SOUTHERN REGIONAL POWER COMMITTEE of the Special Meeting on HVDC Gajuwaka Pole Trippings held on...

Minutes of the Special Meeting on HVDC Gajuwaka Pole Trippings held on 02nd April, 2018 Page 1

SOUTHERN REGIONAL POWER COMMITTEE BENGALURU

Minutes of the Special Meeting on the Trippings of Poles - 1 & 2 at HVDC Gajuwaka SS of POWERGRID (SRTS-I) held on 02nd April, 2018

A. Introduction

A Special Meeting was held on 02nd April, 2018 at RHQ POWERGRID (SRTS-I), Hyderabad to deliberate on

all outstanding issues involved in the recent HVDC Gajuwaka Pole trippings. The list of participants is

enclosed at Annexure-I.

Sh. S.R. Bhat, Member Secretary (MS), SRPC welcomed Officials of POWERGRID, NLDC, and SRLDC to the

meeting and thanked Sh. R.K. Chouhan, ED (Engg- HVDC), POWERGRID, Sh. S. Ravi, GM (AM),

POWRGRID (SRTS-I), Sh. A. Sensarma, AGM, POWERGRID for making it convenient to attend the

meeting. Then, outlining the reasons for calling special meeting, he stated that of all the HVDC pole

trippings that occurred at Gajuwaka SS in the past one year, two categories of trippings, viz., Pole-1

trippings on A.C. auxiliary supply failure and Pole-2 trippings on ferro-resonance detection had been

discussed and high-lighted in various monthly held Protection Coordination Sub-Committee (PCSC)

meetings of SRPC for appropriate action by POWERGRID. However, since the measures required to be

taken to resolve them on a permanent basis had been pending from POWERGRID side, it was decided in

the 72nd meeting of PCSC held on 15.03.2018 to convene a special meeting with participation from

HVDC-Engineering Group of POWERGRID along with representatives of SRTS-I, POWERGRID and HVDC

Gajuwaka SS, NLDC, SRLDC and SRPC to have focused deliberations on the issues involved and to come

out with concrete actionable measures.

He stated that the present meeting had been convened to deliberate on the measures taken/ proposed

by POWERGRID to resolve (i) Pole-1 trippings on AC auxiliary supply failure, and (ii) Pole-2 trippings on

Ferro-resonance detection. In this regard, he briefed the measures already recommended/ suggested by

PCSC forum. While for the former these included providing for full-fledged AC auxiliary supply

redundancy for Poles 1 & 2 (ICT-1 Tertiary, ICT-2 Tertiary + DG), replacing old LT side circuit breakers,

etc., for the latter these included making a comparative study of the settings employed for Pole-2 with

that of Pole-1 along with considering other innovative options for effecting systemic improvements

aimed at removing the cause of ferro-resonance or damping it suitably as and when it occurs.

He requested all members, esp. HVDC Engg. Group of POWERGRID, to actively participate and elaborate

in detail on the way forward so that trippings of the kind experienced at Gajuwaka would become a

thing of the past.

B. Connectivity & Single Line Diagrams

The Connectivity Diagram of Gajuwaka SS and the detailed Single Line Diagrams of HVAC, Pole-1 & Pole-2 of Gajuwaka SS are given at Annexure-II for kind reference.


C. Deliberations on HVDC Pole Trippings

Sl. No.

Details of Event Date &Time Reason Remarks

1

Tripping of HVDC Gajuwaka Pole-1&2 and 400 kV Jeypore-Gazuwaka line-1&2

Pole-1 at 08:44 hrs on 28.04.2017 & Pole-2 at 9:07 hrs on 28.04.2017

Pole-1 got tripped due to failure of auxiliary supply changeover. 400kV Jeypore-Gazuwaka line-2 got tripped on operation of over-voltage protection. Pole-2 east shunt reactor got failed and this in turn resulted in tripping of HVDC pole-2 due to Pole-2 Y block activation. 400kV Jeypore-Gazuwaka line -1 also got tripped during the incident.

PCSC-64 Horn Gap Fuse blowing of ICT-2 Tertiary Unsuccessful auto changeover to DG Pole-1 tripped; Sustained voltage fluctuations Shunt Reactor, W1.WA2.Z11.Z6, failure WA1. Y block operated Pole-2 tripped * Failure of shunt reactor was on eastern side, Y-block on Southern side got operated.

2 Tripping of HVDC Gazuwaka Pole-1 & Pole-2

28.09.2017 at 17:30 hrs

Poles – 1 tripped due to Auxiliary supply failure; Pole-2 tripped due to activation of WA2 Ferro Resonance Detection

PCSC-68 Blowing of Horn-Gap fuse element of ICT-2 Tertiary Pole-1 tripped; Disturbance on Eastern side WA2.FR Pole-2 CN_TX Diff Ph_L2 Trip Z block on Eastern side & Y-block on Southern side operated Pole-2 tripped

3 Tripping of HVDC Gazuwaka Pole-2

08.12.2017 at 19:29 hrs

Pole-2 got tripped due to failure of Valve Cooling system VFD (Variable Freq Drive control)

PCSC-71 Fault in VFD-1 of Pole-2 Blowing of fuses of VFD-1, VFD-2 and LT CB of Transformer connected on the tertiary of ICT-2 Valve Cooling System failed Pole-2 tripped


31.01.2018 at 06:38 hrs

Due to fault in 220 kV line, ICT-1 at Gazuwaka got tripped due to directional OC high set. Since tertiary of ICT-1 was feeding the auxiliary supply to Pole-1, tripping of ICT-1 led to the tripping of Pole-1 due to loss of auxiliary supply

PCSC-72 Fault in 220 kV line (southern side) ICT-1 tripped Unsuccessful auto-changeover from ICT-1 tertiary to DG Pole-1 tripped


01.02.2018 at 00:23 hrs

Pole 2 tripped due to the activation of “WA2 Ferro Resonance Detected" in East side.

PCSC-72 Disturbance on Eastern side WA2.FR Pole-2 CN_TX Diff Ph_L3 Trip Z block on Eastern side & Y-block on Southern side operated Pole-2 tripped



02.03.2018 at 01:38 hrs

Pole 2 tripped due to the activation of “WA2 Ferro Resonance Detected” in East side & Pole 2 tripped at 01:38 on activation of Converter transformer Protection Differential Current Phase L1 Trip

PCSC-72 Disturbance on Southern side WA2.FR Pole-2 CN_TX Diff Ph_L1 Trip Z block on Eastern side operated Pole-2 tripped


07.03.2018 At 01:31 hrs

Pole 2 tripped due to the activation of *WA2 Protective Y-Block Executed" in Pole Control system.

PCSC-72 Disturbance on Southern side Y block on Eastern side operated Pole-2 tripped

The PCSC deliberations of the above Grid Occurrences are given at Annexure-III for kind reference. The presentation made by SRLDC w.r.t. above events is given at Annexure-IV. Before going into the specific issues pertaining to Poles – 1 & 2 trippings, the following general issues were noted: W.r.t. Pole-1 tripping of Events 1 & 2 mentioned above, POWERGRID’s attention was brought to the

fact that it was the blowing out of horn-gap fuse on the tertiary (33 kV) side of ICT-2 that ultimately led to tripping of Pole-1 on loss of auxiliary supply. It was emphasized that had there been a suitable circuit breaker in place of horn gap fuse, more controlled operation with proper time-coordination would have been guaranteed. Accordingly, POWERGRID was asked to see the feasibility of providing suitable Circuit Breaker on the tertiary (33 kV) side of ICT-2, and implement the same, if feasible.

W.r.t. Pole-2 tripping of Event-3 mentioned above, it was pointed out that blowing out of fuses of VFD-1, VFD-2, and LT CB of the Transformer connected on the tertiary of ICT-2 due to fault in VFD-1 was not in order, and the same needed to be reviewed for proper time-coordination. To this, POWERGRID informed that the necessary time coordination had already been taken care of; however, due to excessive fault current in the instant case, all said elements connected to 415 V bus got tripped. The issue was further discussed, and since this kind of tripping happened for the first time, it was agreed to keep LT side (415 V bus connected) CB’s and fuses under observation to ascertain whether they operate with required time discrimination.

W.r.t. Pole-1 tripping of Event-4 mentioned above, it was pointed out that tripping of ICT-1 (high-set operation) due to fault on 220 kV side was not in order since it had essentially operated for a through fault, hence there was a need to revise the settings adopted considering the latest fault levels. In this regard, it was noted that the adopted settings on 220 kV side of ICT-1 for O/C (Inst. & IDMT) and E/F (Inst. & IDMT) protections had not been provided. Accordingly, POWERGRID was asked to look into the correctness of the settings employed on the 220 kV side of ICT-1 and revise them if necessary and provide a copy the settings employed to SRPC Secretariat.

W.r.t. Pole-2 tripping of Event-1 mentioned above, it was noted that failure of shunt reactor on eastern side (W1.WA2.Z11.Z6) and tripping of Pole-2 on Y-block operation on southern side occurred basically due to sustained voltage fluctuations. It was also noted that an internal committee of POWERGRID that went into details of reactor failure opined that higher second harmonic content in the Eastern Bus might have led to failure of the shunt reactor. In this regard, it was pointed out that if ac side disturbances and ferro-resonance were to be experienced recurrently, health of the converter transformers also needed to be suspected, since higher percentage of harmonics on ac side can result in building up of stresses leading to vibrations and consequent mechanical failures. To this, POWERGRID stated that they have a practice of carrying out all mandated tests during annual overhaul, and assured that due diligence is being given to maintain the health of converter transformers.


In this regard, POWERGRID also mentioned that more trippings of 220 kV elements in & around Gajuwaka were resulting in on-set of ferro-resonance phenomenon, and affecting the health of HVDC equipment. Hence, apart from discussing various measures to deal with ferro-resonance, it was also equally important to ensure better line maintenance practices are followed by APTRANSCO in that area. To this, POWERGRID was requested to give an Agenda Item furnishing details of such tripping so that the same could be discussed in the monthly PCSC meetings for appropriate action by all parties concerned.

D. Resolution of Issues w.r.t. Pole-1 Trippings (ALSTOM make)

The schematic of the present AC Auxiliary Supply Changeover for Gajuwaka Station is enclosed at Annexure-V. As informed by POWERGRID, the existing AC auxiliary supply arrangement at Gajuwaka is as follows:

Before November, 2018: HVAC: 33 kV APEPDCL feeder + 250 kVA DG Set

Pole-1: 33 kV Tertiary Supply from of ICT-2 (BHEL make) + 1500 kVA DG Set

Pole-2: 33 kV Tertiary Supply from ICT-2 (BHEL make) + 1000 kVA DG Set

After November, 2018: HVAC: 33 kV APEPDCL feeder + 250 kVA DG Set

Pole-1: 33 kV Tertiary Supply from of ICT-1 (TELK make) + 1500 kVA DG Set


Since the main issue with Pole-1 trippings is related to unsuccessful auto-changeover of its AC Auxiliary Supply to its DG, POWERGRID was enquired about the status of completion of pending PCSC recommendations like providing full-fledged redundancy for Poles-1 & Pole-2 (ICT-1 Tertiary + ICT-2 Tertiary + its corresponding DG), commissioning of new PLC based AMF panel for Pole-1, and replacement of faulty LT side Circuit Breakers. To this, POWERGRID informed that the said works for Pole-1 are already under progress, and are targeted to be completed by April, 2018. However, to test various logics involved, they needed shutdown of Pole-1 for two days. The issue was further discussed. Keeping in view the need to give concerned vendors a 10-day notice to carry out various works, and to ensure least discomfort to the receiving end consumers, the following were agreed: Arrangement for ensuring full-fledged redundancy for Pole-1 (ICT-1 tertiary + ICT-2 tertiary + its

DG), replacement of LT side Circuit Breakers and commissioning of PLC based AMF panel for Pole-1 would be completed well before 28.04.2019.

Verification of various logics in effecting AC supply auto-changeover from ICT-1 tertiary to ICT-2 tertiary to DG set for Pole-1 would be tested and completed by availing shut down on 28th & 29th April, 2018. During this period, mock-exercise would also be conducted for verification of proper functioning / sequence of operation of new PLC. POWERGRID would give the procedure & detailed schedule of testing in the OCC meeting of April, 2018.

W.r.t. Pole-2, POWERGRID stated that with the present arrangement (ICT-2 tertiary + its DG Set) they had not experienced AC supply changeover issues for Pole-2. However, to comply with various PCSC & Protection Audit recommendations of providing full-fledged redundancy (ICT-1 tertiary + ICT-2 tertiary + its DG) for Pole-2, they needed to lay an XLPE power cable of length of about 600 mts. for which procurement action is under progress. POWERGRID informed that it would take another 3 months to complete the job.


E. Resolution of Issues w.r.t. Pole-2 Trippings (ABB make)

The main issue with Pole-2 trippings was noted as occurrence of ferro-resonance which results not only in higher voltages and currents, but also in higher deformity in their corresponding waveforms. In HVDC systems, it can potentially establish itself as an un-damped exchange of energy at sub-synchronous frequencies between the series capacitor and saturating magnetic circuits of the converter transformers. In this regard, the following issues were discussed: 1. Types of blocking employed for ABB Schemes: It was noted that in almost all instances of ferro-resonance detection, Pole-2 got tripped due to operation of X-block/ Y-block/ Z-block protection. When enquired about the same, POWERGRID informed that blocking means removing firing pulses to the thyristors. While doing so, a path is sometimes provided for the DC side current by formation of bypass pairs (BPP). Depending on which side BPP’s are fired or not fired, different protective blocking actions can be obtained as mentioned below:

X-blocking: implies Block without simultaneously firing of Bypass pairs (BPP). It operates for

protections like Valve Short Circuit protection (Rectifier), DC O/V (for high reverse blocking

voltage).

Y-blocking is conditional and implies blocking without Bypass pairs in the rectifier and with

Bypass pair in the inverter. It operates for protections like Commutation failure, DC Under

Voltage, DC O/C, DC Line protection, DC line differential, DC O/V (for high forward voltage)

Z-blocking always implies a blocking with simultaneous firing of Bypass pairs. It operates for

protections like Last Line Disconnect, Converter Transformer Differential, Converter AC Bus and

Conv. transformer O/C, Converter transformer valve side O/C, Gas protection relay, pressure

relay, etc.


2. Adopted Ferro-Resonance detection Logic:

At HVDC Gajuwaka substation, it was noted that first Pole-1 (ALSTOM make) was commissioned followed by Pole-2 (ABB make) followed by Fixed Series Capacitors (FSC’s) at Jeypore end. Hence when Pole-1 was commissioned, there was no instance of ferro-resonance (FR) phenomenon. As such, Pole-1 had not been provided with any ferro-resonance detection logic. However, after commissioning of FSC’s at Jeypore end, the FR phenomenon started making its presence felt by culminating in Pole-2 trippings. When the matter was subsequently referred to ABB (as part of contractual warranty), the following logic had been put in place to overcome ferro-resonance:

From above, it can be seen that whenever AC side voltage waveforms containing dominant frequencies in the range 65 – 135 Hz that exceed the threshold level of 35 kV sustain for a considerable period, ferro-resonance detection would take place, and a carrier signal would be sent to Jeypore end to bypass FSC’s located there.

By above logic, ideally no pole tripping should take place since it essentially removes the cause of ferro-resonance. However, the fact that Pole-2 trippings had taken place in the events under consideration pointed to the need for improved communication and more robust logics.

In this regard, it was noted that in all instances of ferro-resonance detection, Pole-2 got tripped

due to differential protection operation of its converter transformer, which in turn was due to over-coming of bypass logics employed for 2nd harmonic and 5th harmonic restraints. Hence, it was also noted that in order to ensure fool-proof operation of above ferro-resonance logic, it is essential to ensure that the time taken for by-passing FSC from the instant FR is detected would be less than the time required for overcoming harmonic restraints and consequent differential protection operation of Convertor Transformer.

In this context, it was also noted that during the said instances of ferro-resonance, only Pole-2

got tripped while Pole-1 remained intact. When POWERGRID was enquired about the settings employed for harmonic restraints in Pole-1 Converter Transformer, and to consider the possibility of emulating/ replicating them in Pole-2 Converter Transformer, POWERGRID informed that the said settings for Pole-1 had been provided statically, and its OEM had not provided any details w.r.t. the same; hence it was better to focus on the other options available to tackle FR phenomenon.


3. Measures identified/ proposed to overcome Pole-2 trippings on ferro-resonance:

In view of earlier discussions, it was noted that ferro-resonance can be tackled on two fronts: by resorting to various measures aimed at removing the source/ cause of ferro-resonance in

the minimum possible time; by resorting to various measures aimed at nullifying the effect of ferro-resonance by

employing a suitable ferro-resonance damping controller; W.r.t. second alternative, the solution proposed by ABB presented in 2016 IEEE conference (copy enclosed at Annexure-VI) and the substantiation studies carried out by SRLDC showing similar dominant harmonics (DC side 39 ~ 42 Hz; AC side 91.6 Hz) were discussed. Even though the solution suggested seemed to be in order, POWERGRID opined that the tight firing angle control it necessitates for achieving required discrimination, and the cost involved in providing such a specialized controller would simply make the scheme unviable; as such it was appropriate to look for other cost-effective solutions. W.r.t. first alternative, options/ measures such as improving communication between Gajuwaka and Jeypore ends by using DTPC’s (short-term), improving general voltage profile by expediting proposed installation of STATCOM at Jeypore end (medium-term), and employing ferro-resonance detection logic at Jeypore end itself to bypass FSC’s on ferro-resonance detection (long-term) had been discussed. After deliberations, the following were finalized for implementation by POWERGRID:

As a short-term measure for avoiding pole trippings under ferro-resonance conditions, immediate disconnection of FSC would be effected by ensuring fool-proof communication of the ferro-resonance carrier signal from Gajuwaka to Jeypore. To achieve this, it was agreed that the existing PLCC based carrier communication would be replaced by fibre-optic communication by installing Digital Tele-Protection Couplers (DTPCs) at both ends within 2 months (i.e., by May, 2018). It was also agreed that dual communication channels would be provided to ensure reliable bypassing of FSCs. Further, to make above scheme effective, SRPC requested POWERGRID to ensure that the time taken for by-passing FSC from the instant ferro-resonance is detected would be less than the time required for overcoming harmonic restraints and consequent differential protection operation of Convertor Transformer.

As a medium-term measure for avoiding pole trippings under ferro-resonance conditions, it was agreed that POWERGRID would expedite commissioning of the proposed STATCOM at Jeypore end to improve general voltage profile under disturbance/ fault conditions. It was also agreed to study the effect of the proposed STATCOM on improving the voltage profile under transient conditions, to establish its efficacy in arresting ferro-resonance kind of phenomena, and based on the study results to devise a scheme to effect auto-bypassing of FSC for low power order (say 400 MW) in Jeypore – Gajuwaka lines. It was also agreed that POWERGRID, NLDC, and SRLDC would share relevant data with one another, and jointly carry out the requisite studies, and furnish the results to SRPC Secretariat before commissioning of the said STATCOM (expected date: May, 2018) to gauge its performance.


As a long-term measure for avoiding pole trippings under ferro-resonance conditions, it was agreed that POWERGRID would carry out necessary refurbishment works of controller protections at Jeypore end for detecting ferro-resonance locally at FSC location (Jeypore end) and effect automatic by-passing of FSCs by putting necessary infrastructure in place at Jeypore end. POWERGRID was requested to furnish a concrete proposal with suitable time-lines in this regard.

F. Compliance of Protection Audit Recommendations (PAR) pertaining to Gajuwaka (HVDC &

HVAC) Substation

The status of implementation of various pending PAR pertaining to Gajuwaka SS as given at Annexure-VII was discussed in the meeting. POWERGRID was asked to comply with the same including reviewing of stub-protection timing at the earliest, and furnish a compliance report to SRPC Secretariat by 15th May, 2018.

G. Vote of Thanks

MS, SRPC thanked ED, POWERGRID and GM, POWERGRID and other Officials from POWERGRID, NLDC & SRLDC for participating in the Meeting. He hoped that with active cooperation, significant and tangible improvement would be witnessed in the performance of Gajuwaka HVDC Poles operation.

*****

Annexure-I

Connectivity Diagram & Single Line Diagrams:

Annexure-II

Page 1 of 2

Annexure-II

Page 2 of 2

PCSC Deliberations

Sl. No.

Details of Event Date &Time Reason Remarks

1

Tripping of HVDC Gajuwaka Pole-1&2 and 400 kV Jeypore-Gazuwaka line-1&2

Pole-1 at 08:44 hrs on 28.04.2017 & Pole-2 at 09:07 hrs on 28.04.2017

Pole-1 got tripped due to failure of auxiliary supply changeover. 400kV Jeypore-Gazuwaka line-2 got tripped on operation of over-voltage protection. Pole-2 east shunt reactor got failed and this in turn resulted in tripping of HVDC Pole-2 due to Pole-2 Y-block activation. 400kV Jeypore-Gazuwaka line-1 also got tripped during the incident.

PCSC-64

SLD:

Tripping of Pole-1 at 8:44 hrs and Pole-2 at 09:07 hrs:

As per FIR furnished:

Annexure-III

Page 1 of 11

As per TR furnished:

Annexure-III

Page 2 of 11

As per SRLDC Preliminary Report:

PGCIL (SR-I):

Pole-1 tripped due to non-operation of auxiliary supply change over during tertiary supply failure;

Pole-2 tripped on operation of WA1 protective Y-Block execution for fault created by Filter Reactor Bushing failure;

J-G Line 1 tripped on operation of O/V at Jeypore end; and

Annexure-III

Page 3 of 11

J-G Line-2 tripped on operation of O/V at Gajuwaka end. SRPC/ SRLDC:

It was noted that similar trippings of Pole-1 due to failure of auxiliary supply for Pole-1 had been observed in the recent past. When one such event was discussed in PCSC-62 held on 15.03.2017, PGCIL (SR-I) informed that at Gajuwaka, while supply changeover scheme for Pole-2 was fully automatic, that for Pole-1 was not. And, a separate PLC was being proposed to make Pole-1 auxiliary supply changeover scheme fully automatic.

It was also noted that at Gajuwaka, AC auxiliary supply was being arranged as follows: HVAC: 33 kV APEPDCL feeder + 250 kVA DG Set

Pole-1: 33 kV Tertiary Supply from of ICT-2 (BHEL make) + 1500 kVA DG Set


When asked about the reasons for not providing two separate & dedicated AC feeders for HVAC, Pole-1, and Pole-2, PGCIL (SR-I) informed that the reliability of 33 kV APEPDCL feeders was not up to the mark. That was the reason why they had gone for higher capacity DG sets sufficient to provide back-up for extended times.

At this juncture, PGCIL (SR-I)’s attention was brought to suggestion of protection audit team that conducted audit at Gajuwaka (HVAC & HVDC) SS during 28 -30 December, 2015 that tertiary supply from ICT-1 may also be used as redundant AC auxiliary supply. To this, PGCIL (SR-I) informed that ICT-1, which was of TELK make, had already completed 30 years of operation. Hence, a letter had been addressed to its OEM regarding availing tertiary supply from the same.

In this connection, it was also noted that ICTs - 1 & 2 are the main source for VSS of APTRANSCO, on which supply to North Coastal AP including railways is dependent. As such, there was a need to provide an additional Transformer/ augment the capacities of existing transformers. On this, PGCIL (SR-I) informed that there was no space for providing additional transformer at Gajuwaka. However, proposal to replace existing transformers by that of higher capacity was serious being looked into by their management.

When enquired about the time-frame for making auxiliary supply changeover scheme for Pole-1 fully automatic, PGCIL (SR-I) informed that their management had already been seized of the matter, and it may take another 3 months to finish the job.

When enquired about the reasons for East Shunt Reactor failure, PGCIL (SR-I) informed that subsequent to the event, an internal Committee of PGCIL had gone into the details of the issue, and concluded that high percentage of harmonic content experienced in the line voltages might have possibly led to reactor’s failure. The issue had also been referred to OEM.

When enquired about the reasons for Jaipur line-1 tripping on NGR’s Buccholtz relay Stage-II trip, PGCIL (SR-I) replied that the line had tripped due to DT receipt from Jaipur end, where OVR protection had operated. However, since no DT receipt event could be observed from the furnished DR & EL, PGCIL (SR-I) was asked to check with the other end details and investigate the reasons for NGR Buccholtz’s trip operation.

Recommendations:

PGCIL (SR-I) to provide fully automatic supply-changeover scheme for Pole-1 auxiliaries at the earliest.

PGCIL (SR-I) to furnish a detailed report on the reasons for East Shunt Reactor failure including remedial action taken. OEM report in this regard also shall be shared.

PGCIL (SR-I) to provide redundant AC auxiliary supply to both Pole-1 & Pole-2 either by making use of tertiary of ICT-1 or from external dedicated feeder

2 Tripping of HVDC Gazuwaka Pole-1 & Pole-2

28.09.2017 at 17:30 hrs

Poles – 1 tripped due to Auxiliary supply failure; Pole-2 tripped due to activation of WA2 Ferro Resonance Detection

PCSC-68

Annexure-III

Page 4 of 11

Antecedent Conditions:

Tripping of HVDC Pole-1:

As per EL/TR, Pole-1 tripped on VCC1.Cooling system State- Unhealthy at 17:30:38.293 signifying failure of Valve Cooling Motors.

It was stated that DR not available.

In TR, the following were stated as the reasons for Pole-1 tripping:

Tripping of HVDC Pole-2:

As per DR/EL/TR, Pole-2 tripped at 17:30:42.683 on activation of Converter transformer Protection Differential Current Phase L2 Trip.

In TR, the following was stated as reason for Pole-2 tripping:

Tripping of Jeypore- Gajuwaka line-2: Gajuwaka PGCIL (SR-II) end:

As per FIR/EL/TR, line tripped at 17:30:52.327 hrs due to DT receipt from jeypore end.

It was also mentioned in FIR that Gajuwaka - Jeypore line-1 remained in service, and was subsequently hand-tripped at 19:32 hrs from Jeypore end.

DR not furnished. Jeypore (PGCIL-ER1) end:

Tripping files (FIR/DR/EL/TR) not furnished.

Annexure-III

Page 5 of 11

Remedial Action taken:

In this regard, SRLDC brought PGCIL (SR-I)’s attention to an IEEE paper by ABB (enclosed at Annexure-V) on “Mitigation of Ferro-resonance in Line Commutated HVDC Converter Interconnected with Series Compensated Overhead Line Transmission System”. Since the conditions discussed therein closely resemble the ones being experienced at Gajuwaka HVDC station with series compensation at Jaipur end, it is appropriate that PGCIL (SR-I) study the solution discussed therein, viz. – ferro-resonance oscillations encountered in an HVDC system comprising two back-to-back line commutated converter blocks interconnected with long series compensated transmission lines can be effectively damped by modifying the existing current control of the converter by using a dedicated supplementary ferro-resonance damping controller – for implementation feasibility.

In this connection, PGCIL (SR-I)’s attention was also brought to the following pending recommendations given by PCSC forum in their 64th meeting held on 25.05.2017 w.r.t. Gajuwaka HVDC station: PGCIL (SR-I) to provide fully automatic supply-changeover scheme for Pole-1 auxiliaries at the

earliest. PGCIL (SR-I) to furnish a detailed report on the reasons for East Shunt Reactor failure including

remedial action taken. OEM report in this regard also shall be shared. PGCIL (SR-I) to provide redundant AC auxiliary supply to both Pole-1 & Pole-2 either by making

use of tertiary of ICT-1 or from external dedicated feeder.

PGCIL (SR-I) was asked to comply with the above at the earliest. Recommendations:

PGCIL (SR-I) to study the solution proposed by ABB in their paper on “Mitigation of Ferro-resonance in Line Commutated HVDC Converter Interconnected with Series Compensated Overhead Line Transmission System” presented in 2016 IEEE Electrical Power and Energy Conference (EPEC) for implementation feasibility, and furnish an Action Taken Report w.r.t. the same

Annexure-III

Page 6 of 11


08.12.2017 at 19:29 hrs

Pole-2 got tripped due to failure of Valve Cooling system VFD (Variable Freq Drive control)

PCSC-71

As per FIR/DR/EL/TR, Pole-2 tripped at 19:29:46.388 hrs due to failure of Valve Cooling System (of) VFD (Variable Freq Drive control).

As per TR furnished by PGCIL (SR-I):

Annexure-III

Page 7 of 11

Deliberations:

The Connection Diagram of VFD Auxiliary Supply is given below:

It was noted that due to fault in VFD-1, the connecting fuses of VFD-1 and VFD-2 were blown out along with tripping of LT CB of ICT-2’s tertiary. Due to this, the auxiliary bus got de-energized and supply to Valve cooling system went off, resulting in the tripping of Pole-2 ultimately.

However, failure of VFD-2 fuse and tripping of LT CB of ICT-2’s tertiary along with VFD-1 fuse failure seems to be not in order as proper co-ordination is generally required to be ensured for the fuses employed on different elements. Similarly, reasons for VFD-1 failure also needed to be ascertained for taking appropriate action.

Recommendations:

PGCIL (SR-I) to furnish reasons for VFD-1 failure in the instant tripping, and share OEM’s observations w.r.t. the same, along with remedial measures proposed


31.01.2018 at 06:38 hrs

Due to fault in 220kV line, ICT-1 at Gazuwaka got tripped due to directional OC high set. Since tertiary of ICT-1 was feeding the auxiliary supply to pole-1, tripping of ICT-1 led to the tripping of pole-1 due to loss of auxiliary supply

PCSC-72

Tripping of ICT-1:

As per FIR/DR/TR, ICT-1 tripped at 06:38:31.338 hrs on operation of H/s of Directional O/C protection due to permanent fault in 220kV Vizag SS. It was stated that CT failure had been reported at 220 kV Vizag SS.

In EL, 400 kV 67/67N Pick-up was observed at the above time. It’s trip event could not be found.

In DR, only DOC trip was observed (Whether it was due to H/S unit or normal unit was not clear. If it was due to high-set, what was the time delay adopted for its operation?)

What are DOCE settings adopted for ICT-1 at Gajuwaka?

Annexure-III

Page 8 of 11

Tripping of Pole-1:

As per FIR, Pole 1 tripped at 06:38 hrs due to loss of Auxiliary supply due to ICT-1 tripping. Auxiliary supply from ICT-1 is feeding to Pole-1 at Gazuwaka.

In EL, Pole-1 tripping event was not observed.

DR not furnished.

What about Auxiliary supply from ICT-2’s tertiary? What about changeover arrangement to DG Set? What about AMF panel rectification for DG?


01.02.2018 at 00:23 hrs

Pole 2 tripped due to the activation of “WA2 Ferro Resonance Detected" in East side.

PCSC-72

Tripping on 01-02-2018: As per Tripping Report furnished by PGCIL (SR-I):

Annexure-III

Page 9 of 11


02.03.2018 at 01:38 hrs

Pole 2 tripped due to the activation of “WA2 Ferro Resonance Detected” in East side & Pole 2 tripped at 01:38 on activation of Converter transformer Protection Differential Current Phase L1 Trip

PCSC-72


Annexure-III

Page 10 of 11


07.03.2018 At 01:31 hrs

Pole 2 tripped due to the activation of *WA2 Protective Y-Block Executed" in Pole Control system.

PCSC-72


*****

Annexure-III

Page 11 of 11

Special Meeting

HVDC Gajuwaka tripping

02nd April 2018

Annexure-IV

Page 1 of 15

Sr. No.

Details of Event

Date &

Time Reason Remarks

1

Tripping of HVDC

Gazuwaka Pole-1&2 and

400kV Jeypore-

Gazuwaka line-1&2

Pole-1 at 8:44 hrs

& Pole-2 at 09:07

hrs on 28.04.2017

Pole-1 got tripped due to failure of auxiliary supply changeover.

Pole-2 Shunt reactor got failed resulting in tripping of Pole-2 due

to Ploe-2 Y block activation.

400kV Jeypore-Gazuwaka line-2 tripped on overvoltage

protection

PCSC-64

2

Tripping of HVDC

Gazuwaka Pole-1 &

Pole-2

28.09.2017 at

17:30 hrs

Pole-1 tripped due to auxiliary supply failure

Pole 2 Converter transformer Protection Differential Current

Phase L2 Trip due to activation of WA2 ferro-resonance

detection

PCSC-68

3

Tripping of HVDC

Gazuwaka Pole-2

08.12.2017 at

19:29 hrs

Pole-2 got tripped due to failure of VFD of valve cooling

system

PCSC-71

4

Tripping of HVDC

Gazuwaka Pole-1

31.01.2018 at

06:38 hrs

ICT-1 at Gazuwaka tripped due to operation of directional

OC high set. Tertiary of ICT-1 was feeding the aux supply of

Pole-1, tripping of ICT-1 led to tripping of pole-1 due to loss

of auxiliary supply

PCSC-72

5 Tripping of HVDC

Gazuwaka Pole-2

01.02.2018 at

00:23 hrs

Pole 2 Converter transformer Protection Differential Current Phase L3 Trip due to activation of WA2 ferro-resonance

detected in east side

PCSC-72

6 Tripping of HVDC

Gazuwaka Pole-2

02.03.2018 at

1:38hrs

Pole 2 Converter transformer Protection Differential Current Phase L1 Trip due to activation of WA2 ferro-resonance

detected in east side

PCSC-72

7 Tripping of HVDC

Gazuwaka Pole-2

07.03.2018 at

1:31hrs

Pole 2 tripped due to external AC side disturbances leading to initiation of Protective Y-Block from Pole Control.

PCSC-72

Annexure-IV

Page 2 of 15

• HVDC Pole-1 – Auxiliary supply failure - 28.09.2017

– Auxiliary supply change over - 28.04.2017

• HVDC Pole-2 – Operation of convertor transformer differential protection during

ferro-resonance detection - 01.02.2018, 02.03.2018 & 07.03.2018

– Failure of VFD of valve cooling system - 08.12.2017

– Shunt reactor failure - 28.04.2017

Annexure-IV

Page 3 of 15

VFD-1 VFD-2

VC Pump-1 VC Pump-2

DC bus

ICT-2 ICT-1

BC

DG-1 DG-2

Annexure-IV

Page 4 of 15

Sl.

No.

Details of Event Date &

Time PCSC RECOMMENDATIONS Remarks

1

Tripping of HVDC

Gazuwaka Pole-1&2 and

400kV Jeypore-

Gazuwaka line-1&2

Pole-1 at 8:44 hrs

& Pole-2 at 09:07

hrs on 28.04.2017

Fully automatic supply changeover scheme for pole-1 auxiliaries

Redundant AC auxiliary supply to both Pole-1&Pole-2either by

making use of tertiary of ICT-1 or from external dedicated feeder.

Detailed report for East shunt reactor failure along with OEM

report

PCSC-64

2

Tripping of HVDC

Gazuwaka Pole-1 &

Pole-2

28.09.2017 at

17:30 hrs

PG SR-1 to study the solution proposed by ABB in their -

paper on ‘Mitigation of ferro-resonance in line commutated

HVDC convertor inter-connected with series compensated

over-head line’’ for implementation of feasibilty and furnish

action taken report.

PCSC-68

3

Tripping of HVDC

Gazuwaka Pole-2

08.12.2017 at

19:29 hrs

PG SR-1 to furnish reasons for VFD-1 failure and share

OEM’s observations wrt same, along with remedial

measures proposed.

PCSC-71

4

Tripping of HVDC

Gazuwaka Pole-1

31.01.2018 at

06:38 hrs

Review of high set of backup directional OC high of

400/220kV ICT-1.

Tertiary of ICT-1 was feeding the aux supply of Pole-1,

tripping of ICT-1 led to tripping of pole-1 due to loss of

auxiliary supply

PCSC-72

5 Tripping of HVDC

Gazuwaka Pole-2

01.02.2018 at

00:23 hrs

Checking and comparison of differential protection harmonic block setting of Pole-1 & Pole-2 convertor transformers. OEM feedback regarding checking feasibility for providing damping controller for ferro-resonance as discussed in 68th PCSC meeting

PCSC-72

6 Tripping of HVDC

Gazuwaka Pole-2

02.03.2018 at

1:38hrs PCSC-72

7 Tripping of HVDC

Gazuwaka Pole-2

07.03.2018 at

1:31hrs

PCSC-72

Annexure-IV

Page 5 of 15

Log of Gazuwaka Pole tripping due to Ferroresonance from 2015 (as on 08-03-2018)

Date & Time

HVDC pole

Fault in AC grid Indication

31-12-2015 at 9:06 hrs Pole-2

400kV kalpaka Vemagiri line-2 got tripped at both ends on zone-1 due to R-Y phase fault

Pole-2 tripped on operation of "High Harmonic trip“. All filters of Pole-2 got tripped on ‘resistive thermal overload trip’

28-09-2017 at 17:30 hrs Pole-2 ------

Pole 2 Converter transformer Protection Differential Current Phase L2 Trip

01-02-2018 at 00:23 hrs Pole-2 ------


02-03-2018 at 1:38 hrs Pole-2

220kV Kalpaka Brandix line-1 got tripped due to R-N fault


07-03-2018 at 1:31 hrs Pole-2

220kV Kalpaka VTPS line-1 got tripped due to Y-N fault

Pole 2 tripped due to external AC side disturbances leading to initiation of Protective Y-Block from Pole Control.

Annexure-IV

Page 6 of 15

Blocking of pole-2 at 00:23hrs on 01-02-2018


Annexure-IV

Page 7 of 15


Annexure-IV

Page 8 of 15

Log of 400kV Jeypore Gazuwka FSC -I and II Bypass due to Ferroresonance from 2018 onwards (As on 08-03-2018)

Date Time FSC ID Reason Triggering Incident#

08-02-2018 16:02 I & II Ferro- Resonance 765KV Vemagiri Srikakulam line-2 got tripped on Y-B Fault

02-03-2018 02:32 I & II Ferro- Resonance 400KV Khammam Kalpaka got tripped on R-N Fault

Data Log given by ERLDC

Annexure-IV

Page 9 of 15

Bypass of FCSs of both circuits of 400kV Jeypore Gazuwaka at 16:02hrs on 08-02-2018

Bypass of FCSs of both circuits of 400kV Jeypore Gazuwaka at 02:32hrs on 02-03-2018

Annexure-IV

Page 10 of 15

Mitigation of Ferroresonance in Line Commutated HVDC Converter Interconnected with Series Compensated Overhead Line Transmission System

FFT of rectifier side measured α signal FFT of dc current

FFT of rectifier side ac bus voltage

Annexure-IV

Page 11 of 15


DC current of HVDC Gazuwaka from DR

FFT of DC current at HVDC Gazuwaka

indicating presence of 39.2Hz

frequency components during

ferroresonance.

39.2Hz



FFT of DC current at HVDC Gazuwaka

indicating presence of 41.3Hz

frequency components during

ferroresonance.

41.3Hz

Annexure-IV

Page 12 of 15

Blocking of pole-2 at 01:31hrs on 07-03-2018 Observation from DR

Phase voltage of

converter

transformer at 400kV

Gazuwaka East side

FFT of Phase voltage of

converter transformer

at 400kV Gazuwaka East

side indicates presence

of 8.4 Hz, 91.6 Hz and

150Hz frequency

components during

ferro-resonance

DR of Gazuwaka End

DR of Jeypore

End

There is no DR stamping observed at Jeypore end on 07.03.2018 at 01.32hrs.

91.6 Hz

150 Hz

50 Hz

8.4 Hz

Annexure-IV

Page 13 of 15

Bypass of FCSs of both circuits of 400kV Jeypore Gazuwaka at

02:32hrs on 02-03-2018


FFT of DC current at HVDC Gazuwaka indicating the presence of 41.2Hz frequency components during ferroresonance.

Received DR was triggered at 02:32:38.32 and data was

available only till 02:32:40.70. Ferroresonance was

detected at 02:32:42.42. Requested ERLDC to collect DR

triggered during Ferroresonance

DR of Gazuwaka End

DR of Jeypore End

41.2Hz

Annexure-IV

Page 14 of 15

THANK YOU

Annexure-IV

Page 15 of 15

Annexure-V

Mitigation of Ferroresonance in Line Commutated HVDC Converter Interconnected with Series

Compensated Overhead Line Transmission System

Jwala Laxmi Narasimha Rao, Gaurav Bansal, Soubhik Auddy

ABB GISPL, India

Thomas Tulkiewicz, Magnus Ohrstrom

ABB AB Sweden

Abstract— Ferroresonance is a non-linear resonance which can typically occur when a saturable magnetizing inductance forms a resonance circuit with a capacitor. An example of such a resonance condition can be a system configuration where an overhead transmission line with series compensation terminates into a HVDC converter transformer. Such a configuration may lead to post fault ferroresonance oscillations. Mitigation of ferroresonance using converter controls is a simple and economically attractive solution when compared to other solutions such as installing additional equipment in the network. This paper proposes a damping controller to mitigate ferroresonance oscillations for a HVDC system.

Index Terms—Ferroresonance, Ferroresonance Damping Controller (FDC), High Voltage DC (HVDC) system, Fast Fourier Transformation (FFT).

I. NOMENCLATURE

U_AC Converter ac bus voltage Pi Pole i Si Data Set i UD Dc pole to ground voltage PD Power at dc side of pole ID Dc current measured at pole ALPHA Firing angle (α) GAMMA Extinction angle (γ)

II. INTRODUCTION

Ferroresonance is different from the resonance in linear RLC circuits [1]-[2]. In linear circuits, resonance results in high sinusoidal voltages and currents of the resonant frequency. Ferroresonance can also result in high voltages and currents, however, the waveforms are usually irregular in shape. In fact, any operating mode which results in a significantly distorted transformer voltage and current waveform is typically referred to as ferroresonance. More than one response is possible for the same set of parameters, and gradual drifts or transients may cause the response to jump from one steady state response to another [2].

Ferroresonance phenomenon and its mitigation in power system with respect to potential transformers (PTs) is very well studied and reported in the literature [3]-[11]. Substantial research has been done to find mitigation techniques such as adding a resistance on the secondary side of PT [5], installing damping resistors to prevent ferroresonance between circuit breaker grading capacitance and PT [6] etc. Many other ferroresonance examples and their mitigation options are discussed in [7].

However, the ferroresonance in HVDC systems has not been found to be widely reported in literature. The ferroresonance can potentially establish itself as an undamped exchange of energy at subsynchronous frequencies between the series capacitor and saturating magnetic circuits of the converter transformers. It was demonstrated in [8] how the ferroresonance can be damped by converter controls when the converter station is compensated by a series capacitor. Mode shift, i.e., change of control mode from dc voltage control to current control at the inverter side, upon detecting ferroresonance oscillations was proposed as the solution.

The method of mode shift proposed in [8] has been found to be less effective for the scenario where there is an existing series compensated line at the rectifier side of an HVDC system. The reason is that rectifier side current needs to be controlled after the clearance of fault in order not to allow the series capacitor to discharge uncontrollably through the transformer magnetizing reactance. This motivates to propose a supplementary ferroresonance damping controller which would act in addition to the existing current control of the HVDC converters. Such a mitigation technique proposed in this paper is general and should be able to damp ferroresonance occurred due to clearance of the fault at either side of the system, i.e. at rectifier or inverter side.

This paper is organized as follows. Section III of the paper describes study system configuration and its steady state response. Section IV covers the ferroresonance phenomenon in the study system. Section V reports the proposed ferroresonance detection method followed by which the novel damping controller is introduced. Subsequently the case study results demonstrating the performance of the proposed

2016 IEEE Electrical Power and Energy Conference (EPEC)

978-1-5090-1919-9/16/$31.00 ©2016 IEEE

Annexure-VI

Page 1 of 6

controller are reported in Section VII. Finally, conclusions are made in Section VIII.

III. SYSTEM CONFIGURATION

An HVDC Back-to-Back system delivered by ABB and schematically depicted in Fig. 1 is used to perform the study. While conducting the dynamic performance studies, it is observed that when there is a fault at the rectifier/inverter side ac buses, large distortions appear in ac and dc side system variables after clearing the fault. These oscillations are found to be due to ferroresonance interactions between the series compensated line and the converter transformers. In this case study the ferroresonance is due to the rectifier side converter transformer and the series compensated line. The main purpose of this study is to develop a general supplementary control which is effective not only for a back to back system, just used as an example where ferroresonance was reported earlier in dynamic studies, but also for other HVDC configurations such as point to point systems. The study system of Fig. 1 has two HVDC links, namely Pole I and Pole II. Each pole comprises 2x500 MW back to back line commutated HVDC converters. Each converter consists of two 6-pulse converters connected in series (not shown in Fig. 1). The HVDC links interconnect the 400 kV networks of ac system 1 and ac system 2 which operate at 50 Hz nominal frequency. As shown in Fig. 1, at the rectifier side, both the poles are connected by 400 kV double-circuit transmission lines (nearly 225 km in length) having 50% series compensation. Some of the important system parameters are listed in Table 1.

Table 1: Study system parameters

System Parameters Ac System 1 Ac System 2 Ac Voltage 400kV 400 kV Dc Voltage 200 kV Dc Power Pole I rated for 500 MW

Pole II rated for 500 MW Short Circuit MVAmin 1200 MVA 2500 MVA Short Circuit MVAmax 3000 MVA 8000 MVA

A. Steady state response

The system shown in Fig. 1 is modelled in PSCAD with the necessary converter controls, ac networks and relevant generations. Steady state performances of the rectifier and inverter sides of pole I of the study system are shown in Fig. 2 and Fig. 3 respectively. Each pole is operating with a current order of 1 p.u. which is equivalent to 500 MW power output from that pole. The filter configuration, transformer taps and ac voltage set points are adjusted in such a way that the firing angle (α) of the converters and the extinction angle (γ) are close to their nominal values. The ac bus voltage, power output of pole I and the firing angle α of the rectifier end are shown in Fig. 2. The corresponding variables for the inverter side are shown in Fig. 3.

Fig. 1. Study system with 2x500 MW back to back HVDC links with series compensated ac lines.

Fig. 2. Steady state response at the rectifier side for pole I.

Fig. 3. Steady state response at the inverter side for pole I

IV. FERRORESONANCE PHENOMENON

The HVDC system is directly connected to an existing series compensated line at rectifier side and post fault ferroresonance oscillations are observed for certain fault cases. The study results depicted in this section correspond to a 3-phase to ground fault at the rectifier side ac bus as shown in Fig. 4.

0.5 0.52 0.54 0.56 0.58 0.6-500

0

500

U-A

C1

U-A

C2

U-A

C3

y

0.5 0.52 0.54 0.56 0.58 0.6-2

-1

0

1

PD

_P1

[pu]

0.5 0.52 0.54 0.56 0.58 0.65

10

15

20

Time [s]

ALP

HA -P

1

0.5 0.52 0.54 0.56 0.58 0.6-500

0

500

U-A

C1

U-A

C2

U-A

C3

y

0.5 0.52 0.54 0.56 0.58 0.6-2

-1

0

1

PD

_P1

[pu]

0.5 0.52 0.54 0.56 0.58 0.624

25

26

Time [s]

GA

MM

A -P1

(a) Rectifier side ac bus voltage

(b) Dc power (p.u.)

(c) Rectifier side α in degrees

(a) Inverter side ac bus voltage

(b) Dc power (p.u.)

(c) Inverter side ɣ in degrees

2016 IEEE Electrical Power and Energy Conference (EPEC) Annexure-VI

Page 2 of 6

Fig. 4. Study system with the fault at rectifier side ac bus.

A 3-phase to ground fault is applied at the rectifier side ac

bus at t=0.1s for a duration of 100ms. The ac bus voltage, dc side power, and α, γ for rectifier and inverter sides until 1s are depicted in Fig. 5. It is apparent from Fig. 5 that the system is not able to recover after the fault (fault is removed at 0.2s) and has significantly oscillatory response. The FFT of firing angle α, dc current and ac bus voltage at the rectifier side of pole I are shown in Fig. 6, Fig. 7 and Fig 8 respectively, along with their post fault time domain traces captured in a suitable window. From Fig. 6 and Fig. 7, it is apparent that a sub harmonic frequency of 42 Hz is prominent in dc side system variables (ALPHA,P1, ID_P1) whereas a complement of the same frequency (50 – 42 = 8 Hz) is present in ac side voltage which is clearly observed in Fig. 8. In order to confirm that these oscillations are actually due to interaction between series capacitor bank and converter transformer, a test has been performed with the same fault with the series capacitor banks bypassed. The system is able to recover smoothly after the fault with bypassed series capacitor.

V. FERRORESONANCE DETECTION

During ferroresonance, the voltage and current waveforms in ac and dc sides of the HVDC system are distorted and will contain harmonics. Several ferroresonance detection methods have been documented in literature, such as using wavelet-transform [12], over-flux method [13], and frequency identification using sub-harmonic contents [14]. However, these methods require complex computations with longer computational time that may not be suitable in this scenario.

In this paper, ferroresonance is detected using FFT of either ac side signals (bus voltage or current) or dc side signals (α or Idc). Ferroresonance is confirmed whenever the magnitude of an excited frequency exceeds a certain threshold. This threshold level may change from system to system and should be decided based on detailed system studies.

Fig. 5. Plots for Pole I (a): Line to neutral ac voltage (U_AC); (b): Power

measured on dc side in p.u. (PD_P1); (c): Rectifier side α measured (degrees) at Pole I (ALPHA_P1) (d): Inverter side γ measured (degrees) at Pole I

(GAMMA_P1)

Fig. 6. FFT of rectifier side measured α signal for Pole I

Fig. 7. FFT of dc current through Pole I

Fig. 8. FFT of rectifier side ac bus voltage

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

-500

0

500

U-A

C1

U-A

C2

U-A

C3

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-2

-1

0

1

PD

_P1

[pu]

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

0

50

100

150

ALP

HA -P

1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 120

40

60

80

Time [s]

GA

MM

A -P1

0.5 1 1.5 20

50

100

Time [s]

ALP

HA -P

1

20 30 40 50 60 70 800

5

10

15

Frequency [Hz]

Spe

ctra

of

ALP

HA

-P1

0.5 1 1.5 2-0.5

0

0.5

1

1.5

Time [s]

ID

_P1

[pu]

g

20 30 40 50 60 70 80 90 1000

0.1

0.2

0.3

Frequency [Hz]

Spe

ctra

of

ID_P

1 [p

u]

0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7-1000

-500

0

500

Time [s]

U-A

C1

U-A

C2

U-A

C3

g

0 10 20 30 40 50 600

100

200

300

400

Frequency [Hz]

Spe

ctra

of

U-A

C1

Spe

ctra

of

U-A

C2

Spe

ctra

of

U-A

C3

(b) FFT of α shown in plot ‘a’

(a) Rectifier side α in degrees

(b) FFT of dc current shown in plot ‘a’

(a) Dc current through Pole I

(b) FFT of ac voltage shown in plot ‘a’

(a) Rectifier side ac bus voltage

(a) Line to neutral ac voltage

(b) Dc power (p.u.)


(d) Inverter side ɣ in degrees


Page 3 of 6

Fig. 9 shows the block diagram of the detection logic. The

input to this logic may be an ac or a dc signal which can be decided based on the observability of ferroresonance mode that is excited after clearing the fault. Then the input signal is scanned through the FFT block to extract the magnitude of the dominant sub-harmonic component or the excited ferroresonance mode. If the magnitude of the extracted signal containing the ferroresonance mode exceeds a threshold level, ferroresonance signal (FR_ON) will become high based on the hysteresis value. Hysteresis block is used to prevent the ferroresonance signal toggling between ON and OFF states.

Fig. 9. Ferroresonance detection logic

VI. FERROREOSNANCE DAMPING CONTROLLER (FDC)

The basic principle of the proposed damping controller is schematically shown in Fig. 10. The output of the controller is αorder which is generated by adding the alpha modulation signal αmod from the FDC to αcontrol calculated by the existing HVDC current control.

Fig. 10. Ferroresonance damping controller principle

The control block diagram of the FDC is shown in Fig. 11.

The input to the controller is the dc pole current Idc which is filtered through a low pass filter to remove the high frequency components. It has been observed in simulations that the magnitude of 42 Hz frequency component is dominant during the ferroresonance. Hence, a second order Butterworth filter is used to extract this frequency with a sideband of 3 Hz around the center frequency of 42 Hz.

The output of the Butterworth filter is multiplied with a gain and is passed through another low pass filter to avoid the fast transients. Finally, the output is limited in order to avoid high values of αorder which may lead to overshoots in dc current after fault recovery. The αmod output from the damping controller is multiplied with FR_ON signal of ferroresonance detection logic shown in Fig. 9. This is to ensure that FDC only acts upon detection of ferroresonance mode of a particular frequency. It is then added to αcontrol which is calculated from the existing HVDC current control to get the final αorder.

Fig. 11. Ferroresonance damping controller implementation (block diagram)

VII. PERFORMANCE OF FERRORESONANCE DAMPING

CONTROLLER

The performance of the proposed damping controller described in section VI has been tested for 3-phase to ground and 1-phase to ground faults at rectifier and inverter ac buses.

In this section, the study results corresponding to two worst fault cases are presented to show the performance of the proposed damping controller. The first one is the 3-phase to ground fault at the rectifier side ac bus and the other one with the same fault at the inverter side ac bus. The fault is applied at t=0.1s for a duration of 100ms. The plots in the next two sub-sections compare the system response with and without the damping controller for the fault cases mentioned above. In order to avoid multiple plots, only plots related to pole-I are shown. Since both the poles are connected to the same ac buses, the response of Pole-II will be similar to Pole-I.

A. Three Phase to Ground Fault at Rectifier Side

As shown in Fig. 4, a 3-phase to ground fault is applied at the rectifier side ac bus. The post fault system response without a damping controller is shown earlier in Fig. 5. The same signals with the FDC are reported in Fig. 12. In contrary to Fig. 5 where post fault system response were significantly oscillatory and system has eventually failed to recover in several attempts, the FDC controller successfully damps the ferroresonance mode in all the system variables as reported in Fig. 12.

Fig. 12. Plots for Pole I (a): Line to neutral ac voltage (U_AC); (b): Power

measured on dc side in p.u. (PD_P1); (c): Rectifier side α measured (degrees) at Pole I (ALPHA_P1) (d): Inverter side γ measured (degrees) at Pole I

(GAMMA_P1)

In order to further demonstrate the performance of the FDC during post fault, the dc current and converter firing angle α of pole I are plotted in the first two subplots of Fig. 13 with (green curve) and without FDC (blue curve). The output of the FDC i.e., αmod is plotted in third subplot of Fig. 13. It is obvious from

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

-500

0

500

U-A

C1

U-A

C2

U-A

C3

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-2

-1

0

1

PD

_P1

[pu]

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

0

50

100

150

ALP

HA -P

1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 120

40

60

80

Time [s]

GA

MM

A -P1

(b) Dc power (p.u.)



(a)Line to neutral ac voltage


Page 4 of 6

Fig. 13 that the damping controller effectively mitigates the 42Hz oscillations and thus helping the system to recover in less than 0.5s. The tuning of the damping controller will be performed in applicable projects during detailed system design.

Fig. 13. Comparison of dc current and α- with (green) and without (blue)

damping controller

Fig. 14. Comparison of FFT of dc current through Pole I with (green) and

without (blue) damping controller

Fig. 15. Comparison of FFT of α measured at rectifier side of Pole I with

(green) and without (blue) damping controller.

At last FFT of Idc and αmeasured signals with (green curve) and without FDC (blue curve) are shown in Fig. 14 and Fig. 15 respectively in a time window of 0.25s to 0.65s. In both of these signals the presence of 42Hz mode is prominent without the FDC controller. With the FDC controller the FFT magnitudes of the same signals are substantially reduced. This

confirms the successful performance of the FDC in frequency domain as well.

B. Three Phase to Ground Fault at Inverter Side

The performance of the damping controller for a three phase to ground fault at the inverter side is demonstrated in this section. The same signals as reported in section A for rectifier side fault are also reported for inverter side faults in Fig. 16 to Fig. 20. Fig. 16 shows the system performance (converter ac voltage, power, α, γ of pole I) for this fault without FDC. It is also evident from Fig. 17 to Fig. 20 that the FDC is equally effective in damping the ferroresonance after the recovery of a three phase to ground fault at the inverter side.

Fig. 16. Plots for Pole I without damping controller (a): Line to neutral ac

voltage (U_AC); (b): Power measured on dc side in p.u. (PD_P1); (c): Rectifier side α measured (degrees) at Pole I (ALPHA_P1) (d): Inverter side

γ measured (degrees) at Pole I (GAMMA_P1)

Fig. 17. Plots for Pole I with damping controller (a): Line to neutral ac voltage (U_AC); (b): Power measured on dc side in p.u. (PD_P1); (c):

Rectifier side α measured (degrees) at Pole I (ALPHA_P1) (d): Inverter side γ measured (degrees) at Pole I (GAMMA_P1)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-0.5

0

0.5

1

1.5

S1:

ID_P

1 [p

u] S

2:ID

_P1

[pu]

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-50

0

50

100

150

ALP

HA -P

1 A

LPH

A -P1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-30

-20

-10

0

10

20

Time [s]

Alp

ha-M

od

0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65-2

0

2

Time [s]

S1:

ID_P

1 [p

u] S

2:ID

_P1

[pu]

20 30 40 50 60 70 80 90 1000

0.1

0.2

Frequency [Hz] Spe

ctra

of

S1:

ID_P

1 [p

u] S

pect

ra o

f S

2:ID

_P1

[pu]

0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.650

20

40

60

80

100

Time [s]

ALP

HA --

WO

--C

NT

RL

ALP

HA --

W--

CN

TR

L

20 30 40 50 60 70 80 90 1000

5

10

15

Frequency [Hz]

Spe

ctra

of

ALP

HA

--W

O--

CN

TR

L S

pect

ra o

f A

LPH

A--

W--

CN

TR

L

0 0.2 0.4 0.6 0.8 1

-500

0

500

U-A

C1

U-A

C2

U-A

C3

0 0.2 0.4 0.6 0.8 1-2

-1

0

1

S1:

PD

_P1

[pu]

0 0.2 0.4 0.6 0.8 1

0

50

100

150

ALP

HA -P

1

0 0.2 0.4 0.6 0.8 1-100

0

100

200

Time [s]

GA

MM

A -P1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

-500

0

500

U-A

C1

U-A

C2

U-A

C3

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-2

-1

0

1

S1:

PD

_P1

[pu]

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

0

50

100

150

ALP

HA -P

1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-100

0

100

200

Time [s]

GA

MM

A -P1






(b) Rectifier side α in degrees

(c) Modulation signal from the damping controller (a) Line to neutral ac voltage

(b) Dc power (p.u.)



(a) Line to neutral ac voltage

(b) Dc power (p.u.)




Page 5 of 6

Fig. 18. Comparison of dc current and α- with (green) and without (blue)

damping controller

Fig. 19. Comparison of FFT of dc current through Pole I with (green) and

without (blue) damping controller

Fig. 20. Comparison of FFT of α measured at rectifier side of Pole I with

(green) and without (blue) damping controller.

VIII. CONCLUSIONS

In this paper, it is demonstrated that the ferroresonance oscillations encountered in an HVDC system comprising two back to back line commutated converter blocks interconnected with long series compensated transmission lines can be effectively damped by modifying the existing current control of the converter. A dedicated supplementary ferroresonance damping controller is proposed, designed and added to the existing converter current control. The controller is activated only when the ferroresonance mode of the frequency concerned gets excited. The performance of the proposed controller has been extensively tested under different fault scenarios and converter control modes. It is observed that the proposed FDC can satisfactorily damp the 42Hz ferroresonance mode, which gets excited after the clearance of three phase and single phase to ground faults either at the rectifier or at the inverter side ac buses, at varied operating conditions. Even though the chosen study system is a back to back HVDC station, the proposed controller is expected to be effective in damping ferroresonance oscillations in any other point to point line commutated HVDC system.

REFERENCES [1] P. M. Anderson and R.G. Farmer, in Series Compensation of Power

Systems, PBLSH Inc., California, USA, 1996. [2] R. C. Dugan, M. F. McGranaghan, S. Santoso and H. W. Beaty,

Electrical Power Systems Quality, McGraw-Hill, New York, 2000. [3] Slow Transients Task Force of the IEEE Working Group on Modeling

and Analysis of Systems Transients Using Digital Programs, “Modeling and Analysis Guidelines for Slow Transients-Part III: The Study of Ferroresonance,” IEEE Trans. on Power Delivery, vol. 15, no. 1, pp. 255-265, January 2000.

[4] M. Val Escudero, I. Dudurych, M. Redfem, “Understanding ferroresonance,” in Proc. of International Universities Power Engineering Conference (IUPEC), Vol. 2, pp. 1262-1266, September, 2004.

[5] Yunge Li, Wei Shi, Rui Qin and Jilin Yang, “ A Systematical Method for Suppressing Ferroresonance at Neutral-Grounded Substations,” IEEE Trans. on Power Delivery, Vol. 18, No 3, pp 1009-1014, July 2003.

[6] D. A. Jacobson, D. R. Swatek, R. W. Mazur, “Mitigating Potential Transformer Ferroresonance in a 230kV Converter Station,” in proc. of Transmission and Distribution Conference, pp. 269-275, September, 1996.

[7] “Resonance and Ferroresonance in Power Networks,” CIGRE working group C4.307, February 2014.

[8] D. A. Woodford, “Solving the Ferroresonance Problem when Compensating a DC Converter Station with a Series Capacitor”, IEEE Trans. on Power Systems, Vol. 11, No. 3, Aug. 1996.

[9] Ferracci, Ph.,"Ferroresonance", Groupe Schneider: Cahier technique nº 190, pp. 1-28, March 1998.

[10] W. Piasecki, M. Stosur, M. Florkowski, M. Fulczyk, B. Lewandowski, “Mitigating Ferroresonance in HV inductive transformers,” International conference on power systems transients, Japan, June 2009.

[11] David A. N. Jacobson, “Field Testing, Modelling and Analysis of Ferroresonance in a High Voltage Power System” Ph.D. Thesis, Elec. Eng., Univ. of Manitoba, Aug. 2010 , August 2000.

[12] G.Mokryani, M.-R.Haghifam, “Identification of Ferroresonance Based on Wavelet Transform and Artificial Neural Networks”, in Power Engineering Conference, 2007. AUPEC 2007, Dec. 207, pp. 1-6.

[13] X. Dong, X. Li, Z. Bo R. Chatfield and A. Klimek “Method and system for online ferroresonance detection”, US Patent 9346326, Oct. 5, 2010.

[14] J. M. Kern, S. A. Miske and W. H. Sahm, “Sub-harmonic detection and control system”, US Patent 6157552, Dec. 5, 2010.

0 0.5 1 1.5-1

0

1

2

3 S

1:ID

_P1

[pu]

S2:

ID_P

1 [p

u]

0 0.5 1 1.50

50

100

150

ALP

HA -P

1 A

LPH

A -P1

0 0.5 1 1.5-40

-20

0

20

40

Time [s]

Alp

ha-M

od

0.5 1 1.5-0.5

0

0.5

1

1.5

Time [s]

S1:

ID_P

1 [p

u] S

2:ID

_P1

[pu]

20 30 40 50 60 70 80 90 1000

0.1

0.2

0.3

Frequency [Hz]

Spe

ctra

of

S1:

ID_P

1 [p

u] S

pect

ra o

f S

2:ID

_P1

[pu]

0.5 1 1.50

20

40

60

80

100

Time [s]

ALP

HA --

WO

--C

NT

RL

ALP

HA --

W--

CN

TR

L

20 30 40 50 60 70 80 90 1000

5

10

15

Frequency [Hz]

Spe

ctra

of

ALP

HA

--W

O--

CN

TR

L S

pect

ra o

f A

LPH

A--

W--

CN

TR

L





(c) Modulation signal from the damping controller

(b) Rectifier side α in degrees



Page 6 of 6

Sl. No.

Bay

name/Bu

s

Voltage

Protection/Ele

ment/Equipme

nt/

System

auditated

Deficiences/Nonconformity observed

Status of Implementation of Protection

Audit RecommendationsRemarks

1 General

In pole 1 Trip Circuit Supervision relays are

not available in the circuit. However Trip

Circuit Supervision is being monitored by

RPC (Reactive Power Control)

Mail Dated 05-02-2018:

Pole-1 RPC system trip circuit supervision relays

are installed in the month Dec-2017 & presently

in service.Complied

2 General

HVDC pole - 1 & 2 Auxiliaries supply (33

kV) are being fed from ICT-2 (tertiary) only.

In the event of failure of auxiliary supply

DG set 1 & 2 respectively will feed the

supply to the Auxiliaries.


Aux supply provision from ICT-1 (Second Aux

supply) comissioned. Automatic supply

changeover scheme for Pole-1 has

implemented. In the event of failure of auxiliary

supply DG set feed the supply to the auxiliries.

Order placed for retrofitting of LT brakers in

Auxiliary system & installation will complete by

30-April-2018.

Pending

3 General

HVAC S/S Auxiliaries supply (33 kV) are

being fed from APTRANSCO feeder only. In

the event of failure of auxiliary supply DG

set will feed the supply to the Auxiliaries.


Presently HVAC S/S Auxiliaries supply (33 kV)

are feeding from APTRANSCO feeder only. In

the event of failure of auxiliary supply DG set is

feeding the supply to the Auxiliaries.

Pending

4 General

HVAC DG set starting by manual only. Mail Dated 05-02-2018:

HVAC DG set is presently starting by manual in

case of supply failure. The same shall be

changed to auto & will completed in NTAMC

sworks by October-2018.

Pending

5 General

Oil leakage was observed from the line

reactor of Vijayawada line


Oil leakages of Vijayawada L/R rectified in the

month of April-2016.Complied

6 General

Stub Protection time delay is kept as 100

ms.


As per POWERGRID, Corporate

recommendations Stub prot settings are

adopted as 2.0 A ( Sec) if CT ratio is <=1000A

& & time delay 100 msec. For others 1.5 Amp.

Pending

7 General

For 400 kV Kalpakka Line-1, Circuit Breaker

Pole discrepancies kept as 1.0 s


400 kV Kalpakka Line-1, Circuit Breaker Pole

discrepancies changed to 2.5 sec from 1.0 sec

in Janaury-2016.

Complied

8 General

For 400kV Kalpakka line 1& line 2,

Main- 1 relay:

condition.

Main- 2 relay:

condition.


Informed to APTRANSCO for necessary action

& 400kV Gazuwaka-Kalpaka-1&2 line settings

modified as per Ramakrishna Committee

recommendations in December-2017. However

the same relays will be retrifitted with LDP

relays by APTRANSCO. In 71st PCSC meeting

APTRANSCO informed that the LDP relay

installation will complete by April-2018.Pending

PGCIL (SR-I)1) 400 kV & HVDC Gajuwaka SS [28-30 December, 2015]

Annexure-VII

SOUTHERN REGIONAL POWER COMMITTEE of the Special Meeting on HVDC Gajuwaka Pole Trippings held on...

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