* 50Hertz, PSE cooperate on border lines
* Southern interconnector to be equipped better
* Northern line to be upgraded during two-year closure
FRANKFURT, April 13 (Reuters) - German and Polish grid operators are putting measures in place to stabilise excess flows of German renewable energy into Poland and to allow more commercial power trading between the two countries.
Germany's 50Hertz and Poland's PSE will use transformers, or phase shifters, to regulate flows on a southern link from May and have agreed to shut a northern link from June until 2018 for upgrades, the companies said on Wednesday.
"Through these measures, the grid operators expect more possibilities for cross-border power trading," they said.
German industry data shows there were virtually no imports of Polish power in 2015 while 10.7 gigawatts (GW) of German power flowed into Poland, but mostly in unplanned bursts rather than planned transactions.
The upshot is that about 1.3 GW of interconnector capacity is not being used fully, a fact that is not just lamented by traders but is also a reason for Polish criticism of how Germany's green transformation affects its neighbours.
The unregulated electricity flows, mostly from wind farms in Germany's sparsely populated north, can overload outdated cross-border transmission lines, making calculated trading impossible while increasing operating costs and the risk of blackouts.
The current situation also runs counter to the European Union goal to harmonise energy markets and prevents wholesale power markets in both countries from interacting properly.
Last summer, for example, Poland's coal-fired power plants produced barely enough power during a summer heat wave, due to a shortage of cooling water, while German solar power facilities produced at full throttle but could not export to Poland.
Poland will now operate four phase shifters on the southern Hagenwerder to Mikulowa link, which will act as a valve to regulate how much power is received and dispatched, in a bid to inspire trust in the commercial viability of the link.
The northern transmission link from Vierraden to Krajnik will be closed for about two years to be upgraded to the high voltage standard of 380 Kilovolt (kV) and to house the first two of four planned phase shifters at the Vierraden substation.
The link's current 220 kV status is a hangover from communist days when that type of line was commonly used.
Both measures are expected to be completed by 2020, in time to integrate a new intra-German line called the Uckermark link which will bring power from the northern German region to the capital Berlin. (Reporting by Vera Eckert; editing by David Clarke)