Practical implications of frequency regulation

The major practical implication of frequency control is the need to use conventional units to carry out frequency regulation in power systems. Hence, conventional units were considered the major and only methods capable of providing frequency regulation in large interconnected power systems (Ela et al., 2014).

Automatic generation control (AGC) have been researched and implemented extensively over the years. But, battery energy storage system is barely new with a lot of research taking place at the moment with the sole aim of improving the technology and making it available to power utility companies world over. This is informed by the fact that most utility companies are integrating variable renewable energy sources into the power grid. However, the capacity to provide primary control and secondary control has always been an integral part of the requirements for grid integration of both conventional generation units and renewable energy resources. Due to economic constraints, all the units capable of providing frequency regulation/control do not provide on a regular basis and when required. In interconnected systems, only a small portion of the units connected to the grid practically provide frequency regulation. Thus, with the opening of the electricity markets in most countries, and as long as frequency control units are started and stopped at any suitable time, the units that provide power and participate in the provision of frequency control can vary at different intervals not properly controlled (Diouf, 2013). These variance is unique to a specific utility which decides

on the time intervals. This is dictated to by the economic restrictions and the method of acquiring frequency regulation ancillary services. In addition, the units designed to provide frequency regulation do not exclusively respond in the same way and are expected to keep a portion of their capacity as reserve for frequency regulation. The portion kept as reserve aimed for frequency regulation incurs some cost (Rebours et al., 2007). Therefore, deregulation the electricity market, which is based on promoting competitive behaviour amongst producers has strengthened the fact that, reserves aimed at providing frequency regulation must be avoided if not given the necessary incentives or made compulsory. Because of the above concerns raised by producers, they are presently given some form of incentives or adequately remunerated.

3.11.1. Primary control with conventional units

Primary control is achieved only when certain amount of the electrical power generated is kept as primary reserve in some of the generators or energy storage systems that are connected to the grid and the units are able to respond instantaneously to offer primary frequency regulation. The active power generated by a generator is based on the mechanical power output of that particular generator such as wind turbine, steam turbine and hydropower system while for renewable energy resources, it depends on the power rating and the efficiency of the system. However, for primary regulation, a large number of generators have inbuilt additional control loop in order to adjust their electrical power output according to frequency changes and the rotational speed of the system (Ela et al., 2014). The control loop is generally known as the governor control. For any system that provides primary control, the performance of such system must be assessed using the following parameters:

 The insensitivity band

 The droop

 The response time or lag time

3.11.1.1. The insensitivity band

The insensitivity band also referred to as the dead band, is the range in the spectrum where frequency deviations does not affect the units. The insensitivity band can be either not possible to remove or necessary. According to (Rebours et al., 2007) “as the mechanical governor system gets older, the insensitivity band becomes impossible to remove. Actually, contact between the moving parts created insensitivity either necessary or not”. Alternatively, an insensitivity band can be necessary. In addition, an insensitivity band can be fundamentally the outcome of the resolution of frequency measurements.

3.11.1.2. The droop

A generator droop “S”, is the sensitivity of a unit varying its output determined by the frequency variation. The unit droop is represented as:

 ∆𝑃_{𝑢𝑛𝑖𝑡}= MW output change of the unit determined by change in system frequency

 𝑃₀ = Full output power of the generation unit

According to (Diouf, 2013), the drop of a unit is normally 4% to 5%. Smaller droops are more sensitive to frequency changes and the more it impacts the return of the balance between the electric power generated and the power consumed. Actually, the value of the droop determines the frequency reaction of individual unit. In a situation of power and frequency imbalance in a system, the frequency response 𝐾_{𝑢𝑛𝑖𝑡} of individual generators contributes to the restoration of system frequency. Actually, the level of the frequency response 𝐾_{𝑢𝑛𝑖𝑡}, permits the calculation of the level of frequency that will ensure stability. However, the frequency response K and D produced by the frequency variation (i.e.

the influence of each generator offering primary control and value of load frequency sensitivity) is calculated using: The frequency deviation is inversely proportional to the frequency response. An increase in the frequency response of the system “K + D” will result to a smaller frequency deviation. The above point is what differentiates a small standalone system and a large interconnected system. Primarily, the level of the load is comparatively greater in large interconnected systems hence, the load frequency sensitivity D is greater. As a result of the magnitude of these individual systems, the number of generators that normally contribute in primary control is more

than in insulated systems (Rebours et al., 2007). Thus, the value of the frequency response K tends to be higher as a result. In conclusion, the operations, capacities and abilities of units that offer secondary control are functions of the following characteristics:

 The quantity of secondary reserve

 The delay (i.e. the time response of the unit)

 Limitations of the generation rate of the units that offer secondary control.