Frequency Response refers to the MW added to the system, when frequency decreases, or subtracted from the system, when frequency increases. In the “good old days”, when talking about frequency response, the term Self Regulation was used to refer to that component of the response that was strictly due to the intrinsic frequency characteristics of Load and Generation equipment. The term seems to have fallen out of use in North America as I cannot find any references to it in NERC  documents. It is, however, still used in Europe, albeit with the more restrictive meaning of the frequency response from Load (See UCTE “Operation Handbook”). From the discussion below, it becomes evident that Self Regulation is a system characteristic very important to system reliability as it is of critical importance to the ability of the system to recover from disturbances. This discussion explains the meaning of Self Regulation showing that it is due to both generation and load equipment. As the frequency response on our interconnections continues to deteriorate due to emerging generation and load supply technologies, it may get to the point that interconnection of new loads and new generation is allowed only if they provide some minimum amount of Self Regulation. This is not unlike requiring new generators to be able to operate to some minimum power factor and penalizing loads with poor power factors.

Due to disturbances that result in net losses of generation or load, the frequency of the system will deviate from 60 Hz.  Following a loss of generation disturbance, the frequency will decay and reach a minimum (Nadir) at about 5 seconds after the disturbance and  then start recovery  as a result of AGC control and manual deployment of new generation. It is of interest to note that, after the Nadir is reached and recovery has started, the same factors  that contributed to slowing down and finally arresting the frequency decay (Inertia, SelRegulation, and Governor response), will now act to slow down the recovery. During about the first 2 seconds following the disturbance, governor response is practically absent. During this time the rate of frequency decay is slowed down only by Inertia, and the intrinsic frequency characteristics of generation and load equipment. Let’s refer to these intrinsic characteristics as Self Regulation (D).  This D is discussed below with the help of the illustration below under the assumption that these characteristics are linear in the vicinity of 60 Hz.  

In the case of system load, the power demanded by the load increases as the frequency increases and vice versa. This is shown by the blue  “Load” line.

In the case of system generation,  it is a bit more complicated. The black “Generation” line shows the case where generation  decreases as frequency increases and vice versa. In this case, we notice that generation and load are in equilibrium at point “P”. Also, “P” is a stable equilibrium point because: (i) If the frequency is disturbed in the “t” direction, the system develops a generation deficiency that causes the frequency to decay back to point “P”, and (ii) If the frequency is disturbed in the “q” direction, the system develops a generation surplus that causes the frequency to increase back to point “P”.

The red dash-dot “Generation” line shown,  illustrates the case where generation  increases as frequency increases and vice versa. This line has a slope that is is larger than

the slope of the “Load” line. In this case point “P” is not a stable equilibrium point because if the frequency is disturbed from point “P” in either of the “t” or”q” directions, the frequency will run away in that direction and will not return to point “P”.

As a last observation we notice that if the “Generation” line were to be parallel to the “Load” line, there would be no intersecting point between these two lines and, therefore, no equilibrium point would exist.

From the previous three paragraphs, we conclude that for the system to have stable equilibrium points, the slope of the “Generation” line has to be less than the slope of the “Load” line. As our system does have stable operating points, as it is running fine every day, we conclude that our system “Generation” line has a slope that is less than the slope of the “Load” line. For the purpose of the rest of this conceptual discussion, the black line, which has a slope less than that of the load line, is assumed to be the pre-disturbance “Generation” line.

The illustration shows that the system is balanced at 60 Hz. At this frequency, the Load line intersects the Generation line at point P. If the system experiences a generation loss of ΔP,  the generation line will drop to the dotted line, and the frequency will start decaying towards point R at which point it will stop changing. At point R the system is again in equilibrium as S1 (sum of the remaining generation power increase plus the load power decrease due to D) balances out the ΔP generation loss.

The above  illustrates the importance of Self Regulation (D) as it allows the system to regain equilibrium, albeit at a lower frequency, and arrests the frequency decay without the need for any external remedial actions. Once the frequency decay is arrested, there is time for governors and AGC to respond and for manual remedial actions to be implemented. The quantity D = S1/(59.9-60)=-10*S1 Mw/Hz  is referred to as Self Regulation. We notice the resemblance of this to the -10*β in the ACE equation.  The β is larger (in absolute value) than D as it includes governor response and other control considerations.

The above illustration makes it clear why the mechanism currently used to interconnect Solar Generation and Wind Generation to the system,  pose a serious threat to system reliability. Because these generators are connected to the system via DC to AC converters, they provide zero frequency response.

There is also evidence that, after a disturbance, Combined Cycle Units provide negative Frequency Response, that is, Frequency Response in the direction opposit to the one needed.

The emerging use of variable speed drives to supply motor and some other loads, is also decreasing the Frequency Response from loads because these drives rely on AC to DC conversion.

The above discussion points to the need to grant generator and loads connection to the system only if they provide some minimum amount of desired Frequency Response. A good place to start would be to require loads and generators to provide a frequency response equal to that provided by the "classical" loads and generators. That is, response from loads as if there was no variable drives, and from generators as if they where coal units.