1. WG A2 50 Effect of Distributed energy sources and consequentinduced reverse flow on transmission and distribution transformers First Meeting Zürich 11 September 2013

2. A2-50 Members BIRENDRA (BOB) SINGHUtility BOGUSLAW (DR. ) BOCHENSKI3rd Party GABOR GURSZKYManufacturer HIDEYUKI MIYAHARAManufacturer KOJIRO AOKIUtility MOHAMED ZOUITIUtility SAEED KAMALINIAManufacturer YUKIYASU SHIRASAKAManufacturer

3. A2-50 SCOPE The scope of this working group is to address the effect of step up operation on transformers that were not designed for this purpose. The effect on flux density, temperature rise, noise and other performances for both core and shell types designs shall be addressed. Possible tap changer control problems will also be taken into account. Differing transformer locations in final distribution, primary distribution and principal transmission will be examined, as well. The report will address the theoretical consequences for core flux and voltage regulation, risks and limitations to step up operation for each technology and the assessments necessary to allow this mode of network operation.

4. A2-50 Background Originally, transmission and distribution networks were designed for step down operation transmitting energy from the highest voltage to the final user’s voltages. The introduction of more and more renewable or distributed small power generation have created power injection in the lower voltage level which in many cases exceeds the consumption of the local network. This creates an upstream power flow and change the usual step down operation of the transformer into a step up mode in order to evacuate the energy. The design of the transformer and the regulation of the voltages may not be anymore suitable for a safe and long lasting operation. Furthermore, most of those distributed energy generator are base on solid-state inverter, which may produce harmonics on the network, that may also affect the transformer life.

5. A2-50 Table of content Introduction Report of existing problems –Example –Different possible concerns and effects in transformers ( such as harmonics and winding hot spot, overfluxing and core hot spot) linked to reverse power flow –Effect of active and reactive power flow Transformer construction and a few theory –How is flux density fixed Core type Shell type –Stray flux Where does the stray flux go Effect of capacitive load –Voltage drop (and raise) Forecasted load flow evolution on “smarter grid” –Some general consideration on load flow and smart grid

6. A2-50 Table of content Step up step down operation influence on design –Typical configuration for distribution network –double LVs transformers Effect of flux density and stray flux –Temperature rise (core, hot spot) –Noise (due to “overfluxing” due to voltage and current or due to load) –Other performances Tap changer design –Tap changer for both way power flow (asymmetrical pennant cycle ?) –Does it affects the design of the tap changer ? –Differences with conventional tap changer –What about DETC ?

7. A2-50 Table of content Voltage regulation –Tap changer control ( P&C inputs requested) –System view of the transformer control policy U, I both, where LV ? HV? For distribution and transmission –Voltage drop ( effect of high impedance end power factor) –Reactive power control –Reactive power circulation –Tapping range –Regulated voltage (LV with HV taps ?) –Tapping winding position (HV/LV) –DETC ? Effect of harmonics of current and voltage –On no load losses –On load losses Mitigation and usage restrictions –Typical example or real cases

8. A2-50 Table of content Conclusion : advices –Risks on existing fleet –Special requirement for new specification –Recommendation to standardization update ( 76-1 ?) Bibliography