Dealing with the 50.2 Hz problem

By Michael Döring, Ecofys, Berlin, Germany

On 26 July a new ordinance entered into force in Germany requiring existing photovoltaic plants to be retrofitted within three years to address the "50.2 Hz problem."
The problem arises because under requirements introduced in 2005/2006 generating plants connected to the low voltage network, including PV, were required to switch off immediately if system frequency increased to 50.2 Hz.
At that time PV played a negligible role in the German energy mix. But now it is a force to be reckoned with, with PV installed capacity at the time of writing being about 31 GW and rising, of which around 70% is connected to the low voltage network.
Steps towards a solution
As a first step, a transitional arrangement was introduced, in April 2011, to address the issue. Under this arrangement all new PV plants could implement revised frequency settings on a voluntary basis.
In January 2012 an obligatory measure mandating new frequency settings was introduced. Both measures applied to new PV plants.
The new ordinance of July 2012 extends the scope of regulations to existing PV installations and requires their retrofitting.
In a worst case scenario, up to about 14 GW of German PV capacity could be disconnected from the network if system frequency increased to 50.2 Hz.
Reaching a frequency of 50.2 Hz under current conditions is unusual but can happen, for example in situations where the electricity production is higher than the electricity demand due to grid disturbance. In such a situation, the frequency can increase markedly due to oversupply of electrical power.
Examples of this include the European power grid failure of 2006 and the blackout in Italy in 2003. In both cases, in Germany the frequency increased to 50.2 Hz.
The European grid is designed to cope with a maximum sudden loss of 3000 MW of generating capacity. If disturbances similar to those in 2003 and 2006 were to occur on sunny days with much of the current German PV capacity in operation then the potential loss of capacity would greatly exceed 3000 MW (by a factor of more than four) - leading to a high probability of large scale grid failure.
The sequence of initiatives taken to deal with the problem can be summarised as follows:
- Short-term measures for new plants: interim rules for new PV systems (from April 2011 until VDE-AR-N 4105, a new standard developed by VDE (German Association for Electrical, Electronic & Information Technologies), entered into force;
- Longer-term measures for new plants: must meet requirements for new generation facilities set out in VDE-AR-N 4105 (entering into force as from 1 January 2012);
- Measures for existing PV systems: requiring them to be retrofitted between 2012 and 2014.
The short- and longer-term measures for new plants in the low voltage network are set out under rules established by the Forum for network technology/network operation of the VDE (VDE|FNN).
The measures requiring upgrade of existing facilities already in operation are set out in the new ordinance of 26 July 2012.
Joint initiative
The four German transmission system operators (TSO), the distribution network operators, the German Solar Industry Association (BSW-Solar), including representatives of various PV inverter manufacturers, and the VDE|FNN have been working together towards the development of a feasible solution for existing facilities since 2010 under a joint initiative.
This initiative is framed within the system security working group of the 'platform for future networks' by the Federal Ministry for the Environment (BMU), the Federal Ministry of Economics and Technology (BMWi) and the Federal Network Agency (Bundesnetzagentur). The European Network of Transmission System Operators for Electricity (ENTSO-E) also prioritised the issue because of its importance in guaranteeing stable operation of the continental Europe network region. As well as Germany further European countries (eg, Italy) are currently developing retrofit programmes to address the 50.2 Hz issue.
The consulting firm Ecofys and the Institute of Combustion and Power Plant Technology (IFK) at the University of Stuttgart were commissioned by the four German transmission system operators, BSW-Solar and the VDE|FNN, along with BMWi, BMU and the Federal Network Agency, to investigate whether and to what extent a technical upgrade of the PV equipment inventory was necessary and possible. The VDE|FNN also initiated an additional investigation of the impact that a retrofit of existing PV installations would have on distribution network operations, examining in particular the temporary operation of sub-networks with so-called emergency standby power systems.
The study confirmed that it was indeed necessary to retrofit a significant proportion of the existing PV plants that switch off when system frequency increases to 50.2 Hz. A total of about 400 000 PV plants have to be retrofitted.
The analysis also showed that other distributed generation systems, eg, wind power plants, may have to address the issue and that other countries, notably, Denmark and Italy, and maybe also Belgium and France, may have to take comparable measures in the future.
Beside the over frequency setting of 50.2 Hz it was shown that the under frequency setting of 49.5 Hz of other distributed generation systems represents a risk to the stability of power system, as well.
Consequently a further study was commissioned in 2012 to analyse the "49.5 Hz problem". First results can be expected early in 2013.
Retrofit options
As a result of the 2011 study it was recommended that all PV systems be retrofitted that began operating after 1 September 2005 and which are larger than 10 kWp. These minimum requirements were suggested so as to achieve a cost effective retrofit programme. It was also recommended that, in view of the different retrofitting solutions available, the concerned PV power plant operators should have the liberty to choose among the following options:
I. Update to VDE|FNN application guide VDE-AR-N 4105;
II. Update to BDEW technical guideline for generating plants connected to the medium-voltage network; or
III. Change parameters according to VDE|FNN instructions (of April 2011) for temporary measures.
The following order of priority was recommended: Option I comes before Option II, and this precedes Option III. Replacement of the inverter should be avoided in all cases.
It was also recommended that, in compliance with the order of priority for the inverter retrofit, inverter manufacturers should be required to provide recommendations for their products, and the electrical installers should be required to comply with these recommendations when retrofitting PV systems.
As of the end of 2012 the distribution system operators have started to contact the affected unit owners and begun to execute the practical retrofitting measures. In most cases these consist of updating the software or changing the frequency settings of the inverter itself depending on the type of inverter. Major updates of the hardware or an exchange of the inverter are not intended. If appropriate, the frequency settings of the external frequency protection at grid junction points will be adjusted as well. Due to system security considerations, only competent installers or the inverter companies themselves are allowed to carry out the retrofits.
The basic procedure to be adopted in the retrofit programme is defined by the July 2012 ordinance. In general the distribution network system operators are responsible for the execution of the programme. They contact the unit owner and contract the electrical installers to retrofit the PV systems. The unit owner has the obligation to co-operate and provide information about the inverter, eg serial number, otherwise he can lose his feed-in tariff. The unit owner doesn't bear the cost, they are recovered via network charges and the Renewable Energy Act (EEG) levy.
Under the July 2012 ordinance all units must be retrofitted by 31 December 2014.
Summary of the impact
- Approximately 400 000 PV systems (> 10 kWp) must be retrofitted.
- The effort required to implement this limits the number of retrofits to about 8500 to 11000 per month.
- In the 2011 study, the total cost for the retrofitting of the PV systems is estimated at 175 million euro, plus associated administrative costs for inverter manufacturers and distribution network operators.

CAPTION

PV installed capacity in Germany, up to 30 June 2012
(Source: DGS e.V. (energymap.info) and German Federal Network Agency)