- freely available
Inventions 2017, 2(3), 23; doi:10.3390/inventions2030023
- the operating characteristics of sources and their generated power;
- the characteristics of the electrical grid where these sources are connected;
- the operating characteristics of the classical fuelled plants.
- analysing the possibilities, variants, conditions for system integration, in the location area—a local analysis involving a delimitation of the location area and verifying the operating conditions in steady-state regime, level of short-circuit currents etc.;
- analysing the operating conditions of the network after connecting the new renewable sources—local area analysis involving the study of network area where the new sources are connected;
- assuring the adequacy in case of high bandwidth variation in large limits of the sources’ generated power, from zero to maximum power, for evaluating the operating conditions during one year, by analysing the momentary generated powers with respect to forecasted values on short time intervals.
- the requirement for infrastructure reinforcements;
- difficulties in balancing the production/consumption, requiring to shut down some priority sources (like hydro power plants) and the cross-border export that is not practically possible;
- the necessity to disconnect the RESs on the occurrence of outages in the Romanian power system, until the implementation of reinforcements;
- informing the producers on the current necessity to procure the fast tertiary reserve, used for balancing the sudden variation of powers generated by wind power plants and photovoltaic installations.
- In 2010, WPP in operation reached a capacity of approximately 370 MW without causing problems in the electrical distribution/transmission network;
- In 2011, WPP in operation reached a capacity of approximately 826 MW; at the end of 2011 WPP with connection agreements reached a capacity of approximately 8534 MW (power approximately equal to the power consumption at peak load); WPP with connection technical approval  reached a capacity of 9032 MW. The installed capacity of the photovoltaic installations was 1 MW in 2011. Transmission and distribution network reinforcements were proposed, with the obvious disadvantage that these reinforcements were not considered within the operator’s development plans and the achievement of these reinforcements will be possible in a quite long period;
- In 2012, WPP in operation reached a power of approximately 1822 MW; at the end of 2012, WPP with connection agreements reached a capacity of approximately 14,573 MW and connection technical approval reached a capacity of 7850 MW. Photovoltaic installations received connection agreements and connection technical approval (ATRs), totalling 1000 MW of installed capacity. The important increase of renewable sources connection was determined by the governmental incentives, as green certificates, granted for each 1 MWh generation from renewable sources (3 green certificates for wind, and 6 green certificates for photovoltaic). The most strongly influenced by the interconnection of these power sources was the transmission system operator (TSO), because, without any reinforcement of the network, must accept approximately 3000 MW of WPP.
- In 2013, WPP in operation reached a capacity of approximately 2782 MW; at the end of 2013, WPP with connection agreements totalling approximately 14,604 MW and ATRs totalling 4000 MW. Photovoltaic installations received connection agreements and ATRs totalling the installed capacity of 6000 MW, and by the end of the year 1000 MW were put in operation. The important increase of PV was determined by the governmental incentives, as green certificates, granted for each 1 MWh generated from renewable sources (6 green certificates for photovoltaic). A significant share of installed wind power plants was concentrated in Dobrogea, south-east of Romania, and connected to the transmission and distribution networks. The 100 kV lines from districts Constanta and Tulcea (the most favourable for WPP due to high wind speeds) faced important challenges, the connection decision imposing to the local distribution system operator the construction of over 11 new lines of 110 kV and/or the reconductoring of the existing lines. The end of 2013 brought changes in the legislation concerning the connection of sources to the public grid, determining the owners of these RESs to participate in reinforcing the grid. On long term, the changes in legislation can discourage investors, as shown by the trend in Figure 1;
- In 2014, WPP in operation had a slight growth and the number of sources that obtained connection agreements and connection technical approvals decreased. In this year, PVs of small capacity located in areas not experiencing the problem of reinforcement requirements were introduced. In addition, at the end of the year a governmental regulation decreasing the number of green certificates and rolling the incentives until 2018 was adopted.
- In 2015, no RESs were put in operation.
- The impact of congestions occurring in the power system on the voltage level in the most relevant buses of the Romanian power system connected with the analysed network area;
- The impact of congestions in the transmission system occurring during periods of high power injected by the renewable energy sources;
- The dynamic studies of wind generators after short circuits occurring in the power system.
- Influence of 70% of WPP installed power on the voltage level in the connection bus and vicinity buses (that means 420 (2 × 210) MW from the 600 MW installed in the WPP analysed);
- The impact of disconnecting a 400 kV line within the analysed area on the adjacent lines power flows, an impact which may lead to new power flows limiting the outgoing power from the WPP within the network;
- Dynamic studies analysing the voltage and frequency variations following the disconnection of the analysed WPP (210 MW);
- the dynamic studies of wind generators due to after short circuits occurring in the power system in the analysed area.
2. Transmission Expansion Planning (TEP)
- —hydro energy generation units;
- —thermal energy production units;
- —wind energy production units;
- —transmission lines;
- —annualised cost for investing in new transmission lines l [Euro];
- —binary variable that is 1 if a possible line l is built, and 0 if a possible line l is not built;
- —time period [hours];
- —cost of production of thermal unit [€/MWh];
- —cost of production of hydro unit [€/MWh];
- —cost of production of wind unit [€/MWh];
- —power produced by hydro unit [MW];
- —power produced by hydro unit [MW];
- —power produced by hydro unit [MW];
- —load shedding cost of demand [€/MWh];
- —load shedding of demand [MW].
- —annualised budget for investing in construction of new transmission lines [Euro];
- —transmission lines possible to be build.
- —power flowing through transmission line [MW];
- —consumption of demand [MW].
- —susceptance of transmission line l [p.u.]
- —receiving node
- —sending node
- —voltage angle at bus b [rad].
- —maximum capacity for transmission line l [MW]
- —generation capacity of thermal unit [MW].
- —generation capacity of hydro unit [MW].
- —generation capacity of wind farm [MW].
3. Materials and Methods
- establishing the network capacities required for the power take in;
- balancing production/consumption, under the specific conditions of the national power plants (nuclear power plant, combined heat and power plant, hydro power plants, thermal power plants, gas-fired turbines);
- feasible trades with neighbouring countries (the transfer limit amounts to 2000 MW), and for which contracts should be established;
- implementing the necessary network reinforcements for connecting RES under the conditions of volatility of primary source , uncertainties on requirements for these consolidations and the time non-correlation between investments and necessities;
- falling under the national targets on the production from RES;
- the annual quotas of green certificate acquisitions ;
- establishing the contribution method for the reinforcement investments.
4.1. Voltage Profile Variation and Overloading of Network Elements under Congestions
4.2. Verifying the Transient Stability
- wind generator model with power converter, modelled as synchronous wind power generators (connected to the network through power converters) ;
5.1. Voltage, Grid Load and Frequency Analysis after Disconnection of a 210 MW WPP
- when the WPP is disconnected (at time instant t = 2 s), the voltage has a slight increase and afterwards decreases by approximately 0.02 p.u.;
- after about 3 s from the disconnection of the 210 MW WPP, the reactive power control occurs in other available groups, restoring the voltage to a new value.
5.2. Verifying the Transient Stability Conditions of Generators in Case of Faults
- classical generators are stable (like the unit from thermal power plant connected at bus RPALAST), and the variation of rotor angle is smoothed after fault clearance (as shown in Figure 12);
- doubly-fed induction generators have the rotor angle strongly influenced by a fault in the network, but this influence is not transmitted to the voltage at generator terminals or to the 110 kV bus where the generator is connected (as shown in Figure 13);
- synchronous wind generators with power converters are maintaining their rotor angle constant during the fault, considering the galvanic separation from the network (as shown in Figure 14).
- Voltage levels in case of faults in the power system;
- Overloading of power system elements in case of unavailability of lines and transformers;
- The dynamic behaviour of voltage and frequency when a 210 MW wind farm is disconnected;
- The transient stability conditions of classic and wind power plant generators.
- in case the WPP produces 90% of the installed power, overloading of the transmission network can occur;
- if a higher share of renewable energy sources present within the power system is considered, then network reinforcement actions are required;
- in case of a short-circuit, the synchronous wind generators connected to the network through converters keep their rotor angle constant during the fault, while the doubly-fed induction generator have the rotor angle heavily influenced by a fault occurrence.
Conflicts of Interest
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|Source Type||Installed Power * (MW)||Net Available Power ** (MW)|
|Hydroelectric power plant (HPP)||6731||6368|
|Nuclear power plant (NPP)||1413||1413|
|Combined heat and power plant (CHP)||11,997||10,256|
|Wind power plant (WPP)||2980||2944|
|Photovoltaic plant (PV)||1302||1176|
|Biomass power plant (BPP)||121||99|
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