110

Study Committee B5 Colloquium2005 September 14-16

Calgary, CANADA

* TNB Transmission Division, Level 4 Crystal Plaza, Lot 4 Jalan 51A/223, 46100 Petaling Jaya, Selangor, MALAYSIA. Tel: +603-79654888 Fax: +603-79654777 Email: zainorens@tnb.com.my

NOTE: The subject of this paper was contributed in CIGRE SC-B5 2003 Colloquium in Sydney, Australia but was not published nor circulated (Section 1-6). An update is presented as Section 7 in this paper.

110 - 1

Application of Distance Function as Back-up Protection for Grid Transformer in TNB 500/275/132kV System (Updated)

Zainoren Shukri* Engineering (Protection)

TNB Transmission Division Tenaga Nasional Berhad

Mohd. Hendra Hairi Prof. Dr. Abdullah Asuhaimi Mohd Zin

Electrical Engineering Faculty Universiti Teknologi Malaysia

MALAYSIA

Summary: Tenaga Nasional Berhad (TNB) is currently using plain overcurrent relay as backup protection for grid power transformer. Based on the experiences of the previous disturbances and joint studies with TEPCO (Tokyo Electric Power Company), it was concluded that the overcurrent backup is not adequate to satisfy overall fault clearance requirements for the 500/275/132kV system, and hence distance function was recommended also on grid power transformers in place of the overcurrent relay.

This paper describes the differences between the overcurrent and distance function characteristics and expected performance as backup protection. It presented the methodology of studies conducted and simulation results to verify the recommendation and identify possible problems. Furthermore, it also suggested the best strategy for settings application and selection of relevant functions (e.g., self-supervision, monitoring and disturbance recording) within the distance protection to effectively activate the required backup functions.

Keywords: Autotransformer – Protection – Backup –Distance – Settings

1. INTRODUCTION

Peninsular Malaysia’s national grid is operated at the system voltage level of 500kV and 275kV while 132kV is operated as sub-transmission radial system with open-points. Although majority of the existing generation stations are connected to the 275kV system, newer and larger generating stations are constructed for connection to the new 500kV backbone systems.

For grid transmission, power generated at the 500/275kV is transformed to the lower voltage level using autotransformers. Autotransformers with 33kV delta tertiary and standard

capacities of 750MVA and 240MVA are being used for 500/275kV and 275/132kV transformation, respectively. These autotransformers are solidly grounded; hence the 500/275/132kV transmission grid is effectively a common system (electrically). Earthing transformer with zig-zag-wye vector group is usually connected to the tertiary to provide the 415V substation supply.

1.1 Current Practice

Figure 1 illustrated the general practice for protecting the autotransformer [1]. The two main protections are based on unit differential philosophies that are biased differential and high impedance differential protection scheme types, besides the typical transformer guards (Buchholz, Pressure Relief, Winding and Oil Temperature). High impedance restricted earth-fault is supplemented to the tertiary winding with earthing transformer.

The backup protection is typically a non-directional overcurrent protection on both high-voltage and low-voltage terminals. On an early installation (1970’s), directional overcurrent was provided normally meant to avoid parallel transformer tripping during low-voltage side faults.

Figure 1. 500/275kV or 275/132kV Grid Autotransformer Protection Scheme

87TB

87TH

64REF

51OC-HV

51OC-LV

51OCEF-TV

64SBEF

64SBEF

500/275kV

275/132kV

PROTECTION SCHEME LEGEND: 87TB – Biased Differential 87TH – High Impedance Differential 51OC – Non-directional Overcurrent 64REF – Restricted Earthfault 64SBEF – Standby Earthfault

21Z-HV

21Z-LV

50BF

TRANSFORMER GUARD LEGEND: 26W – Winding Temperature 26O – Oil Temperature 63B – Main tank Buccholz 63TCB – Tap Changer Buccholz 63PRD – Pressure Relief Device 71OL – Oil Level

71OL

63PRD

63B

26W

63B

71OL 63PRD

63TCB

26O

33kV

1.2 Overcurrent versus Distance Characteristics

Typical overcurrent protection relays in TNB Transmission system are set 150% of the autotransformer rating with time multiplier of 0.4 and 0.5 for LV-side and HV-side, respectively. Relay characteristic type IEC255 Standard Inverse is being used. Figure 2 illustrates such characteristics that are tightly below the transformer damage curve [3][4].

0.1

1

10

100

1000

10000

1000 10000 100000

TIM

E in

Sec

onds

CURRENT in P.Amps

Clo

se-in

Fau

lt

AA

1. H10Z_5150HV "THREE PHASE" Model:MCGGBranch Main CT: 12381-123805 Ckt 1 (275.0 kV); CTR:600Pickup:1.25 R.amps; Time dial:0.500Characteristic:STANDARD INVERSE(SI)Test Time:3.96 seconds Test Current:3 R.Amps

2. 180Z_5150LV "THREE PHASE" Model:MCGGTransformer CT: on 11381-123807 Ckt 1; CTR:1200Pickup:1.3 R.amps; Time dial:0.400Characteristic:STANDARD INVERSE(SI)Test Time:3.32 seconds Test Current:3 R.Amps

R1 . ZM1 (37.29ohms@88.37deg)

R2 . ZM2 (62.58ohms@86.01deg)

R3 . ZM3 (151.88ohms@81.76deg)

R4 . Transformer thermal damage curve

R5 . Mechanical damage curve

For comparison, a three zones distance relay reach on the same location (HV-side) was also plotted in Figure 2. Reach settings of 80% and 120% of autotransformer (Zone 1 and Zone 2, respectively) and 120% of next longest adjacent line (Zone 3) were applied for example. By simulating sliding faults [2] along the distance relay direction and obtaining the relevant relaying current at each zone reach points, the plots were as shown.

Figure 2. Overcurrent vs Distance Protection Reach Coordination

110 - 2

The overcurrent relays were aligned for fault on the LV (Point A). This gives the correct normalization for both types of protections for faults along the direction of the distance relay. By observing the Time-Current Curve (TCC) in Figure 2, the distance relay has deterministic fault coverage and definite operating times. Meanwhile, the overcurrent relay has far less fault reach due to limitation of the autotransformer overload settings and slower operating time (not less than 1 second; to maintain coordination with down-stream LV overcurrent relays). Hence, it is apparent that distance relay has better fault clearing requirement especially for the backup protection zones.

1.3 General Review of Backup Protection Application on Autotransformer

Generally, Backup Protection takes-over the tripping function in the event failure of the Main Protection, with a discrimination delay. At the 500/275kV, the Local and Remote Backup Protection are used while only Remote Backup Protection at 132kV system. The Local Backup is the Breaker Failure Protection type while Remote Backups are either Distance Overreached Zones or IDMT-Overcurrent types.

In recent joint-studies with Tokyo Electric Power Company (TEPCO) [5], a renewed philosophy that autotransformer should be treated as transmission line was conceded. Although overcurrent protection could barely act as local backup for the autotransformer only (as shown in Figure 2), it could not satisfy the remote backup fault clearance requirement for the grid system. In place, the distance protection was proposed for its selectivity and definite characteristic to achieve coordinated backup protection on the transmission system. The proposed application of distance protection as shown by dashed line in Figure 1 was proposed.

Table I. Worst Case Operating Time (Simulation) of Overcurrent Backup Protection on Autotransformer

Relay Operating Time (s) Case Fault Location 51OC-HV 51OC-LV Fault Clearing

Requirement (s) A Close-in fault at LV relay location 3.27 (2.53) 1.14 (X) <3 B Close-in fault at HV relay location 1.02 (X) 28.2 (2.32) <3 C Busbar fault at LV bus 3.27 2.61 <3 D Busbar fault at HV bus 35.85 28.21 <3

Table I tabulates results of relay simulation using CAPE [2] that support the proposal. Faults were simulated close to the autotransformer for worst cases (bolted faults). Fault currents beyond that within the backup protection zones should not be more onerous than the ones tabulated. In Cases A and B, values in brackets indicate sequential fault clearance after the faster relay tripped and fault current redistributed. Cases A, B and C shows that total fault clearing times are still less than 3 seconds maximum allowed. However, Case D cannot be allowed since the fault clearance is too long (>28 s) as in case of failure of the main busbar protection. If high resistance busbar fault occurred, the fault may not be cleared by the desensitized and non-selective overcurrent relay.

TNB is considering reducing the present 3-seconds equipment rating to 1-second. The used of overcurrent relay as shown in all of the above cases may no longer satisfy this stringent fault clearance requirement. Hence, the application of distance relay as the backup protection seemed to be supported. This paper described the methodology used in investigating the details of the proposal, suggesting the best strategy for settings and configuration, functional selection and other requirements for the distance relay to meet the intended application.

110 - 3

2. METHODOLOGY OF STUDIES

In order to determine the best settings strategy, short-circuit analyses are used to evaluate performance of the distance relay to various fault conditions simulated. The power system equipments were model as pi-branches according to its sequence impedances equivalence [7]. Commercial power system analysis software [2] was used to perform sliding faults along the protected zones. The fault loci were plotted on RX-Diagram to graphically evaluate the operation of the distance zones.

2.1 Modeling the Autotransformer

Although the generalized N-circuit transformer model is available in the software, the basic pi-branch model for three-winding transformer was initially utilized to model the autotransformer [2][7][6]. The impedances were derived from typical transformer test results. In order to allow sliding faults to be applied within transformer winding branches, ideal transformers were included for voltage level correction. Figure 3 shows the equivalent model used in the analyses.

Figure 3. Autotransformer Model for Short Circuit Analysis (a) Pi-M275/132/33kV ANynd1 Impedance Data (b) Modified Model

LV HV

TV

Zb=Zlv = -2.13%Zb=Zhv = 16.91%

Zb=Ztv = 29.46%

HV

∆

Υ

Zt << small

Υ Υ

Zt << small

HV

(a) T m)est Data in Delta (top), Equivalent Star (botto

HV LV

TV

Ztv = 29.46%

Zhv = 16.91% Zlv = -2.13% TV

LV HV

Zlv-tv = 27.33% Zhv-tv = 46.37%

Zhv-lv = 14.78%

Take note that delta-star winding impedance conversion resuthe low-voltage winding. Typical values of this impedance arof transformer rated impedance. It represents a capacitance inimpose underreaching problem for distance relaying. This analysis.

Furthermore, internal faults in the autotransformer, e.g., earthfaults, cannot be simulated with accuracy using the abocircuit model did not include flux equations as the flux patterncould effect the actual fault current at the transformer terminal

However, since the distance backup is intended more for thethan transformer internal faults, the model chosen is adequatebecomes the basis for setting the first zone of the distanInvestigation for the latter case is being proposed for future sttransient type analysis or real time digital simulator.

110 - 4

(b)

odel with Typical 15% 240MVA with Ideal Transformers

lted in negative impedance on e in the range of 1.5% to 2.2% the RX-Diagram and thus may will be illustrated later in the

interturn faults and winding ve short circuit model. Short- changes during internal faults

s [8][9].

grid backup protection rather enough for this purpose. This ce relay as mentioned later. udies utilizing electromagnetic

2.2 Impedance Seen Through Transformation Ratio & Tap Changer

Basically, system impedance connected to one terminal of a transformer is seen from the other terminal as the squared of the turns ratio (or voltage ratio) [11]. For example, an impedance of 1 ohm on the 132kV terminal of a 275/132kV transformer will be seen as 4.34 ohms (1 x 275²/132²) from the 275kV terminal. Consequently for transformer with tap changer, the impedance transformation factors will vary with the tap changer steps.

Typical TNB autotransformers have tap changer ratios in the range of ±5% from nominal ratio. Table II shows the transformation factor at maximum, nominal and minimum tap steps for typical for TNB 500/275kV and 275/132kV autotransformers. Since the nominal impedance and tap step are normally used in power system model, the impedance margin from the nominal deviation (as tabulated) must be included in the distance relay reach to avoid under reaching due to tap changer operation.

Table II. Impedance Transformation Factor for TNB Autotransformers

Type Tap Steps (kV) Voltage Impedance (%)

Transformation Factor

Percentage from Nominal (%)

525/275 10.7 3.645 500/275 12.0 3.306 500/275kV 475/275 13.6 2.983

±10

300/132 12.5 5.165 275/132 15.0 4.340 275/132kV 245/132 19.0 3.445

±25

2.3 Earthfault Compensation Factor

Based on typical positive and zero sequence impedances of typical autotransformer as shown in Table III, earthfault compensation for distance zones within the autotransformer’s impedance (e.g., Zone 1) can be negligible. However for the overreaching zone (e.g., Zone 2), using no compensation will result in relay underreaching (Figure 4). On the other hand, using the overhead line compensation factor as an overall compensation parameter may result in serious overreaching into the grid system (Figure 5). In the figures, cross-mark “A” indicated three-phase fault location, whereas, cross-mark “B” indicated earth-fault location on the RX-diagram.

This is a problem for conventional distance relay using common K0 compensation factor (i.e., magnitude only, or magnitude and angle parameters). Correct factor cannot be achieved in such case. Therefore, newer distance relays that have separate K0 compensation factors for each zones or the ones that specifies zone settings in terms of separate phase-fault and earth-fault loop impedances are more suitable for this application.

Table III. Earthfault Compensation Factors for TNB Autotransformers and Overhead Lines

Type Z1 Z0 K0=(Z0/Z1-1)/3 Remarks 500/275kV AT 1.0 p.u. 1.0 p.u. 0 3 x 1-Phase Banks 275/132kV AT 1.0 p.u. 0.85 p.u. 0.05 3-Phase Bank

500kV OHL 0.2654 |86.3° 0.7922 |73.6° 0.6738 |-18.9° 4x500mm2 Curlew – Ω/km 275kV OHL 0.2887 |83.1° 0.8636 |74.0° 0.6701 |-13.6° 2x400mm2 Zebra – Ω/km 132kV OHL 0.2742 |80.4° 0.8827 |73.0° 0.7473 |-10.7° 2x300mm2 Batang – Ω/km

110 - 5

-10

10

20

30

-20 -10 10 20

X in P.Ohms

R in P.Ohms

11381+

123807123805

12381+

12541

12521+

-10

10

20

30

-20 -10 10 20

X in P.Ohms

R in P.Ohms

11381+

123807123805

12381+

12541

12521+

2.4 Identification of Critical Relaying Conditions

As a standard practice, distance relays applied in TNB grid transmission system should be set to cover at least the types of faults as listed in Table IV. Initial relay settings based on conventional philosophy as mentioned in 1.2, were further evaluated by applying those fault types in short circuit simulations onto RX-Diagram [2]. Final settings should be simulated under normal and contingency power system operation to include effects of remote infeeds, power swing and zero-sequence mutual coupling, as well as sequential fault clearing under backup protection reactions. Close-in fault, sliding faults and line-end fault were necessary to test the relay’s reach. The relay responses were observed especially at boundary cases, where finer adjustments of the final settings are made interactively. Some graphical examples of the results will be shown later.

Table IV. Fault Types for Critical Relaying Conditions (Boundary Case)

Fault Types Requirement of Conditions Remarks Three-phase-fault Correct Phase-Phase zone reach as

reference Check consistency of positive sequence

reach in RX-Diagram Earth-fault Correct Phase-Earth reach setting with

zero-sequence compensation, infeeds from multiple grounding system effects and zero sequence mutual coupling effects.

Check consistency of zero-sequence compensation relative (normalized) to

positive sequence reach in RX-Diagram

Two-phase-to-earth Correct selection of Phase-Phase or Phase-Earth measuring loops

Difficult relaying conditions depending on fault loop selection [11]

Arching (Resistive) faults

Correct resistive reach coverage for critical faults with fault resistances:

Phase-Phase = 10Ω Phase-Earth = 10Ω (15Ω for 132kV)

Tower Footing = 10Ω (4 Ω for 500kV)

Check adequacy of resistive coverage due to arching faults

Power swing Ensure tripping zones are not affected by power swing loci.

Decide to block power swing or trip under pole-slip conditions.

1 (Z1 U1) 1 (Z1 U2)

2 (Z2 U1) 2 (Z2 U2)

3 (Z3 U1) 3 (Z3 U2)

A (1 2 3)

B (1 2 3)

A (1 2 3)

B (1 2 3)

A (1 2 3)

B (1 2 3)

1 ++ 180Z_21LV,Z12 ++ 180Z_21LV,Z23 ++ 180Z_21LV,Z3

Figure 4. Relay Reach for Earthfault without K0-compensation (Underreaching)

1 (Z1 U1) 1 (Z1 U2)

2 (Z2 U1) 2 (Z2 U2)

3 (Z3 U1) 3 (Z3 U2)

A (1 2 3)

B (1 2 3)

A (1 2 3)

B (1 2 3)

A (1 2 3)

B (1 2 3)

1 ++ 180Z_21LV,Z12 ++ 180Z_21LV,Z23 ++ 180Z_21LV,Z3

Figure 5. Relay Reach for Earthfault with OHL K0-compensation (Overreaching)

110 - 6

2.5 Strategy for Fault Clearing by Autotransformer Backup Distance Protection

Basically, the distance relay should provide backup protection for the autotransformer under internal faults (primary or local zone) as well as backup protection for grid system for external faults (backup or remote zones), up to adjacent substations. Time delays were set to coordinate with the respective primary main protections and equipment ratings.

Figure 6. Distance Zone 2 as the Grid System Backup Protection

~ ~

~ ~

RELAYFAILED

Z2 Z2

Z2 Z2

BSEP

Z2 Z2

Figure 6 shows how the overreaching zone (Z2) of the remote relays including the autotransformer’s tripped (indicated by clear boxes) when the local relay failed to operate. This is true provided a bus separation scheme (BSEP) utilizing two distance relays on the bus coupler looking towards their opposite busbar sides, is in place. Otherwise more breakers on the parallel lines or autotransformers are tripped as indicated by grey boxes. Such scheme is not discussed here.

This forward overreaching zone as mentioned above served as the local backup for autotransformer, remote busbar and partly grid system (for close-in faults). Further consideration for backup zone (i.e., fault external to the autotransformer), two strategies of relay setting can be adopted, namely forward backup or reverse backup zones. In the forward backup strategy, additional overreaching zones of low-voltage side distance were set to protect the high-voltage side grid system. Conversely, the high-voltage side distance is set to protect the low-voltage side grid system.

In the reverse backup strategy, additional zones of low-voltage side distance are set reverse to backup the low-voltage side grid system, while the high-voltage side distance are set reverse to backup the high-voltage side grid system. One of the zones backups the local reverse circuits and the other backups to the adjacent reverse busbar. The normal delays for the overreaching Zone 2 are 450ms (500 & 275kV) and 300ms (132kV), while for Zone 3 and 3R are 1 second (500 & 275kV) and 3 seconds (132kV).

Figure 7 and Figure 8 below; illustrate the polar plots of these settings strategies. All of the options above are practical except for reverse low-voltage distance relay because it is assumed not able differentiate between faults on the grid and faults on the negative impedance portion of the autotransformer.

Figure 7. Reverse Reach Settings

Figure 8. Forward Reach Settings

~

Z

Z

~Z1

Z2

Z2 Z3

~~

Z

Z

Z1 Z2

Z1Z2Z3

Z3

Z1

Z3

110 - 7

3. FAULT SIMULATION

3.1 Strategy 1: Forward Back-up Zones

The fault types mentioned in 2.4 were applied on a numerical distance relay with forward zones as shown in Figure 9. In this case, uncompensated earth-fault zone reach was used, therefore each zones has separate reach settings for phase-to-phase and phase-to-earth measurement loops. A 20 ohms fault resistance was also included in the simulation. Also the load encroachment plots were computed based on 200% overload condition allowable.

-160

-120

-80

-40

40

80

120

160

200

-280 -240 -200 -160 -120 -80 -40 40 80 120 160 200 240 280

X in

P.O

hms

R in P.Ohms

12381+

123805123807113

The results onto this relay are within expectation. In another simulations not shown using a relay with separate K0-compensation settings for each zones, similar results were observed. However, setting calculations are not as straightforward.

3.2 Strategy 2: Reverse Back-up Zones

Similar to above case, another numerical relay with reverse zones was simulated. In this case, conventional K0-compensation factor setting was applied for all reverse zones as overall. The forward zone for autotransformer backup was set without compensation as indicated in Table III.

The results were also within expectation. However, simulated faults in the negative impedance part of the autotransformer were detected by the forward Z1. This means that autotransformer faults within that portion were appropriately detected, even though the apparent angle of the impedance observed was negative. Although this was not expected as mentioned in 2.1, the type of distance algorithm used in the relay might have influenced the result and need further detailed investigation. The transient studies as suggested in [8][9] may be appropriate to verify this.

4. ADDITIONAL FUNCTIONS

Additional functions within the distance relay required to ensure proper operation are summarized as in the following table. Generally, they are common functions within a numerical distance relay but some remarks are made to some specific issues concerned.

81

11231+

R

R2

1 (Z1 U1) 1 (Z1 U2)

2 (Z1 U1)

3 (Z2 U1)

3 (Z2 U2)

4 (Z2 U1)

5 (Z3 U1)

5 (Z3 U2)

6 (Z3 U1)

A (1 2 3 4 5 6)B (1 2 3 4 5 6) C (1 2 3 4 5 6)

D (1 2 3 4 5 6)E (1 2 3 4 5 6) F (1 2 4 6)

G (1 2 3 4 5 6)

H (1) I (1)

A (1 2 3 4 5 6)B (1 2 3 4 5 6) C (1 2 3 4 5 6)

D (1 2 3 4 5 6)E (1 2 3 4 5 6) F (1 2 3 4 5 6)

G (1 2 3 4 5 6)

H (2 3 4 5 6) I (2 3 4 5 6)

A (1 2 3 4 5 6)B (1 2 3 4 5 6) C (1 2 3 4 5 6)

D (1 2 3 4 5 6)E (1 2 3 4 5 6) F (2 3 4 5 6)

G (1 2 3 4 5 6)

H (2 3 4 5 6) I (2 3 4 5 6)

A (1 2 3 4 5 6)B (1 2 3 4 5 6) C (1 2 3 4 5 6)

D (1 2 3 4 5 6)E (1 2 3 4 5 6) F (1 2 3 4 5 6)

G (1 2 3 4 5 6)

H (2 3 4 5 6) I (2 3 4 5 6)

A (1 2 3 4 5 6)B (1 2 3 4 5 6) C (1 2 3 4 5 6)

D (1 2 3 4 5 6)E (1 2 3 4 5 6) F (2 3 4 5 6)

G (1 2 3 4 5 6)

H (2 3 4 5 6) I (2 3 4 5 6)

A (1 2 3 4 5 6)B (1 2 3 4 5 6) C (1 2 3 4 5 6)

D (1 2 3 4 5 6)E (1 2 3 4 5 6) F (1 2 3 4 5 6)

G (1 2 3 4 5 6)

H (2 3 4 5 6) I (2 3 4 5 6)

1 ++ H10Z_21HV,Z1G2 + H10Z_21HV,Z1P3 ++ H10Z_21HV,Z2G4 + H10Z_21HV,Z2P5 ++ H10Z_21HV,Z3G6 + H10Z_21HV,Z3PR1 R2

Figure 9. Forward Backup Distance Simulation

-40

-30

-20

-10

10

20

30

40

-70 -60 -50 -40 -30 -20 -10 10 20 30 40 50 60 70

X in

P.O

hms

R in P.Ohms

11381123807123805

11231+

12381+

R1

R2

1 (Z1 U1) 1 (Z1 U2) 2 (Z2 U1) 2 (Z2 U2) 3 (Z3 U1) 3 (Z3 U2)A (1 2 3)B (1 2 3)C (1 2 3)

D (1 2 3)E (1 2 3)G (1 2 3)

H (1 2 3)I (1 2 3)

A (1 2 3)B (1 2 3)C (1 2 3)

D (1 2 3)E (1 2 3)G (1 2 3)

H (1 2 3)I (1 2 3)

A (1 2 3)B (1 2 3)C (1 2 3)

D (1 2 3)E (1 2 3)G (1 2 3)

H (1 2 3)I (1 2 3)

1 ++ 180Z_21LV,Z12 ++ 180Z_21LV,Z23 ++ 180Z_21LV,Z3R1 R2

Figure 10. Reverse Backup Distance Simulation

110 - 8

Table V. Summary of Additional Functions within Distance Protection

Functions Purpose Remarks on Application Voltage Transformer

Supervision To block operation of relay due to loss of voltage transformer input

Conventional scheme may be applicable

Switch-on-to-fault To ensure instantaneous tripping of local zone in case of closing in circuit breaker onto fault.

Not necessary as to avoid mal-operation during switching in the nearby circuit breaker in the system. This type of fault should be effectively covered by the differential transformer protection.

Fault & Event Recorder

To record events in the relays input or output including contact assignment and Oscillography.

Important for post-event analysis, both for the autotransformer as well as the grid system. Triggering of this function must be by fault detection zone.

Self-supervision To monitor the condition of the relay internal processes.

Required for condition-based maintenance as well as monitoring the steady state power system quality.

Power Swing Blocking

To block distance relay operation due to power swings in the system during major disturbance.

Further studies are required to identify criticality of power swing to occur within the autotransformer impedance.

Pole-slipping To trip relevant distance zone when a pole slip condition occurred during major disturbance

Same as power swing blocking.

Backup overcurrent To provide backup function in case of distance relay function being blocked.

Proper activation of function must be foolproof.

5. RECOMMENDATION AND CONCLUSION

5.1 Recommendation

From the analysis and simulations made above, the following conclusions can be made:

• The use of overcurrent relay as backup protection for the autotransformer as well as the grid system cannot fulfill deterministic fault clearance requirement. The worst case would be in the case of busbar fault at high-voltage side and busbar protection failed to isolate the fault, where the overcurrent relay take too long to operate or possibly not operating at all under higher fault resistance.

• The use of distance relay is more deterministic; such as the zone reaches into the grid system are more definitive, higher fault resistance coverage and coordinated backup timing zones.

• In order to protect 100% of autotransformer, the distance Zone 2 must be set overreaching the opposite terminal of the relaying point and including the negative impedance (12%) of the low-voltage terminal as mentioned in 2.1 and effect of tap changer and transformation ratio as mentioned in 2.2. Therefore reach of at least 120% or 150% of autotransformer impedances for 500/275kV or 275/132kV respectively, are recommended.

• For remote backup into the grid system, Forward Zone 3 must reach 120% of the longest adjacent lines to cover up to all adjacent substations. Finally for local busbar backup, Reverse (or Offset) Zone 3 must be set to cover at least 20% autotransformer impedance.

110 - 9

• For earth-fault coverage, either relay with independent zones K0-compensation factors or independent phase-phase-fault and phase-earth loop zones settings shall be used. Un-compensated distance zones can also be used if appropriate setting can be calculated.

• Forward zone settings strategy is more preferable as its philosophy is consistent and easier to coordinate with the conventional distance relay presently protecting the lines. As the protected equipment impedances are relatively large, benefits of quadrilateral and polarized mho expansion characteristics can be maximize to cove higher fault resistance for earth faults.

• Final adjustment of relay settings shall be made through steady state short circuit simulation to ensure appropriate coverage under actual system conditions.

• The effect of negative impedance on the autotransformer need to be further verified. Also further studies are required to ensure proper application of power swing blocking and pole-slip functions.

5.2 Conclusion

The application of distance relay as backup protection for autotransformer is justified considering the requirements for consistent and predictable fault clearing time that cannot be ascertained using the present overcurrent protection. As the autotransformer can be treated as the transmission line, similar approach or methodology used in transmission line can be used to set the distance relay for autotransformer with consideration mentioned earlier. Actual implementation may require a long-term replacement program; hence risk analysis must be performed to prioritize the critical area in the system.

6. REFERENCES

[1] Tenaga Nasional Berhad Protection and Control Code of Practice, 2nd. Edition, 2003.

[2] Computer-Aided Protection Engineering (CAPETM), User Manual, Electrocon International, Inc., January 2003.

[3] J. L. Blackburn, Protective Relaying: Principles and Application, Marcel & Dekker Inc., 1987.

[4] Institute of Electrical & Electronic Engineers, IEEE Guide for Protective Relay Application to Power Transformers, ANSI/IEEE Standard C57.109, 1985.

[5] Final Report of TNB/TEPCO Joint Studies on Protection System Studies and Coordination, Project No.5, May 2001.

[6] Electrical Transmission and Distribution Reference Book, Westinghouse Electric Corporation, 1964.

[7] J. L. Blackburn, Symmetrical Components for Power System Engineering, Marcel & Dekker, 1991, pp 236-243.

[8] P. Bastard, P. Bertrand, M. Meunier, A Transformer Model for Winding Fault Studies, (IEEE Transaction on Power Delivery, Vol. 9, No. 2, April 1994, pp 690-699).

110 - 10

[9] A.I. Megahed, A Model for Simulating Internal Earth Faults in Transformer, (IEE Conference Publication No. 479, 7th. International Conference on Developments in Power System Protection, Amsterdam - Netherlands, 9-12 April 2001).

[10] M.H. Hendra, Penilaian Antara Penggunaan Geganti Aruslebih Berbanding Geganti Jarak Sebagai Perlindungan Sokongan untuk Pengubah-Oto (Evaluation of Overcurrent versus Distance Protection for Back-up Protection of an Autotransformer), M.E.E. Dissertation, Universiti Teknologi Malaysia, Mac 2003.

[11] G. Ziegler, Numerical Distance Protection: Principles and Application, Publicis MCD, 1999, pp 121-124.

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7. APPENDIX: UPDATE OF SUBJECT

Summary: In this section, applications of selected additional functions within a distance relay as in Table V were further elaborated. These functions include switch-on-to-fault, voltage transformer supervision, backup overcurrent, load encroachment and power swing blocking. Each function’s philosophy related to its specific application on autotransformer was revisited and their settings criteria were suggested. This paper also includes a R-X plot of an actual power swing that occurred during the 13th January 2005 system disturbance that provided additional hints for setting the backup distance relay for an autotransformer.

Keywords: Switch-on-to-Fault – Voltage Transformer Supervision – Backup Overcurrent – Load Encroachment – Power Swing – Pole Slip

7.1 Switch-On-To-Fault Function

Switch-On-To-Fault (SOTF) function is considered as a main protection to allow instantaneous tripping when closing a line on to fault such as forgotten earthed line link after maintenance. Therefore, it is normally activated with the distance relay main protection.

However in the application of distance relay as backup protection, it is crucial for such function to be defeated to avoid indiscriminate tripping. As mentioned in Table V, SOTF within the autotransformer protection zone has already been covered by the transformer differential protection schemes.

7.2 Voltage Transformer Supervision

The Voltage Transformer Supervision (VTS) supervised the loss of the voltage input to the distance relay. The apparent impedance of load current with no voltage could be small enough to encroach into any of the distance zone characteristics. Therefore it imposed risk to trip the primary circuit (autotransformer) for a non-system fault condition. Hence the detection of secondary voltage loss should block the distance protection function. In TNB, monitoring the zero sequence voltage and zero sequence current typically does this. The secondary VT circuits are also protected using phase segregated miniature circuit breakers. Upon detecting a voltage transformer failure, the distance function should be blocked instantaneously but alarm after 2 seconds.

As a backup protection, it is necessary to implement this function to increase the security of the backup distance function, i.e., not to trip unnecessarily. However, it is also important to have high availability of the backup protection function. Therefore a permissive backup overcurrent is proposed as described next.

7.3 Backup Overcurrent

In the event that the VTS blocks the distance relays function, there would be no backup protection to the autotransformer. Therefore to ensure that a backup protection is always available, the backup overcurrent function within the distance relay should be enabled after the VTS alarm had locked-in. Such scheme is shown in Figure 11.

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The settings for this overcurrent function should be more tightly coordinated to the backup distance characteristic, as shown in Figure 12. A pickup current setting of more than 150% might be necessary to allow higher overloading limit. Therefore, a setting of 200% is recommended. The time multiplier is then set nearest to the Zone 2 distance backup delay of 0.45 second.

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E in

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onds

CURRENT in P.Amps

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lt

x 80% OL

7.4 Load Encroachment Compensation

On 13th January 2005, TNB 500/275/132kV transmission grid experienced cascaded circuits overloading that eventually caused power system collapse in the Central-Southern Region. At a critical substation, disturbance records of 275/132kV autotransformers captured the entire event until they were tripped by the present backup directional overcurrent relay, severing one of the last links to the sub-transmission system. This events gave a valuable information on how would a backup distance protection behave during such disturbance. Such disturbance records are shown in Figure 13 and explained in the next paragraph.

Figure 11. Backup Overcurrent Enabling by VTS Function

OTHER TRIP CONDITIONS

&

VTS

51OC R

Y B

51OC TRIP

TRIP 3 POLE

VTS ALARM

ENABLE 51OC

BLOCK 21Z

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1. TOC; H10Z_5150HV; MCGG; Tag:5370THREE PHASE; CTR:600; PU:1.65 R. AmpsTD:0.2; STANDARD INVERSE(SI)

R1 . Z1:37.3ohms@88.3deg

R2 . Z2:60ohms@85deg

R3 . Z3:143ohms@80deg

R4 . Transformer thermal damage curve

R5 . Mechanical damage curve

600% OLx

275% OL x

350% OL x

900 s

275% OL x

350% OL x

x 600% OL

Figure 12. Revised Overcurrent Backup Settings & Coordination

t / s1 2 3 4 5 6 7 8 9 1 0 1 1

U R H V A T 1 A /K V

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01 0 0

t / s1 2 3 4 5 6 7 8 9 1 0 1 1

U Y H V A T 1 B / K V

- 2 0 0- 1 0 0

01 0 0

t / s1 2 3 4 5 6 7 8 9 1 0 1 1

U B H V A T 1 C /K V

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t / s1 2 3 4 5 6 7 8 9 1 0 1 1

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t / s1 2 3 4 5 6 7 8 9 1 0 1 1

I B H V A T 1 C /K A

- 3- 2- 101

t / s1 2 3 4 5 6 7 8 9 1 0 1 1

I N H V A T 1 N / K A

- 3- 2- 101

Figure 13. Disturbance Record on 275/132kV Autotransformer During 13-January-2005 Central-South Regional System Collapse

A

~0.5 sec ~1 sec ~1 sec ~4 sec

EDC B

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From the start of the Event A, the 2 units of 275/132kV 180MVA autotransformer were each carrying approximately 80% of capacity. When a critical 275kV overhead line tripped due to prolonged overloading causing the lowest conductor to flashover to ground, power increased through these autotransformers to 275% of its capacity within 0.5 seconds (Event B). The power oscillation stabilize at this overload level for about 4 seconds before another 275kV overhead lines tripped due to overloading (Event C). After which, these autotransformers suffered further overloading up to about 600% (Event D) before a pole slip occurred in the system (Event E). Before these autotransformers tripped on the backup directional overcurrent relay, two pole slips had occurred with a maximum overloading of 350% autotransformer capacity.

The impedance trajectory during the load encroachment period (Events ABCD) on the proposed backup distance relay is shown in Figure 14. The following are some key observations:

• The impedance came from the left-hand side of the RX Diagram, therefore the power was imported from the LV-side (132kV) into the 275kV system.

-80

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-560 -480 -400 -320 -240 -160 -80 80 160R in P.Ohms12381 KKSR275_M+

123805 KK

• The new stability point enforces the autotransformers to be overloaded about 275% of its capacity. At point C of Figure 14, the Zone 3 distance characteristic was well below the limit, hence no danger of tripping. From the transformer thermal/mechanical damage point of view (refer to Figure 12), maximum of 15 minutes (900 seconds) can be tolerated by the autotransformer at that level of extreme overloading before it must be tripped.

• Beyond 300% of capacity overload, the load encroachment can enter the backup distance protection zones, even down to Zone 1 reach. However, it is not safe to allow the autotransformer to operate beyond this limit based on the transformer’s 2 seconds damage characteristics. Therefore, tripping the autotransformer beyond this overload limit would potentially safe the autotransformer from permanent damage.

Severe load encroachment can cause unexpected distance relay tripping. On the other hand, actual tripping may have reduced the impact to the protected equipment and probably the

SR275F1

123807 KKSR132F1

11381 KKSR132_M+

123807 KKSR132F1

11231 PAPN132_M+

1. H10Z_21HV "Z1" Zone:1

2. H10Z_21HV "Z2" Zone:2

3. H10Z_21HV "Z3" Zone:3R1 . ZAB Load Encroachment on 13-Jan-2005TNB Central-South Regional Collapse

B A

CD

R3 .150% OL @1.0pu V

R4 .600% OL @0.52pu V

R5 .200% OL @1.0pu V

R6 .275% OL @0.94pu V

Figure 14. Load Encroachment on 275/132kV Autotransformer During 13-January-2005 Central-South Regional System Collapse

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power system stability itself. Therefore as shown in Figure 15 and in addition to the recommendations stated in 5.1, it is also recommended that the Zone 3 should be set to include the 8s mechanical damage (R4 – 300% overload) and 50s thermal damage (R6 – 500% overload) characteristics. They can tolerate longer tripping time. The Zone 2 should be set to cover the 2s mechanical damage (R8 – 600% overload) and 2s thermal damage (R10 – 2500% overload) characteristics that require faster operating time.

In summary, the 300% overload shall become the maximum loadability limit for the autotransformer, and tripping on distance relay is required. For overload range of 200% – 300%, a thermal overload relay or manual load shedding shall be best applied. A further observation of Figure 15, an ideal distance characteristic for protecting the autotransformer from thermal and mechanical damages is a impedance relay with directional blinder (refer to C1). However, in a modern distance relay, a quadrilateral characteristic can be more flexible to set (refer to C2). A load compensation function may be necessary for a larger capacity autotransformer (>750MVA).

7.5 Power Swing Blocking

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R5

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1. H10Z_21HV "Z1" Zone:12. H10Z_21HV "Z2" Zone:23. H10Z_21HV "Z3" Zone:3

R2 .Thermal Damage 1800s (200%)R4 .Mechanical Damage 8s (300%)R6 .Thermal Damage 50s (500%)R8 .Mechanical Damage 2s (600%)R10.Thermal Damage 2s (2500%)

R11. ZAB Load Encroachment

Figure 15. Plotting Autotransformer Damage Curve ANSI/IEEE C57.109-1985 onto RX-Diagram

C1

C2

During load encroachment as shown in above case, typical power swing blocking elements based on impedance blinder crossing time will cause blocking of the distance zone tripping. Permanent blocking until the power swing trajectory goes out of its detecting zones is not desirous, as we may not know how long will the swing stays in the damage zone. As recommended above, the zone tripping shall be allowed for maximum protection of the autotransformer.

Therefore, a definite delay of 1-second (i.e., half the 2 seconds transformer damage characteristic) after power swing detection shall be set to release zone tripping. Based on the disturbance referred in this subject, in one second the power swing had gone into Zone 1 or 2 reach where fast tripping would be necessary.

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7.6 Pole Slipping

In the actual disturbance event as referred above, there was an actual pole slip condition occurred as shown in Figure 16 and Figure 17. The initial severe overloading condition developed into a slip condition passing through the distance relay twice before the autotransformer tripped by its backup directional overcurrent relay.

It is possible to use a pole slip protection to allow longer operation of the autotransformer before a definite trip is necessary. In such case then, the power swing blocking shall be set to longer inhibit delay. Further studies need to be conducted to evaluate the impact of pole slipping to the damage characteristic of the autotransformer.

7.7 Summary & Conclusion

The following are summary and conclusion of this Section:

• Switch-On-To-Fault function shall be defeated.

• Voltage Transformer Supervision shall be activated.

• Backup Overcurrent Protection shall be automatically activated when VTS operated.

• Distance zone reach to be coordinated with transformer damage characteristics (ANSI/IEEE C57.109-1985) and its maximum reach to be less than 300% overload.

• Power Swing Blocking for only 1-second duration after detection (300% inner zone).

• Pole Slip Protection can be used; subject to further studies on its impact onto transformer damage characteristics.

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R1 . ZAB Load Encroachment

R2 . ZAB Pobecame P

wer Swingsole Slip

Figure 16. Pole Slipping Traces on 275/132kV Autotransformer During 13-January-2005 Central-South Regional System Collapse

12381 KKSR275_M+

11381 KKSR132_M+

11231 PAPN132_M+

R1 . ZAB Load Encroachment

R2 . ZAB Power Swingsbecame Pole Slip

Figure 17. Zoomed Figure 16 Relative to Distance Relay’s Power Swing Outer and Inner Zones

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