EPRI (Electric power research institute) and other participating power domain companies had conducted a survey to improve the design, performance, and maintenance of substation emergency power systems. EPRI has reported results of surveys [10], and a study on utility practices for substation batteries [11], and measurements of loads at actual substations compared to loads projected during design [12]. The analysis was given in regards to replacement or augmentation of lead-acid batteries (LABs) with new technologies when beneficial from a performance or cost view point.
The results focused in the survey conducted by the institutions on the utility practices adopted for the sizing, installation, and maintenance of the substation’s emergency energy stand-by system. This survey found that the dominant technology utilized for these systems is the vented lead-acid battery. Valve-regulated lead-acid (VRLA) batteries are still used at significant number of sites installed, but are generally being replaced with vented lead-acid systems due to continuously degrading life performance. Nickel-cadmium batteries are used in few of locations mentioned in survey.
Most of the users seem to be contented with the vented lead-acid batteries (VLABs) which meet the 15-20 years of life cycle and the discharging requirements during the occasional AC power outage. However there observed were some issues regarding the performance of the batteries and the efforts taken for the proper and regular maintenance. The unfavorable feedback for the storage technologies received from the customers were more related to the preventive measures taken for their maintenance rather that for the technology itself making it hard quantify time and maintenance resources. Further this, the batteries lasted only for 5-6 years due to the continuous discharging and charging action in the Utility stations due to the fluctuating load requirements. One major concern was also about how to monitor the State of Health and performance of the batteries. When vented lead-acid batteries could be monitored using electrolyte level and specific gravity measurement, the health of VRLA batteries could be accessed only by undergoing a full capacity test. This annoying experience has made variety of users reluctant to use newer technologies to replace traditional lead-acid batteries in this application.
The research report in [12] includes the monitoring of substation equipment to analyze the varying load profile in the Utility stations. Substations of different periods were embedded in the study, so that equipment change trend could be studied in a more sophisticated way. It was found that air and spring-controlled systems were used for switches and circuit breakers in older substations, they have been substituted by Motor (AC or DC) driven equipment in newer substations, for the reason that the later systems are much easier to install and maintain. The result is that the loads on modern substation power supplies are somewhat larger than on systems built 5-6 decades ago.
It is thus mandatory to consider the nature of the ever-changing load profile. The dc/ac load profile in a substation is classified into three types by IEEE Standard 485 [24], the sizing document for substation batteries.
‘ Continuous loads: Those loads remain energized all along throughout the battery duty cycle, such as relays, continuously operating motors, inverters, emergency lighting, energized coils, and control and communications systems.
‘ Non-continuous loads: These loads are energized only during a segment of the battery cycle. These loads may come online at any time within the duty cycle, and may be ON for a due period and then become OFF automatically or by operator’s action, or may continue till the end of the battery duty cycle. These loads include pump motors, ventilation motors, fire protection actuators, motor driven valves, and lighting etc.
‘ Momentary Loads: These are loads which do come alive most of the times during the battery duty cycle, but last only for a short duration say 1 minute. Although most momentary loads last significantly for less than a minute, or even a second, it is customary to assume that the momentary loads last one minute because the initial voltage drop often determines the battery’s 1-minute rating. Momentary operations include switchgear and circuit breaker operation, motor-driven valves, isolating switches, field flashing of generators, motor starting currents, and inrush currents of transformers, switchover to redundant systems and so on.
The majority of the load increment has been observed in the substations has occurred significantly only in the Momentary loads. This suggests that the batteries or the power supplies have to be robust and overrated from an Ampere-Hour viewpoint so as to ensure that the batteries could maintain a healthy voltage level about the requisites of the fed system, which is determined by the low voltage electronics. Thus it is visible that there is an appreciable requirement of the power sources which can handle the large and instant current requirements for a brief period of time to serve the majority of momentary loads.
Inferring from the above discussion, the next step in our further research is to identify technologies which comply with the requirements for an energy stand-by system and evaluate them from technical and market perspectives to determine which has the most potential in this regard. In particular, we are looking for a technology which gives a system, which is easier to maintain and appreciate in terms of reliability, availability and robustness.
Nowadays the non-battery energy sources, such as Ultracapacitors, flywheels and fuel cells, also show some great potential in this area but need to be supplemented. Hence the hybrid systems of Ultracapacitors may make sense even with existing lead-acid batteries. A substation battery in general is sized according to a power profile, to ensure that it can support the current required by all loads that are likely to occur simultaneously, even if those loads last only for short time span. This type of oversizing may be sometimes not appreciable. An Ultracapacitor on the other hand, can more easily support high currents for a short period of time and can perform efficiently as per the load variation. Also they can withstand millions of charge-discharge cycles proving their resistance to fatigue. For this reason, it is possible to build a Battery-Ultracapacitor hybrid system significantly smaller than a lead-acid battery sized for the equivalent loads, capable enough to support a redundant system for power supply in a Utility substation.