Page 44 - Energize September 2021 HR

TECHNICAL

Obtaining power quality grid-code

compliance for a wind farm

by Kyle Lass, RWW Engineering; Dirk van der Bank, ADAee; and Aki Tiira, Merus Power Dynamics Oy

Grid-code compliance was recently achieved for a wind farm in the Eastern Cape, after the installation of
active harmonic filtering (AHF) to mitigate plant generated harmonic emissions. This is one of the largest
installations of its kind in the world. Some of the requirements and technical challenges overcome are
highlighted in this article.

he 132 MW wind farm, comprising of sixty 2,3 MW wind level with the transformer impedance used as a buffer between plant
turbines, went into commercial operation after being granted generated and grid related flicker content.
Ttemporary exemption on the harmonic current emissions The wind turbine generators provide power at a nominal 690 V
from the plant into the network. After fine tuning the wind turbine level. This voltage is transformed from 690 V to the distribution
inverters, it was still required to mitigate high levels of current 33 kV via transformers local to each wind turbine. The Merus A2
emissions at the second and fifth harmonic orders. This was active harmonic filter generates the compensation currents at 400V
measured at the point of coupling to the 132 kV network. and is connected to the 33 kV network via a 5000 kVA, 400 V/33 kV
A 2700 A (at 400 V) Merus A2 active harmonic filter was supplied three-winding transformer.
and installed by RWW Engineering to fulfill the harmonic cancellation Effectively this translates the seemingly modest level of
requirements and achieve compliance to the South African Grid harmonic current mitigation requirement at grid level to very high
Code for renewable power plants (RPP). Grid-code compliance levels of injected harmonic current at the 400 V level. For example,
recordings and confirmation studies for the project were performed the need for a mere 5,5 A of harmonic current compensation at
by ADAee. grid 132 kV level equates to more than 1815 A of harmonic current
injection required at 400 V.
Regulations and standards
Renewable power plants wishing to connect to the South African Control systems to interface at various bus voltage levels
national grid must comply with regulatory specifications as set out The wind farm in question previously achieved compliance to all
by the national energy regulator. The relevant power quality (PQ) PQ parameters according to agreed limits with the national energy
parameters are defined in the grid code. For this plant, the category regulator, other than current harmonic emissions at the second and
C (20 MVA and larger) provisions are applicable. The latest update fifth harmonic orders. These emissions were in the region of 5,5 A
is Revision 3, dated August 2019. In addition, the specific limits at 132 kV. Various proven methodologies were undertaken prior to
for each of the defined parameters are also contractually agreed implementing the active filter to ascertain whether these harmonic
and assigned to each individual RPP. Several further guidelines currents were indeed coming from the RPP or whether the RPP was
and standards support the above specification and ensure a absorbing these harmonics from the grid network. After exhaustive
standardised approach in collecting and analysing data from testing, simulation and online plant controller modifications, it was
operations, including standards and guidelines as per various NRS, proven that the second and fifth harmonic currents were mostly
IEC and Cigré papers. being generated within the RPP and exported to the grid network.

Case study of wind farm challenges and solutions
Control systems to interface at various bus voltage levels
The point of connection (POC) for the WF (the point at which full
PQ compliance must be demonstrated) is on the national grid at
a 132 kV bus voltage level (Figure 1). The permanently installed
highly accurate metering PTs and CTs are used to record voltage
and current trends. These recordings effectively determine harmonic
voltages, harmonic currents, flicker, resonance sensitivity and
voltage unbalance conformance to the contractual limits.
The plant itself collects power at the 33 kV level from the wind
turbines. Therefore, there is a need for a step-up transformer
between the internal wind farm level at 33 kV and the grid at 132 kV.
Flicker compliance is determined from this internal 33 kV voltage Figure 1: High level SLD wind farm with Merus A2 active harmonic filters

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