System Requirements for Wind Farms and Distributed Generation. Giuseppe Di Marzio

Transcription

1 ystem Requirements for Wind Farms and Distributed Generation Giuseppe Di Marzio 1

2 Contents Grid interconnection schemes Power quality requirements Fault Level considerations Protection schemes New grid code connection requirements My contribution within the project 2

3 Interconnection to MV and HV grids < 5 MW Existing MV feeder (e.g. 15 or 20 V) reinforcement possibly required < 20 MW directly to MV busbar of MV/HV sub station via dedicated feeder > 20 MW to HV system via dedicated TR in existing MV/HV sub station or via dedicated sub station ource tavaros A. Papathanassiou 3

4 teady tate Voltage Variations implified evaluation 10 min average values EN stipulates ±10% limit for MV systems 2% limit is set by utilities ε (%) 100 = 100 R cos(ψ n cos(ψ + Φ) 2% + Φ) = Where the short-circuit ratio R is defined as: R = n 4

5 teady tate Voltage Variations Detailed evaluation Procedure: Perform the load flow in 4 cases 1. Min load Min generation 2. Min load Max generation 3. Max load Min generation 4. Max load Max generation Determine U max, U min for each node Chec the deviation limits for all the nodes Median value within ±5% of the nominal voltage Deviation around the median value < ± 2% Can be corrected via the fixed taps of MV/HV transformers Ensures that LV networ voltage always remains within 10% of the nominal voltage 5

6 Rapid Voltage Variations witching operation of a single WT (IEC ) d max (%) = 100 u (ψ ) 100 = R (ψ Where (ψ ) is the voltage change factor u n u ); If the voltage change factor is unavailable, the starting current can be used: Magnitude limits (IEC ) r (hour -1 ) r 1 1 r r r 1000 MV d max (%) HV u ( ψ ) I I start n for ynch. gen for Asynch. gen for DFIG No summation effect needs to be taen into account 6

7 Harmonics & flicer emissions Evaluating principle: 1. pecify acceptable disturbance level (planning level) Acceptable distortion or fluctuation level of system voltage, caused by all consumers or producers connected to the system. Generic values, specified by the utility; 2. Allocate them to individual WFs The limit for each DG (or consumer) is a fraction of the planning level, proportional to the ratio of its agreed power to the available system capacity; 3. Chec the emissions of a specific WF against its allocated limit Calculate expected harmonic distortion or flicer of the system voltage at the PCC due to a specific DG and compare to its allocated limit; If the system is fully loaded and all the consumers are injecting their individual limits, the total disturbance level will be equal to the planning levels. 7

8 Harmonic emissions Examined only in normal operation Planning level for LV, MV and HV networs (IEC ) Compatibility levels for harmonic voltages in LV,MV and HV systems in % of the nominal voltage: At high frequencies (>2 Hz), no standards exist. Utilities demand U h < 0.2 regardless of the type of generator 8

9 Harmonics emissions in MV systems For each harmonic order, the actual harmonic voltage in MV system results from the vectorial combination of the harmonic voltage from the upstream HV system and the harmonic voltage resulting from all the non-linear loads connected to the MV system. It should hot exceed the planning level of the MV system T hhm L hhm HV G hmv = α L α hmv (T hhm L hhv ) α Distortion transmitted from HV system ummation exponent WF MV Global emission in the MV system (may be assessed from the harmonic current times source impedance Planning level of the hth Harmonic in the MV system Rated (agreed) power of the WF Individual emission limit E Uhi = G hmv α i MV 1 F MV Coincident factor for MV Loads (0.4 1) Total power of the loads supplied at MV/HV transformer 9

10 Flicer emissions in MV systems Examined for normal and switching operations MV compatibility planning levels P st =0.9 and P lt =0.7 Assessment of flicer emissions from WTs during normal operations ingle WT Networ impedance angle at PCC Annual average wind speed P st = Plt = c ( ψ, v a ) n Rated apparent power of the WT hort circuit apparent power at PCC Flicer coefficient (WT certificate) P 1 Aggregate wind farm N 2 st Σ = Plt Σ = ( c i ( ψ, va ) n,i) i= 1 10

11 Fault level: tatement of the problem Distribution networs are characterized by a design short-circuit capacity (max fault current) Distributed generation resources, as wind farms connected to distribution networs contribute to the total fault level of the networ (sum of upstream grid + local contributions) The short-circuit capacity of existing distribution networs is close to the design maximum value Little margin for the connection of new power installation Fault level frequently is the main limitation for high WP penetration into the grid The at PCC to the MV-networ shall be at least 50 times the apparent rated power of the wind power installation 11

12 The IEC method for the calculation of the short-circuit currents Advantages tandardized, reliable, widely accepted Easily applicable in radial networs Pre-fault operating conditions not needed Disadvantages Not adapted to distributed generation as Wind power Lac of standardized treatment of the DG sources Correction factors derived for conventional generation 12

13 Fault current contribution of the DG sources (IEC 60909) ynchronous generators directly coupled to grid: Z = K Z = K R + jx ) GK G G G ( G d Asynchronous generators directly coupled to grid: Z G = I LR 1 I rg U rg 3I DFIG with power in the rotor circuit K 2 rg U c n max G = (impedance correction factor) UrG 1+ x d sinϕrg ame as for asynchronous generator (I 5 I rated ) Power converter interfaced units rg = I LR 1 I rg U rg As constant current injection (I 1.2 p.u. usually) 13

14 olutions for fault level management Increase the design maximum fault level Reduce the prospective short-circuit current of the grid Networ reconfiguration Install current limiting reactors Increase transformer impedance Reduce the prospective short-circuit current of the DG Use equipment (gen) with increased short current impedance Install current limiting reactors Connect the grid via power converters 14

15 Protection functions Objectives Over-current protection (phase and ground protection, as in any consumer installation) Protect the DG installation from abnormal operating conditions (voltage/frequency) Prevent islanding of the DGs with part of the networ Prevent operating the DGs from experiencing re-closing of networ breaers upervise the re-synchronization of the installation with the grid 15

16 Protection schemes: Basic functions Inst Opening 0.1s Automatic re-closing after 0.5 s Relaxed settings ource tavaros A. Papathanassiou 16

17 Protection schemes Considerations and remars Increasing the sensitivity and speed of disconnection protects the installation more effectively but increases the number of unnecessary trips Networ with re-closing schemes The DG must be disconnected before the first re-closing of the line breaer Preferably the DG should be disconnected as soon as the line breaer trips, to avoid feeding the fault In networ with short interruption/fast re-closing schemes, the DG must be disconnected in less than 0.5 sec. mall island networs with large WP penetration is a special case The protection system settings are relaxed in order to avoid the loss of large amounts of generations 17

18 Grid code requirements EON requirements Deepest voltage allowed on gen. terminal side about 0.3 p.u. Frequency band Hz Different power factors (over/under-excited mode) FRT capability at PCC (relay setting at WT 0.3 p.u. ) Reactive Current into the networ 18

19 My contribution within the project Main focus on voltage and angle stability constraints, and special emphasis is put on power systems incorporating large scale wind power integration. Large cale Wind Power Integration, Voltage stability Limits and Modal Analysis (PCC05) Fundamental nature of low frequency inter-area modes of oscillations and power system damping; mall signal and voltage stability are performed as a combined study; WF modeled as an asynchronous machine improves the damping of the inter-area mode of oscillation; With synchronous generators local mode of oscillations might be the limiting factor in relation to the power produced; A Proposed Model for tability Assessment of Electrical Power ystem with Large cale Wind Power Integration (WWEC05) The method adopted is based on the systematic analysis both static and dynamic of power systems; It gives some insights into a systematic method to follow in WP stability assessments which enhances the WP integration in electrical power systems; It is shown that in some cases the disconnection of the WF can be convenient reducing the reactive power consumption and stabilizing the voltage; 19