Fundamentals of the secure electric supply


At the beginning we had direct current (d.c.), the use of which would have required a power station at each street corner. With the use of the alternating current (a.c.) the problem of long buses and high of voltages was solved. One had to face the problems of accessibility and not foreseeable failures. 

But a number of other factors limited the use of electric current.

Only a private power station could put things right, which the user could attend and service. Over decades the good old Diesel served us well for all types of emergency supply.

However, mains failure and mains return usually accompanied network interruptions.


Modern electronics can do without such dinosaurs of electro-technology and take shiftings and short-time interruptions (SI) amiss. 

As a consequence, uninterrupted power supplies (USV or UPS) were developed in order to supply energy for the consumers during a short malfunction. 

The most substantial media were batteries and kinetic energy, which are used up to highest power stages.

In the data processing technology the philosophy of security was raised to a religion and led to a great number of solutions, which were limited only by financial access. 


UPS types:


-- rotary power plant:                     short time storage:  kinetic energy

                                                      long term storage:  Diesel (modified)


-- static power plant:                      short time storage:  Battery

                                                      Long term storage:  Diesel (modified)

                                                      Types: Double converter, delta converter

                                                                  Off- and online systems


Main versions:


a)    net-parallel redundance (secured A-supply)                                                                                                                       

                                                                                                                                                                   Parallel to the network a system is constantly held ready, which adjusts a malfunction immediately. A bypass becomes active with error or overload.  (image 1)






b)   by parallel connecting of several units and an oversizing (n+1) further security is achieved. One UPS-unit can always break down.  (image 2)

       A static switch (SS) takes over the switching co-ordination.








c)    part-parallel redundance

       (secured A-supply, secured or unsecured B-supply)

A second supply way is set up to the load, that alternatively will be secured as an UPS branch. 

At the load now a fast alternation switch can choose between two supplies.

Interpretation: Load = Ĺ UPS1 + Ĺ UPS2 

       Each load with a static switch is secured.  (image 3)



d)   separated redundance

With this system a redundance is designated for N-plants, which replaces a plant at a time. Basis for the interpretation is a view of the malfunction-probability and a view of economy. A redundant system for max. six main facilitieses makes good technic and economic sense. The continuing off-voltage stand-by mode of the redundant system is a disadvantage. Each load with a static switch is secured.  (image 4)



e)    integrated redundance

With this system the redundance is integrated into each plant and must not be used. The height of the not usable load depends on the number of parallel systems and is not changeable any longer.


       That means: 

       2 facilities:         50% not usable reserve supply,

       3 facilities:         33% not usable reserve supply,

       4 facilities:         25% not usable reserve supply, 

       5 facilities:         20% not usable reserve supply etc.  (image 5)




f)     UPS- and STS systems

A system of parallel UPS produces two UPS networks, the critical load is coupled over STS. In a failure scenario the balance of loads is to be considered.  (image 6)








g)   self-contained redundance

With this system the bypass is supported additionally by an UPS. Thus also load priorities are possible. (image 7)








The meaning of A and B systems

We talk about A and B systems, if two independent networks are available. For use a STS ( fast static transfer switch) is absolutely necessary. Up to the STS a double installation is to be planned. Only in special cases there can be an A and B system downstream of central STS. Here load variations are to be considered. Without a registration of the load flux all systems are not operable in borderline cases.


The meaning of a fast static transfer switch (STS) and a static switch (SS) :


Circuit speeds of circuit-breakers are usually too long and cause short-time interruption (SI), in particular if one switches over to another network. 

For that fast static switches (STS) were developed, which make the shifting between networks. 

Both networks (A and B) should be almost in phase, in order to avoid current peaks during the shifting. 

Thus STS is absolutely necessary with the supply with 2 networks.

Basically the A- and B-network must be interpreted for the full load of the running current,                    thus:  LOAD = A = B.

Static switches (SS) are components of an UPS and take over the function of automatic bypasses. A hand bypass is additionally necessary.



Version 1:     load-referred STS with A- and B-adapter (shown in image 3)

                     (separated full installation for A and B close to STS and load)



Version 2:    central STS for 100% A and 100% B and 100% load (shown in image 4 and 5)

                                                                                                                                                     For A + B at the STS a hand bypass must be available.

Load adapters with A and B are possible, but not necessary. Here the load balance is to be considered.



Version 3:    double central STS for 50% A and 50% B and 50% load, a load-referred (smaller) STS is absolutely necessary, that enabled the complete shifting from A to B or in reverse.



                     (system:  2A+2B  shown in image 8)

                     Without a load-referred (small) STS the system is useless and involves a certain risk.



The combination of those versions is a question of philosophy and depends on the amount of the investment. An expansion of level is always possible (except version e), but not an exchange of versions. 

Donít mix different versions of redundant systems.

The general non-critical load is to be considered separately.


The planner/advisor is responsible for the operation of the facilities from mains supply to load considering the following items:


-- traffic routes, weights

-- stages of development

-- availability of space according to the importance of the facilities

-- conditions given through the construction certificate

-- network configurations

-- locks

-- compensation

-- harmonic wave measurement and controlling

-- synchronization

-- fault signals

-- emergency shut-downs

-- load management/load lines

-- letting conditions

-- maintenances

-- load test in the network

-- overvoltage protection/lightning protection

-- short circuit and selectivity

-- emission of noise and waste gas

-- refueling systems

-- facility cooling

-- escape and emergency routes

-- security management


Copyright  2002