and one earth fault scheme is inadequate protection for this particular case.

India





24

FIGURE:-2

India





25

Figure-3

RECOMMENDATION: IT IS BETTER TO PUT 3-OC & 1-E/F SCHEME INSTEAD OF 2-OC & 1-E/F SCHEME FOR CRITICAL TRANSFORMER WHICH HAVE DY-1 OR DY-11 VECTOR GROUP.

RECOMMENDED RELAYS FOR 3-OC & 1-E/F SCHEME: -CDG-61 & CDG-11 LOWER COST ELECTROMECHANICAL RELAYS -ICM-21 LOWER COST ELECTROMECHANICAL RELAY -SPAJ-141/142 ABB MAKE NUMERICAL RELAYS -7SJ61X SIEMENS MAKE NUMERICAL RELAYS -P124/125/126 ALSTOM MAKE NUMERICAL RELAYS

India





26

1.4.2 Non-directional IDMT RELAYS WITH HIGHSET INSTANTANEOUS OVER CURRENT UNIT APPLICATION FOR TRANSFORMER PRIMARY FEEDER Particularly on any Transformer primary feeder required Two Over current elements; [A] Inverse definite Minimum time lag (IDMTL) Over current Unit Application of IDMTL Unit is for Backup Protection of any down stream faults

on Secondary side & primary faults on primary side. It is always co-ordinate with

IDMTL relay of transformer secondary feeders for through faults or down

stream faults only.

Here as shown in figure-1, IDMTL unit should be operated as backup protection

on fault (F1 & F2).

[B] Highset or Instantaneous Over current Unit Application of Highset Unit is for Primary protection of fault on primary side.

Due to the co-ordination problem, operation of IDMTL unit is very sluggish

(take more time) for fault on primary side & it may chance to damage the

transformer to sustain the fault current for longer time (Say 400ms or more). That

is why the highest unit is required to operate instantaneously (Say 50ms) for

primary feeder.

Here as shown in figure-4, highest unit should be operated as primary protection

on fault (F1).

The setting of an instantaneous over current unit on the primary side of transformer should be little above asymmetrical value of the fault current for three phase fault on the secondary of the transformer & it should be lesser than minimum value of the fault current on the primary side.

India





27

EXAMPLE SETTING OF HIGHSET UNIT: CTR = 150 / 1A RELAY TYPE = CDG-61 PLUG SETTING RANGE OF IDMETL UNIT = 0.5 2.0A PLUG SETTING OF IDMTL UNIT = 1.0A PLUG SETTING RANGE OF HIGHSET UNIT = 2.0 20A FAULT CURRENT (F2) = 40Ka on 415V BUS F2 FAULT CURRENT REFLECTED ON 11Kv = 1.509kA [A] PLUG SETTING MULTIPLIER DURING F2 FAULT = 1.509 *1000 / 150 PSM = 10.06 [B] PLUG SETTING MULTIPLIER DURING F1 FAULT = 4.000 *1000 / 150 PSM = 26.66 [C] HIGHSET UNIT SHOULD NOT BE OPERATED ON FAULT F2. THEREFORE, HIGHSET SETTING is > PSM 10.06 other wise it will immediately tripped on through fault (F2) & isolate whole system. HIGHSET SETTING is < PSM 26.66. RECOMMENDED HIGHSET SETTING = 10.06 * 1.3 (Safety margin) = 13A

India





28

Figure-4: Example system diagram RECOMMENDED RELAY (R2) ON PRIMARY SIDE IS CDG-61 or EQUIVALENT.

India





29

1.5 Directional Phase & Earth fault Over current relays: Directional Over current relays are combinations of directional & over current units in the same enclosing case. Any combination of directional relay, inverse-time over current relay, and instantaneous over current relay is available for phase- or ground-fault protection. When fault current can flow in both directions through the relay location, it may be necessary to make the response of the relay directional by the introduction of a directional control facility. The facility is provided by use of additional voltage inputs or Polarizing voltage inputs to the relay. Relays that must respond to power are generally used for protecting against conditions other than short circuits. Such relays are connected to be polarized by a voltage of a circuit, and the current connections and the relay characteristics are chosen so that maximum torque in the relay occurs when unity-power-factor load is carried by the circuit. The relay will then pick up for power flowing in one direction through the circuit and will reset for the opposite direction of power flow. If a single-phase circuit is involved, a directional relay is used having maximum torque when the relay current is in phase with the relay voltage. The same relay can be used on a three-phase circuit if the load is sufficiently well balanced; in that event, the polarizing voltage must be in phase with the current in one of the three phases at unity-power-factor load. (For simplicity, the term phase will be used frequently where the term phase conductor would be more strictly correct.) Such an in-phase voltage will be available if phase voltage is available; otherwise, a connection like that in neutral voltage is not available. Directional Element of OC relay checks the phase angle between current & voltage and threshold magnitude of current. If the phase angle is reverse & fault magnitude crosses its threshold then it allows to operation of relay. Application:

Generally the Protection is applied for parallel feeders, cables, lines & transformers in ring main system.

Sometimes it is more helpful while relay co-ordination is not possible through non-directional IDMTL OC relays.

Protection should be able to offered with higher sensitivity while using the directional control. This is not possible with non-directional OC relays.

Selective tripping

India





30

Protection is not applied for radial distribution system.

Where, R1 = Directional OC relay on Line-1 R2 = Directional OC relay on Line-2 F1 = In zone fault of Line-1 F2 = In zone fault of Line-2

F1

R1

G1

F1 FAULT FEED BY SOURCE G2

F2

R2

CB

F2 FAULT FEED BY SOURCE G1

G2

India





31

Here as shown in figure below; relay R1 is connected on line-1 & relay R2 is connected on line-2. Relay R1 seen fault current at F1 location & R2 seen fault current at F2 location.

1.5.1 CONNECTION FOR DIRECTIONAL OVERCURRENT RELAYS: Two separate Elements:

1) DIRECTIONAL PHASE FAULT OVER CURRENT ELEMENT 2) DIRECTIONAL EARTH FAULT OVER CURRENT ELEMENT

1.5.1a DIRECTIONAL PHASE FAULT OVER CURRENT ELEMENT: RELAY CONNECTIONS: There are mainly three possibilities for a connectional connections of voltage and current inputs & these connections are dependent on the phase angle, at unity system power factor, by which the current and voltage applied to the relay are displaced.

1) 90o RELAY QUADRATURE CONNECTION

Ia current & Vbc voltage.

India





32

2) 30o RELAY ADJACENT CONNECTION

Ia current & Vac voltage.

3) 60o RELAY CONNECTION

Ia current & (Vbc+Vac) voltage. Circuit diagram of most commonly method for phase over current element is 90o quadrature connection as shown below;

India





33

Fig.5 Connections and vector diagram for a directional relay where phase-to-neutral voltage is not available.

This is the standard connection for Electromechanical, static, digital or numerical relays. Depending on the angle by which the applied voltage is shifted to produce maximum relay sensitivity (the Relay Characteristic Angle, or RCA for numerical relays & MTA for Electromechanical & static relays ) two types are available.

India





34

1) 30o MTA LEAD (Maximum torque angle) or 30o RCA LEAD (Relay

characteristic angle)

Fig.6 Vector diagram for the 30 connection (phase A element)

The A phase relay element is supplied with Ia current and Vbc voltage displaced by 30 in an anti-clockwise direction. In this case, the relay maximum sensitivity is produced when the current lags the system phase to neutral voltage by 60. This connection gives a correct directional tripping zone over the current range of 30 leading to 150 lagging; see Figure 6. The relay sensitivity at unity power factor is 50% of the relay maximum sensitivity and 86.6% at zero power factor lagging. This characteristic is recommended when the relay is used for the protection of plain feeders with the zero sequence source behind the relaying point.

India





35

2) 45o MTA LEAD (Maximum torque angle) or 30o RCA LEAD (Relay

characteristic angle)

Fig.7 Vector diagram for the 45 connection (phase A element) The A phase relay element is supplied with current Ia and voltage Vbc displaced by 45 in an anti-clockwise direction. The relay maximum sensitivity is produced when the current lags the system phase to neutral voltage by 45. This connection gives a correct directional tripping zone over the current range of 45 leading to 135 lagging. The relay sensitivity at unity power factor is 70.7% of the maximum torque and the same at zero power factor lagging; see Figure 7. This connection is recommended for the protection of transformer feeders or feeders that have a zero sequence source in front of the relay. It is essential in the case of parallel transformers or transformer feeders, in order to ensure correct relay operation for faults beyond the star/delta transformer. This connection should also be used whenever single-phase directional relays are applied to a circuit where a current distribution.

India





36

1.5.2 DIRECTIONAL EARTH FAULT OVER CURRENT ELEMENT: PRINCILAL: The directional earth-fault unit measures the neutral current I0, the residual voltage U0 from open delta PT and the phase angle(pHi) between residual voltage(U0) and neutral current(I0). An earth-fault stage starts if all of the three criteria below are fulfilled at the same time:

- the residual voltage U0 exceeds the threshold or set level - the neutral current I0 exceeds the set value - if the phase angle between residual voltage and neutral current falls

within the operation area RELAY CONNECTIONS: The residual current is extracted as shown in Figure 8. Since this current may be derived from any phase, in order to obtain a directional response it is necessary to obtain an appropriate quantity to polarize the relay. Residual voltage: A suitable quantity is the residual voltage of the system. This is the vector sum of the individual phase voltages. If the secondary windings of a three-phase, five limb voltage transformer or three single-phase units are connected in broken delta, the voltage developed across its terminals will be the vector sum of the phase to ground voltages and hence the residual voltage of the system, as illustrated in Figure 8. The primary star point of the VT must be earthed. However, a three-phase, three limb VT is not suitable, as there is no path for the residual magnetic flux.

India





37

Figure 8. Residual voltage connection for Earth fault element by using open delta PT

India





38

RESIDUAL CONNECTION DIAGRAM OF CDD21 EARTH FAULT UNIT:

India





39

CHARACTERISTIC ANGLES FOR EARTH FAULT UNIT: The characteristic angles are varies from relay to relay but, most commonly are as shown below; Characteristics are available in ELECTROMECHANICAL & STATIC RELAYS (CDD21, 2TJM12 etc) are;

1) 30o MTA LAG or 30o RCA LAG FOR SOLIDLY EARTHED SYSTEM 2) 45o MTA LAG or 45o RCA LAG FOR SOLIDLY EARTHED SYSTEM 3) 12.5o MTA or 14o MTA FOR RESISTIVE EARTHED SYSTEM

Characteristics are available in NUMERICAL RELAYS (SPAS348, MICOMP140 etc) are;

1) o to 90 degree RCA LAG.

India





40

2) WATTMETRIC Io*COS(pHi) FOR RESISTIVE EARTHED SYSTEM

If the watt-metric component of zero sequence power is detected in the forward direction, it indicates a fault on that feeder, while a power in the reverse direction indicates a fault elsewhere on the system. This method of protection is more popular than the sensitive earth fault method while system is resistive earthed, and can provide greater security against false operation due to spurious CBCT output under non-earth fault conditions. Watt-metric power is calculated in practice using residual quantities instead of zero sequence ones. The resulting values are therefore nine times the zero sequence quantities as the residual values of current and voltage are each three times the corresponding zero sequence values. The equation used is: 3Vo * 3Io * COS(- c) = Vres * Ires * COS( -c) where: Vres = residual voltage Ires = residual current

India





41

Vo = zero sequence voltage Io = zero sequence current = angle between Vres and Ires c = relay characteristic angle setting

3) VARMETRIC Io*SIN(pHi) FOR REACTIVE EARTHED SYSTEM

If the Var-metric component of zero sequence power is detected in the forward direction, it indicates a fault on that feeder, while a power in the reverse direction indicates a fault elsewhere on the system. This method of protection is more popular than the sensitive earth fault method while system is reactive earthed, and can provide greater security against false operation due to spurious CBCT output under non-earth fault conditions. Watt-metric power is calculated in practice using residual quantities instead of zero sequence ones. The resulting values are therefore nine times the zero sequence quantities as the residual values of current and voltage are each three times the corresponding zero sequence values. The equation used is: 3Vo * 3Io * SIN(- c) = Vres * Ires * SIN( -c) where: Vres = residual voltage Ires = residual current Vo = zero sequence voltage Io = zero sequence current = angle between Vres and Ires c = relay characteristic angle setting

India





42

1.6 Non-directional stand by earth fault Protection:

Protection is mainly used for Star-Star transformer or Star side of Delta-Star transformer.

Time dial setting of Protection is require to co-ordinate with down stream feeders relay.

Protection is work as a main protection for small transformer & backup protection of medium & large transformer.

CTR is independent of load current.

R-ph

Y-ph

B-ph

IDMT / DMT E/F

3Iao

India





43

1.7 Sensitive Earth fault protection using Core Balance C.T. (CBCTs):

Normally applied when the earth fault current magnitude is very very low (10mA or 20mA).

Where low infeed earth faults are excepted, requiring a very sensitive earth fault protection, CBCTs can be used in conjunction with sensitive earth fault relay.

The CBCT surrounds all 3-phases (and neutral also in case of 4-core cables) and is excited by the primary residual current. Better sensitivities are achieved due to reduced number of secondary turns since the turns ratio is independent of the load current. Usually static relays with sensitive setting and low ohmic burdens should be used with CBCTs to limit the output voltage requirement in view of low turns ratio. The CBCTs are used on cables which are usually armoured. The earthing of the cable sheath/armour should be such that the earth return current carried by the armour should not offset the earth fault current in the phase conductor as it would adversely affect the sensitivity. Typical earthing arrangement of the sheath/armour at load and source end is shown in figure ANNEXURE:-1

India





44

1.8 Earth Fault Protection in 3-phase 4-wire System:

3-phase 4-wire means, 3-phase + Neutral. Normally, it is seen in LT (415V) distribution system. 4-CT connections are require because of high unbalanced loads. In 3-phase 4-wire system having substantial single phase

unbalanced loads, conventional residually connected earth fault relays using 3-phase CTs cannot provide adequate sensitive against earth faults. This is because the earth fault relay is required to be set above the max. single phase unbalanced current. If a sensitive E/F setting is desirable, independent of the single phase unbalanced loads a 4th C.T. in the neutral becomes necessary as shown in figure below;

O/C E/F

R

Y

B

N

India





45

1.9 Non-operation of two O/C and one E/F scheme for Parallel Feeder: Here as shown in figure below; two O/C and E/F scheme of protection as applied to a parallel feeder where the phase relays are provided in R & B phases. Let us consider R-Y phase fault as shown in figure below. The instantaneous directions of currents are as shown. For the case of the R-Y fault as shown in figure(), the relay in R-phase gets practically zero current (if fault current is of the order of the normal load current of feeder) while Y-phase carries practically double the fault current. The relay R will not operate because of insignificance current flow through its element and since there is no relay element in Y-phase this fault is not cleared. Solution: Three O/C & one E/F relay protection scheme take care of this.

IF2 IF1

IF2

IF2 = IF1

2IF2

IF2

India





46

2.0 Pilot wire Unit Protection: There are mainly three types of pilot relaying relays:

1) DC Pilot Wire relay 2) AC Pilot Wire relay

1) DC PILOT WIRE RELAYING SCHEME: Mainly two schemes: 1) Series DC PILOT wire relaying scheme 2) Shunt DC PILOT wire relaying scheme 1) Series DC Pilot Wire Relaying scheme: Here as shown in figure below; Where, D = Poly-phase mho relay (Directional comparison) T = Tripping relay O = N/C contact of Over current fault detector S = Alarm relay for pilot wire open circuit indication

The supervisory alarm relays (S) are energized through DC circuit. Relay (S) de-energized while pilot wire circuit break & it gives an

alarm. Scheme for Internal Faults:

When the internal fault occurs, the over current fault detector pick up.

Contact O will open De-energized S relay Mho relay contact D will close while power flow in to the protected

section Finally Relay T will energize to gives a trip command to their

circuit breakers

India





47

Scheme for External Faults:

When external fault occurs then one end Mho relay will energize but other end relay will not energize because of power flow direction & it will not gives a trip command to circuit breakers

2) Shunt DC Pilot Wire Relaying Scheme: Here as shown in figure below; Where, D = Poly-phase mho relay (Directional comparison) TC = Tripping relay or trip coil of CB O = N/C contact of Over current fault detector B = Blocking relay PW = Pilot Wire Scheme for External Faults:

External faults or Normal condition (Under load), Main relay (both end) contact D will energize because of the power flow directions

Blocking relay B will pick up Now, N/C contact of B will open & it blocks the tripping

India





48

Scheme for Internal Faults: Internal fault condition, one end directional comparison relay will

de-energize. Now, DC supply at relay B will break. Relay B contact close & it permits a trip command to circuit

breakers. 2) AC Pilot Wire relaying scheme: The principal of unit protection was first established by merz-price. This fundamental differential system have formed the basis of many highly developed protective arrangements for feeders & many plant equipments. Two forms of Pilot differential schemes are available.

a) Circulating current system b) Balanced voltage system

2.1 Circulating current system: In this arrangement current transformers of identical ratio & ratings are provided at each end of the protected zone & are interconnected by secondary pilot as shown in figure:2.1 below;

India





49

Figure:2.1 For external faults, the two end CTs see equal inflow & out flow producing a circulating current between the CT secondary & Pilots, with no differential current through the relay. For an in zone fault, however, the secondary current have an additive polarity & hence the summated current flows through the relay, causing operation. In practice, unequal saturation of the CTs can cause increased spill current through the relay on external faults, producing instability. The problem is normally overcome by making the relay branch high impedance by adding series stabilizing resistor. 2.2 Balanced voltage system: In balanced voltage system, CT secondary outputs are opposed for through fault so that no current flows in series connected relays. An in zone fault however, produce a circulating current, causing operation. The arrangement is shown in figure 2.2

87 R1 87 R2

CT1 CT2

CABLE

PILOT CHANNEL

India





50

Figure: 2.2

In above arrangement, external faults would, in effect, cause a CT open circuit conditions, as no secondary current would flow. To avoid excessive saturation of the core. The core is provided with non-magnetic gaps to absorb the maximum primary m.m.f. the secondary winding therefore would produce an e.m.f. & can

87 R1 87 R2

CT1 CT2

CABLE

PILOT CHANNEL

India





51

be regarded a voltage source. The inherent CT error & Pilot capacitance would produce a substantial spill current through the relay on external fault, causing instability. The problem is overcome by providing a through current bias (restraint) which increases the differential pickup, approximately proportional to the through fault current, there by ensuring stability. 2.3 Summation arrangement: In three phase system, independent protection can be provided for each phase using phase by phase comparison of the two end currents. This would however a require a minimum for core pilot adding up to the cost. An alternative is to combine the separate phase current in to a single quantity for comparison over a pair of Pilots. This achieved by using summation current transformer. A typical summation CT is shown in figure 2.3

The interface section of the summation winding (i.e. A-B & B-C) usually have equal number of turns & the neutral end winding (C-N) having greater number of turns. The above summation arrangement would produce output for balanced as well as un balanced faults. However, the relay offers a different sensitive for different types of faults depending upon the phases involved. In the summation arrangement illustrated, the associated relay will have highest sensitive for A-C & A-N faults.

R-ph

Y-ph

B-ph

N

India





52

2.4 Supervision of Pilots: The pilot circuits are subjected to various hazards, which can cause open circuit or short circuit of the pilot cores. While overhead pilots are vulnerable to storms, buried pilots may be damaged during excavation. The pilot failure may lead to either mal operation or non-operation of the protection and hence continuous supervision of the healthiness of the pilots becomes necessary. This is achieved by injecting a small d.c. current through the pilot from one end and monitoring its presence at the other end by energizing an auxiliary relay. The auxiliary relay resets in the event of any discrepancy in the pilots and sounds an alarm. A small time delay is introduced to prevent transient operation due to primary system faults causing momentary dip in the auxiliary supply. Over current check feature may also be incorporated to prevent tripping on load in the event of a pilot open circuit condition as it may lead to instability. A typical pilot supervision arrangement is shown in figure-ANAXURE-2. The supervision arrangement detects any discrepancy in the pilot including open circuit, short circuit & cross pilot conditions.

India





53

MULTIPLICATION OF THIS DOCUMENT IS STRICTLY PROHIBITED

India





54

TABLE OF CONTENTS:

TITLE Sr.No. PAGE NUMBER Introduction 3.1 55 Parameter Conversions 3.2 55 Two winding Transformer Parameter conversions

3.2.1 58

3-winding Transformer Parameter conversions

3.2.2 62

Transmission line Parameter conversions

3.2.3 66

Motor Parameter conversions

3.2.4 67

Series Reactor Parameter conversions

3.2.5 70

Shunt Capacitor Parameter conversions

3.2.6 70

Shunt Reactor Parameter conversions

3.2.7 71

Series Capacitor Parameter conversions

3.2.8 71

Generator Parameter conversions

3.2.9 71

Perunit & Percentage Qty 3.2.10 76 Basic consideration for short circuit study

3.3 78

Asymmetrical fault current 3.3.1 79 3-ph SC on unloaded synchronous m/c

3.3.2 81

Symmetrical fault current 3.3.3 84 Effect of low zero seq. impedance of generators

3.3.4 88

ANSI/IEEE calculations 3.3.5 94 Importance of X/R ratio 3.3.6 102 PU method for SC calculations

3.3.7.1 108

Ohmic method for SC calculations

3.3.7.2 125

India





55

TITLE Sr.No. PAGE NUMBER Point-to-Point method for SC calculations

3.3.7.3 134

1-ph SC current for 1-ph TR system

3.3.7.4 139

Introduction relay co-ordination study

3.3.7.5 149

Functional characteristics 3.3.7.6 154 Criterias for Pickup setting & time dial

3.3.7.7 157

Data required for O/C & E/F relay co-ordination

3.3.7.8 159

Example for Phase fault relay setting

3.3.7.9 162

Example calculation for DMT relays

3.3.3.10 170

India





56

3.1 INTRODUCTION: Successful Operation of a power system depends largely on the engineers ability to provide reliable and uninterrupted service to loads. The reliability of the power supply implies much more than merely being available. Ideally, the loads must be fed at constant voltage and frequency at all times. In practical terms this means that consumers equipment may operate satisfactorily. For example, a drop in voltage of 10-15% or a reduction of the system frequency of only a few hertz may lead to stalling of the motor loads on the system. As electrical utilities have grown in size, and the number of interconnections has increased, planning for future expansion has become increasingly complex. The increasing cost of additions and modifications has made it imperative that utilities consider a range of design options, and perform detailed studies of the effects on the system of each option, based on the number of assumptions: like normal and abnormal operating conditions, peak and off-peak loadings, and present and future years of handled. Future transmission and distribution systems will be far more complex than those of today. This means that the power system planners task will be more complex. If the systems being planned are to be optional with respect to construction cost, performance, and operating efficiency, better planning tools are required. 3.2 Parameter Conversion: Power transmission lines are operated at voltage levels where kilovolts is the most convenient unit to express voltage. The amount of power transmitted is in terms of kilowatts or megawatts and kilo amperes or mega amperes. However the quantities, current and Ohms are often expressed as a percent or per unit of base value. The per unit value of any quantity is defined as the ratio of the quantity to its base value expressed as a decimal. Both the per unit (p.u.) and percent methods of calculation are simpler than the use of actual amperes, Ohms, and voltage values. The per unit method has an advantage over the percent method because the product of two quantities expressed in per unit is expressed in per unit itself, but the product of two quantities expressed in percent must be divided by 100 to obtain the result in percent. The per unit value of a line to neutral voltage on the line to neutral voltage base is equal to the per unit value of the line to line voltage at the same point on the line to line voltage base if the system is balanced. Similarly, the three-phase kVA is three times the kVA/phase and the three-phase kVA base is three times the

India





57

base kVA per phase. Therefore the per unit value of the three-phase kVA on the three-phase kVA base is identical to the per unit value of the kVA per phase on the kVA per phase base. Base impedance and base current value can be computed directly from three-phase values of base kilovolts and base kilo-amperes.

Base Current = {Base kVA (3-ph) / 1.7325*Base kV} Where, Base kV is the line-to-line voltage.

Base Z = { (Base kV / 1.7325)2*1000 / (Base kVA) / 1.7325 } Base Z = { (Base kV)2 / (Base MVA) } Sometimes the per unit impedance of a component of a system is expressed on a base other than the one selected as base for the part of the system in which the component is located. Since all impedances in any one part of a system must be expressed on the same impedance base when making computations, it is necessary to have a means of converting per unit impedances from one base to another. The per unit impedance is given by following equation; Per unit Z = { ( Actual Z in Ohms*Base MVA ) / ( Base kV )2 } Which shows that per unit impedance is directly proportional to base MVA and inversely proportional to the square of the base voltage. Therefore, to change from per unit impedance on a given base to per unit impedance on a new base, the following equation is used, Per unit Znew = ( Per unit Zgiven )*( Base kVgiven / Base kVnew )2*( Base MVAnew / Base MKVAgiven ) The Ohmic values of resistance and leakage reactance of a transformer depends on whether they are measured from the LT side or HT side of a transformer. If they are expressed in p.u., the base MVA rating of the transformer which is same as referred from HT side or LT side. The base kV is selected as the voltage of LT winding, if the ohmic values are referring to LT side, else it is selected as voltage of HT winding, if the ohmic values are referring to HT side of transformer. Whereas the PU values remains same regardless of whether they are determined from HT side or LT side.

India





58

The advantages of the PU method are;

The PU impedance of machines of same type and widely different ratings usually lie within a narrow range, although the ohmic values differ for machines of different ratings. For this reason, when the impedance is not known definitely, it is generally possible to select from the tabulated values a PU impedance which will be reasonably correct.

When impedance in ohms is specified in an equivalent circuit, each impedance

must be referred to the same circuit by multiplying it by the square of the ratio of rated voltages of the two sides of a transformer. The PU impedance, once expressed in proper base, remains same either referring from HT side or LT side.

The way in which transformers are connected in three phase circuits does not

affect the PU impedances of the equivalent circuit, although the transformer connection does determine the relation between the voltage bases on the two sides of the transformer.

3.2.1 Two winding Transformer Parameter Conversions: Manufacturers usually specify the impedance of a piece of apparatus in percent or per on the base of the nameplate rating. It is converted to common base using MVA rating and the voltage rating of transformer. Sometimes the voltage ratings of the transformer does not match exactly with the base voltage on their respective sides, in case the transformer parameters are converted to the base values of voltage and MVA. To begin with, assuming that transformer tap is on primary side ( HV side ), the given impedance is converted to common base as; Znewpu = { (Zoldpu)*(MVAnew / MVAold)*(Rated kVsec / Base kVsec)2 } If the transformer parameters are given in actual units (ohms). Then the values are converted to common base as; Zpu = (Zohms)*(Base MVA / BasekV2) Base kV is the voltage referred to the side at which measurements are made. The transformer R/X ratio is used to separate the transformer resistance and reactance values from the impedance. If number of units are in parallel then the effective equivalent impedance is computed by dividing the impedance by units. X = [ {Z2 / { 1 + ( R / X )2 }} / Units ] R = (X)*(R / X)

India





59

The minimum tap value is computed as; Tapmin(pu) = [ { VTap min kV / Base kVpri }*{ Base kVsec / Rated kVsec } ] The maximum tap value is computed as; Tapmax(pu) = [ { VTap max kV / Base kVpri }*{ Base kVsec / Rated kVsec } ] The tap step value (pu) is computed as;

Tap step(pu) = [ { VTap max kV VTap min kV } / { NTap max NTap min } ] Nominal tap value (pu) is computed as;

Tap nom = [ { VTap min kV + { ( NTap nom NTap min )*Tap step kV } } ]*[ Base kV sec / Rated kV sec ] The vector groups shows the connection of phases of two windings of a transformer and the numerical index for the phase displacement of the vectors of the two star-voltages. The numerical index shows by what multiples of 30o the low voltage vector lags ( anti-clockwise rotation of vectors ) behind the high voltage vector with the corresponding terminal designation. For example the groups are interpreted as;

Vector Group Dy5 High voltage in Delta and low voltage in star connection. Vector Group Yz11 High voltage in Star and low voltage in zigzag connection.

The transformer vector group information is required for 3-phase load flow and unbalanced fault studies. The different vector groups used are;

Star with neutral isolated. Star with neutral grounded. Star with neutral impedance. Delta connected.

The zero sequence impedances differ greatly depending on the type of connection and the construction of the transformers. Conductors connected to transformer windings with delta connection or with star with an insulated neutral point cannot carry a zero sequence current. The zero sequence impedance is therefore infinite. When the neutral point of star winding is earthed or connected, zero sequence can flow in the associated system. If the transformer is star

India





60

connected on primary side and delta connected on secondary side, then shunt impedance will exists from primary node to ground and vice-versa. The neutral impedance given in ohms, converted to common base as; Base Zpri = ( Base kVpri2 / Base MVA ) in Ohm Base Zsec = ( Base kVsec2 / Base MVA ) in Ohm Rpu neutral pri = ( ROhm neutral pri / Base Z pri ) Rpu neutral sec = ( ROhms neutral sec / Base Zsec ) EXAMPLE: Rated MVA = 315 Primary Voltage = 420 kV Secondary Voltage = 240 kV Positive sequence impedance = 0.125PU or 12.5% Zero sequence impedance = 0.100PU or 10% TAPmin = 1.0 TAPmax = 17.0 TAPnormal = 12.0 Minimum TAP Voltage = 360 kV Maximum TAP Voltage = 440 kV Neutral Rpri = Rsec = 2.0 Ohm. Connection = YnYn0 The transformer is connected to a bus on HT side with voltage 400 kV and on LT side is connected to a bus with voltage 220 kV. Hence primary base voltage = 400 kV and the secondary base voltage is 220 kV. The common base MVA = 100 Zpositive seq. in PU = (%Z / 100)*(Base MVA / Rated MVA)*(Rated kV / Base kV)2 = (12.5 / 100)*(100 / 315)*(240/220)2 = 0.047225 PU Xpositive seq. in PU = [ {Z2 / { 1 + ( R / X )2 }} / Units ] = [ {(0.047225)2 } / { 1 + (0.05)2 } ] = 0.047166 PU Rpositive seq. in PU = (0.04766 * 0.05) = 0.002358 PU Zzero seq. in PU = (%Z / 100)*(Base MVA / Rated MVA)*(Rated kV / Base kV)2 = (10 / 100)*(100 / 315)*(240/220)2

India





61

= 0.0377804 PU Xzero seq. in PU = [ {Z2 / { 1 + ( R / X )2 }} / Units ] = [ {(0.0377804)2 } / { 1 + (0.05)2 } ] = 0.0377332 PU Rzero seq. in PU = (0.0377332 * 0.05) = 0.00188666 PU Tapmin(pu) = [ { VTap min kV / Base kVpri }*{ Base kVsec / Rated kVsec } ] = [ { 360 / 400 }*{ 220 / 240 } ] = 0.82500 PU Tapmax(pu) = [ { VTap max kV / Base kVpri }*{ Base kVsec / Rated kVsec } ] = [ { 440 / 400 }*{ 220 / 240 } ] = 1.00833 PU Tap step(pu) = [ { VTap max kV VTap min kV } / { NTap max NTap min } ] = [ { 440 360 } / { (17 1)*400 } ] = 0.0125 PU

Tap nom = [ { VTap min kV + { ( NTap nom NTap min )*Tap step kV } } ]*[ Base kV sec / Rated kV sec ] = [ { 360 + { (12 1)*5 } } / 400 ]*[ 220 / 240 ] = 0.95104 PU The neutral impedance values are computed as; Base Zpri = ( Base kVpri2 / Base MVA ) in Ohm = ( 400 )2 / 100 = 1600 Ohm. Base Zsec = ( Base kVsec2 / Base MVA ) in Ohm = ( 220 )2 / 100 = 484 Ohm. Rpu neutral pri = ( ROhm neutral pri / Base Z pri ) = ( 2 / 1600 ) = 0.00125 PU Rpu neutral sec = ( ROhms neutral sec / Base Zsec ) = ( 2 / 484 ) = 0.004132 PU

India





62

3.2.2 Three Winding Transformer Parameter Conversions: The MVA rating of a two-winding transformer is same on primary and secondary side, whereas all the three windings of a three winding transformer may have different MVA ratings. The impedance of each winding of a three-winding transformer may be given in percent or PU based on the rating of its own winding. The transformer impedance values, which are measured by short circuit test, are

Impedance measured in primary with secondary short circuited and tertiary open (Z ps).

Impedance measured in primary with tertiary short circuited and secondary open (Z pt).

Impedance measured in secondary with tertiary short circuited and primary open (Z st).

If the three impedances measured in Ohms are referred to the voltage of one of the windings, the impedance of each separate winding referred to that same winding are related to the measured impedances as; Z ps = Zp + Zs Z pt = Zp + Zt Zst = Zs + Zt Where Zp, Zs, and Zt are the impedance of primary, secondary and tertiary windings referred to primary circuit. If Zps, Zpt, Zst are the measured impedances refer to primary circuit, the real and reactive parts are separated as; X ps = [ { Z ps2 } / {( R / X ps )2 + 1} ] / Units X pt = [ { Z pt2 } / {( R / X pt )2 + 1} ] / Units X st = [ { Z st2 } / {( R / X st )2 + 1} ] / Units Solving the above impedance simultaneous equations for Rp, Rs, & Rt, Xp, Xs & Xt yields,

Rp = [ R ps + R pt R st ] / 2

Rs = [ R ps + R st R pt ] / 2 Rt = [ R pt + R st R ps ] / 2 Xp = [ X ps + X pt X st ] / 2

India





63

Xs = [ X ps + X st X pt ] / 2 Xt = [ X pt + X st X ps ] / 2 The impedance of the three windings are connected in star (Y) to represent the single phase equivalent circuit of the three winding transformer. Since the ohmic values of the impedances must be referred to the same voltage, it follows that conversion to PU requires the same MVA base for all the three circuits and requires voltage bases in three circuits of the transformer.

The neutral impedances, if present is converted to per unit values on common base as; Base Z pri = ( Base kV pri )2 / Base MVA Base Z sec = ( Base kV sec )2 / Base MVA Base Z ter = ( Base kV ter )2 / Base MVA Rpu neutral pri = Rohm neural pri / Base Z pri Rpu neutral sec = Rohm neutral sec / Base Z sec Rpu neutral ter = Rohm neutral ter / Base Z ter EXAMPLE: Rated Primary MVA = 15 Primary Voltage = 66 kV Rated Secondary MVA = 10 Secondary Voltage = 13.2 kV Rated tertiary MVA = 5.0 Tertiary Voltage = 2.3 kV Zps = 7% on 15 MVA, 66 kV RX-Ratio-ps = 0.05 Zpt = 9% on 15 MVA, 66 kV RX-Ratio-pt = 0.05 Zst = 8% on 15 MVA, 66 kV RX-Ratio-st = 0.05 TAPmin = 1.0 TAPmax = 17.0 TAPnormal = 12.0 Minimum TAP Voltage = 59.4 kV Maximum TAP Voltage = 72.6 kV Rpri = Rsec = Rtertiary = 2.0 Ohm. Connection = YYnYn0

India





64

Assuming the common base values as 15MVA and 66 kV. First step is to convert all the impedance to common base on primary side. Zps & Zpt are measured at primary ratings, need no conversion while the Zst measured at different ratings it is converted to common base values as; Zst in PU = (%Z / 100)*(Base MVA / Rated MVA)*(Rated kV / Base kV)2 = (8.0 / 100)*(15 / 10)*(13.2 / 13.2 )2 = 0.12 PU Zps in PU = (%Z / 100)*(Base MVA / Rated MVA)*(Rated kV / Base kV)2 = (7.0 / 100)*(15 / 15)*(13.2 / 13.2 )2 = 0.07 PU Zpt in PU = (%Z / 100)*(Base MVA / Rated MVA)*(Rated kV / Base kV)2 = (9.0 / 100)*(15 / 15)*(13.2 / 13.2 )2 = 0.09 PU X ps = [ { Z ps2 } / {( R / X ps )2 + 1} ] / Units = [

135692909-Dobble

Documents

Transcript of 135692909-Dobble