The Electromagnetic Transient Analysis in the decision making process at planning stage Andrea Mansoldo, Mark Norton Technology & Standards Eirgrid, Ireland 160 Shelbourne Road Ballsbridge Dublin 4, Ireland Andrea.mansoldo@eirgrid.com [email protected]
Abstract - Following the 2020 targets for CO2 reduction and the subsequent large planned RES deployment in Europe, there is evidence that the present HV Grid is not sufficient to cope with the new future environment. Therefore, TSOs are thoroughly investigating Grid Transmission needs for the future. Nowadays, a large variety of Transmission Technoloies (TT) are available on the market with different capital and operating costs. The location of many of the RES and the increase of environmental sensitivity is obliging TSOs to considering often and often the undergrounding option using HVAC or HVDC cable technology to allow for the Grid upgrading. Because of the aggressive 2020 targets, 40% of RES energy by 2020 and the consequence of large amount of RES, about 5000 MW of wind, coupled with their remote location and the weaknesses of the existing network, Eirgrid, the Irish TSO, is trying to conceive a comprehensive methodology for the Grid Expansion planning aiming at identifying the most efficient, reliable and cost effective development strategy of the network in the long term.
The paper describes the implementation of an extensive EMT power system analysis in the Grid Planning department aiming at investigating any low frequency transient issue that can put system at risk. In particular Transmission Technology Alternatives EMT performances is the focus of the analysis. It is shown how in the planning process the EMT investigation can provide technical insights that are still fundamental for the decision of infrastructure investment. Some examples are considered, where the EMT technical performances of the best alternatives are compared .
Keywords: System Planning, TOVs analysis, Cables, ATPDRAW.
1. Generals With a 40% RES penetration to be fulfilled by 2020 Republic of Ireland and Northern Ireland (ROI and NI, All Island, AI) are facing a huge challenge in order to make this strategy to happen. Indeed the 5 GW request of connection onshore, especially for wind, and offshore wind, are at a level that allow the target to be covered; however, If NI transmission grid is a strong double circuit 275 kV Grid, this is not the case for ROI, with a meshed 220 kV, and a radial 400 kV East-West. Moreover, the integrations of the two jurisdictions is also weak, with only one double circuit 220 kV lines and about 400 MW of Net Transfer Capacity (NTC); congestions are expected in the future has the wind beahvior and distribution may dictate frequent North-South transfers. Because wind resources location, mostly on the West Coast, is far from the main load consumptions in the East and South, a considerable Long Term grid development plan is currently in progress, [1]. On the other end, Offshore wind developer have asked consensus in Irish territorial water and are willing to either increase or request further connection to the Irish Transmission System (ITS). Eirgrid has therefore further investigated potentials and advantages to optimize infrastructure investment to both fulfill onshore/offshore request as well as better integrate the two AI jurisdiction together and with the British and European markets, towards a 2030 horizon year.
Among the focuses are the understanding the future backbone voltage level, the grid structure, the transmission technology and the best long term dynamic expansion strategy to deploy RES connection request as well as integrate the isle of Ireland with neighbouring countries [2]. Eirgrid assumption for onshore grid developments is to consider the OHL based strategy; however, environmental sensitivity in many Irish countryside compels to investigate mitigations measures to make the projects to be delivered. Undergrounding is among the second best alternatives which finds better acceptance.
2. Introduction The paper describes the generalities of the Long Term Planning (LTP) approach to fulfill a comprehensive design of a future grid; it focuses on the EMT studies which have been incorporated at the end of the technical/economical investigations. A Case of study is shown, starting from a potential Grid structure Expansion Solution (GES) towards 2030, based on 5GW + 5GW wind deployment onshore/offshore respectively thoroughly described in [2]. The GES is submitted to an ATP-EMTP analysis to investigate whether they are technically feasible in some switching operational performances. In particular TOVs are of major concerns at this stage as they can generate overvoltages that jeopardize system reliability on a large extension of the transmission system. Mitigation are discussed in terms of options which may include the change to a second best economic planning solution. 3. The Proposed Long Term Planning (LTP) methodology With reference to fig.1, the proposed LTP methodology is shown. A first screening among all possible Alternatives is performed providing incremental GES y1, y2 .. y3. This analysis is based on a linear Load Flow (DCLF) approach. They are first all submitted to two steady refinement; reliability analysis where possible extra reinforcements are introduced and AC load-flow calculations. At this stage possible extra Compensation is located and a short-circuit analysis is performed to verify the compliance of the future grid with present Transmission Planning Criteria (TPC).
Following a Transient Stability calculation, an EMT study is performed with the main purpose of evaluating whether the GES withstands some operational switching procedures. In particular the analysis concentrates on low frequency resonances phenomena which may be triggered by transients following some switching procedure.
2 3 4 This is of particular concerns because the phenomena: 2010 2015 2020 2030
y1 y2 y3 a) has an high energy contents b) may not be mitigated or mitigation may not be Reliability, Market and VAR Planning y1 (1 year statistical simulation) cost effective by traditional devices, i.e. surge Specific Steady-state Scenarios arresters, pre-insertion resistors, harmonic filters y1 and Short-Circuit evaluations c) may be disruptive on long distances, i.e. 300-400 y1 Transient Stability. Worst case Scenarios km d) may be due to the proposed specific transmission y1 EMT Studies. Worst case Scenarios on a portion of the system technology
Fig.1. Proposed LTP for future Grid design
4. The EMT analysis using ATP-EMTP
To perform such investigation Eirgrid Technology & Standards proposed the ATP-EMTP code [], which is already in use in the Operation Department. Because of the focus of the analysis it has been proposed to represent the ITS 400/220 kV system thoroughly into detail. This may be necessary to investigate TOVs propagation area, should any resonance occurs.
4.1. Resonance phenomena
The resonance phenomenum has always attracted attention to the Power system planner because of its unpredictable occurrances and the corresponding system outage it can produce. Moreover, cascading events may also be triggered in, causing the outage to extend to a large part of the system. Two possibility can be singled out, a parallel resonance and a series resonance. 4.1.1 Parallel
Parallel resonance may occur when energy stored into the system inductance encounters favourable conditions to exchange with the grid capacitances, see fig. 6.
4.1.2 Series
4.2. Modelling set-up
The focus of the analysis is the low frequency resonance and therefore it may be expected a propagation over long distances in particular in case of parallel resonance. This is a consequence of the lower dumping losses, which are voltage dependant, i.e. skin effect, iron core transformer losses, and the energy exchanges between capacitance and inductances.
That said, an ATP-EMTP circuit of the 400/220 kV grid of the All Island Transmission System (AITS) has been implemented, see fig.2a. Y Y SAT 110kV
Y Y SAT
25km STRB2 COOL2
0.2km 0.17km CAC2A CAC2B 60km ABC TURL4 56.09km 56.09km
Y Y SAT MAGF2 42km 26.04km 275kV 19mH 31.69km TAMN2 Y Y SAT 110kV 5.63km Dummy source GROUP turl tr Y Y 19.94km SAT GROUP 220kV turl tr BLLC2 123km
Y TURL2
SAT GROUP Y Y turl tr V
Y 220kV SAT DM136 0.0L 0.0 400V 0.0L 0.0
V 400V
TURL4 Load
TAND2 LCC 50km 50km
LOUTX Y Y 0.0L 0.0 SAT
V Dummy source
LCC
90km 220kV 30mH Y Y LOUTY FLAG2 SAT CASH2 Y Y Y Y standard SAT
20km LCC LOUT2 KING2 Y Y I-Phi
LCC V
0.0L 0.0 OLDS2
LCC LCC
X0215 MYNT2 V
0.0LX0185 0.0
17kV LCC 61km Y Y MNYPG OLDS2 Y Y SAT Load 0.0L 0.0 18.43km standard V CORD2 0.0L 0.0 0.0L 0.0 50MVA V LCC LCC Y Y V 17.83km Load MNPT4 OLDS4 I-Phi
V WDLD4 WDLD2 GROUP 220kV 105. km Y Y 30mH reactor 0.0L 0.0 80MVar I-Phi 4.5km RCC HNST2 WDLD4 Dummy source
WDLD2 3.73km UI
SHANB LCC 0.0L 0.0 22.5km
V 50MVA V LCC 1.4km Load TRST4 LCC
0.0L 0.0 LCC FING2 V
0.0L 0.0 0.0L 0.0 UI TRBT2 LCC LCC LCC LCC UI V V
LAOI4 DNST4 MYNT4 MYNT2 V Y Y WMDL4 MNPT4 MNPT2 85. km 85.23 km 44.77 km standard Y Y 30.55km Load 0.0L 0.0 WMDL2 LAOI2 DNST2 MYNT2 I-Phi Y Y Y Y Y Y MNPT4 MNPT2 Y Y standard standard standard I-Phi Load MNPT4 MNPT2 Load Y Y LCC LCC Load Reactor 80MVar I-Phi
LCC LCC KILP2 KILP2 60. km
LCC KELL4 LCC CARR2 LCC LCC KELL2 Y Y KILP2 KILL2 DNST2 BABG2 standard 20km
wdl-kin TRST4
GROUP LCC
Load LCC V X0101 KNOC2 Y Y KILP1 0.0L 0.0 Y Y SAT ARKL2 I-Phi LCC
KILP2
Y Y
standard Y Y SAT
LCC LCC KNRH2 LODG2 LCC
LCC LCC LCC GREI2 LODG2
LCC LCC LCC LCC LCC LCC
KISK2 BAVK2 CLAS2 KNRH2 CULL2 GREI2
LCC LCC
2202Y 2202
SAT
Y WMDL4 BAVK2 X0169 85. km 17kV Aghada a) ATP-EMTP AITS b) 400 kV OHL c) 220 kV OHL
Flux [Wb] 1600 1400 1200 1000 MNPT4Y Y MNPT2 800 600 SAT 400 200 0 -5 15 35 55 75 95 115 135 [A]
d) 400/220 kV Transformer Model
LCC
V UI
UI
e) submarine cable model f) underground cable model
Fig.2. ATP modelling set up
In figg. b, c and d model components are also shown according to ATPDRAW representation; in particular:
1. 400 and 220 kV Overhead lines have been modelled using the J-marti frequency dependent option in the frequency range 0-500 kHz. 2. Transformers have been modelled using BCTRAN option. In the area of investigation, the model has also been considered with the relevant magnetising characteristics. 3. Cables have been modelled using the Cable Parmeters (CP) constant frequency option, using 250 Hz has tuning frequency. Has shown in fig. 2e,f, submarine cable has been considered as one sections with sheath solid bonded connected, whereas for underground cable a cross-bonded shetah connection have been considered with relevant minor/major cross-bonded sections.
4.3. Frequency Scan (FS) and Time Domain (TD) simulations
An extensive N and N-1 contingency, frequency scan is performed in order to identify critical areas/grid assets. In particular the following criteria have been used to single out for each busbar the worst case scenario:
1) lowest resonance frequency 2) corresponding impedance module at that frequency
At present, FS calculations have been perfomed using an alternative tool, IPSA-Power []; suitable additionals scripts allows the automatisation of topology and current injections changes, as well as the store of the results. Investigation are currently in progress to implement the same methodology using ATP-EMTP.
Frequency Scan analysis allows to singled out, grid assets that may be prone to trigger low frequency resonances. These scenarios have to be submitted to a Time Domain (TD) simulation, where evidence of TOVs existence is given. In particular, TOVs is quantified and compared with IEC standards with the purpose of:
1. verify the withstand of existing grid components 2. identify suitable requirements for new components 3. indetify field test parameter for new components
Should IEC standrads limits be exceeded, the TDS can also be used to identify mitigation measures like:
1. additional protection device, i.e. surge arresters, pre-insertion resistors, synchronised switching 2. additional filtering device to detune the grid asset 3. change in the transmission technology to detune the grid asset
5. A Case of Study: an AI GES to 2030. The described methodology have been applied to a potential development of the AITS to 2030. Using and 'ad hoc' Expansion Planning tool developed by Eirgrid in Cooperation with Ricerca sul Sistema Energetico (RSE) Italy, an offshore grid study have been performed and is thoroughly described in []. An onshore/offshore wind deployment of 5+5 GW has been assumed according to the GATE 3 process results, and information from National Offshore Wind (NOW) association for future offshore installation.
A number of GES has been obtained, also in intermediate steps, as a result of a analysis which optimise Operation and Investment costs, by using a DCLF approach. Following, an AC analysis have been considered in order to investigate the need and location of var supports. With reference to fig.3, GES for 2015, 2020 and 2030 are shown. The onshore grid is assumed to be reinforced using OHL technology; for offshore connection both AC and DC technology have been allowed for connection. Selection have been performed by the Optimisation process.
a) 2015 b) 2020 c) 2030 Fig.3. Grid Expansion Solution(GES) to 2030
.With reference to Figg.3, in early stages, only radial connection are considered for offshore wind, whereas some 400 kV reinforcements have been selected onshore, South east and North. More interconnections is then selected towards England following increase in Wind deployment onshore (up to 2020) and offshore. By 2030, a meshed Offshore Grid id obtained, meshing the AITS on the East Cost, using AC technology. Moreover, an Offshore DC is also selected from the Offshore space to further interconnect the country using VSC technology. This latter solution is an example of how synergies can be exploited between Interconnections and Wind farm connections.
5.1. Results
An extensive FS analysis have been performed in N and N-1 conditions on the previously describe GES. Following, some scenarios have been selected to investigate whether any potential TOVs, in some switching operation occurr.
5.2. Frequency Scan
With reference to fig.4, the FS results are shown both for N and N-1 (worst case) conditions.
a) Impedance Module b) Natural System Frequency (NSF)
Fig.4. FS results for Grid Expansion Solution(GES) to 2030
From fig. 4b, it is possible to identify areas with the same beahavior in terms of NSF; in particular, in Area A, NSF tend to decrease as a consequence of increase amount of Capacitances installed in the area, submarine cables and SVC for LCC technology.
5.2.1 Criterium for Worst Cases (WC) screening
Area A
2015 With reference to fig.5, all the nodes show a low NSF, except Dunstown in 2015, (out of scale). Initially, 2015 they also show high impedance module which make all scenarios at risk. In 2030 all nodes are in low NSF but alsoin low IM,
2030 so no scenarios are at risk. This is due to the prevailing effect of network reinforcements to the increase capacitance (East Offshore Grid AC).
Fig.5. Area A NSF vs. Ohm
Area B Area B WF3O211 _380.0kV ARKCDC1_380.0kV CARCDC1_380.0kV DUNCDC1_380.0kVWith reference to fig.10, the NSF behaviour is the KISODC2 _380.0kV MAYCDC1 _380.0kV AGSB111 _380.0kV KNOB111 _380.0kV AGSBDC1_380.0kV CAVAN4 _380.0kV CAVADC1_380.0kV COLD111 _380.0kV ISL0211 _380.0kV MAYC111 _380.0kV following for the nodes; Cavan shows a lowering of
1000 NSF but with a contemporary lowering of the IM, 937.66 900 which keeps a safe combination parameters. 800 743.66740.8 700 ISLE offshore and Coleraine in NI and Offshore 638.53643.67 600 575.56575.57 Scotland are at risk over trhe entire period of
Ohm 500 295.84 analysis, with a low NSF, below 100 Hz and an high 400 336.71 IM, above 500 Ohm. This is due to the use of AC 300 294.44 200 203.1 offshore cables combined with a radial structure of 164.04166.45 100 118.94119.47 the grid. 0 0 100 200 300 400 500 600 700 800 Hz All the other nodes are confined in a low NSF/low IM area which make them not at risk during the system development. Fig.6. Area B NSF vs. Ohm
Area C
With reference to fig.7, different dynamic behaviour Area C ARG0211 _380.0kV KIN0211 _380.0kV KN1B111 _380.0kV can be singled out in 2015-2030 GES: MNYB111 _380.0kV MNYPG1 _380.0kV MNYPG3 _380.0kV TASB111 _380.0kV BALLYVOU_380.0kV KNOCKANU_380.0kV 1400 The Offshore development in Scotland, Argyll and
1200 N-1 Isle, connected to NI, shows potential risk all over 1000 the period with NSF as low as 300 Hz and IM
800 Ohm between 800 and 1200 Ohms. 600 The North Kerry, Moneypoint Area moving towards 400 lower NSF it is not at risk because of the low IM; 200 2015-2030 0 however temporary high IM in N-1 conditions are 0 100 200 300 400 500 600 700 800 Hz shown in some scenarios that require TD simulations.
Fig.7. Area C NSF vs. Ohm Area D
Area D
LOUA221 _380.0kV ORIO211 _380.0kV BALA211 _380.0kV With reference to fig.12, the D grouping shows a
1000
900 starting point of potential risk, orange area, moving 842.58842.56 800
700 to a risk asset with low NSF and high IM, 900 Ohm 600 and than evolving to a safer grid arrangement. Ohm 500 400 391.11 The final stage is due to the large offshore meshed 300 2015-2030 200 GES, cfr fig.16, a and b. 100 112.39
0 0 50 100 150 200 250 300 350 Hz Fig.12. Area D NSF vs. Ohm
Area E Area E HUNTSTOW_380.0kV WOODLAND_380.0kV WALES_IN_380.0kV INT_CAP _380.0kV WOOCDC1 _380.0kV HUNC111 _380.0kV 300 With reference to fig.13, none of the nodes shows 250 247.62247.91 particular critical conditions, in particular because 200 205.32 206.28 180.34180.46 IM is kept low over the GEX 2015-2030. 150 2015-2030 126.11127.32 Because of the relatively low NSF, 200 Hz, an early 100 stage TD is suggested 50 0 0 100 200 300 400 500 600
Fig.13. Area E NSF vs. Ohm
Area F Area F KNOCKRAH_380.0kV CAHIR _380.0kV FLAA111 _380.0kV OMAD111 _380.0kV SHAA111 _380.0kV With reference to fig.14, none of the nodes shows 3500
3000 particular critical conditions, in particular because 2500 NSF is kept above 500 Hz over the GEX 2015-2030. 2000 Ohm 1500 Because of the high IM, some 2000-3000 Ohm, 1000 some TDs are suggested for Flagford and Cahir 380 500 kV. 0 0 100 200 300 400 500 600 700 Hz Fig.14. Area E NSF vs. Ohm
Area G Area G
KILB111 _380.0kV LOUGHTEE_380.0kV WEMB111 _380.0kV BVKB111 _380.0kV CARRICKM_380.0kV MAYNOOTH_380.0kV MNYPG2 _380.0kV TURLEENA_380.0kV OLDSTREE_380.0kV With reference to fig.15, none of the nodes shows 2500 particular critical conditions; both NSF and IM put 2000 grid assets out of potential risks for resonances and 1500 Ohm TOVs over the GEX 2015-2030. 1000
500
0 0 100 200 300 400 500 600 700 800 Hz Fig.15. Area E NSF vs. Ohm
C C B B
F F D B B D E E G B G B
A A C C G F G F B B
a) 2015 b) 2030
Fig.16. Evolution of critical areas for Resonance phenomena
5.3 Example:North-Kerry reinforcement
One of the areas that can be critical, according to fig.16a is area C, in North-Kerry. The area is submitted to some development projects, aiming at solving congestions, using an extra interconnection between Moneypoint power station and Knocanurha This is also higlighted below in the ATP model, fig.17.
Y Y SAT 110kV
Y Y SAT The link foresees a submarine cable to
25km STRB2
60km ABC X0467 57.29km 56.26km
Y Y SAT MAGF2 42km 26.04km 275kV 19mH cross the Shannon bay of about 7 km, 31.69km TAMN2 Y Y SAT 110kV 5.63km
Dummy source GROUP turl tr Y Y 19.94km SAT GROUP 220kV turl tr BLLC2 123km
Y TURL2
SAT GROUP Y Y turl tr V
Y 220kV SAT DM136 0.0L 0.0 400V TURL4 0.0L 0.0
V from Moneypoint to Tarbert shore; LCC 400V
X0467 Load TAND2
82km 82km
kin-tur kin-tur GROUP GROUP 50km 50km
X0479 LOUTX Y Y 0.0L 0.0 SAT
V Dummy source
LCC
90km 220kV 30mH Y Y LOUTY following an OHL based solution has FLAG2 SAT CASH2 Y Y Y Y standard SAT
20km LCC LOUT2 KING2 Y Y I-Phi been proposed by Eirgrid to connect
LCC
X0191
OLDS2 LCC
LCC LCC 30mH
X0511 MYNT2 Tarbert shore with Knockanurha V 220kV 0.0L 0.0 0.0L 0.0 58km 58km
V wdl-kin
17kV wdl-kin 61km
GROUP GROUP Y Y MNYPG OLDS2 Y Y SAT Load WDLD4 0.0L 0.0 18.43km standard V CORD2 0.0L 0.0 0.0L 0.0 LCC LCC 50MVA V I Y Y V 17.83km Load MNPT4 OLDS4 X0428 I-Phi
V WDLD4 WDLD2 GROUP 220kV 105. km 125. km Y Y 30mH reactor 0.0L 0.0 80MVar I-Phi substation at 400 kV. However, 4.5km HNST2 Dummy source WDLD2 3.73km
SHANB LCC 0.0L 0.0 22.5km
V 50MVA V LCC 1.4km Load TRST4 LCC
0.0L 0.0 LCC FING230mH V
0.0L 0.0 0.0L 0.0 220kV
UI TRBT2 LCC LCC LCC LCC UI I V V
MNPT4 X0452 LAOI4 DNST4 MYNT2 V Y Y WMDL4 X0001MNPT4 85. km 85.23 km 44.77 km standard
Y Y Inrush X0488
I I Y Y 30.55km Load 0.0L 0.0 WMDL2 LAOI2 DNST2 MYNT2 I-Phi feasibility of 400 kV cable options is X0046MNPT4 Y Y Y Y Y Y
Y Y X0488 Inrush I Y Y standard standard standard I-Phi Load X0047MNPT4 Load
Y Y X0488 Inrush I Y Y LCC LCC Load Reactor 80MVar I-Phi
LCC LCC
KILP2 KILP2 LCC LCC CARR2 LCC LCC X0518 X0519LCC KELL2Y Y Y Y KILP2 KILL2 I-Phi DNST2 BABG2 standard also evaluated, should any opposition 20km
wdl-kin Load LCC GROUP
V 220kV KNOC4 110kV 30mH KNOC2 Y Y KILP1 0.0L 0.0 Y Y SAT ARKL2 I-Phi LCC
KILP2
Y Y
standard Y Y SAT
LCC LCC raise for the OHL solution. Tab.1 KNRH2 LODG2
LCC LCC
GREI2 LODG2
30mH 220kV
LCC LCC LCC LCC LCC LCC describes the combination of examined 30mH KISK2 BAVK2 CLAS2 KNRH2 CULL2 GREI2
220kV
LCC LCC
2202Y 2202
SAT
Y X0205 options
Fig.17. North Kerry reinforcement plan
tab.1. Description of cases Existing Grid MNP-TARB SUBC TARB-KNOCKANURHA OHL OLDST-WOODLAND OUTAGE
Basecase YES NO NO NO Case1 YES YES YES NO Case1_1 YES YES YES YES Case2 YES YES NO NO Case2_1 YES YES NO YES
5.3.1 Frequency Scan
Area evidenced already conditions suitable for resonances; the additional reinforcements might have counteracting effects:
1) reinforce the system thus increasing NSF 2) introducing extra large capacitance, submarine cable and UGC in Case 2, thus decreasing NSF
400 kV Cable option 400 kV OHL option It is worth notice that: 2000 A. Basecase has already a low NSF, about 1600 180 Hz
1200 B. the additional reinforcement a. keeps resonance the same if OHL+SUBC 800 b. reduces NSF to 150 Hz if it is UGC+SUBC 400 c. reduces NSF further, increasing IM in contingency of a 400 kV circuit. 0 0 50 100 150 200 250 300 350 400 f s-mny -tarb-case1.pl4: v :MNPT4A f s-mny -tarb-case2.pl4: v :MNPT4A f s-mny -tarb-basecase.pl4: v :MNPT4A f s-mny -tarb-case2_1.pl4: v :MNPT4A Fig.18. FS with different options reinforcements Vs. without
5.3.2 Time Domain Simulation
Time domain simulation is performed in order to verify if Resonance occurs and whether determines TOVs that put system at risk. Transformer energisation is considered critical in particular weak grid assets due to low harmonic order injected into the system through the inrush current. This is the most severe case and it is likely to triggered large TOVs in system with natural frequency around the 2nd harmonics. Different switching time will be considered.
500 *103 375 From fig.19 it can be seen that no TOVs are
250 triggered from the switching however a
125 voltage higher the nominal, about 1.1Uo. It is
0 worth notice that they are sustained for more than 1 second, which means that a slight -125 resonance effect is hidden underneath. -250
-375 -500 0.0 0.3 0.6 0.9 1.2 1.5 td-mny-tarb-basecase_ener_18.pl4: v:MNPT4B td-mny-tarb-basecase_ener_17.pl4: v:MNPT4B td-mny-tarb-basecase_ener_14.pl4: v:MNPT4B Fig.19. TD in the base case.Phase to Ground Voltage in Moneypoint
When introducing the Project as mixed Submarine Xlpe cable and OHL, case1, results are shown in fig.3.5, with also the contingency on the OLD-WOOD. 400 *103 300 Values are still around the 1.1Uo, sustained for more than 1 sec. In fig.20, 200 same results are shown for a longer lasting 100 simulation interval. 0 The trend to resonate is evident from the -100 picture. The voltage amplifies after 1 -200 second remaining however in a 1.1Uo
-300 range.
-400 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 td-mny -tarb-case1_1_ener_18.pl4: v :MNPT4C td-mny -tarb-case1_ener_18.pl4: v :MNPT4C Fig.20. TD in Case1.Phase to Ground Voltage in Moneypoint
If TARB-KNOCKANURHA is connected using an UGC system results are shown in fig.21.
600 It is note worthy the effect of the *103 Oldstreet-Woodland contingency. In N 400 conditions, the overvoltages after
200 energisation is kept within 420 kV peak phase to ground which is 1.3p.u. This is 0 acceptable with stand voltage for the cable
-200 provided that a suitable after laying testing voltage is chosen. Note that length -400 of cable may determine some difficulties for the AC testing voltage to be performed -600 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 in one length,i.e. 20 km. td-mny -tarb-case2_ener_18.pl4: v :MNPT4A td-mny -tarb-case2_1_ener_18.pl4: v :MNPT4B Fig.21. TD in Case2.Phase to Ground Voltage in Moneypoint
A contingency on OLDStreet-Woodland make the system weaker and the amplitude to rise at 1600 Ohm, see Frequency scan fig.18. As a consequence resonances are triggered with a sustaine TOVs of about 543 kV peak, about 1.7Uo. This would put the cable at risk unless using a 500 kV AC cable. 500 *103 Furthermore, insulation coordination 280 review of nearby equipments has to be undertaken as the overvoltage spreads still 60 dangerous as far as Dunstown and Oldstreet, see fig.22. -160
-380
-600 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 (f ile td-mny -tarb-case2_1_ener_18.pl4; x-v ar t) v :OLDS4B v :DNST4B Fig.22. Case2. Phase to Ground Voltage in Dunstown and Oldstreet
6. Conclusions
A methodology has been described aiming at identifying areas that are potentially critical for low frequency Electromagentic transient Overvoltages, TOVs. The methodology has been applied within the process a Long Term Planning Analysis to techno/economic solutions of Grid Expansion within years 2015 to 2030. An extensive Harmonic analysis have been carried on to single out potential system assets at risk for low frequency resonances.
Results have shown that, the broad initial frequency scan indications may be suitable to focus on specific planned reinforcement technologies to identify feasibility of different Transmission technology options. The additional information using Time domain simultations to identify critical grid assets can further evaluate whether the system can be put at risk using different Transmission Technology options and therfore to integrate the decision making process of the planned reinforcement. It has also been verified the counteracting influence on NSF of grid reinforcements which depends not only on the Transmission Technology but also on the Grid topology. Infact, whether it is meshed or radial plays and important role in the changing of NSF for each area as well as whether ist is OHL or UGC based. As general trends for the specific GES, it looks that early stages assets are more likely to provide critical scenarios to be investigated.
7. References [1]:Grid25.http://www.eirgrid.com/media/Grid25%20Newsletter%20Dec%202012%20Issue %2010.pdf [2]:Offshore_Grid_Study.http://www.eirgrid.com/media/EirGrid%20Offshore%20Grid%20St udy.pdf [3]: ATP-EMTP:BPA Adminisration, Portland, 1994. [4] http://www.ipsa-power.com