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Low Voltage Ride Through with Software Simulation using Digsilent PowerFactory

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Mohan Kumar's picture
Power Systems Engineer Power Projects

𝐷𝑒𝑡𝑎𝑖𝑙𝑒𝑑-𝑜𝑟𝑖𝑒𝑛𝑡𝑒𝑑, 𝑟𝑒𝑠𝑝𝑜𝑛𝑠𝑖𝑏𝑙𝑒 𝑎𝑛𝑑 𝑐𝑜𝑚𝑚𝑖𝑡𝑡𝑒𝑑 𝑒𝑛𝑔𝑖𝑛𝑒𝑒𝑟, 𝑤𝑖𝑡ℎ 𝑎 𝑔𝑒𝑡 𝑖𝑡 𝑑𝑜𝑛𝑒, 𝑜𝑛 𝑡𝑖𝑚𝑒 𝑎𝑛𝑑 ℎ𝑖𝑔ℎ 𝑞𝑢𝑎𝑙𝑖𝑡𝑦 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 𝑠𝑝𝑖𝑟𝑖𝑡 𝑖𝑛 𝑝𝑜𝑤𝑒𝑟 𝑠𝑦𝑠𝑡𝑒𝑚 𝑠𝑡𝑢𝑑𝑖𝑒𝑠 ⚐ Knowledge of various 𝐈𝐄𝐂 𝐚𝐧𝐝 𝐀𝐍𝐒𝐈/𝐈𝐄𝐄𝐄 𝐬𝐭𝐚𝐧𝐝𝐚𝐫𝐝𝐬 for...

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  • Oct 8, 2021 4:21 pm GMT
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Introduction:

  • A drastic increase in penetration of renewables into the existing grid network, there is necessity to maintain grid stability,security and reliability. Grid stability is one of the main aspect needs to be monitor in the power system. Power generarting plants should have control capabilities and protection mechanisms inorder to avoid power outages.
  • In the past, these requirements are fulfilled by the conventional generating stations. In the recent years a steady rise in the grid integration of renewables so there is a new requirement in the grid codes to maintain the stability of the grid. It has led to the requirement of low voltage ride through (LVRT) capability in wind turbines.
  • Grid codes for LVRT capability of wind turbines are proposed in all major wind-energy-producing countries. The testing of LVRT capability compliance of the wind generators as per the specified grid codes during different fault conditions is essential to ensure its reliability.
  • The testing process is generally done by the manufacturers and crucial for certification as per the IEC 61400-21 international standard for measurement of power quality characteristics of wind turbines.

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Definition of LVRT:

LVRT requirements can be described as voltage versus timecharacteristic, which denotes the minimum period required for withstanding a particular voltage sag level, even up to zero voltage by the wind generator. 

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In other words,

LVRT is a short-form for Low Voltage Ride-Through and it describes the requirement that generating plants must continue to operate through short periods of low-grid voltage that does not disconnect from the grid.

Note: Short-term voltage dips may occur, for example, when large loads are connected to the grid or as a result of grid faults like lightning strikes or short-circuits.

LVRT Capability:

LVRT capability, which is a regulation for interconnection of wind turbines with grid, was established for wind turbines to mitigate the adverse impacts of loss of generation due to any disturbance. Loss of generation had not been an issue when the wind power generation was small, but due to significant growth of wind power the sudden loss of generation may lead to the risk of instability in grid. Since LVRT is one of the most demanding and stringent grid codes imposed on renewable power generation, it receives a major focus in the field of renewable energy research

The guidelines for LVRT capability may vary with respect to the TSO guidelines but they can be simplified by grouping them under the categories of voltage sag tolerance, control of active power, control of reactive power, protection and power quality.

Generating plants can support the grid by feeding reactive current into the network and so raise the voltage. Immediately after fault clearance, the active power output must be increased again to the value prior to the occurrence of the fault within a specified period of time. These requirements which at the beginning only applied to wind turbines, now also have to be fulfilled by photo-voltaic systems (PV) and most recently by combined heat and power plants (CHP).

Overview of Indian Electricity Grid Code (IEGC) and Need of Indian Wind Grid Code (IWGC):

  • IEGC provides the technical rules to facilitate the operation, maintenance, development, and planning of electricity grid.
  • IEGC were originally developed considering the synchronous generators generally used in conventional power plants. WTG do not have the same characteristics as synchronous generators and hence modification in the grid code is necessary.
  • IWGC has been developed for the reliable and secure operation for the reliable and secure operation of wind farms and their integration into the electrical network. This grid code can be used in tandem with IEGC/State Grid Code can be amended with the provisions

Central Electricity Authority-Technical Standard for Connectivity to the Grid:

Wind generating stations connected at voltage level of 66 kV and above shall remain connected to the grid when voltage at the interconnection·point on any or all phases dips up to the levels depicted by the thick lines in the following curve: 

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Where, Vr/Vn is the ratio of the actual voltage to the nominal system voltage at the interconnection point 

Provided that during the voltage dip, the individual wind generating units in the generating station shall generate active power in proportion to the retained voltage; Provided further that during the voltage dip, the generating station shall maximise supply of reactive current till the time voltage starts recovering or for 300 ms, which ever time is lower.

LVRT Simulation:

The simulation of voltage dips requires special technology. Most grid codes and guidelines have specific requirements for the test equipment. According to the international standard for the measurement of power quality characteristics of wind turbines (IEC 61400-21) for example, an inductive voltage divider is recommended which is to be connected ahead of the plant to be tested.

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The basis of LVRT validation and certification requires a sag generator. Since, the voltage sag due to fault is artificially simulated or generated through hardware to test the performance of the LVRT solution and the wind turbine andto evaluatethe performance and reliability. The principal design, operation and measurement of the sag generator (LVRT testing tool) is done as specified in IEC 61400-21.

The sag generator generally consists of a voltage divider inserted into the medium voltage feeder and configured such that it does not cause significant short circuit current disturbance for the upstream wind park installation.

For example, an inductive voltage divider is recommended which is to be connected ahead of the plant to be tested. This voltage divider consists of a longitudinal impedance(coil) L1 and a short-circuit impedance L2. The figure above shows a simplified view of the test equipment. The impedance L1 and L2 can consist of several coils each (series and parallel connection). By changing the ratio L1 to L2 the depth of the voltage dip can be configured.

Depending on the respective grid code, different depths of voltage dips have to be simulated, for wind turbines usually dips to 15%, 25%, 50% and 75% of the rated voltage are required. The duration of the dip depends on the depth and ranges from several hundred milliseconds (deep dips) to several seconds(flat dips). In some cases the duration can also extended to several minutes. German and international guidelines demand the simulation of LLL as well as LL faults. In England, guidelines additionally demand LG fault.

The simulation results of LVRT Study of 2.3 MW WTG is shown below for various cases:

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Single Line Diagram of Impedance - based LVRT Testing Equipment

 

Case 1: 15% Voltage dip for 0.3 seconds:

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Created a 3 phase fault with the voltage dip of 0.15 p.u and the fault is continued for 0.3 seconds as per the Indian Grid Code. From the results it is seen that it Capable of supporting reactive power to grid.

 

Case 2: 25% Voltage dip for 0.8 seconds:

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Created a 3 phase fault with the voltage dip of 0.25 p.u and the fault is continued for 0.8 seconds as per the Indian Grid Code. From the results it is seen that it Capable of supporting reactive power to grid.

 

Case 3: 50% Voltage dip for 1.5 seconds:

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Created a 3 phase fault with the voltage dip of 0.5 p.u and the fault is continued for 1.5 seconds as per the Indian Grid Code. From the results it is seen that it Capable of supporting reactive power to grid.

 

 

Case 4: 75% Voltage dip for 2.0 seconds:

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Created a 3 phase fault with the voltage dip of 0.75p.u and the fault is continued for 2.0 seconds as per the Indian Grid Code. From the results it is seen that it Capable of supporting reactive power to grid.

 

References:

  1. Indian Wind Grid Code (IWGC)
  2. Central Electricity Authority, Technical Standards for Connectivity to the Grid, (Amendment), regulations, 2019
  3. IEC 61400 - 21: Measurement and assessment of power quality characteristics of gridconnected wind turbines
  4. Testing of Low Voltage Ride Through Capability Compliance of Wind Turbines- A Review
Mohan Kumar's picture
Thank Mohan for the Post!
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Matt Chester's picture
Matt Chester on Oct 8, 2021

The testing of LVRT capability compliance of the wind generators as per the specified grid codes during different fault conditions is essential to ensure its reliability.

How much do these codes vary from jurisdiction to jurisdiction? 

Mohan Kumar's picture
Mohan Kumar on Oct 9, 2021

For example, In India, windfarm has to ride through the fault for 300 ms when voltage goes 0.15p.u. and when comes to Western Power (Australia), windfarm has to ride through the fault for 450 ms when voltage goes to 0 p.u.

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