REE 6052

L.Ngoyi

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How Wind Farms Affect The Grid Performance

Originally by Eng. Fatma NadaStudies Consultant – EETC

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Large wind farms are typically located where good wind resources exist, and are often far away from the main load centres and strong AC network connections.

Large wind farms can result in major changes to the load flow within the network, causing real and reactive power flows that were not experienced before.

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Wind power stations have some features that make their operational behaviour different from that of conventional power stations, which since many years have been the main source of electrical energy.

In line with such a spectacular increase of wind power, there are several questions, regarding the Integration of large scale wind power into AC power systems that must be clarified.

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This presentation will focus on the important issues related to system operation, stability, and protection when large scale wind power plants are integrated into power systems.

1. The requirements for wind power plants for transmission networks will be described.

2. Challenges and grid code’s technical requirements in worldwide courtiers having considerable share of wind power.

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These cover fault ride through, voltage and frequency operating range, reactive power range/voltage control, wind power forecasts requirements, as well as remote operation requirements.

From that point, the need to have different control and protection philosophy in big electrical networks is emphasized through using Real-Time Wide Area Monitoring, Protection and Control systems (RT WAMPAC).

Within the ongoing rise in electricity demand, wind energy will play a major role in the race to satisfy the demand.

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The world wide trend is to utilize the renewable energy resources since the expected life time for fossil fuel is about 30 -> 50 years

Large wind farms can result in major changes to the load flow within the network, causing real and reactive power flows that were not experienced before.

In line with such a large increase of wind power, there are several questions, regarding the Integration of large

scale wind power into AC power systems that must be clarified.

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How should wind farms be expected to behave and perform?

Is it realistic to expect wind turbines and wind farms to behave as any other conventional power plant?

What is the behaviour during grid faults/Fault ride through?

which requirements should be imposed in order to expect wind farms to support the system?

What about reactive power compensation and voltage control, voltage and frequency operating range, remote data transfer and forecast issues?

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In order to accommodate and safely operate a high level of variable and uncontrollable wind power generation on a power system, many challenges on both wind power transmission technologies and transmission grid arise. They include:

1. Wind power forecasts requirements.2.Fault ride through (FRT) requirements;3.System frequency and frequency response requirements;4.Transmission system voltage and reactive powercapability requirements;5.Remote operation requirements;

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A forecast of wind generation is an additional input to the pre-dispatch demand forecasting processes. Grid Codes specify that controllable wind farms should provide their wind power output forecasts at least once a day for the following 48 hours for, as an example, each 30-minute interval. A forecast update must also be available in National Control Center (NCC). The accuracy of the wind power forecasts depends of a number of factors, the most important being the wind speed forecast.

Wind power forecast requirementsWind power forecast requirements

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Large Scale Wind Generation Energy Forecast

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1. REQUIREMENTS FOR WIND POWER PLANTS IN TRANSMISSION NETWORKS

There are several issues should be taken into consideration in grid integration of wind turbines, guaranteeing a stable, profitable and highly qualitative supply of wind energy : 1.Wind turbines have to be able to remain in operation without reducing performance and without time limits even with considerable voltage and frequency fluctuations.

 

 

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2. If voltage dips occur due to grid problems, wind turbines have to remain connected to the grid for a defined period of time.

 3. Short-circuit current power feed-in may be requested

during a grid fault. Depending on the grid, the turbine has to be able to feed in primarily active or reactive power to the grid.

4. Abrupt grid frequency changes should not cause the wind turbine to shut down.

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5. During a failure and while a grid fault is being cleared, reactive power absorption by wind generators is restricted or not permissible at all.

6. After a fault has been remedied, a wind farm should resume power feed as quickly as possible within a specified maximum time range.

7. Wind farms should be able to operate with reduced power output with no time restrictions.

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8. For coordinated load distribution in the ride, the increase in power output (power gradient), for example when the wind farm is starting, should be able to be restricted in accordance with the grid operator's specifications.

9. Wind farms have to be able to contribute reserve energy within the grid. If grid frequency increases, the power output of a wind farm should be reduced.

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10. If necessary, wind farms should be able to contribute to maintaining voltage stability in the grid by supplying or absorbing reactive power with no time restrictions. Dynamic criteria to maintain grid stability must be met.

11. Wind farms must be able to be integrated into the grid control system for remote monitoring and control of all components in the grid

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2. CHALLENGES FOR GRID CONNECTION OF LARGE WIND FARMS

In order to accommodate and safely operate a high level of variable and uncontrollable wind power generation on power system, many challengeson both wind power transmission technologiesand transmission gird operation arise. These include:

1. Fault ride through requirements;2. System frequency and frequency response

requirements;3. Transmission system voltage and reactive power

capability requirements;4. Wind power forecasts requirements.5. Remote operation requirements.

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The large increase in the installed wind capacity in transmission systems necessitates that wind generation remains in operation in the event of network disturbances.

For this reason, grid codes issued during the last years invariably demand that wind farms (especially those connected to HV grids) must withstand voltage dips to a certain percentage of the nominal voltage (down to 0% in some cases) and for a specified duration.

Such requirements are known as Fault Ride Through (FRT) or Low Voltage Ride Through (LVRT) and they are described by a voltage vs. time characteristic, denoting the minimum required immunity of the wind power station.

2.1 Fault (low voltage) ride through (FRT) requirements.

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All LBRT requirements cited in the different grid codes. The requirements depend on specific characteristics of each power system and the protection employed , so they deviate significantly from each other.

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An example from Ireland represent the response of one grid-connected double feed induction generator (DFIG)-wind power station (WPS) during a 3-phase fault on an adjacent 110 kV line resulting in a 30% voltage dip at the CCP (note that the WPS still connected with the grid). This particular WPS has separately installed capacitor banks that support voltage at common coupling point (CCP) and enhance WPS FRT capability. This WPS also stayed connected for 70% voltage dip at CCP. For a high wind penetration scenario, if the wind power plant is removed from the generation pool during the slightest fault event, it may induce the cascading effect commonly associated with a pre-blackout event.

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2.2 Voltage and frequency operating range

Wind farms must be capable of operating continuously within the voltage and frequency variation limits encountered in normal operation of the system.

Further, they should remain in operation in case of voltage and frequency excursions outside the normal operation limits, for a limited time and in some cases at reduced output power capability.

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The frequency responses of some of transmission /distribution system operator (TSO/DSO) connected WPSs during the tripping of one of the large units on the system.

All TSO connected WPSs stayed connected during the frequency excursion; while some of the DSO connected WPSs were tripped.

Recorded under-frequency responses of some WPS

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2.3 Reactive Power Range / Voltage Control

A common requirement is that the wind farm shall be able to operate with a power factor anywhere between defined leading and lagging power factors at the grid connection point.

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The decision of the working point that is generally commanded by the grid operator.

Reactive power requirements in the UK grid code specified by National GridPoint A is equivalent (in Mvar) to: 0.95 leading Power Factor at Rated MW outputPoint B is equivalent (in Mvar) to: 0.95 lagging Power Factor at Rated MW outputPoint C is equivalent (in Mvar) to: -5% of Rated MW outputPoint D is equivalent (in Mvar) to: +5% of Rated MW outputPoint C is equivalent (in Mvar) to: -12% of Rated MW output

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2.4 Wind power forecasts requirements.

A forecast of wind generation is an additional input to the pre-dispatch demand forecasting processes.

Grid Codes specify that controllable wind farms should provide their wind power output forecasts at least once a day for the following 48 hours for, as an example, each 30-minute interval. A forecast update must also be available in National Control Center (NCC).

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2.5 Remote operation requirements.

These requirements include the feasibility to exchange signals between WPS and TSO. Among the signals that WPS should make available to the TSO’s remote terminal units (RTU) are:

1. Grid connected transformer (GCT) tap positions;2. Voltage at the GCT low voltage terminals;3. Active and reactive power output at the LV side of the

GCT;4. Voltage regulation system set-point (in kV);5. On/Off status indications for reactive power devices;6. MV Circuit-breaker position indications.

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Remote operation requirements.

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3. THE NEED FOR WAMPAC Wide Area Monitoring, Protection and Control SCADA-systems provide only a steady state slow picture

in much longer time intervals.

However, Synchrophasor technology in power systems has opened up new possibilities for better real-time monitoring and control of system wide area events.

These measurements can be used as system snapshots

and therefore, show the dynamics of the power system.

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• Synchrophasor is considered to be the main core of Real Time Wide Area Monitoring, Protection and Control (RT WAMPAC)

• The Synchrophasors system is designed to operate in parallel with the existing SCADA/EMS system and maximizes the mutual benefits in power system RT WAM PAC.

• The new system of RT WAM PAC fills the coordination and speed gaps between the very fast dynamic local protection systems and the slow static SCADA/EMS systems.

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The new system of RT WAM PAC fills the coordination and speed gaps between the very fast dynamic local protection systems and the slow static SCADA/EMS systems .

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Example

Remote wind generation is transmitting power over a simplified cross-country two-terminal line.

Each end is monitored by a phasor measurement ( PM) unit and sending time-synchronized measurement data to the master station in order to visualize the relative phase angle and frequency variations between the source and load

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CONCLUSIONS

WP differs from the traditional power generation sources because of its intermittent varying nature. WP imposes many technical requirements and challenges of different aspects.

The fast growing penetration of the WP within the existing grids has to be faced by adding new equipments and technologies to adapt the grid capabilities

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CONCLUSIONS (cont.)

New Grid Code requirements are necessary. New philosophy and tools for the protection and control strategy of the grids utilizing phasor measurement ( PM )has to be developed and applied

The relatively fast performance of the closed control loops using synchrophasor data via fast and robust communication permit facing the effect of the intermittent and variable nature of the WPS. This can be achieved by fast-acting equipment such as SVC, HVDC, FACTS, PSS.

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