Wind and solar use power electronic converters to create the sine shaped current, which can be supplied to the network. The shape is determined by control algorithms only and, for that reason, generation is called 'non-synchronous'.

The share of generation based on power electronic converters in the total instantaneous supply is also called System Non-Synchronous Penetration (SNSP). This share is relevant because the behaviour of power electronic converters differs from synchronous generators:

• By design, power electronic converters are not able to energise a network without some pre-existing voltage meeting the quality standards of the network. They do not support system restoration in case of interruptions.
• With nowadays designs, they represent a current source. Their peak current output is just slightly higher than the rated current. As a consequence:
  A) They do not help mainaining voltage quality in case non-linear loads introduce harmonic distortion. This may affect the quality of supply for the customers.
  B) They have a lower short circuit capacity than synchronous generators. This may affect protection schemes.
• With current designs, they do not add inertia to the system. Changes of system frequency may become more rapid.

With these characteristics, the penetration of power electronic converters in the instantaneous supply, i.e. SNSP has to be limited. As a consequence, as of today, vRES generation has to be curtailed as soon as the share exceeds a certain portion of total generation. Of course, this is a serious barrier for decarbonising electricity supply.

Note 1: The mentioned characteristics of power electronic converters are design features. If required they can be changed. The difficulty is that specifying the desired behaviour needs a complex compromise between partially conflicting interests.
Note 2: HVDC transmission links also use power electronic converters. They contribute to the total SNSP and, hence, in a way compete with vRES generation. This may reduce the advantages of interconnection.
Note 3: The trend to power electronic converters applies to loads as well. This makes related issues even more challenging.

With current technology, this is a key issue as soon as instantaneous vRES penetration (SNSP) may pass 50%...60%. Examples are Ireland (DS3 programme of Eirgrid) and similarly UK (Frequency Changes during Large Disturbances and their effect on the total system).
However, relevant are not the political, but the electrical system boundaries.
That's the advantage of countries like Denmark or Germany being part of a large interconnected system. Their high instantaneous vRES penetrations are possible thanks to the neighbours. Within the continental European system, vRES penetration by now did not exceed 30%.
Once the technical solutions are commercially available, the issue may be relaxed for every country.

There are systems which will rely on high shares of dRES and, hence, synchronous generation also in future. Countries with favourable hydro or geothermal resources, may never touch this restriction.

Note 1: This restriction does not apply at distribution level. Distribution networks are embedded into larger, interconnected systems. As long as you can export power, you cannot measure an isolated SNSP.
Note 2: There are many examples of small scale isolated systems, running reliably at 100% SNSP. However, their concepts cannot be easily copied to large scale power systems, mostly because of economics. Generally, they use storage batteries, rating of power electronic converters is substantially higher than the peak load and balancing load and supply relies heavily on curtailment. Scaling these concepts up would result in excessive cost of electricity.

Attention is required whenever system-wide, instantaneous penetration of vRES approaches 50%. This is likely to happen in Phase3. In order to further increase SNSP, technical capabilities of the vRES plants as well of the existing power plants need to be enhanced.

Important remark: the contribution of a synchronous generator to system ineratia does not depend on output. As long as a unit is spinning and connected to the network, it provides exactly the same amount of inertia. That means, if synchronous generators are operating in part load, due to reduced residual load, but do not disconnect, inertia is not really affected.

Planning
There is no single technical solution for all related issues. Some important steps are:
Development of concepts for power electronic converters emulating both, voltage source behaviour and inertia. These concepts are referred to as 'grid forming converters'. Consequently, these concepts have to be integrated in technical codes and standards (Grid Codes). The current challenge for the power industry is to specify and agree on the desired behaviour.
In parallel, new ancillary services like 'fast frequency response' help to maintain frequency stability with enhanced SNSP levels. Establishing this kind of services is a matter of regulation.

Operation
With current common concepts of power electronic converters, limitation of SNSP and, hence, vRES generation is inevitable.
In the transition phase, existing technologies synchronous condensers may help maintain minimum system inertia and, thus, allow to increase the feasible SNSP level. Dedicated battery storage is also an option. Current battery-inverters, however, don't differ from power converters of PV- or windplants. They can provide fast frequency response, without depending on resources like sun and wind. Currently, they also are part of SNSP and, hence, only marginally help increasing instantaneous vRES penetration.