Meshed LV grids

[piaulous]

A very rare case of a meshed lv grid has been detected (one grid at the moment).
TODO's

1. check origin of this problem
2. if necessary right a function to get rid of the mesh

[joda9]

Measured the scope on a full ding0 export — 3648 MV grids, 352768 LV grids.

159 LV grids are meshed, in 153 MV grids. So it is rare per LV grid (0.045 %)
but hits 4.2 % of MV grids, and a calculation always processes a whole MV grid.

21 of those 159 are not real meshes but a line whose two ends are the same bus — a
separate artifact, filed as #406. The remaining 138 are genuine rings.

An example, LVGrid_8829100000 in mvgd_32064: 41 buses on 41 lines, one triangle of
three cable cabinets.

line	length	type
KVS 3 – KVS 4	374.5 m	NAYY 4x1x50
KVS 3 – KVS 5	152.6 m	NAYY 4x1x50
KVS 4 – KVS 5	222.0 m	NAYY 4x1x50

No switch, no open point, nothing marking it as a ring meant to be operated open.
Worth contrasting with MV, where ding0 is consistent: that grid's MV level has 120
buses on 119 lines and no cycle at all, because the rings are modelled open via the
circuit breakers.

It does cause real breakage downstream. eDisGo's pseudo-coordinate layout assumes
a tree; all 138 genuine rings made it abort with
NetworkXError: The node ... is not in the graph, taking the whole spatial complexity
reduction with them — 132 MV grids, roughly one in 28. Fixed on the eDisGo side in
openego/eDisGo#765, but the grids still carry the rings.

[joda9]

These are the 138 genuine rings — the 21 self-loop lines of #406 are excluded, so this is the set that actually makes a tree-based traversal fail.

138 meshed LV grids in 132 MV grids

Genuine meshed LV grids in the ding0 export
138 LV grids in 132 MV grids
(self-loop artifacts of openego/ding0#406 excluded)

mv_grid   lv_grid           buses  lines  rings
30945     2186900000           43     43      1
30946     6770000000           52     52      1
30961     3272500009          132    132      1
30990     2127040000           71     71      1
30991     9988600032           74     74      1
31050     1560650004          214    215      2
31099     6839000004           56     56      1
31170     1458110001          175    175      1
31267     6245000099          212    212      1
31291     6281700067           42     45      4
31326     1193300000          175    175      1
31344     5963500002           69     69      1
31345     5978100011          144    144      1
31412     2096950006          193    193      1
31466     8935000000           27     27      1
31470     6468900000           55     55      1
31524     1695270001          329    329      1
31575     1902110000          316    316      1
31576     1987680005           58     58      1
31577     1510200000          200    200      1
31585     7718000000          146    147      2
31590     2359100001           21     21      1
31590     2359400000           35     36      2
31592     1532300000           56     56      1
31592     1612700000          114    115      2
31593     1650900002           47     47      1
31655     6284400000           98     98      1
31790     1000850045          114    114      1
31791     5691300014          141    141      1
31792     4556700013           63     63      1
31809     6967900000           27     27      1
31864     3897000000           17     17      1
31867     3289000000           18     18      1
31868     1289100001          101    101      1
31943     1325040001          222    222      1
31972     4272300000           31     31      1
32004     2110800007          373    373      1
32036     4324400000          106    106      1
32064     8829100000           41     41      1
32238     3231300000           51     52      2
32247     8216000000          107    107      1
32261     8021900002          151    151      1
32288     1259760000           28     28      1
32362     6948400093          162    162      1
32396     1173560000           43     43      1
32452     1293200001          193    193      1
32453     1697900000           48     48      1
32501     9164700000           19     19      1
32508     4185300000          177    177      1
32542     1917460078           38     38      1
32545     1169950000           50     50      1
32591     1949160004           59     59      1
32636     1635330000           83     83      1
32744     2658100000          279    279      1
32744     2946400054           33     33      1
32746     7577000041          181    181      1
32794     1869040002           29     29      1
32806     1542700002          220    220      1
32862     1132080001           76     77      2
32975     9628200000          106    106      1
33003     1333200001           25     25      1
33004     1187750019           49     49      1
33065     1990730000          301    301      1
33100     8029000002          203    203      1
33110     1859160001           93     93      1
33117     8731600001           82     82      1
33130     2001550004          120    120      1
33132     1806240000           58     58      1
33134     1730840000           49     49      1
33238     1204860001           38     38      1
33252     8749500000           69     69      1
33344     5171100000           45     45      1
33370     6507000027           20     20      1
33416     3395500093           73     73      1
33431     5401200002           57     57      1
33431     5583700010          104    104      1
33434     5525000000           44     44      1
33448     1524710000          107    107      1
33467     3557400001           45     45      1
33470     8359500001          145    145      1
33478     7301000000           41     41      1
33493     1026750000           66     66      1
33533     2174020000           43     43      1
33583     1094820000           31     31      1
33584     2077870000           60     60      1
33601     1191000000           23     23      1
33649     2027940000           69     69      1
33661     1128320001           96     96      1
33732     1426420000           46     46      1
33736     3784500024           52     52      1
33787     1948000019           52     52      1
33802     2142130000          372    372      1
33912     6640700009           54     55      2
33912     6649400000          141    141      1
33913     7219800000           32     32      1
33920     1862300000          130    130      1
33938     5975800000          159    159      1
33941     1241040000          127    127      1
34042     5900100002           83     83      1
34047     5278400002          106    106      1
34047     5278400006           47     47      1
34058     2109350000           27     27      1
34083     2700700002          202    202      1
34136     4946400002          411    411      1
34137     4646400000           53     53      1
34200     1034260015           50     50      1
34266     1186300000          202    202      1
34315     2051140001          169    169      1
34317     1181080000          112    112      1
34337     1939610000          155    155      1
34381     1858280009           72     72      1
34402     1760600000           61     62      2
34426     3821800000           74     74      1
34471     3571500000          194    194      1
34524     4089600000          112    112      1
34556     7293800001           77     77      1
34632     6846200000           14     14      1
34644     7832800005          205    205      1
34735     3178400050           89     89      1
34786     1879650000          126    126      1
34812     1015490010           97     97      1
34873     9339200000           40     40      1
34917     3502200005           32     32      1
35133     3561000033           18     18      1
35134     7171700000          107    107      1
35152     3040700013           38     38      1
35157     1367800000          101    101      1
35165     2889400000          240    240      1
35168     1947900002          142    142      1
35208     1332620012           87     87      1
35659     1076000006           59     59      1
35670     8315000000          168    168      1
35733     3087400000          101    101      1
35801     1777170003          213    214      2
36067     2079760000          171    171      1
36121     2184490002          667    667      1
36137     4639600000          211    211      1
36165     1890860000           82     82      1