As solar and wind generation sources increase their penetration into the bulk electric system (BES), the overall reliability of the BES may be impacted.  One aspect of BES operation that may be reduced is BES stability.  BES stability is the ability of generators synchronized to the BES to remain synchronized when there is an upset to the BES, commonly referred to as a contingency.  Examples of contingencies are the trip of a large generator and the loss of a large load, such as a big factory.

Transient stability is a function of three other types of stability, rotor angle stability, frequency stability, and voltage stability.  These three types of stability are interrelated, for instance, transient stability, which is the ability of generators to remain synchronized to the BES.  Transient stability is lost when the rotor angle becomes too large, a situation that can happen because of loss of frequency stability.  The two types of stability that are directly related to Generating Units are frequency stability and voltage stability.

Frequency Stability

When there is a contingency, there is a sudden large imbalance between real power generation and demand.  The result is a large deviation in the BES frequency.  The response of the BES to the frequency deviation determines its transient stability.

The response of the BES occurs in three stages:

Inertial response – the collective rotational inertia of all Generating units and synchronous motors synchronized to the BES resist changes in the BES frequency, thus the speed of the generators.  Additionally, kinetic energy is stored in the rotors of all of the machines synchronized to the BES that provide energy to help reduce the imbalance between BES demand and generation.  The inertial response is vital in about the first 0.5 seconds of the contingency.

Primary Response – as soon as the droop governors of all of the generating unit prime movers sense the change in speed (and thus BES frequency), they respond through regulation by using a control device (such as the fuel valve for a gas turbine) to increase or decrease the energy to the prime mover, which moves the speed/frequency back towards the nominal value, which is the governor speed setpoint.  However, there are two caveats; the governor’s speed of response is slower than inertial response, and second, the droop governor always has a speed error that remains after the frequency stabilized.

Secondary response (also called tertiary response) – secondary response eliminates the speed error in the nature of droop governors by either increasing or decreasing output of some generating units to return the BES frequency to nominal (60 Hz); the secondary response is usually made by the RTO or ISO sending AGC )Automatic Generator Control) signals to some generating unit AGCs to adjust outputs as required.  If the units involved in the initial secondary response do not have the capacity needed to correct the speed error, BES generation is increased further using the operating reserves, some of which respond within 10 minutes and other within 30 minutes.

No early photovoltaic (PV) solar or Wind Generating Units could participate in response to deviations in BES frequency.  PV solar generating units have no rotating mass in the rotor to provide an inertial response.  Wind generating units have some kinetic energy to supply to the system, but since their generators were not connected directly to the system, as a rule, there was no contribution to rotational inertia.  For both PV solar and wind units, there was no droop governor to provide primary response, and they were not dispatchable using AGC or any other means and so could not participate in secondary response.

This lack of rotational inertia and inability to participate in either primary or secondary response to deviations in BES frequency made the deviations in BES frequency greater than they would have been otherwise, and thus reduced the transient stability of the BES.  In those early days, however, the penetration of non-dispatchable EGUs was small, and so the degradation of frequency response was small as well.  With larger penetrations (say 50%), however, this problem could be expected to degrade BES frequency stability and the BES transient stability, seriously.

Fortunately, this problem has been recognized.  Accordingly, manufacturers have worked to develop a feature used in most newer PV solar and wind generating units called fast frequency response (FFR), which allows the generating unit to respond to deviations in BES frequency with regulation like a prime mover droop governor.  Regulators have mandated that new generating units “install, maintain, and operate equipment capable of providing primary frequency response.”

The regulation explicitly says that the operation of the generating unit to provide headroom is not mandated and that there is no mandate for generating unit owners to be compensated for providing primary frequency response.

Voltage Stability

Generating units have a direct effect on BES voltage stability.  In normal operation of the BES, the Automatic Voltage Regulators (AVRs) for all generating units that are synchronized to the BES are operated to maintain a constant BES frequency.  Because of the relationship between reactive power and voltage, when the AVR maintains BES voltage, it is also responsible for the generation of reactive power from the generator.  When reactive power generation is balanced with the demand, the BES voltage is a nominal value.  There are deviations in BES voltage from nominal when there is imbalance between reactive power demand and generation; inadequate reactive generation reduces voltage and vice versa.

While voltage deviations both above and below nominal can damage BES equipment, drops in voltage are generally more serious and more likely than deviation above nominal.  One reason that excessive drops in voltage are potentially a bigger problem than voltage increases is the fact that there is a low voltage stability limit.  If BES voltage falls below this stability limit, the BES voltage becomes unstable, and a phenomenon called a voltage collapse occurs, which results in a BES blackout.

There was concern about early solar and wind generating units regarding support of BES voltage because these EGUs produced no reactive power.  However, newer solar and wind generating units have power electronics capable of generating and absorbing reactive power.

It is expected that many new solar and wind generating units will be located far from load centers.  This is a problem because there are considerable losses of reactive power in transmissions lines, and those losses increase as the length of the transmission line increases.  Further, current must flow through the transmission lines to transmit reactive power.  Still, as the current flow in the transmission lines increases, the reactive power demand from the transmission lines themselves increases.  This means that it is difficult for these distant solar and wind generating units to supply reactive power to the load centers.

The solution to this problem is to generate reactive power near the load centers.  Fortunately, there are usually already generating units installed near load centers; those units are conventional fossil-fuel generating units.  Some of those units are generating units that would be in service in any event during periods of high reactive power demand.  In many cases, these generating units can increase reactive power output by increasing voltage.  There are however, limits to the reactive power generation that depend, in part, on the real power output; reducing real power output generally allows the generating unit to generate more reactive power.  Since high real power demand generally accompanies high reactive power demand, reducing real power output may not be an operation.

The solution would then be to start additional generating units to supply that reactive power.  Gas turbines are often used in this circumstance.

From this discussion of voltage stability, it should be apparent that conventional fossil-fuel generating units are needed to support reactive power generation in the BES in order to provide voltage/reactive power support for operation of the BES, and that the need is likely to be greater with increased penetration of solar and wind generating units.  Other possible solutions include the use of capacitor banks and static VAR compensators.

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