Let's get back to fundamentals here. I want to make the point that there is no such thing as "DC Harmonics", as DC is defined as zero frequency. However, there can be DC output current from your wind inverter that flows into the grid. This would be caused by slightly unequal positive versus negative half cycle waveforms of your sine wave output. This in turn, can be caused by your control ckt or by some unequal part of your power circuit switches. If you have a 3-phase output, the situation is not different than single phase, but there can be inequalities causing DC output current between any of the 3-phases. Normally, in an inverter such as this, there will always be some DC output current, but it's usually small enough to ignore, like in the low mA range or less on a several kVA or higher inverter.  If you are using a digital scope, you should be able to take a measurement of the output voltage waveform from Line-to-Line, then download the waveform to a computer using perhaps MS Excel or other math program and you can separate the positive and negative half-cycles and compare them to see how unequal they are and this is a first step to debugging your inverter design to minimize the DC output current. Notice I didn't say to eliminate it, as that would be very hard, but getting the DC output down to a reasonably low enough level shouldn't be that hard.

The wind turbine is directly connected to the rectifier side of the converter. The capacitive coupling by the DC bus through the wind turbine is composed of the path between the rectifier side and ground because of the high harmonic current component imposed by the switching actions, whereas the capacitive coupling seen through the grid is represented by the inverter side, the filter and the underground cable. It is observed that the harmonics components near the switching

frequency are considerably higher than the fundamental component. Harmonics components 70 (3500 Hz) is 575% of fundament component magnitude which is 3.05 V. 

Ref: Harmonic Distortion in Renewable Energy Systems: Capacitive Couplings

Miguel García-Gracia, Nabil El Halabi, Adrián Alonso and M.Paz Comech

CIRCE (Centre of Research for Energy Resources and Consumption)

University of Zaragoza ;Spain 

Any electrical system contains a power electronics device, such as a converters or inverters, draws  a distorted current ( non sinusoidal) from the main supply. The Fourier analysis shows that any non sinusoidal signal will contains a harmonics in addition to the fundamental. If there are a dc harmonics, that is indicated to distorted output voltage waveform.   

Let's get back to fundamentals here. I want to make the point that there is no such thing as "DC Harmonics", as DC is defined as zero frequency. However, there can be DC output current from your wind inverter that flows into the grid. This would be caused by slightly unequal positive versus negative half cycle waveforms of your sine wave output. This in turn, can be caused by your control ckt or by some unequal part of your power circuit switches. If you have a 3-phase output, the situation is not different than single phase, but there can be inequalities causing DC output current between any of the 3-phases. Normally, in an inverter such as this, there will always be some DC output current, but it's usually small enough to ignore, like in the low mA range or less on a several kVA or higher inverter.  If you are using a digital scope, you should be able to take a measurement of the output voltage waveform from Line-to-Line, then download the waveform to a computer using perhaps MS Excel or other math program and you can separate the positive and negative half-cycles and compare them to see how unequal they are and this is a first step to debugging your inverter design to minimize the DC output current. Notice I didn't say to eliminate it, as that would be very hard, but getting the DC output down to a reasonably low enough level shouldn't be that hard.

True that DC is not a multiple harmonic but it is the zero sequence harmonic. There are typical ways to have DC: 1) Even harmonics and a non-linear element such as a rectifier 2) A disturbance (capacitive, inductive) in the feedback signals that is equal to the controlled frequency 3) Even when you want to compensate that offset, DC transducers have offset, and temperature coefficient of the offset. 4) Not perfect functioning appliances such as a CFL where one diode is de-soldered. 5) Some soldering irons and hair dryers use diodes as a cheap way to choose 50% power level. 

DC is harmful in transformers if it is a significant fraction of the magnetising current, let say 20A in 1MVA distribution transformers, and 0.02A in accurate 50Hz measurement transformers. A saturating transformer draws an exaggerated magnetising current and makes noise. In England distributing companies ask limits of milliamps DC. This is a way to keep away renewable energy by making it costly and to avoid import of devices from foreign countries. In fact every time an instrument is started, a DC fraction is injected in the grid. Milliamps make no sense, except for measurement, but measurement equipment can be adapted to be insensitive to it. 

DC is created in the current of a PWM converter or inverter due to:

1) inequal positive & negative areas of the voltage created; This may be due to control pulse width generation problem, inequal delays in top & bottom gate drive circuits, inequal dead-time or blanking periods between top & bottom devices, inequal voltage drops in switching devices (IGBTs)

2) load characteristics; like presence of a half-wave rectifier connected to the AC line that draws DC component of current from the AC supply

3) existing DC offset of the AC grid waveform; If the existing grid already has a DC  voltage offset due to its line impedance & connected load (note that DC current flow through a line creates DC voltage drop and hence DC offset at the load end), then as the connected converter waveform tries to synchronize with it, a DC current will flow.

The most common reason for DC component of supply current is inequality of positive and negative half cycles of converter output. If you are sure that the both cycles are equal, you should know that for some converter topologies, dc component can be seen as a natural cause of conversion especially working in capacitive region.  If you give a detailed information regarding your inverter/rectifier system, I can propose you a solution to reduce dc currents (you can never guarantee zero dc component, it will always be there) to negligible values. The most basic solution to decrease dc currents is simply adding an intentional dc voltage reference to the converter output which gives an opposite effect to existing dc component. Most probably you have step-up transformers to connect the grid, you should notice that the transformer most probably subject to core saturation due to dc current components(especially when your current has leading angle because leading angle causes a voltage rise in a transformer terminal). You may verify the core saturation by observing a current waveform which has a triangular shape instead of sinusoidal. It is obvious that the dc component can not pass through high voltage side due to transformer nature so you have nothing to worry for power system, but it is dangerous to transformer when it has high values.

I have to disagree with some of the answers here. Mathematically speaking, DC has every frequency in it, since a constant function repeats itself on every subset of its domain.

Mathematicians discovered the number "zero". DC is a zero order harmonic. It can be treated similar if for harmonics, rms values are used. But also special effects occur if you sample at the same frequency as an harmonic. So DC is not an exception. Some distribution companies do not like DC injection if they use current transformers. But that is a measurement problem.

Yes you can blow up a transformer by injecting dc...