"Reducing spinning reserve" was written for the January 1998 issue of
International Power Generation.
To contribute an article, news story or product review contact:
Editor, Bill Evett or Assistant Editor, James Luckey
Tel: +44 (0) 1737 768611, Fax: +44 (0) 1737 855475

Reducing spinning reserve

Each year, European Power News organises the Young Engineer of the Year Award. Highly recommended this year was a paper by Patrick O'Kane, research assistant at Queen's University of Belfast, UK. We publish an abridged version of his paper.

The UK's 1983 Energy Act promoted pollution control and energy conservation. It encouraged the use of smaller, cleaner generators with high fuel efficiencies close to the local load. Many of these installations, when operated as part of a combined heat and power scheme, have efficiencies of 80 per cent and above. These independent generators are usually privately owned and have the capability to operate in parallel with the distribution network.

At present, the UK has more capacity available from independent generation than ever before and the private sector's ability to influence the system is becoming more significant.

A more intelligent control strategy needs to be devised, allowing remote units to act in a predictable and co-ordinated fashion, making the contribution from embedded plant 'utility friendly'.

Independent generators have transformed a traditionally passive distribution network into an active system, bringing a range of benefits that include postponement of network reinforcement, reduced transmission losses and voltage support. They also introduce a number of problems. One of the more prevalent, is loss of mains detection. It is widely acknowledged that providing protection against the loss of mains condition for private generators is one of the most challenging aspects of electrical system design. The main problem is 'nuisance tripping'.

Nuisance trips generally occur through the mal-operation of 'loss of mains' protection relays, which are normally the 'rate of change of frequency' (ROCOF) type. They protect against 'islanding', a condition that occurs when an embedded generator becomes disconnected from the grid and operates independently from it. When islanded, the independent generator may supply power to a segment of network load. Islanding usually occurs through the operation of utility circuit-breakers, normally the 'auto-reclose' type. Since 70 to 80 per cent of faults on the network are non-persistent these circuit-breakers, connected at distribution voltage levels, open when a fault is detected and reclose after a few seconds. The reclosure can result in an out-of-phase reconnection with the system that can damage the generator's equipment.

Above: Independent power producer operating while islanded.

Loss of mains relays are used to detect the condition and trips the interconnecting circuit-breaker, separating the independent generator from the utility. This action allows a controlled reconnection when network conditions permit.

The ROCOF relay is currently the preferred option but its detection method is fundamentally flawed. Choosing a calibration setting for the relay requires a compromise between the ability to detect the condition, and the possibility of an erroneous trip which adversely affects the relay's performance. The result is that independent generators will be nuisance tripped from the grid during frequency transients and can fail to operate if islanding does not produce the required frequency deviation.

From a utility standpoint, the control of frequency following a sudden loss of generation is a significant enough problem, without being exacerbated by loss of capacity from available embedded generators by the common operation of ROCOF relays. This will increasingly become a problem in isolated power systems, which generally have a high ratio of largest on-line unit to total generation, and consequently a significant frequency dip when any of the main system generators are tripped.

Loss of mains can be detected by sensing the increase in impedance that occurs at a private generator's site when it becomes disconnected from the low impedance utility network. A typical example of this is illustrated here.

Above: Voltage divider

The unit uses an oscillator and low power audio amplifier to superimpose a small high frequency (hf) signal onto the mains via a coupling capacitor. The coupling capacitor acts as Z1 of the voltage divider and the system impedance acts as Z2. By monitoring the effect that the system has on the hf signal, with the use a high-pass filter circuit, the relay can detect the step change in impedance that occurs when an independent generator's site becomes disconnected from the system.

The magnitude of the detected signal will change with variations of system impedance and when the increase is above a predetermined value, the unit trips.

The problem is more prevalent in isolated power systems since they require a higher percentage reserve margin than an interconnected system due to a high ratio between largest on-line unit and total generation. Load relief is a natural phenomenon that occurs during low frequency and voltage conditions and reduces the real and reactive power demand from a load. It is inexpensive and causes negligible inconvenience to the customer. By using independent generators to reduce local voltage levels during a contingency, load relief can be enhanced, thus facilitating operational cost savings and reducing the reserve requirement and/or load shedding.

Enhanced load relief can thus be regarded as a source of non-spinning reserve that can be used to contribute to the overall spinning reserve requirement of the system. The relationship between spinning reserve, load relief and load shedding is dictated by the equation.

SR + LR = Pi - LS
where
Pi = loss of i-th generator
SR= spinning reserve
LR= load relief
LS= load shedding

The concept of reducing network voltage to decrease demand is not new and was used in the USA by many utilities to reduce peak demand during critical loading periods. It is known as Conservation Voltage Reduction (CVR) and test programs demonstrated that a 5 per cent reduction in voltage resulted in a 4 per cent drop in load. These results indicate that a significant source of emergency reserve can be released from the system by using independent generators to control local network voltage levels and provide load relief. Permitted voltage deviation from nominal of typically minus 5 per cent is respected during the procedure.

The problem with CVR is that many heating type loads are designed to work at a rated voltage and in some cases a 5 per cent decrease in voltage can result in a 10 per cent reduction in heat output. In thermostat controlled equipment this results in a reduced demand but with the load switched on for longer periods of time, any short-term energy savings are eventually negated. By using power-factor control to reduce voltage levels during frequency transients a certain degree of load relief can be achieved.

The technique is more suitable for this application because the voltage will only need to be suppressed during the period of time that it takes the system to recover after a contingency, which is normally in the order of tens of seconds. Since this time period is small the control strategy should not have any adverse effects on heating type loads or on the tap-changing control of local distribution transformers.

ROCOF relays cannot detect loss of mains under all operating conditions and leave the embedded generator and utility exposed to the problems of 'nuisance tripping' and 'block tripping' during frequency transients. Measurement of system impedance provides a more reliable approach to identification of the condition.

The new method presented, provides fast identification of the condition and correctly determines loss of mains during operating conditions in which more established forms of protection fail to operate, and it remains stable during system disturbances.

In terms of spinning reserve, embedded generation has traditionally been considered 'negative reserve' due to the probability of erroneous trips from multiple sets of ROCOF relays during grid frequency transients. Since the proposed method remains stable under all frequency and fault conditions, it presents the opportunity of using embedded generation to contribute to spinning reserve. The proposed development is to use independent generators for enhancement of system load relief during low frequency situations by lowering local network voltage levels, until the system recovers.