High availability through 3-in-1-Monitoring
In years gone by a brief voltage dip would have caused the lights to flicker slightly - nowadays it can bring entire operations to a standstill. Accurate monitoring is therefore mandatory. Interruptions can also be home-made - in the true sense of the word. In best case scenarios, defects can be detected and eliminated literally as they arise. The user is not required to work with a vast range of instruments in order to monitor the entire infrastructure. A single modern monitoring system can take on this task conveniently and reliably.
Highly automated production systems, computer centres and systems with constant processes (e.g. food sector, cable fabrication, paper production) require a reliable power supply - often even high availability, i.e. an availability of at least 99.9%. The numerous servers, monitors, storage media and network components rarely tolerate voltage dips or other deviations in power quality from the standard (e.g. EN 50160). However, electrical energy does not only need to be reliably available for information and communication technology; this is also the case with infrastructure tasks such as air-conditioning, fire prevention, EMC, safety engineering, lighting, lifts and drives.
3-in-1 monitoring for safety and efficiency
It is no wonder, with all of these applications, that the demand for a safe power supply comes even before the ubiquitous energy efficiency. Constant monitoring with corresponding integrated measuring equipment for energy management, power quality and residual current monitoring fulfils this requirement; indeed it serves both purposes. At the same time, residual current monitoring also improves preventative fire protection. However, in practice it is highly complex to acquire, evaluate and document all of the measurement data. All of this must take place extremely quickly, e.g. if one wishes to detect an insulation fault that has just arisen before a system failure occurs.
Janitza - the specialist when it comes to digital measuring technology and monitoring systems in energy supply - has specially developed its new UMG 512, UMG 96RM-E and UMG 20CM ranges here, for monitoring over 3 levels (see section „Monitoring solutions in practice“). Together with the GridVis® software and the integrated alarm management, solutions for three areas are united within a common system environment and just one measuring device per measurement point:
- Energy management according to ISO 50001 (acquisition of V, A, Hz, kWh, kW, kVArh, kvar …)
- Power quality monitoring (harmonics, flicker, voltage dips, transients, etc.)
- Residual current monitoring (in short RCM)
This consolidation of the three different functions within a single measuring device brings with it the major advantage that both the assembly and installation, as well as the remaining infrastructure (current transformer, communication lines and equipment, database, software, analysis tools and reporting software, etc.) are only required once. Furthermore, all data is logged centrally in a database and can be conveniently processed with a single software. This not only saves direct costs during purchasing but also simplifies integration: No interfaces are required between the various systems - because there is just one system. This also reduces the scope of training measures and induction required, which in turn increases the acceptance amongst the electrical engineers responsible.
Signal before failure
A significant advantage of this integrated data acquisition is its speed and the comprehensive overview of all data. This facilitates the detection of faults, which would only be partially perceived - or even entirely missed - by a single system. The user is therefore able to react before fuses or residual current devices (RCD) switch off affected systems or socket power circuits. This applies in particular to quietly rising residual currents (e.g. triggered by an insulation fault), overly high operating currents and any other overloading of system parts and loads (image 1).
Other sources of faults are massive grid feedback effects or resonance effects due to a growing number of non-linear electrical loads. If one detects irregular grid parameters such as excessively high harmonics or residual currents in a timely manner, it is still possible to commission repair measures before a device fails and in doing so avoid downtimes, or at least plan for these and reduce them.
Universal tool RCM: Increased safety, increased system availability, reduced risk of fire
As previously mentioned, RCM is playing an increasingly important role with high availability power supplies, which are now found in almost all market segments. Constant processes and especially sensitive applications such as computer centres, hospitals and semiconductor factories are depending on RCM in particular. Furthermore, RCM measurement offers a good alternative in all areas in which it is not possible to utilise insulation resistance measurements and residual current devices due to local or operational circumstances. The „foresighted“ monitoring described also helps to reduce alarms, as required for example with alarm management according to EEMUA 191 or NAMUR NA 102.
However, RCM can do even more - namely reduce the risk of fire! Residual current, triggered by defective insulation, can be treacherous. The current level is determined by the power of the supply network, the insulation fault resistance and the resistance to ground. With a sufficiently high current flow (with a dead earth short or corresponding low-resistance short) the upstream protective device disconnects the electrical consumers from the mains. However, if the residual current is too low then the protective device will not trigger. If the recorded fault power exceeds a value of approx. 60 Watt (approx. 261 mA at 230 V), a risk of fire exists. Residual current monitoring therefore also serves as fire prevention. The next section explains how RCM works in detail.
RCM – the functionality
The basic functionality of the residual current principle is shown in image 2. Here, the phase and neutral conductor of the protected output are fed through the summation current transformer, the ground wire is left out. The image provides a better overview due to the highly simplified wiring. In practical terms, all three phases and the neutral conductor run through the summation current transformer. If the system is in fault-free condition, the summation current is zero or close to zero (within a tolerable range), meaning that the current induced in the secondary circuit is also zero or close to zero. If, however, residual current flows away to ground due to a fault, the current differential in the secondary circuit will result in a current being logged and evaluated by the RCM measuring device (image 3).
Modern RCM devices accept different threshold value settings here (image 4). A static threshold value has the disadvantage that it is either too high with a part load, or too low with a full load, i.e. either insufficient protection is provided or erroneous alarms are issued, which may have negative effects on the attentiveness of the monitoring personnel over time. For this reason it is advisable to use RCM measuring devices with dynamic threshold value formation. In this case the residual current threshold value is formed on the basis of the actual load conditions and is therefore optimally aligned with the respective applicable load (image 5).
Through parameterisation (i.e. stipulation of the typical residual current in „GOOD“ condition) of the system in new condition and constant monitoring, all changes to the system state after the point of start-up can be detected. This also enables detection of creeping residual currents
New technology, new fault sources
Examples of „modern fault sources“ include collapsing polypropylene PFC capa-citors. These serve to com-pensate for reactive currents, which can be generated for example with three-phase motors. Paradoxically, a fault therefore arises due to equipment that is actually intended to improve the energy supply. With these capacitors, an overload or excessively high tempera-ture frequently results in a melting of the PP winding. The melt in turn causes a high-resistance short circuit to ground. It is not possible to shut off such short circuits to ground with conventional protection measures (HRC fuse, circuit breaker). The constant residual current usually leads in the mid-term to a dead earth short circuit and may pose a considerable risk of fire or endanger safety under certain circumstances (image 6). The residual current measurement detects such faults and enables rapid countermeasures. In this way it is possible to avoid costly and dangerous system failures.
Errors such as impermissible connections between the N and PE phase also frequently arise during installation. The two are sometimes simply interchanged. Image 7 shows a typical connection error, which can easily result in a residual current of 5000 mA. With RCM, such errors are detected immediately during the installation phase and are reported via the alarm management.
A further and rather more recent fault source is a large number of single-phase loads, such as switched mode power supplies from servers in computer centres or PCs in office buildings. These generate a high proportion of 3rd harmonics. These harmonic portions bring with them the significant disadvantage that they superimpose themselves on the neutral conductor rather than being nullified via the transformer windings. This can result in overloads on the N phase. Integrated measuring devices, such as the UMG 96RM-E, enable comprehensive monitoring of all phases and are therefore able to report increased neutral conductor currents in a timely manner.
In this context, reference is also made to the safety specifications of the VdS (association of insurers in Germany) for electrical systems up to 1000 Volt:
„VdS 2046 : 2010-06 (11)
3.2.4 In order to increase the safety of electrical systems in which numerous non-linear loads (such as frequency converters, phase angle-controls e.g. in lighting systems) are operated, measurement of the current in the neutral conductor should take place regularly - e.g. once annually and additionally after any significant changes to the electrical system or the type and quantity of electrical loads. If the safety of the system is at risk due to excessively high harmonic currents, measures must be implemented in order to protect the harmonics according to the publication „Low-fault electrical installations“ (VdS 2349).“
Challenge of high availability
IT technology itself places high demands on the supply. However, particularly critical are applications in which the loss of data simply cannot be allowed to occur. BITKOM therefore writes the following in its guidelines for „Operationally reliable computer centres“: „In computer centres the maximum availability requirements apply. The energy supply must therefore be permanently guaranteed. Therefore comprehensible is the requirement that the power supply to the computer centre itself, and to all areas in the same building to which data cables run, must be designed as a TN-S system. Essential for assured operation is permanent self-monitoring of a “clean“ TN-S system and the issuance of signals to a permanently manned desk, e.g. in the control centre. The electrical engineer will then detect any action requirements on the basis of signals received, and can avoid damages through targeted service measures.“
With the Janitza solution, the safety criteria „RCM residual current monitoring“ can be realised through this type of EMC-optimised TN-S system (image 8).
Reduced testing costs with RCM
Recurrent testing, as prescribed for example in BGV A3 – Electrical systems and operating equipment, is time-intensive and therefore costly. RCM monitoring systems can reduce these test costs, whilst also ensuring increased safety. Fixed electrical systems and operating equipment are considered to be monitored constantly if they are permanently maintained by electrical engineers and tested by measuring equipment within the framework of operations (e.g. monitoring of the insulation resistance). Through permanent RCM measurement, monitoring systems are able to deliver the required degree of constant testing.
Particularly noteworthy here is that RCM renders the cost-intensive measurement of insulation resistances at least partially superfluous, whilst constant testing of the insulation characteristics takes place. In order to carry out conventional insulation measurements, fixed systems or loads must be switched off and the neutral conductor disconnected. Furthermore, there is a risk that the high test voltage used for the insulation measurement may damage sensitive electronic components. The test accuracy and scope can be reduced by constant monitoring. However, this must be determined on an application-specific basis. Discussions with the operator and if necessary also with experts and / or the employers‘ liability insurance association are essential here!
It is also explicitly noted at this point that the following work must be carried out despite constant RCM measurement:
- Visual inspection for externally visible defects
- Protective measures and switch-off conditions
- Loop resistances and testing of the continuity of ground wires
- Functional testing
The association of insurers (Germany) requires RCM
The VdS has said the following on the subject of harmonics / the installation of power supply systems:
„In the case of power supply systems with PEN phase, operational currents - which may cause damage - flow through the entire ground and potential equalisation system (see section 3.3). With new electrical system installations it is therefore necessary to plan TN systems as TN-S systems. In the case of existing TN-C systems, modification to a TS-S system is advised. TN-S systems must be realised from the supply (handover) point where possible.
In order to guarantee the functionality of a TN-S system on a permanent basis (no conductor short between the N and PE phase, interchanging of the N and PE phase) this must be monitored by a residual current measurement device (RCM).
If the set trigger value is reached, a perceivable optical and acoustic error signal must be issued, in order that the defect can be eliminated immediately. In order that signal issuance is successful, this should be sent to a manned desk where applicable. If signalling is dispensed with then the forced shut-down of the faulty current circuit is required...“
Elsewhere, with respect to the safety regulations for electrical systems up to 1000 Volt, the VdS prescribes:
„VdS 2046 : 2010-06 (11)
3.2 Compliance with proper condition
3.2.3 In order to guarantee safety in electrical systems on a permanent basis, if it is not possible to carry out insulation resistance measurements due to local or operational circumstances then it is necessary to implement substitute measures. Such measures are described in the publication „Protection with insulation faults“ (VdS 2349).
An adequate substitute measure here is permanent RCM monitoring!
Energy measurement and electrical standard parameters
RCM plays a dominant role in system monitoring by the Janitza system. Despite this, the following additional points should not go unmentioned: In addition to a safe energy supply, energy efficiency is playing an increasingly significant role. A milestone was set in place here with the implementation of the ISO 50001 standard. ISO 50001 is the standardised basis for the introduction of an energy management system - whereby the focus here lies on the term management system. This is a methodology, applied in conjunction with other management systems such as ISO 9001 or ISO 14001, through which to set objectives, implement these systematically and in doing so eliminate the chance factor insofar as possible. The term „objective“ should essentially be understood here in the sense of „the route is the objective“. As an example, the following is a quote from the resolution of the IT representatives council from February 2013:
(Page 2, Resolution No. 2013/2, Point 2)
„The IT council shall continue to strive towards a high proportion of constant measurements by the end of 2013 and asks the division to continue promoting the use of permanent measuring devices with consideration to the principle of cost efficiency.“ With all of its UMG measuring devices and electricity meters, Janitza offers the possibility of capturing and recording standard electrical parameters, as well as power and energy consumptions (image 9).
Monitoring the power quality
RCM, as well as the requirements of Bitkom and the association of insurers, were dealt with in the first two parts. The final point of 3-in-1 monitoring is the power quality. The reliable operation of modern plants and systems always demands a high degree of supply reliability and good power quality. However, in modern energy supply a wide range of single and three-phase, non-linear loads are used in industrial networks right through to office blocks. These include lighting equipment such as lighting controls for headlamps or low energy bulbs, numerous frequency converters for heating, air-conditioning and ventilation systems, frequency converters for automation technology or lifts, as well as the entire IT infrastructure with the typically used regulated switched mode power supplies.
Today, one also commonly finds inverters for photovoltaic systems (PV) and uninterruptible power supplies (UPS). All of these non-linear electrical loads cause grid feedback effects to a greater or lesser extent with a distortion of the original „clean“ sinusoidal form. This results in the current or voltage waveform being distorted in the same way (image 10 and image 11).
The load on the network infrastructure through the described electrical and electronic loads with grid feedback effects has increased significantly in recent years. Depending on the type of generation system and the operating equipment (mains feed with converter, generator), mains rigidity at the connection point and the relative size of the non-linear loads, varying grid feedback effects and influences arise. For safeguarded power supplies in computer centres, the power quality must reflect EN 61000-2-4 (Class 1).
With its broad palette of UMG measuring devices, Janitza offers the option of capturing and analysing the various parameters of power quality. Standardised power quality reports in the GridVis® software (e.g. for EN 50160, EN 61000-2-4 and ITIC: “CBEMA Curve“) facilitate report generation for conventional standards at the touch of a button.
Monitoring solutions in practice
The aim of 3-in-1 monitoring solutions - the integrated measurement of energy, power quality and RCM - requires the measurement of all phases (L1, L2, L3, N) + CEP (central earth point) + RCM with a single measuring device.
A high performance measuring device with 6 measuring current inputs for the 3-in-1 measurement is the UMG 96RM-E for intermediate distributors, or the UMG 512 for main nodes and CEP from Janitza. The IP-based measuring devices can be easily integrated into existing communication networks via Ethernet. Numerous IP protocols, on-board homepage and SNMP protocol simplify the work of administrators.
The 20-channel UMG 20CM is ideal for complex electrical installations with a large number of monitoring points. The measuring devices are able to acquire (in arbitrary combinations), constantly log and analyse residual, earth leakage and operating currents via the associated measuring current transformers (e.g. CT-6-20).
Special residual current transformers in practical special designs are also suitable for cost-efficient retrofitting to existing systems, without the need to switch off electrical consumers.
Alarm in the right place
Alarms must never sound unheard. An acoustic signal from the switch cabinet in the main distribution is of little use in the control room.
Through the integration of the RCM measuring devices in the GridVis® software, with its comprehensive alarm management signalling options, it is possible to ensure that the signal quickly reaches the right recipient. With arbitrary escalation levels and logbook function, the monitoring control room has access to all the tools required for efficient monitoring. In this way it is possible for the responsible electrical engineer to detect and evaluate any residual current increases, and if necessary initiate remedial measures as quickly as possible.
Stray currents impair EMC
Connections between the N and PE phase result in „stray“ operating currents being distributed across the PE system, via data lines and all metal building parts. Because these currents are not equalised, they generate electromagnetic fields. Diverse currents in the electrical systems, IT networks and pipe systems of building installations are the consequence. Image 12 shows how the operating current can distribute at the PEN bridge and flow back via multiple paths, whereby the sum of the supply and return conductor current is no longer 0. This can bring the following faults with it:
- Change in the operating behaviour of frequency-dependent parts (e.g. capacitors draw increased current)
- Data transfer disturbances due to magnetic and inductive influences
- Transfer of lightening influences to the electrical system
- Corrosion of metal lines
- Adverse effects on personnel
The supply and return conductors, also in distribution systems, must be positioned close to each other in order to minimise magnetic fields. At every node point in a current circuit the sum of the currents must be equal to zero, in order to avoid residual currents. Additionally, the sub-distribution or current circuit should be monitored by an RCM. The UMG 96RM-E is very well suited for monitoring sub-distribution or larger loads. Individual current circuits, in which no residual current circuit breakers can be used for operational reasons, can be monitored with the UMG 20CM. A signalling RCM in combination with the specialist personnel on location provides for the maximum alternative safety.
Neutral conductor and CEP (Central earthing point)
The neutral conductor (operating current return conductor) has become the most important phase. It is to be treated as a phase conductor. In order that the earthing system remains „clean“, the current-loaded
N phase must be positioned far from the PE phase. No galvanic operating currents may be permitted to flow via the earthing system because these would cause inductive couplings. These measures must be implemented right to the supply source.
In the TN-S system, the N phase must only be connected at a suitable point with the earthing system once - at the so-called CEP (central earth point from N to PE) - and monitored. Undesirable insulation faults or galvanic connections between N and PE are detected immediately with monitoring of the CEP. Deviations are reported in a timely manner and analysed with temporal dependencies.
It is possible to check that the TN-S system is functioning fault-free, e.g. with the UMG 512. This allows a holistic appraisal of the power quality and EMC. It is even possible to record and analyse the trigger phase of an earth short fault. The phase current increases in parallel to the CEP current in this case. The current at the CEP must always be appraised depending on the overall power of the TN-S system. On the one hand this means that operation-dependent leakage currents are tolerated, whilst abnormal deviations at the CEP are reported by the RCM.
Summary and outlook
Increasingly high demands will continue to be placed on future power supplies, because power failures result in high costs and huge disruption! Constant RCM monitoring for high availability power supplies with high EMC demands and also for preventative fire protection is becoming increasingly established. In order to account for this trend, Janitza brought the new 20-channel UMG 20CM range to the market in 2013, and will be presenting two further products in 2014 - in the form of the UMG 509 and UMG 512. The aim here is RCM monitoring of the power supply across all four levels (supply [PCC], main distribution [transformer outputs], sub-distribution, individual loads [e.g. server cabinets]).
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Janitza® electronics GmbH
Janitza electronics GmbH is a German company and has been active for 50 years in the manufacturing of systems for efficient power application, energy measurement and cost savings. As a globally renowned manufacturer of network monitoring and energy management systems, digital integrated measurement devices, power factor controllers and compensation systems, the company stands for the highest quality standards and innovations.
Products are manufactured according to leading-edge expertise with state-of-the-art production technology. At Janitza, quality management is an ongoing managerial task (e.g. ISO 9001). Comprehensive know-how, competent consultancy and concept generation, right through to the commissioning of tailored solutions, ensure fulfilment of customer wishes and requirements.