Adoption of advancements and standardization of

switchyard equipment specification

to improve system reliability

Shivaprasad Lakhapati, Pramod Tupe, G P Sastry

The Tata Power Company Limited, Mumbai

25th July 2019, CBIP

Outline

2

Introduction

Engineering Interventions

Improvements in specifications

Conclusion

Introduction

Private and Confidential | 01

About Tata Power

Tata Power, India's largest integrated power company has installed generation

capacity of 10,757MW supported through Thermal, Hydel, Wind, Solar power plants.

Transmission operations in Mumbai License with 1,125 CKT KM of transmission

network (comprising of 995 CKT KM of O/H lines and 130 CKT KM of UG cables).

Tata Power work on ensuring uninterrupted power supply to Mumbai (TPCL-D), Delhi

(TPDDL) and Ajmer (TPADL) cities.

A strong portfolio of 2168MW of operating capacity under renewable energy. 64 grid-

connected solar PV sites with capacity of 1237MW at 68 locations in 10 states.

Introduction

Switchyard equipment mainly Circuit Breaker, Instrument Transformers,

Isolators, Lightning Arresters are critical component in any EHV electrical

installation

Plays a significant role in the management for power system.

As an utility engineers, it is expected that these equipment gives trouble-free

and safe service life of minimum 25 years.

Engineering Interventions

Private and Confidential | 01

Engineering Interventions

Based on equipment failure data for last 4-5 years, it is found that-

Many switchyard equipment has failed during service without any major pre-

failure indications.

In some instance, external causes (like improper equipment earthing, cable /

line faults, externally and operational induced factors etc.) were involved.

Many times, the catastrophic failure of switchyard equipment involves damage

to the other nearby equipment. This leads to forced outage and resulting into

costly reconditioning / repairs / replacement of the defective equipment.

In order to mitigate the above issues, efforts were made to relook / revisit the

current specifications of the switchyard equipment considering the nature and

severity of the faults experienced.

Engineering interventions

8

Case Study - 220kV CT failure

9

Occurrence details: On 14.06.15 at 02:03 Hrs, 220kV bus

Zone-VI Bus fault operated and opened out all connected

breakers (including Unit-7 GTG and STG breaker no- II) due to

shattering of C-ph. CT of GTG 220KV bay-II.

„C‟ phase CT flash-over occurred and CT caught fire.

Fragments/ splinters of the CT hit and damaged the adjoining

220kV breaker, „A‟ phase CT and Main Bus VI support

insulators. Fire was extinguished by Fire personnel.

Analysis: During Inspection of failed CT , it was noticed that

CT Tap point (DF measurement) came out from terminal link.

Also the grounding link between Tap & ground was found open.

CT was 24 years old & its Dissipation Factor was showing

increasing trend (value was just above the permissible limit).

Case Study - 220kV CVT failure

10

Occurrence details: On 16.10.15 at 00:10 Hrs, 220kV Kalwa–Salsette # 4 line tripped due

to failure of “A” phase CVT. Simultaneously, 220kV Bus fault Zone 2 relay operated at

Salsette R/S, thereby tripping all the connected lines and ICT leading to major disturbance

in Mumbai power supply network.

Analysis: “No Voltage” appeared on SCADA repeatedly from 00:07:00 onwards. Line

tripped at 00:10 hrs by the action of distance relay due to failure of A-phase CVT. The CVT

failed before any corrective action could be taken. Some internal problem in CVT. The CVT

was tested in 2014 and was found to be healthy.

Improvements in Specifications

Private and Confidential | 01

Instrument Transformers (IT)

The population of 145 / 245 / 400kV Instrument Transformers @ 1500+, which

includes Oil filled CTs, PTs and CVTs as well as SF6 filled CT.

Majority of CTs are of Live tank design. However, old Dead type design CTs are

available in some locations; which are getting replaced by live tank design.

Polymeric housing SF6 filled CTs were introduced in the year 2010. Initially,

there were multiple CT failures reported on account of design / manufacturing

issues and the same were replaced with improved design.

Subsequently as on today, satisfactory overall performance observed for SF6

filled CTs and selected for 400 kV application.

Polymeric housed Oil filled Live tank CTs were introduced in the year 2017.

Improvements in Specifications for IT

Composite insulator housing in place of Porcelain insulator

housing for 145-245kV CT

Standardization of Instrument Transformer parameters

(ratio, no of cores / windings, VA burden, accuracy /

protection class etc.)

Use of PT in place of CVT (except where PLCC facility

required)

Inclusion of Oil sampling kit for DGA

Stringent Tan Delta acceptable limit during FAT/SAT

(0.4% for CT and 0.5% for PT)

Ref: CG

Composite insulator housed

SF6 filled CT for 220 / 400kV class

1. REPTURING DISC STAINLESS STEEL

2. CORE HOUSING AL ALLOY

3. CORES CRGO / MU METAL

4. PRIMARY CONDUCTOR AL ALLOY

5. HV SPACER EPOXY

6. ELECTRODE AL ALLOY

7. OUTLET TUBE AL ALLOY

8. COMPOSITE INSULATOR SILICON RUBBER / FRP TUBE

9. CABLE OUTLET EPOXY

10.BASE PLATE (FUNDAMENT) MS (HDG)

11.BASE FRAME MS (HDG)

12.SEALING POINT (O-RING) EPOM

BASIC CONSTRUCTION

Ref: Siemens

Instrument Transformer - CT

Core No I II III IV

Primary Current (A) 800-1200-2000 800-1200-2000 800-1200-2000 800-1200-2000

Sec Current (A) 5 5 5 5

Primary Terminal P1-P2 P1-P2 P1-P2 P1-P2

Sec-Terminal 1S1-1S2-1S3-1S4 2S1-2S2-2S3-2S4 3S1-3S2-3S3-3S4 4S1-4S2-4S3-4S4

Accuracy class 5P 5P 0.2S PS

Burden 40-60-100 40-60-100 40-60-100 ---

ISF --- --- <5 ---

ALF 20 20 --- ---

Knee Point Voltage (V) --- --- --- >=800/1200/2000

I exc (Max) at Vk/2

(mA) --- --- --- <= 30 mA @2000/5

RCT at 75 oC (Ohms) --- --- --- <= 2 Ohms @2000/5

Application Main Protection Main Protection Metering Bus-Fault Protection

Instrument Transformer - PT Winding no. I II III

Primary winding 110 kV / √3

Primary connection Star

Secondary winding I II III

Rating 110 V / √3 110 V / √3 110 V / √3

Connection Star Star Star

Accuracy class 0.2 3P 3P

Burden 100 100 100

Application Metering Protection Protection

Rated Power Factor 0.8

Simultaneous Burden 200 VA

Rated voltage factor 1.2 Continuous, 1.5 for 30 seconds

Neutral terminal of HV winding Should be brought out through 3kV bushing

Creepage Distance 31 mm / kV

Surge Arrestor (SA)

Around 700 nos.145 / 245 / 400kV Surge Arrestors (SA) are installed, which

includes Gapless porcelain and polymer type.

Polymeric housing SAs are introduced in the year 2017.

During last few years, it is observed that the main reason of failure of SAs

i) Moisture entry through sealing system/ gasket/ explosion vent leading to

degradation of ZnO block

ii) Accelerated degradation of ZnO block indicated by LCM monitoring

iii) Discharge due to surface contamination.

Moreover, it is reported that shattering of porcelain insulators during SA failure

resulted into damages to nearby equipment and injury to personal in some cases.

Improvements in Specifications for

SA Composite insulator housing in place of Porcelain insulator

housing for 145 & above SA

Standardization of Surge arrestor parameters (Nominal

discharge current, Discharge Class, Energy Discharge Capacity

etc.) based on Transient Over Voltage Study report.

Sealing of explosion vent and sealing plate by Silicon sealant to

prevent moisture ingress.

11kV base insulators in place of 1.1kV insulators used to avoid /

eliminate premature mechanical and electrical failures

Digital surge counter in place of conventional counters on pilot

basis. Ref: CG

Parameters 110kV 220kV

Arrestor voltage (kV) 96 198

Short circuit current (KA) for 0.2 sec 40 40

Nominal Discharge Current 10 20

Pressure relief class (kAp) Class – A Class – A

Discharge class Class – III Class – IV

M.C.O.V (kV) 81 168

Nominal Creepage Distance (mm) 31 mm/ kV 31 mm/ kV

Energy Discharge capacity (kJ/kV) 8 12

Base plate insulator rating Min 11 kV Min 11 kV

Surge Arrestor

Ref: CG

Surge Arrestor

Ref: CG

11kV Base Insulators for SA

Circuit Breaker (CB)

Spring/ Hydraulic / pneumatic outdoor type SF6 filled

Circuit breakers (Population around 600+, including PASS

units).

Few breaker failures were observed over past 5 years,

mostly due to pole burst during fault clearing occasions

and stuck breaker condition during breaker operation.

Apart from the major failures, unwanted breaker tripping

and forced outages observed on account of moisture/

water ingression in operating mechanism box and density

monitoring device, SF6 leak, low hydraulic oil pressure,

Suplhation of SF6 gas supervision relay contacts,

improper damping devices etc. 145kV, 2500Amp Breaker

Circuit Breaker (CB)

Recently SF6 gas quality assessment of new as well

as old breakers were carried out after occurrence of

breaker flashover at our critical substation.

Following issues were observed in 50% breakers out

of tested samples

i) High Moisture

ii) High SO2

iii) Low % Purity

which is an area of concern and need attention…

Breaker C Ph - Fixed Contact

Breaker C Ph – Moving Contact

Breaker C Ph – Operating rod

Improvements in Specifications for

CB Preferably to use Spring- Spring breakers with three stage SF6 monitoring

(Alarm-1 Low, Alarm- 2 Lo-Lo, Lockout)

Contact travel transducers for Dynamic Contact Resistance Measurement along

with breaker.

Emergency Mechanical trip outside marshaling box with flap/ cover.

Disconnecting type droppable links for all AC / DC external cable for easy

isolation of supply points.

Foldable / Removable and lockable operating platform.

Additional name plate inside the marshalling box.

Quality requirement for SF6 gas.

Isolator (ISO)

The population of 145/245/400kV Isolator in Tata Power system is around 1500+.

The major issues with Isolators are

i) hot spots at hinge portion

ii) Misalignment

iii) Quality of aux contacts.

Especially, hot spots generation on terminal stem assembly portion were

experienced in 400kV single break isolator design. After detailed analysis and

interaction with OEM, GT side Isolators were replaced by Double break design.

Similar, issues were also reported on 145kV Isolators with tulip contact design at

our 110kV Receiving station. Ever after, replacement of damaged / faulty terminal

steam assemblies, the above issue is still occurring after some period.

145kV, 1250Amp Isolator

Improvements in Specifications for ISO

Change in 400kV Isolator design - Double Break in place of single break

GI pipe attached to the base frame/ channel for anchoring safety harness during

maintenance.

Relays for AC & DC monitoring with potential free contacts.

Disconnecting type droppable links for all AC / DC external cable for easy

isolation of supply points.

Additional name plate inside the marshalling box

Clamps / Connectors along with the equipment

Stringent Qualification Requirement and approved Vendor list

Review of Type Test Reports (not older than 5 years) during Bid evaluation.

Minimum 5 years of guarantee for the equipment supplied

Issues to be addressed

Limited vendors for CTs and PTs with composite Insulators.

Also, SF6 filled CTs, breakers and isolators with composite insulators offered by

few OEMs only at high cost proposition

Effective Monsoon protection, Moisture ingress prevention & Vermin proofing for

Junction Box / Marshalling Box / Panel and SF6 monitoring device -

Requirement of IP 55 class with rain protection

Provision of Pressure relief device for SF6 EHV breaker to avoid bursting.

Proper sealing of SAs explosion vent to avoid Moisture / Water ingress.

Maintaining SF6 quality in SF6 filled equipment (especially Breakers)

Guidelines for SF6 quality monitoring / reconditioning / replacement

Conclusion

Private and Confidential | 01

Conclusion

In the recent past, there have been some failures of Switchyard equipment

experienced in Tata Power System. Detailed study of these failures was carried

out (using RCA, FMEA and ZFO initiatives).

Technical cum administrative approaches were brainstormed and suggested /

adopted technical specification improvements to address the above issues

considering new advancements and safety interventions, which will lead to

improve system reliability.

Thank You! Website: www.tatapower.com

Email Id: pbtupe@tatapower.com

Contact: (+91) 022 – 6717 3703

Private and Confidential

Disclaimer: The contents of this presentation are private & confidential.

Please do not duplicate, circulate or distribute without prior permission.