1. RES-E Impact on Transmission Grid and Power System Reserves
TRANSELECTRICA - DEN
RES-E Impact on Transmission Grid and Power System Reserves
Florin Balasiu
Director of Operational Planning Division
Update Romanian Renewable Market
02 apr 2012

2. 1. Benefits and Challenges to Integrate RES
Content
1. Benefits and Challenges to Integrate RES
2. Technical Topics Regarding RES Integration
Connection schemes. Network development
Power system stability topics
Shortcircuit withstand
Protective schemes
Power quality
3. Conclusions

3. 1. Benefits and Challenges to Integrate RES-E
Developing and integration of RES (wind and photovoltaic) contribute to:
Actions to mitigate climate changes
Secure energy requirements
Increase competitiveness and wealth
New challenges arise from:
Variable nature of the RES
Distributed nature
Different electrical technologies
Larger forecast errors

4. Fluctuating Nature of RES
Peak MW
Increase 340 MW; 2 h
Decrease 390 MW; 1 h

5. Distributed Nature of RES

6. 2. Technical Topics on RES Integration
Connection scheme depends on: region, installed capacity, existing lines, active power generation-load balance of the region
Power System Stability – rotor angle, frequency and voltage stability
Frequency stability – balance between total active power generation and load at Romanian power grid level, including load, export and storage
Voltage stability – balance between reactive power generation and absorption capability
Shortcircuit withstand – symmetrical and asymmetrical (SLG) faults
Protective schemes of the generating units and of the network
Power quality – voltage drops, swings, flicker, operational performance indexes
Total investment costs 4

7. Power System Stability

8. Transmission lines reinforcement vs new lines
Network development
S/s modernization;
Transmission lines reinforcement vs new lines
increase transmission capacity by use of high temperature conductors
optimal power flow management
New lines: 400 kV Cernavoda-Gura Ialomitei-Stalpu
400 kV Suceava-Gadalin
400 kV Gutinas-Smardan
400 kV Portile de Fier-Resita-(Pancevo)-Timisoara-Arad
Typical 110 kV connection diagrams 4

9. Romanian Transmission Network – new 400 kV lines

10. Typical 110 kV Connection Layouts for Renewable Medium Size (1)
Visible for system operators
Operational security => N-1 fulfilled
Reliable protective system:
Dependability
Security
Critical clearing time

11. Typical 110 kV Connection Layouts for Renewable Medium Size (2)
Visible for system operators
Operational security => N-1 not fulfilled
Islanding issues
The protective system:
Difficulties to avoid unwanted trips or delayed ones
Difficulties to provide remote back-up
thus not recommended

12. Power System Stability Topics
Frequency control – balance between load and generation
Aim – to maintain the balance between load and generation within a synchronous area
Based on three control actions:
Primary frequency control is a local control to maintain load-generation balance and stabilise frequency after large disturbances
Secondary frequency control – centralised automatic generation control (AGC) to bring back the frequency to its reference value. It restores the interchanges with surrounding power systems
Tertiary frequency control – to restore primary and secondary reserves
Secure system operation is only possible by close cooperation between owners of Power Generating Facilities and the Network Operators. In particular, the system behavior in disturbed operating conditions depends upon the response of Power Generating Facilities to deviations from nominal values of voltage and frequency.

13. Effects – frequency range of operation

14. Effects – active power frequency response
P = the change in MW output from the generator
f = the frequency deviation in the network
Pmax = the max capacity to which P is related
Active Power frequency response of Generating Units in Frequency Sensitive Mode

15. Effects – tertiary reserves
Tertiary frequency control is performed manually in order to restore primary and secondary
reserves.
Tertiary control may be achieved by:
connection of generating units – fast startup, enough generating units
changes in the dispatching of units
changes in the interchanges program
load control – energy storage
Actual limit for RES in the grid – about 3000 MW installed capacity

16. Power System Stability Topics
Voltage stability – reactive power control
Voltage stability refers to the ability of the power system to maintain voltages at all busbars
within the operational ranges during normal operation as well as after being subjected to
disturbances in the network.
Sufficient reactive power support is the most important part for voltage control and voltage stability in a transmission and distribution network. As reactive power can not be transported over long distances, the reactive power has to be supplied where required.
Consequently, increasing amount of wind generation reduces reactive power reserves in the
transmission system, which is defined by available reactive power of the synchronous
generators, SVCs and shunt capacitors minus reactive power consumption in the network
including reactive power of the switchable inductors.

17. Effects – voltage range of operation

18. Effects – fault ride through (FRT)
Ability of non-conventional generators to stay connected in the case of network faults
It is of particular importance to transmission system operators, that wind farms and photo-voltaic generators stay connected, in case of faults at transmission or distribution levels that lead to a voltage dips in a wide area.
It is mandatory for RES to be equipped with FRT-capability
FRT profile of a RES connected at 110 kV or above voltage levels. Boundaries for a voltage-against-time profile at the Connection Point

19. Effects – the P-Q Capability Diagram
P-Q/Pmax-profile of a Power Park Module
When operating at an Active Power output below the Maximum Capacity (P<Pmax), each Power Park Module shall be able to be operated in every possible operating point inside the outer envelope

20. Power System Stability Topics
Transient stability – critical fault clearing time
The main aspects having a possible impact on transient stability issues are:
RES are usually connected at different locations than conventional power stations. Hence, power flows are considerable different in the presence of a high amount of renewable power and actual power systems are typically not optimized for renewable power transport. This aspect is relevant for Dobrogea area
Renewable generating units are usually based on different technologies than conventional synchronous generators and new tools for modeling and calculations are needed
Based on these differences two phenomena can be distinguished, which can be affected by RES generation:
Global effects which can result in loss of synchronism of generators:
Transient stability (large-disturbance effect)
Local effects
Trip of RES generators after subjected to a disturbance (w/o FLTR)

21. Transient stability – critical fault clearing time
Transient stability = the ability of the power system to maintain synchronism during and after severe disturbances, for example short circuits or generator trips
The behaviour of the system is highly dependent on the type and duration of disturbance, thus to ensure transient stability in a system often a number of critical contingencies have to be simulated at different locations
For transient stability, the Critical Clearing Time is of utmost importance and represents an useful measure for characterizing the transient stability performance of a given dispatch scenario
To counteract against transient stability problems:
Fast protection tripping time + fast CB opening time => teleprotection schemes
Fast bus-bar faults clearing time => BBP and BFP

22. 3. Conclusions
Large scale renewable power integration in the Romanian Power System is possible by taking plenty technical measures
The installed RES (wind and photovoltaic) integration capability depends on transmission and distribution lines and S/s development as well as balancing capabilities and available reserves
Secure system operation is only possible by close cooperation among owners of Power Generating Facilities, Network Operators and ANRE

23. Update Romanian Renewable Market
Thank You !
Florin Balasiu
Update Romanian Renewable Market
02 apr 2012