Challenges for Offshore Wind Grid Integration

In the course of the past few years, various schemes for Offshore Wind Grid Interconnection have experienced enormous progress and the industry is moving forward with the deployment of several GW of projects globally.

Although the penetration of large-scale Offshore Wind Power Plants (OWPP) in power system results in increased Power Quality and System Stability Issues which must be studied in detail. Let’s review the mail challenges:

POWER QUALITY ISSUES

Power Quality disturbances at OWPP are regularly monitored to check whether, output complies with the permitted local grid regulations. Power Quality issues include the folllowing:

Flickers.

Flickers can be defined as fast variation in supplied voltage that may last for a certain period so that the variation in electric light can be recognized visually. IEC standard 61400-21 establishes codes for defining Power Quality requirements of wind generation. A portion of the mentioned standard makes use of the time domain readings of current and voltage taken at terminals of the wind turbine to measure Flickers due to the switching action. A wind turbine produces more flickers in weak grids because of inverse relationship of flickers emission and short-circuit capacity.

Voltage sags/swell

Temporary reduction of the RMS voltage between 0.9 and 0.1 pu of nominal value from half-cycle to few seconds is termed as a voltage sag, while the voltage swell or over voltage is an increase in the RMS (effective) voltage to between 1.1 and 1.8 pu of normal value from half-cycle to one minute. A voltage sag due to wind generator penetration may result in disconnection if voltage drops under a certain value. This voltage dip can also result in a high current through IGBT inverter which connects a permanent magnet synchronous generator (Type IV WTG) to the grid. Fast removal of sag and swell in such cases events is crucial.

Harmonics

In an OWPP the voltage source inverter stimulates some harmonics while the other harmonics are from grid background, which are reflected back to the turbine farm terminals, and therefore at the point of common coupling. In addition, the large capacitance in the form of subsea cables and compensating capacitor banks resonant frequencies influences the resonant frequencies of the system.

STABILITY ISSUES

As wind power penetration level increases in electric grid, researchers focus more towards the problem of power system stability. Stability issue in OWPP is mostly caused, either by any uncertainty occurring at wind turbine side, or a fault/ disturbance at the grid side. With an increasing number of wind farms connected, controlling dynamic characteristics of overall power systems is becoming more challenging.

A related phenomenon of sub synchronous resonance (SSR) has gained attention in recent years. It has been reported to cause oscillations below system's rates frequency in grid connected wind farm system in vicinity of series compensated transmission line. ?R&D in the areas of small signal stability and transient stability for big OWPP grid integration is a hot topic for power system researchers and transmission system operators.

VOLTAGE AND FREQUENCY ISSUES

In an electrical power system, the reactive power is related to the voltage variation, while the frequency change corresponds to a change in the active power in the system.

Voltage Control

With increasing level of offshore wind energy penetrations into the grid, voltage stability is becoming a challenge. Voltage control of a wind farm need to be designed in such a way that demand of reactive power output to be met is in accordance with the dispatch instructions form grid side to support the voltage at the Point of Interconnection. In this regard, voltage and reactive power control must meet the following requirements

• Voltage at PoI should be maintained normally ranging from 95% to 105% of the nominal voltage at grid side
• Its speed of regulation and control accuracy should meet the demand of voltage control of grid operation.

Impacts of Frequency Variation

OWPP penetration in the grid proportionally increases the unpredictability and frequency variation of the power output. The operating conditions change due to these deviations in frequency, because it alters the frequency related parameters like reactance and the slip of the wind turbines. Furthermore, modern variable speed wind turbines such as type III and IV are connected to the grid with end-to-end power converters. The intermediate DC voltage in this AC-DC-AC interface creates electrical decoupling between OWPP and the grid. Consequently, the system inertia reduces, as the generator rotor does not see the changes in frequency.

Ramp Rate Requirements

Wind power changes continuously and as a result, an important issue related to grid integration is upholding the economic cost of wind energy and the system resiliency. This intermittency of wind energy is countered by calling fast ramping or peak-load units, so that balance of supply and demand must be kept. An OWPP need to have the ability to limit increased and decreased power rates. By and large, the ramp-rate limit (RRL) for a short duration of 1-minute addresses system regulation capability. A longer duration of 10-minute typically is applied on grid load-following capability.

All these electrical disturbances and power quality issues are met with strict requirements imposed to OWPP by grid code and grid connection standards and regulations. In this way Transmission System Operators (TSOs) share the responsibility of mantaining electrical grid parameters (Voltage, frequency, etc) under aceeptable/design values.

In a following short article we will review the main industry solutions/tecniques to address power quality problems.