Index: trunk/resources/data/validator/geometry.mapcss
===================================================================
--- trunk/resources/data/validator/geometry.mapcss	(revision 19570)
+++ trunk/resources/data/validator/geometry.mapcss	(revision 19590)
@@ -430,4 +430,11 @@
 }
 
+/* 20421 */
+area[power=substation] ⊇ *[transformer=distribution] {
+  throwWarning: tr("A distribution transformer shouldn't be contained in a substation.");
+  suggestAlternative: "transformer=main";
+  fixAdd: "transformer=main";
+}
+
 /* #21801 */
 way[railway][railway !~ /^(turntable|traverser|platform_edge|roundhouse|workshop|platform)$/] > node[/railway$/ !~ /^(switch|railway_crossing)$/][count(parent_osm_primitives("railway")) > 2]:connection,