In a general sense, emergency supply systems refer to full standby power supply systems which can completely take over the power supply to delimited units, such as supply and waste disposal plants, public authorities, IT infrastructure and companies, in the event of a power failure, even for longer periods of time. The necessity for emergency supply systems is often called into question. This is because virtually no longer-term power outages have ever occurred, to date at least, and providing such systems not only incurs investment costs, but also on-going expense for maintenance and operation. As a result, decision-makers not familiar with the issue often find it very difficult to reach objective decisions. Showing interrelationships, explaining the main system types and weighing up the pros and cons should provide impartial guidance for making decisions on this extensive subject matter in relation to water supply and waste water disposal.
You don't necessarily need to read Marc Elsberg's engaging "Blackout: tomorrow will be too late", a power outage disaster novel, to vividly imagine the effects of a longer-term power failure. The catastrophic scenarios in the report for the German Bundestag are rather sobering but no less informative. The report comes to the grave conclusion that a wide-area power outage would quickly turn into a national catastrophe. Although the consequences of a longer lasting power outage are highly complex with very few areas of life remaining unaffected, the vital importance of a stable, reliable power supply is grossly underestimated.
One reason is that power supply has functioned relatively reliably in past decades. The prevailing opinion is that our power grid has become more secure due to the many decentralised power generations systems such as photovoltaic arrays or wind turbines and that we are no longer dependent on power stations. If we examine the circumstances more closely, this conclusion is misguided. This is because all decentralised systems must disconnect from the grid for safety reasons in the event of a power outage – this may entail a mains cut-off, for example.
Since the grid has been relatively stable to date at least, many believe that the danger does not exist and there is thus no need for special precautions. Termed a vulnerability paradox, this behaviour poses a risk that the effects will be that much more dramatic if supply is, in fact, interrupted for a longer period of time. Basically, it can be said that power outages over longer periods of time may also affect our supply infrastructure for the mid-term. There are enough examples on a national and international level: natural disasters, wars, terrorist attacks, targeted sabotage, cyberattacks and similar incidents to name just a few. It would therefore also seem sensible to make suitable preparations and true provision in Germany with its comparatively high population density. There are already plenty of advisers who address the issue, but what is needed is action.
Water supply and waste water disposal
Critical infrastructures include both public water supply and public waste water disposal. Critical infrastructures refer to facilities and institutions which have key importance for the state. Their failure or impairment would result in serious, long-lasting supply shortages, significant disruption to public safety or other dramatic consequences.
As a general rule, a failure in the power supply is noticed immediately and the consequences soon become apparent. However, a failure in the water supply is not noticed straight away, unless you happen to be under the shower. This reason alone is sufficient for people to view the need for water supply as less important than power supply. If we look more closely, we can see how mistaken this conclusion is: Many only associate water supply with supplying drinking water to the general population, but it is much more than that. A power failure implies:
- Pump shutdown, i.e. tanks can no longer be filled
- Pressure boost systems and installation facilities stop
- Water supply breaks down
- Sewage transport grinds to a halt
- Pipelines block up
- Sewage treatment plants overspill
- There is no extinguishing water provision.
In many regions, the drinking water supply provides extinguishing water needed in the event of the fire, for example. A breakdown in the drinking water supply could lead to huge major fires. This is all the more likely if many people resort to lighting open fires in cold weather from necessity if the power supply should fail (also see vulnerability paradox). Waste water disposal also depends directly on the drinking water supply. Sewage conveyance grinds to a halt by the time pipes are empty, if not before. Sewage remains in gravity pipelines while pumping stations can no longer work either and clog up or overflow. Problematic situations can also be expected to arise quickly in medical centres, hospitals and care and retirement homes when water supplies fail. Some of these institutions have emergency supply systems to provide power, but the water supply is not safeguarded. At best it is buffered. Productive livestock operations are also reliant on a dependable water supply. They not only need drinking water to feed livestock, but also to maintain hygienic conditions for milking.
Without water supply, production will also come to a halt in many workshops and production plants in trade and industry. A lack of water also seriously affects the food industry and craft-based food suppliers such as bakers and butchers, which play an essential role in basic food supply.
Besides the aforementioned breakdown in the waste water disposal, there will also be a massive deterioration in hygiene standards. As toilets no longer work in homes, people will relieve themselves outside buildings. Body hygiene will also be adversely affected. It will only be a question of a few days or weeks before epidemics break out.
Basically, it can be said that a longer breakdown in water supply systems can cause massive supply and disposal problems. Under these circumstances, failing to provide an emergency supply system is extremely questionable, if not downright irresponsible.
UPS versus emergency supply system
What is known as uninterruptible power supply (UPS) has been widely adopted in control engineering. UPS systems are powered by rechargeable batteries. Their purpose is essentially to protect sensitive technical systems such as servers and control systems for a short period of time, sufficient to ensure that the protected equipment can be shut down properly or a standby power source can be started.
The best emergency supply systems ensure fast, automatic switch-on in seconds when a stationary system is used, thus ensuring local power supply is re-established to the facility. Emergency supply systems are generally fully automatic, diesel engine-powered generator systems. Supply systems can be portable or stationary. Portable systems need to be turned on manually at a suitable feed-in point with a transfer switch.
Designing an emergency supply system
When designing an emergency supply system, planners should first consider all factors and strike a balance between system vulnerability and system resilience. Local switching devices for pumps, pressure booster systems and valves should always be provided. All the tasks required for emergency operation also need to be established in this first step. These also form the basis for calculating the capacity for emergency operation. The system can thus take over power supply automatically on a regular basis. The engine and generator are scheduled to come into operation at specific times and fuel is renewed on a regular basis as a consequence.
A water supply system with elevated tanks or water towers, which can at least supply water on a temporary basis thanks to geodetic pressure, should be categorised differently to ground water tank with downstream pressure booster systems. Interconnected water supply networks also need to be checked individually. Such a grouping for mutual emergency supply also requires emergency power supply systems on both sides. The volume of a tank for drinking water is generally based on daily need, meaning that elevated tanks will be able to maintain supplies for hours. However, there is no guarantee of supply for several days even with elevated tanks. Today's water supply systems are often networked through an IT infrastructure. A general power failure will also bring such systems to a standstill . Restrictions on comfort and convenience can definitely also be taken into account here as should staff availabilities.
Another constantly debated issue is how much fuel should be kept in reserve. The author considers this debate as pretty irrelevant, particularly in the context of a major power outage, since fuel supply also needs to be assured. In all probability, fuel provision can no longer be guaranteed either in the event of a wide-area power outage. The issue will arise when the emergency supply system is discussed, if not before. One argument very often used against reserve supply is that fuel ages.
The emergency system simply needs to be integrated into normal operations – it must be capable of operating parallel to the grid. In the case of well systems in water protection areas, the emergency supply system does not need to be installed in or next to the well building. An external transformer station or power transmission station can also be used to house such systems. This may also be necessary to provide protection against high water or flooding, a required measure for all water supply systems, such as wells, tanks, water treatment installations and pressure booster systems. Switchgear assemblies should be installed at the highest point in the building in flood-proof designs to be absolutely sure that they are protected.
Special features of an emergency supply system
Unlike in the integrated public grid, significantly greater voltage and frequency fluctuations may arise when loads change during generator operation. A sudden load increase can cause a voltage and frequency drop at the consumer end while load shedding can intensify the frequency or voltages. Frequent switching of large generators should thus be eliminated at the planning stage by using power-up and -down increments. Conventional protection devices such as fuses or circuit breakers only offer limited protection against short circuits or need to be adapted.
Frequency converters are increasingly used to operate pumps these days. Idle currents also have an impact on generators and must be taken into account when designing the generator. Alternatively, passive filters must be used to reduce such currents. Systems can also be operated without a frequency converter if necessary for emergency operation. This should also be included in the system design since other engine types need to be provided in such cases. Plant operation with an emergency supply system should thus be included at an early stage of planning.
As a basic rule, water protection area regulations and DIN VDE standards must be observed when generators are used. The generator supplier must be informed of its intended use.
Portable or stationary
Main pumping stations, pressure booster systems, all crucial systems and high power requirements must be equipped with a stationary generator which automatically switches on in the event of a power failure. This is the only way to guarantee immediate operation at all times.
Stationary units also have advantage that they can be safely operated fully automatically or unmanned in secured, enclosed spaces. This advantage takes on an importance which should not be underestimated in the event of a catastrophe since normal operating staff are exposed to a great deal of stress and physical and psychological strain in such situations.
Emergency power generators with single-axle, tandem-axle or 2-axle chassis can be used for portable use. Such generators can be used at different points. However, they need to be monitored a great deal and are only permitted where their operation is required for hours rather than days. The amount of work required to look after, refill and run these generators should not be undervalued. 24-hour monitoring is required, possibly with armed staff. The limitations of such solutions are revealed when operation needs to be maintained for several days or weeks and at different locations at the latest; this is without taking unsecured fuel supply into account.
Power take-off generators are somewhat less conventional portable solutions which may only be used as an extreme emergency solution for small supply needs. Their fuel supply is limited to the contents in the tractor's tank. A power take-off generator must not be used without someone constantly monitoring its operation. It should be noted that a tractor cannot maintain full engine output on a sustained basis as this can cause serious damage to its gears. The following design criteria can be used for a power take-off generator:
P erf x 2 = P min Generator x 2 = P min tractor
A generator output of 40 kW and a tractor output of at least 80 kW are required for a power consumption of 20 kW. It is recommended to fit an electronic speed control to the engine; this is essential when a frequency converter is used. Idle currents must be taken into account when dimensioning the generator. A longer period of trial operation is essential for power take-off generators. A power take-off generator cannot be regarded as a fully-fledged emergency supply system.